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fia_/cffia-;h4zip7      ,,conversations 
,,on ,,natural ,,philosophy1  ,,in      
,,which ,,the ,,elements ,,of ,,that    
,,science ,,are ,,familiarly            
,,explained4   ,illustrated with        
,plates4   ,,by ,,the ,,author ,,of     
,,conversations ,,on ,,chemistry1 c4    
,,with ,,corrections1 ,,improvements1   
,,and ,,considerable ,,additions ,,in   
,,the ,,body ,,of ,,the ,,work2         
,appropriate ,questions1 and a          
,glossary3  ,,by ,,dr4 ,,thomas ,p4     
,,jones1 ,,professor ,,of ,,mechanics1  
,,in ,,the ,,franklin ,,institute ,,of  
,,the ,,state ,,of ,,pennsylvania4      
,philadelphia3 ,published and ,sold by  
,john ,grigg1 ,no4 #i ,north ,fourth    
,street4 ,stereotyped by ,l4 ,johnson4  

#ahbf4  ,eastern ,district of           
,pennsylvania1 to wit3  ,be it          
remembered1 that1 on the twenty-fourth  
day of ,april1 in the ,fiftieth year of 
the ,independence of the ,united ,states
 of ,america1 ,a4 ,d4 #ahbf1 ,john      
,grigg1 of the said ,district1 hath     
deposited in this office the title of a 
book1 the right whereof he claims as    
proprietor1 in the words following1 to  
wit3       ,conversations on ,natural   
,philosophy1 in which the ,elements of  
that ,science are familiarly explained4 
,illustrated with ,plates4      ,by the 
,author of ,conversations on ,chemistry1
 c4 ,with      ,corrections1            
,improvements1 and considerable         
,additions1 in the      ,body of the    
,work2 appropriate ,questions1 and a    
,glossary3 ,by ,dr4      ,thomas ,p4    
,jones1 ,professor of ,mechanics1 in the
 ,franklin      ,institute1 of the      
,state of ,pennsylvania4  ,in conformity
 to the ,act of the ,congress of the    
,united ,states1 entitled ,an ,act for  

the ,encouragement of ,learning1 by     
securing the ,copies of ,maps1 ,charts1 
and ,books1 to the ,authors and         
,proprietors of such ,copies1 during the
 times therein mentioned2--,and also to 
the ,act1 entitled1 ,an ,act            
supplementary to an ,act1 entitled1 ',an
 ,act for the ,encouragement of         
,learning1 by securing the ,copies of   
,maps1 ,charts1 and ,books1 to the      
,authors and ,proprietors of such       
,copies during the times therein        
mentioned1' and extending the benefits  
thereof to the arts of designing1       
engraving1 and etching1 historical and  
other prints4                           
,d4 ,,caldwell1                         
,clerk of the ,eastern ,district of     
,pennsylvania4     ,,preface4           
,notwithstanding the great number of    
books which are written1 expressly for  
the use of schools1 and which embrace   
every subject on which instruction is   
given1 it is a lamentable fact1 that the
 catalogue of those which are well      

adapted to the intended purpose1 is a   
very short one4 ,almost all of them have
 been written1 either by those who are  
without experience as teachers1 or by   
teachers1 deficient in a competent      
knowledge of the subjects1 on which they
 treat4 ,every intelligent person1 who  
has devoted himself to the instruction  
of youth1 must have felt and deplored1  
the truth of these observations4  ,in   
most instances1 the improvement of a    
work already in use1 will be more       
acceptable1 than one of equal merit     
would be1 which is entirely new2 the    
introduction of a book into schools1    
being always attended with some         
difficulty4  ,the ,conversations on     
,chemistry1 written by ,mrs4 ,marcet1   
had obtained a well-merited celebrity1  
and was very extensively adopted as a   
school-book1 before the publication of  
her ,conversations on ,natural          
,philosophy4 ,this1 also1 has been much 
used for the same purpose2 but1 the     
observation has been very general1 among

 intelligent teachers1 that1 in its     
execution1 it is very inferior to the   
former work4  ,the editor of the edition
 now presented to the public1 had       
undertaken to add to the work1          
questions1 for the examination of       
learners2 and notes1 where he deemed    
them necessary4 ,he soon found1 however1
 that the latter undertaking would be a 
very unpleasant one1 as he must have    
pointed out at the bottom of many of the
 pages1 the defects and mistakes in the 
text2 whilst numerous modes of          
illustration1 or forms of expression1   
which his experience as a teacher1 had  
convinced him would not be clear to the 
learner1 must1 of necessity1 have       
remained unaltered4 ,he therefore       
determined to revise the whole work1 and
 with the most perfect freedom1 to make 
such alterations in the body of it1 as  
should1 in his opinion1 best adapt it to
 the purpose for which it was designed4 
,were the book1 as it now stands1       
carefully compared with the original1 it

 would be found1 that1 in conformity    
with this determination1 scarcely a page
 of the latter1 remains unchanged4      
,verbal alterations have been made1     
errors1 in points of fact1 have been    
corrected2 and new modes of illustration
 have been introduced1 whenever it was  
thought that those already employed1    
could be improved2 or when it was known1
 that1 from local causes1 they are not  
familiar1 in this country4  ,the editor 
feels assured1 that1 in performing this 
task1 he has rendered the book more     
valuable to the teacher1 and more useful
 to the pupil2 and he doubts not that   
the intelligent author of it1 would     
prefer the mode which has been adopted1 
to that which was at first proposed4    
,the judicious teacher will1 of course1 
vary the questions according to         
circumstances2 and those who may not    
employ them at all1 as questions1 will  
still find them useful1 in directing the
 pupil to the most important points1 in 
every page4  ,the ,glossary has been    

confined to such terms of science as    
occur in the work2 and is believed to   
include all those1 of which a clear     
definition cannot be found in our common
 dictionaries4     ,,contents4          
,,conversation ,i4       ,,on ,,general 
,,properties ,,of ,,bodies4             
#i       ,,introduction4 ,general       
,properties of ,bodies4                 
,impenetrability4      ,extension4      
,figure4 ,divisibility4 ,inertia4       
,attraction4      ,attraction of        
,cohesion4 ,density4 ,rarity4 ,heat4    
,attraction of      ,gravitation4       
,,conversation ,,ii4       ,,on ,,the   
,,attraction ,,of ,,gravity4            
#bb       ,attraction of ,gravitation1  
continued4 ,of ,weight4 ,of the ,fall of
      ,bodies4 ,of the ,resistance of   
the ,air4 ,of the ,ascent of ,light     
,bodies4   ,,conversation ,,iii4        
,,on ,,the ,,laws ,,of ,,motion4        
#cb       ,of ,motion4 ,of the ,inertia 
of ,bodies4 ,of ,force to produce       
,motion4 ,direction of ,motion4         

,velocity1 absolute and relative4       
,uniform ,motion4 ,retarded ,motion4    
,accelerated ,motion4 ,velocity      of 
,falling ,bodies4 ,momentum4 ,action and
 ,reaction equal4      ,elasticity of   
,bodies4 ,porosity of ,bodies4          
,reflected ,motion4      ,angles of     
,incidence and ,reflection4             
,,conversation ,,iv4       ,,on         
,,compound ,,motion4                    
#df       ,compound ,motion1 the result 
of two opposite forces4 ,of             
,curvilinear ,motion1 the result of two 
forces4 ,centre of ,motion1      the    
point at rest1 while the other parts of 
the body move round      it4 ,centre of 
,magnitude1 the middle of a body4       
,centripetal      ,force1 that which    
impels a body towards a fixed central   
point4      ,centrifugal ,force1 that   
which impels a body to fly from the     
centre4 ,fall of ,bodies in a ,parabola4
 ,centre of ,gravity1 the      point    
about which the parts balance each      
other4   ,,conversation ,v4       ,,on  

,,the ,,mechanical ,,powers4            
#ed       ,of the ,power of ,machines4  
,of the ,lever in general4 ,of the      
,lever      of the first kind1 having   
the ,fulcrum between the power and the  
weight4 ,of the ,lever of the second    
kind1 having the ,weight      between   
the power and the fulcrum4 ,of the      
,lever of the third      kind1 having   
the ,power between the fulcrum and the  
weight4 ,of      the ,pulley4 ,of the   
,wheel and ,axle4 ,of the ,inclined     
,plane4 ,of      the ,wedge4 ,of the    
,screw4   ,,conversation ,,vi4          
,,astronomy4       ,,causes ,,of ,,the  
,,motion ,,of ,,the ,,heavenly ,,bodies4
                  #gj       ,of the     
,earth's annual motion4 ,of the         
,planets1 and their motion4      ,of the
 ,diurnal motion of the ,earth and      
,planets4   ,,conversation ,,vii4       
,,on ,,the ,,planets4                   
#hj       ,of the ,satellites and       
,moons4 ,gravity diminishes as the      
,square      of the ,distance4 ,of the  

,solar ,system4 ,of ,comets4            
,constellations1 signs of the ,zodiac4  
,of ,copernicus1 ,newton1 c4            
,,conversation ,,viii4       ,,on ,,the 
,,earth4                                
#ia       ,of the ,terrestrial ,globe4  
,of the ,figure of the ,earth4 ,of the  
,pendulum4 ,of the ,variation of the    
,seasons1 and of the ,length of         
,days and ,nights4 ,of the ,causes of   
the ,heat of ,summer4 ,of ,solar1       
,siderial1 and ,equal or ,mean ,time4   
,,conversation ,,ix4       ,,on ,,the   
,,moon4                                 
#ajh       ,of the ,moon's ,motion4     
,phases of the ,moon4 ,eclipses of the  
,moon4      ,eclipses of ,jupiter's     
,moons4 ,of ,latitude and ,longitude4   
,of the      ,transits of the inferior  
,planets4 ,of the ,tides4               
,,conversation ,x4       ,,hydrostatics4
       ,,on ,,the ,,mechanical          
,,properties ,,of ,,fluids4             
#aah       ,definition of a ,fluid4     
,distinction between ,fluids and        

,liquids4      ,of ,non-,elastic        
,fluids1 scarcely susceptible of        
,compression4 ,of      the ,cohesion of 
,fluids4 ,of their ,gravitation4 ,of    
their      ,equilibrium4 ,of their      
,pressure4 ,of ,specific ,gravity4 ,of  
the      ,specific ,gravity of ,bodies  
heavier than ,water4 ,of those of the   
same weight as ,water4 ,of those lighter
 than ,water4 ,of the      ,specific    
,gravity of ,fluids4   ,,conversation   
,,xi4       ,,of ,,springs1 ,,fountains1
 c4                                     
#abh       ,of the ,ascent of ,vapour   
and the ,formation of ,clouds4 ,of the  
,formation and ,fall of ,rain1 c4 ,of   
the ,formation of ,springs4 ,of         
,rivers and ,lakes4 ,of ,fountains4     
,,conversation ,,xii4                   
,,pneumatics4       ,,on ,,the          
,,mechanical ,,properties ,,of ,,air4   
#acf       ,of the ,spring or           
,elasticity of the ,air4 ,of the ,weight
 of the      ,air4 ,experiments with the
 ,air ,pump4 ,of the ,barometer4 ,mode  

of      ,weighing ,air4 ,specific       
,gravity of ,air4 ,of ,pumps4           
,description of      the ,sucking ,pump4
 ,description of the ,forcing ,pump4    
,,conversation ,,xiii4       ,,on ,,wind
 ,,and ,,sound4                         
#adf       ,of ,wind in ,general4 ,of   
the ,trade ,wind4 ,of the ,periodical   
,trade      ,winds4 ,of the ,aerial     
,tides4 ,of ,sound in ,general4 ,of     
,sonorous      ,bodies4 ,of ,musical    
,sounds4 ,of ,concord or ,harmony1 and  
,melody4   ,,conversation ,,xiv4        
,,on ,,optics4                          
#aeg       ,of ,luminous1 ,transparent1 
and ,opaque ,bodies4 ,of the ,radiation 
of ,light4 ,of ,shadows4 ,of the        
,reflection of ,light4 ,opaque ,bodies  
seen only by ,reflected ,light4 ,vision 
,explained4 ,camera ,obscura4           
,image of ,objects on the ,retina4      
,,conversation ,,xv4                    
,,optics--continued4       ,,of ,,the   
,,angle ,,of ,,vision1 ,,and            
,,reflection ,,of ,,mirrors4            

#afh       ,angle of ,vision4           
,reflection of ,plain ,mirrors4         
,reflection of      ,convex ,mirrors4   
,reflection of ,concave ,mirrors4       
,,conversation ,,xvi4       ,,on        
,,refraction ,,and ,,colours4           
#agi       ,transmission of ,light by   
,transparent ,bodies4 ,refraction4      
,refraction by the ,atmosphere4         
,refraction by a ,lens4 ,refraction     
by the ,prism4 ,of ,colour from the     
,rays of ,light4 ,of the ,colours       
of ,bodies4   ,,conversation ,,xvii4    
,,on ,,the ,,structure ,,of ,,the ,,eye1
 ,,and ,,optical ,,instruments4         
#aie       ,description of the ,eye4 ,of
 the ,image on the ,retina4 ,refraction 
by the ,humours of the ,eye4 ,of the use
 of ,spectacles4 ,of the      ,single   
,microscope4 ,of the ,double            
,microscope4 ,of the ,solar             
,microscope4 ,magic ,lanthorn4          
,refracting ,telescope4 ,reflecting     
,telescope4   ,,glossary1               
#bje     ,,conversation ,i4  ,,on       

,,general ,,properties ,,of ,,bodies4   
,,introduction4 ,,general ,,properties  
,,of ,,bodies4 ,,impenetrability4       
,,extension4 ,,figure4 ,,divisibility4  
,,inertia4 ,,attraction4 ,,attraction   
,,of ,,cohesion4 ,,density4 ,,rarity4   
,,heat4 ,,attraction ,,of ,,gravitation4
   ,,emily4  ,i must request your       
assistance1 my ,dear ,mrs4 ,b41 in a    
charge which ,i have lately undertaken3 
it is that of instructing my youngest   
sister1 a task1 which ,i find proves    
more difficult than ,i had at first     
imagined4 ,i can teach her the common   
routine of children's lessons tolerably 
well2 but she is such an inquisitive    
little creature1 that she is not        
satisfied without an explanation of     
every difficulty that occurs to her1 and
 frequently asks me questions which ,i  
am at a loss to answer4 ,this morning1  
for instance1 when ,i had explained to  
her that the world was round like a     
ball1 instead of being flat as she had  
supposed1 and that it was surrounded by 

the air1 she asked me what supported it4
 ,i told her that it required no        
support2 she then inquired why it did   
not fall as every thing else did8 ,this 
,i confess perplexed me2 for ,i had     
myself been satisfied with learning that
 the world floated in the air1 without  
considering how unnatural it was that so
 heavy a body1 bearing the weight of all
 other things1 should be able to support
 itself4  ,mrs4 ,b4 ,i make no doubt1 my
 dear1 but that ,i shall be able to     
explain this difficulty to you2 but ,i  
believe that it would be almost         
impossible to render it intelligible to 
the comprehension of so young a child as
 your sister ,sophia4 ,you1 who are now 
in your thirteenth year1 may1 ,i think1 
with great propriety1 learn not only the
 cause of this particular fact1 but     
acquire a general knowledge of the laws 
by which the natural world is governed4 
,emily4 ,of all things1 it is what ,i   
should most like to learn2 but ,i was   
afraid it was too difficult a study even

 at my age4  ,mrs4 ,b4 ,not when        
familiarly explained3 if you have       
patience to attend1 ,i will most        
willingly give you all the information  
in my power4 ,you may perhaps find the  
subject rather dry at first2 but if ,i  
succeed in explaining the laws of       
nature1 so as to make you understand    
them1 ,i am sure that you will derive   
not only instruction1 but great         
amusement from that study4  ,emily4 ,i  
make no doubt of it1 ,mrs4 ,b42 and pray
 begin by explaining why the earth      
requires no support2 for that is the    
point which just now most strongly      
excites my curiosity4  ,mrs4 ,b4 ,my    
dear ,emily1 if ,i am to attempt to give
 you a general idea of the laws of      
nature1 which is no less than to        
introduce you to a knowledge of the     
science of natural philosophy1 it will  
be necessary for us to proceed with some
 degree of regularity4 ,i do not wish to
 confine you to the systematic order of 
a scientific treatise1 but if we were   

merely to examine every vague question  
that may chance to occur1 our progress  
would be but very slow4 ,let us1        
therefore1 begin by taking a short      
survey of the general properties of     
bodies1 some of which must necessarily  
be explained before ,i can attempt to   
make you understand why the earth       
requires no support4  ,when ,i speak of 
bodies1 ,i mean substances1 of whatever 
nature1 whether solid or fluid2 and     
matter is the general term used to      
denote the substance1 whatever its      
nature be1 of which the different bodies
 are composed4 ,thus1 the wood of which 
this table is made1 the water with which
 this glass is filled1 and the air which
 we continually breathe1 are each of    
them matter4  ,emily4 ,i am very glad   
you have explained the meaning of the   
word matter1 as it has corrected an     
erroneous conception ,i had formed of   
it3 ,i thought that it was applicable to
 solid bodies only4  ,mrs4 ,b4 ,there   
are certain properties which appear to  

be common to all bodies1 and are hence  
called the essential or inherent        
properties of bodies2 these are         
,impenetrability1 ,extension1 ,figure1  
,divisibility1 ,inertia and ,attraction4
 ,these are also called the general     
properties of bodies1 as we do not      
suppose any body to exist without them4 
,by impenetrability is meant the        
property which bodies have of occupying 
a certain space1 so that where one body 
is1 another can not be1 without         
displacing the former2 for two bodies   
can not exist in the same place at the  
same time4 ,a liquid may be more easily 
removed than a solid body2 yet it is not
 the less substantial1 since it is as   
impossible for a liquid and a solid to  
occupy the same space at the same time1 
as for two solid bodies to do so4 ,for  
instance1 if you put a spoon into a     
glass full of water1 the water will flow
 over to make room for the spoon4       
,emily4 ,i understand this perfectly4   
,liquids are in reality as substantial  

or as impenetrable as solid bodies1 and 
they appear less so1 only because they  
are more easily displaced4  ,mrs4 ,b4   
,the air is a fluid differing in its    
nature from liquids1 but no less        
impenetrable4 ,if ,i endeavour to fill  
this phial by plunging it into this     
bason of water1 the air1 you see1 rushes
 out of the phial in bubbles1 in order  
to make way for the water1 for the air  
and the water can not exist together in 
the same space1 any more than two hard  
bodies2 and if ,i reverse this goblet1  
and plunge it perpendicularly into the  
water1 so that the air will not be able 
to escape1 the water will no longer be  
able to fill the goblet4  ,emily4 ,but  
it rises some way into the glass4  ,mrs4
 ,b4 ,because the water compresses or   
squeezes the air into a smaller space in
 the upper part of the glass2 but1 as   
long as it remains there1 no other body 
can occupy the same place4  ,emily4 ,a  
difficulty has just occurred to me1 with
 regard to the impenetrability of solid 

bodies2 if a nail is driven into a piece
 of wood1 it penetrates it1 and both the
 wood and the nail occupy the same space
 that the wood alone did before8  ,mrs4 
,b4 ,the nail penetrates between the    
particles of the wood1 by forcing them  
to make way for it2 for you know that   
not a single atom of wood can remain in 
the space which the nail occupies2 and  
if the wood is not increased in size by 
the addition of the nail1 it is because 
wood is a porous substance1 like sponge1
 the particles of which may be          
compressed or squeezed closer together2 
and it is thus that they make way for   
the nail4  ,we may now proceed to the   
next general property of bodies1        
extension4 ,a body which occupies a     
certain space must necessarily have     
extension2 that is to say1 length1      
breadth and depth or thickness2 these   
are called the dimensions of extension3 
can you form an idea of any body without
 them8  ,emily4 ,no2 certainly ,i can   
not2 though these dimensions must1 of   

course vary extremely in different      
bodies4 ,the length1 breadth and depth  
of a box1 or of a thimble1 are very     
different from those of a walking stick1
 or of a hair4  ,but is not height also 
a dimension of extension8  ,mrs ,b4     
,height and depth are the same          
dimension1 considered in different      
points of view2 if you measure a body1  
or a space1 from the top to the bottom1 
you call it depth2 if from the bottom   
upwards1 you call it height2 thus the   
depth and height of a box are1 in fact1 
the same thing4  ,emily4 ,very true2 a  
moment's consideration would have       
enabled me to discover that2 and breadth
 and width are also the same dimension4 
,mrs4 ,b4 ,yes2 the limits of extension 
constitute figure or shape4 ,you        
conceive that a body having length1     
breadth and depth1 can not be without   
form1 either symmetrical or irregular8  
,emily4 ,undoubtedly2 and this property 
admits of almost an infinite variety4   
,mrs4 ,b4 ,nature has assigned regular  

forms to many of her productions4 ,the  
natural form of various mineral         
substances is that of crystals1 of which
 there is a great variety4 ,many of them
 are very beautiful1 and no less        
remarkable by their transparency or     
colour1 than by the perfect regularity  
of their forms1 as may be seen in the   
various museums and collections of      
natural history4 ,the vegetable and     
animal creation appears less            
symmetrical1 but is still more          
diversified in figure than the mineral  
kingdom4 ,manufactured substances assume
 the various arbitrary forms which the  
art of man designs for them2 and an     
infinite number of irregular forms are  
produced by fractures and by the        
dismemberment of the parts of bodies4   
,emily4 ,such as a piece of broken      
china1 or glass8  ,mrs4 ,b4 ,or the     
masses and fragments of stone1 and other
 mineral substances1 which are dug out  
of the earth1 or found upon its surface2
 many of which1 although composed of    

minute crystals1 are in the lump of an  
irregular form4  ,we may now proceed to 
divisibility2 that is to say1 a         
susceptibility of being divided into an 
indefinite number of parts4 ,take any   
small quantity of matter1 a grain of    
sand for instance1 and cut it into two  
parts2 these two parts might be again   
divided1 had we instruments sufficiently
 fine for the purpose2 and if by means  
of pounding1 grinding1 and other similar
 methods1 we carry this division to the 
greatest possible extent1 and reduce the
 body to its finest imaginable          
particles1 yet not one of the particles 
will be destroyed1 but will each contain
 as many halves and quarters1 as did the
 whole grain4  ,the dissolving of a     
solid body in a liquid1 affords a very  
striking example of the extreme         
divisibility of matter2 when you sweeten
 a cup of tea1 for instance1 with what  
minuteness the sugar must be divided to 
be diffused throughout the whole of the 
liquid4  ,emily4 ,and if you pour a few 

drops of red wine into a glass of water1
 they immediately tinge the whole of the
 water1 and must therefore be diffused  
throughout it4  ,mrs4 ,b4 ,exactly so2  
and the perfume of this lavender water  
will be almost as instantaneously       
diffused throughout the room1 if ,i take
 out the stopper4  ,emily4 ,but in this 
case it is only the perfume of the      
lavender1 and not the water itself that 
is diffused in the room4  ,mrs4 ,b4 ,the
 odour or smell of a body is part of the
 body itself1 and is produced by very   
minute particles or exhalations which   
escape from the odoriferous bodies4 ,it 
would be impossible that you should     
smell the lavender water1 if particles  
of it did not come in actual contact    
with your nose4  ,emily4 ,but when ,i   
smell a flower1 ,i see no vapour rise   
from it2 and yet ,i perceive the smell  
at a considerable distance4  ,mrs4 ,b4  
,you could1 ,i assure you1 no more smell
 a flower1 the odoriferous particles of 
which did not touch your nose1 than you 

could taste a fruit1 the flavoured      
particles of which did not come in      
contact with your tongue4  ,emily4 ,that
 is wonderful indeed2 the particles     
then1 which exhale from the flower and  
from the lavender water1 are1 ,i        
suppose1 too small to be visible8  ,mrs4
 ,b4 ,certainly3 you may form some idea 
of their extreme minuteness1 from the   
immense number which must have escaped  
in order to perfume the whole room2 and 
yet there is no sensible diminution of  
the liquid in the phial4  ,emily4 ,but  
the quantity must really be diminished8 
,mrs4 ,b4 ,undoubtedly2 and were you to 
leave the bottle open a sufficient      
length of time1 the whole of the water  
would evaporate and disappear4 ,but     
though so minutely subdivided as to be  
imperceptible to any of our senses1 each
 particle would continue to exist2 for  
it is not within the power of man to    
destroy a single particle of matter3 nor
 is there any reason to suppose that in 
nature an atom is ever annihilated4     

,emily4 ,yet1 when a body is burnt to   
ashes1 part of it1 at least1 appears to 
be effectually destroyed3 look how small
 is the residue of ashes in the fire    
place1 from all the fuel which has been 
consumed within it4  ,mrs4 ,b4 ,that    
part of the fuel1 which you suppose to  
be destroyed1 evaporates in the form of 
smoke1 and vapour1 and air1 whilst the  
remainder is reduced to ashes4 ,a body1 
in burning1 undergoes no doubt very     
remarkable changes2 it is generally     
subdivided2 its form and colour altered2
 its extension increased3 but the       
various parts1 into which it has been   
separated by combustion1 continue in    
existence1 and retain all the essential 
properties of bodies4  ,emily4 ,but that
 part of a burnt body which evaporates  
in smoke has no figure2 smoke1 it is    
true1 ascends in columns into the air1  
but it is soon so much diffused as to   
lose all form2 it becomes indeed        
invisible4  ,mrs4 ,b4 ,invisible1 ,i    
allow2 but we must not imagine that what

 we no longer see no longer exists4     
,were every particle of matter that     
becomes invisible annihilated1 the world
 itself would in the course of time be  
destroyed4 ,the particles of smoke1 when
 diffused in the air1 continue still to 
be particles of matter as well as when  
more closely united in the form of      
coals3 they are really as substantial in
 the one state as in the other1 and     
equally so when by their extreme        
subdivision they become invisible4 ,no  
particle of matter is ever destroyed3   
this is a principle you must constantly 
remember4 ,every thing in nature decays 
and corrupts in the lapse of time4 ,we  
die1 and our bodies moulder to dust2 but
 not a single atom of them is lost2 they
 serve to nourish the earth1 whence1    
while living1 they drew their support4  
,the next essential property of matter  
is called inertia or inactivity2 this   
word expresses the resistance which     
matter makes to a change from a state of
 rest1 to that of motion1 or from a     

state of motion to that of rest4 ,bodies
 are equally incapable of changing their
 actual state1 whether it be of motion  
or of rest4 ,you know that it requires  
force to put a body which is at rest in 
motion2 an exertion of strength is also 
requisite to stop a body which is       
already in motion4 ,the resistance of   
the body to a change of state1 in either
 case1 arises from its inertia4  ,emily4
 ,in playing at base-ball ,i am obliged 
to use all my strength to give a rapid  
motion to the ball2 and when ,i have to 
catch it1 ,i am sure ,i feel the        
resistance it makes to being stopped4   
,but if ,i did not catch it1 it would   
soon fall to the ground and stop of     
itself4  ,mrs4 ,b4 ,matter being inert  
it is as incapable of stopping of itself
 as it is of putting itself into motion3
 when the ball ceases to move1          
therefore1 it must be stopped by some   
other cause or power2 but as it is one  
with which you are yet unacquainted1 we 
can not at present investigate its      

effects4  ,the last property which      
appears to be common to all bodies is   
attraction4 ,all bodies consist of      
infinitely small particles of matter1   
each of which possesses the power of    
attracting or drawing towards it1 and   
uniting with any other particle         
sufficiently near to be within the      
influence of its attraction2 but in     
minute particles this power extends to  
so very small a distance around them1   
that its effect is not sensible1 unless 
they are 7or at least appear to be7 in  
contact2 it then makes them stick or    
adhere together1 and is hence called the
 attraction of cohesion4 ,without this  
power1 solid bodies would fall in       
pieces1 or rather crumble to atoms4     
,emily4 ,i am so much accustomed to see 
bodies firm and solid1 that it never    
occurred to me that any power was       
requisite to unite the particles of     
which they are composed4 ,but the       
attraction of cohesion does not1 ,i     
suppose1 exist in liquids2 for the      

particles of liquids do not remain      
together so as to form a body1 unless   
confined in a vessel8  ,mrs4 ,b4 ,i beg 
your pardon2 it is the attraction of    
cohesion which holds this drop of water 
suspended at the end of my finger1 and  
keeps the minute watery particles of    
which it is composed united4 ,but as    
this power is stronger in proportion as 
the particles of bodies are more closely
 united1 the cohesive attraction of     
solid bodies is much greater than that  
of fluids4  ,the thinner and lighter a  
fluid is1 the less is the cohesive      
attraction of its particles1 because    
they are further apart2 and in elastic  
fluids1 such as air1 there is no        
cohesive attraction among the particles4
  ,emily4 ,that is very fortunate2 for  
it would be impossible to breathe the   
air in a solid mass2 or even in a liquid
 state4  ,but is the air a body of the  
same nature as other bodies8  ,mrs4 ,b4 
,undoubtedly1 in all essential          
properties4  ,emily4 ,yet you say that  

it does not possess one of the general  
properties of bodies--attraction4  ,mrs4
 ,b4 ,the particles of air are not      
destitute of the power of attraction1   
but they are too far distant from each  
other to be influenced by it so as to   
produce cohesion3 and the utmost efforts
 of human art have proved ineffectual in
 the attempt to compress them1 so as to 
bring them within the sphere of each    
other's attraction1 and make them       
cohere4  ,emily4 ,if so1 how is it      
possible to prove that they are endowed 
with this power8  ,mrs4 ,b4 ,the air is 
formed of particles precisely of the    
same nature as those which enter into   
the composition of liquid and solid     
bodies1 in each of which we have a proof
 of their attraction4  ,emily4 ,it is   
then1 ,i suppose1 owing to the different
 degrees of cohesive attraction in      
different substances1 that they are hard
 or soft1 and that liquids are thick or 
thin4  ,mrs4 ,b4 ,yes2 but you would    
express your meaning better by the term 

density1 which denotes the degree of    
closeness and compactness of the        
particles of a body4 ,in philosophical  
language1 density is said to be that    
property of bodies by which they contain
 a certain quantity of matter1 under a  
certain bulk or magnitude4 ,rarity is   
the contrary of density2 it denotes the 
thinness and subtilty of bodies3 thus   
you would say that mercury or           
quicksilver was a very dense fluid2     
ether1 a very rare one4 ,those bodies   
which are the most dense1 do not always 
cohere the most strongly2 lead is more  
dense than iron1 yet its particles are  
more easily separated4  ,caroline4 ,but 
how are we to judge of the quantity of  
matter contained in a certain bulk8     
,mrs4 ,b4 ,by the weight3 under the same
 bulk bodies are said to be dense in    
proportion as they are heavy4  ,emily4  
,then we may say that metals are dense  
bodies1 wood comparatively a rare one1  
c4 ,but1 ,mrs4 ,b41 when the particles  
of a body are so near as to attract each

 other1 the effect of this power must   
increase as they are brought by it      
closer together2 so that one would      
suppose that the body would gradually   
augment in density1 till it was         
impossible for its particles to be more 
closely united4 ,now1 we know that this 
is not the case2 for soft bodies1 such  
as cork1 sponge1 or butter1 never       
become1 in consequence of the increasing
 attraction of their particles1 as hard 
as iron8  ,mrs4 ,b4 ,in such bodies as  
cork and sponge1 the particles which    
come in contact are so few as to produce
 but a slight degree of cohesion3 they  
are porous bodies1 which1 owing to the  
peculiar arrangement of their particles1
 abound with interstices1 or pores1     
which separate the particles4 ,but there
 is also a fluid much more subtile than 
air1 which pervades all bodies1 this is 
heat4 ,heat insinuates itself more or   
less between the particles of all       
bodies1 and forces them asunder2 you may
 therefore consider heat1 and the       

attraction of cohesion1 as constantly   
acting in opposition to each other4     
,emily4 ,the one endeavouring to rend a 
body to pieces1 the other to keep its   
parts firmly united4  ,mrs4 ,b4 ,and it 
is this struggle between the contending 
forces of heat and attraction1 which    
prevents the extreme degree of density  
which would result from the sole        
influence of the attraction of cohesion4
  ,emily4 ,the more a body is heated    
then1 the more its particles will be    
separated4  ,mrs4 ,b4 ,certainly3 we    
find that bodies not only swell or      
dilate1 but lose their cohesion1 by     
heat3 this effect is very sensible in   
butter1 for instance1 which expands by  
the application of heat1 till at length 
the attraction of cohesion is so far    
diminished that the particles separate1 
and the butter becomes liquid4 ,a       
similar effect is produced by heat on   
metals1 and all bodies susceptible of   
being melted4 ,liquids1 you know1 are   
made to boil by the application of heat2

 the attraction of cohesion then yields 
entirely to the repulsive power2 the    
particles are totally separated and     
converted into steam or vapour4 ,but the
 agency of heat is in no body more      
sensible than in air1 which dilates and 
contracts by its increase or diminution 
in a very remarkable degree4  ,emily4   
,the effects of heat appear to be one of
 the most interesting parts of natural  
philosophy4  ,mrs4 ,b4 ,that is true2   
but heat is so intimately connected with
 chemistry1 that you must allow me to   
defer the investigation of its          
properties till you become acquainted   
with that science4  ,to return to its   
antagonist1 the attraction of cohesion2 
it is this power which restores to      
vapour its liquid form1 which unites it 
into drops when it falls to earth in a  
shower of rain1 which gathers the dew   
into brilliant gems on the blades of    
grass4  ,emily4 ,and ,i have often      
observed that after a shower1 the water 
collects into large drops on the leaves 

of plants2 but ,i cannot say that ,i    
perfectly understand how the attraction 
of cohesion produces this effect4  ,mrs4
 ,b4 ,rain1 when it first leaves the    
clouds1 is not in the form of drops1 but
 in that of mist or vapour1 which is    
composed of very small watery particles2
 these in their descent mutually attract
 each other1 and those that are         
sufficiently near in consequence unite  
and form a drop1 and thus the mist is   
transformed into a shower4 ,the dew also
 was originally in a state of vapour1   
but is1 by the mutual attraction of the 
particles1 formed into small globules on
 the blades of grass3 in a similar      
manner the rain upon the leaf collects  
into large drops1 which when they become
 too heavy for the leaf to support1 fall
 to the ground4  ,emily4 ,all this is   
wonderfully curious6 ,i am almost       
bewildered with surprise and admiration 
at the number of new ideas ,i have      
already acquired4  ,mrs4 ,b4 ,every step
 that you advance in the pursuit of     

natural science1 will fill your mind    
with admiration and gratitude towards   
its ,divine ,author4 ,in the study of   
natural philosophy1 we must consider    
ourselves as reading the book of nature1
 in which the bountiful goodness and    
wisdom of ,god are revealed to all      
mankind2 no study can tend more to      
purify the heart1 and raise it to a     
religious contemplation of the ,divine  
perfections4  ,there is another curious 
effect of the attraction of cohesion    
which ,i must point out to you2 this is 
called capillary attraction4 ,it enables
 liquids to rise above their ordinary   
level in capillary tubes3 these are     
tubes1 the bores of which are so        
extremely small that liquids ascend     
within them1 from the cohesive          
attraction between the particles of the 
liquid and the interior surface of the  
tube4 ,do you perceive the water rising 
in this small glass tube1 above its     
level in the goblet of water1 into which
 ,i have put one end of it8  ,emily4 ,oh

 yes2 ,i see it slowly creeping up the  
tube1 but now it is stationary3 will it 
rise no higher8  ,mrs4 ,b4 ,no2 because 
the cohesive attraction between the     
water and the internal surface of the   
tube is now balanced by the weight of   
the water within it2 if the bore of the 
tube were narrower the water would rise 
higher2 and if you immerse several tubes
 of bores of different sizes1 you will  
see it rise to different heights in each
 of them4 ,in making this experiment1   
you should colour the water with a      
little red wine1 in order to render the 
effect more obvious4  ,all porous       
substances1 such as sponge1 bread1      
linen1 c4 may be considered as          
collections of capillary tubes3 if you  
dip one end of a lump of sugar into     
water1 the fluid will rise in it1 and   
wet it considerably above the surface of
 the water into which you dip it4       
,emily4 ,in making tea ,i have often    
observed that effect1 without being able
 to account for it4  ,mrs4 ,b4 ,now that

 you are acquainted with the attraction 
of cohesion1 ,i must endeavour to       
explain to you that of ,gravitation1    
which is probably a modification of the 
same power2 the first is perceptible    
only in very minute particles1 and at   
very small distances2 the other acts on 
the largest bodies1 and extends to      
immense distances4  ,emily4 ,you        
astonish me3 surely you do not mean to  
say that large bodies attract each      
other8  ,mrs4 ,b4 ,indeed ,i do3 let us 
take1 for example1 one of the largest   
bodies in nature1 and observe whether it
 does not attract other bodies4 ,what is
 it that occasions the fall of this     
book1 when ,i no longer support it8     
,emily4 ,can it be the attraction of the
 earth8 ,i thought that all bodies had a
 natural tendency to fall4  ,mrs4 ,b4   
,they have a natural tendency to fall1  
it is true2 but that tendency is        
produced entirely by the attraction of  
the earth3 the earth being so much      
larger than any body on its surface1    

forces every body1 which is not         
supported1 to fall upon it4  ,emily4 ,if
 the tendency which bodies have to fall 
results from the earth's attractive     
power1 the earth itself can have no such
 tendency1 since it cannot attract      
itself1 and therefore it requires no    
support to prevent it from falling4 ,yet
 the idea that bodies do not fall of    
their own accord1 but that they are     
drawn towards the earth by its          
attraction1 is so new and strange to me1
 that ,i know not how to reconcile      
myself to it4  ,mrs4 ,b4 ,when you are  
accustomed to consider the fall of      
bodies as depending on this cause1 it   
will appear to you as natural1 and      
surely much more satisfactory1 than if  
the cause of their tendency to fall were
 totally unknown4 ,thus you understand  
that all matter is attractive1 from the 
smallest particle to the largest mass2  
and that bodies attract each other with 
a force proportional to the quantity of 
matter they contain4  ,emily4 ,i do not 

perceive any difference between the     
attraction of cohesion and that of      
gravitation2 is it not because every    
particle of matter is endowed with an   
attractive power1 that large bodies     
consisting of a great number of         
particles1 are so strongly attractive8  
,mrs4 ,b4 ,true4 ,there is1 however1    
this difference between the attraction  
of particles and that of masses1 that   
the former takes place only when the    
particles are contiguous1 whilst the    
latter is exerted when the masses are   
far from each other4 ,the attraction of 
particles frequently counteracts the    
attraction of gravitation4 ,of this you 
have an instance in the attraction of   
capillary tubes1 in which liquids ascend
 by the attraction of cohesion1 in      
opposition to that of gravity4 ,it is on
 this account that it is necessary that 
the bore of the tube should be extremely
 small2 for if the column of water      
within the tube is not very minute1 the 
attraction of cohesion would not be able

 either to raise or support it in       
opposition to its gravity2 because the  
increase of weight1 in a column of water
 of a given height1 is much greater than
 the increase in the attracting surface 
of the tube1 when its size is increased4
  ,you may observe also1 that all solid 
bodies are enabled by the force of the  
cohesive attraction of their particles  
to resist that of gravity1 which would  
otherwise disunite them1 and bring them 
to a level with the ground1 as it does  
in the case of a liquid1 the cohesive   
attraction of which is not sufficient to
 enable it to resist the power of       
gravity4  ,emily4 ,and some solid bodies
 appear to be of this nature1 as sand1  
and powder for instance3 there is no    
attraction of cohesion between their    
particles8  ,mrs4 ,b4 ,every grain of   
powder1 or sand1 is composed of a great 
number of other more minute particles1  
firmly united by the attraction of      
cohesion2 but amongst the separate      
grains there is no sensible attraction1 

because they are not in sufficiently    
close contact4  ,emily4 ,yet they       
actually touch each other8  ,mrs4 ,b4   
,the surface of bodies is in general so 
rough and uneven1 that when in apparent 
contact1 they touch each other only by a
 few points4 ,thus1 when ,i lay this    
book upon the table1 the binding of     
which appears perfectly smooth1 so few  
of the particles of its under surface   
come in contact with the table1 that no 
sensible degree of cohesive attraction  
takes place2 for you see that it does   
not stick or cohere to the table1 and ,i
 find no difficulty in lifting it off4  
,it is only when surfaces1 perfectly    
flat and well polished1 are placed in   
contact1 that the particles approach in 
sufficient number1 and closely enough1  
to produce a sensible degree of cohesive
 attraction4 ,here are two plates of    
polished metal1 ,i press their flat     
surfaces together1 having previously    
interposed a few drops of oil1 to fill  
up every little porous vacancy4 ,now try

 to separate them4  ,emily4 ,it requires
 an effort beyond my strength1 though   
there are handles for the purpose of    
pulling them asunder4 ,is the firm      
adhesion of the two plates merely owing 
to the attraction of cohesion8  ,mrs4   
,b4 ,there is no force more powerful1   
since it is by this that the particles  
of the hardest bodies are held together4
 ,it would require a weight of several  
pounds to separate these plates4 ,in the
 present example1 however1 much of the  
cohesive force is due to the attraction 
subsisting between the metal and the oil
 which is interposed2 as without this1  
or some other fluid1 the points of      
contact would still be comparatively    
few1 although we may have employed our  
utmost art1 in giving flat surfaces to  
the plates4  ,emily4 ,in making a       
kaleidoscope1 ,i recollect that the two 
plates of glass1 which were to serve as 
mirrors1 stuck so fast together1 that ,i
 imagined some of the gum ,i had been   
using had by chance been interposed     

between them2 but ,i am now convinced   
that it was their own natural cohesive  
attraction which produced this effect4  
,mrs4 ,b4 ,very probably it was so2 for 
plate-glass has an extremely smooth1    
flat surface1 admitting of the contact  
of a great number of particles1 when two
 plates are laid upon each other4       
,emily4 ,but1 ,mrs4 ,b41 the cohesive   
attraction of some substances is much   
greater than that of others2 thus glue1 
gum and paste1 cohere with singular     
tenacity4  ,mrs4 ,b4 ,bodies which      
differ in their natures in other        
respects1 differ also in their cohesive 
attraction2 it is probable that there   
are no two bodies1 the particles of     
which attract each other with precisely 
the same force4  ,there are some other  
modifications of attraction peculiar to 
certain bodies2 namely1 that of         
magnetism1 of electricity1 and of       
affinity1 or chemical attraction2 but we
 shall confine our attention merely to  
the attraction of cohesion and of       

gravity2 the examination of the latter  
we shall resume at our next meeting4    
,questions  #a4 7,pg4 #aj7 ,what is     
intended by the term bodies8  #b4 7,pg4 
#aj7 ,is the term matter1 restricted to 
substances of a particular kind8  #c4   
7,pg4 #aj7 ,name those properties of    
bodies1 which are called inherent4  #d4 
7,pg4 #aj7 ,what is meant by            
impenetrability8  #e4 7,pg4 #aj7 ,can a 
liquid be said to be impenetrable8  #f4 
7,pg4 #aa7 ,how can you prove that air  
is impenetrable8  #g4 7,pg4 #aa7 ,if air
 is impenetrable1 what causes the water 
to rise some way into a goblet1 if ,i   
plunge it into water with its mouth     
downward8  #h4 7,pg4 #aa7 ,when ,i drive
 a nail into wood1 do not both the iron 
and the wood occupy the same space8  #i4
 7,pg4 #aa7 ,in how many directions1 is 
a body said to have extension8  #aj4    
7,pg4 #aa7 ,how do we distinguish the   
terms height and depth8  #aa4 7,pg4 #ab7
 ,what constitutes the figure1 or form  
of a body8  #ab4 7,pg4 #ab7 ,what is    

said respecting the form of minerals8   
#ac4 7,pg4 #ab7 ,what of the vegetable  
and animal creation8  #ad4 7,pg4 #ab7   
,what of artificial1 and accidental     
forms8  #ae4 7,pg4 #ab7 ,what is meant  
by divisibility8  #af4 7,pg4 #ab7 ,what 
examples can you give1 to prove that the
 particles of a body are minute in the  
extreme8  #ag4 7,pg4 #ac7 ,what produces
 the odour of bodies8  #ah4 7,pg4 #ac7  
,how do odours exemplify the minuteness 
of the particles of matter8  #ai4 7,pg4 
#ac7 ,can matter be in any way          
annihilated8  #bj4 7,pg4 #ac7 ,what     
becomes of the fuel1 which disappears in
 our fires8  #ba4 7,pg4 #ad7 ,how can   
that part which evaporates1 be still    
said to possess a substantial form8     
#bb4 7,pg4 #ad7 ,what do we mean by     
inertia8  #bc4 7,pg4 #ad7 ,give an      
example to prove that force is          
necessary1 either to give or to stop    
motion4  #bd4 7,pg4 #ad7 ,what general  
power do the particles of matter exert  
upon other particles8  #be4 7,pg4 #ae7  

,what is that species of attraction     
called1 which keeps bodies in a solid   
state8  #bf4 7,pg4 #ae7 ,does the       
attraction of cohesion exist in liquids1
 and how is its existence proved8  #bg4 
7,pg4 #ae7 ,if the particles of air     
attract each other1 why do they not     
cohere8  #bh4 7,pg4 #ae7 ,from what then
 do you infer that they possess         
attraction8  #bi4 7,pg4 #ae7 ,how do you
 account for some bodies being hard and 
others soft8  #cj4 7,pg4 #af7 ,what is  
meant by the term density8  #ca4 7,pg4  
#af7 ,do the most dense bodies always   
cohere the most strongly8  #cb4 7,pg4   
#af7 ,how do we know that one body is   
more dense than another8  #cc4 7,pg4    
#af7 ,what is there which acts in       
opposition to cohesive attraction1      
tending to separate the particles of    
bodies8  #cd4 7,pg4 #ag7 ,what would be 
the consequence if the repulsive power  
of heat were not exerted8  #ce4 7,pg4   
#ag7 ,if we continue to increase the    
heat1 what effects will it produce on   

bodies8  #cf4 7,pg4 #ag7 ,what body has 
its dimensions most sensibly affected by
 change of temperature8  #cg4 7,pg4 #ag7
 ,what power restores vapours to the    
liquid form8  #ch4 7,pg4 #ag7 ,what     
examples can you give8  #ci4 7,pg4 #ag7 
,how are drops of rain and of dew said  
to be formed8  #dj4 7,pg4 #ah7 ,what is 
meant by a capillary tube8  #da4 7,pg4  
#ah7 ,what effect does attraction       
produce when these are immersed in      
water8  #db4 7,pg4 #ah7 ,what is the    
reason that the water rises to a certain
 height only8  #dc4 7,pg4 #ah7 ,give    
some familiar examples of capillary     
attraction4  #dd4 7,pg4 #ah7 ,in what   
does gravitation differ from cohesive   
attraction8  #de4 7,pg4 #ah7 ,what      
causes bodies near the earth's surface1 
to have a tendency to fall towards it8  
#df4 7,pg4 #ai7 ,what remarkable        
difference is there between the         
attraction of gravitation1 and that of  
cohesion8  #dg4 7,pg4 #ai7 ,in what     
instances does the power of cohesion    

counteract that of gravitation8  #dh4   
7,pg4 #ai7 ,why will water rise to a    
less height1 if the size of the tube is 
increased8  #di4 7,pg4 #bj7 ,why do not 
two bodies cohere1 when laid upon each  
other8  #ej4 7,pg4 #bj7 ,can two bodies 
be made sufficiently flat to cohere with
 considerable force8  #ea4 7,pg4 #bj7   
,what is the reason that the adhesion is
 greater when oil is interposed8  #eb4  
7,pg4 #ba7 ,what other modifications of 
attraction are there1 besides those of  
cohesion and of gravitation8            
,,conversation ,,ii4  ,,on ,,the        
,,attraction ,,of ,,gravity4            
,,attraction ,,of ,,gravitation1        
,,continued4 ,,of ,,weight4 ,,of ,,the  
,,fall ,,of ,,bodies4 ,,of ,,the        
,,resistance ,,of ,,the ,,air4 ,,of     
,,the ,,ascent ,,of ,,light ,,bodies4   
,,emily4  ,i have related to my sister  
,caroline all that you have taught me of
 natural philosophy1 and she has been so
 much delighted by it1 that she hopes   
you will have the goodness to admit her 

to your lessons4  ,mrs4 ,b4 ,very       
willingly2 but ,i did not think you had 
any taste for studies of this nature1   
,caroline4  ,caroline4 ,i confess1 ,mrs4
 ,b41 that hitherto ,i had formed no    
very agreeable idea either of           
philosophy1 or philosophers2 but what   
,emily has told me has excited my       
curiosity so much1 that ,i shall be     
highly pleased if you will allow me to  
become one of your pupils4  ,mrs4 ,b4 ,i
 fear that ,i shall not find you so     
tractable a scholar as ,emily2 ,i know  
that you are much biased in favour of   
your own opinions4  ,caroline4 ,then you
 will have the greater merit in         
reforming them1 ,mrs4 ,b42 and after all
 the wonders that ,emily has related to 
me1 ,i think ,i stand but little chance 
against you and your attractions4  ,mrs4
 ,b4 ,you will1 ,i doubt not1 advance a 
number of objections2 but these ,i shall
 willingly admit1 as they will afford an
 opportunity of elucidating the subject4
 ,emily1 do you recollect the names of  

the general properties of bodies8       
,emily4 ,impenetrability1 extension1    
figure1 divisibility1 inertia and       
attraction4  ,mrs4 ,b4 ,very well4 ,you 
must remember that these are properties 
common to all bodies1 and of which they 
cannot be deprived2 all other properties
 of bodies are called accidental1       
because they depend on the relation or  
connexion of one body to another4       
,caroline4 ,yet surely1 ,mrs4 ,b41 there
 are other properties which are         
essential to bodies1 besides those you  
have enumerated4 ,colour and weight1 for
 instance1 are common to all bodies1 and
 do not arise from their connexion with 
each other1 but exist in the bodies     
themselves2 these1 therefore1 cannot be 
accidental qualities8  ,mrs4 ,b4 ,i beg 
your pardon2 these properties do not    
exist in bodies independently of their  
connexion with other bodies4  ,caroline4
 ,what6 have bodies no weight8 ,does not
 this table weigh heavier than this     
book2 and1 if one thing weighs heavier  

than another1 must there not be such a  
thing as weight8  ,mrs4 ,b4 ,no doubt3  
but this property does not appear to be 
essential to bodies2 it depends upon    
their connexion with each other4 ,weight
 is an effect of the power of           
attraction1 without which the table and 
the book would have no weight whatever4 
,emily4 ,i think ,i understand you2 it  
is the attraction of gravity which makes
 bodies heavy4  ,mrs4 ,b4 ,you are      
right4 ,i told you that the attraction  
of gravity was proportioned to the      
quantity of matter which bodies contain3
 now the earth consisting of a much     
greater quantity of matter than any body
 upon its surface1 the force of its     
attraction must necessarily be greatest1
 and must draw every thing so situated  
towards it2 in consequence of which1    
bodies that are unsupported fall to the 
ground1 whilst those that are supported1
 press upon the object which prevents   
their fall1 with a weight equal to the  
force with which they gravitate towards 

the earth4  ,caroline4 ,the same cause  
then which occasions the fall of bodies1
 produces their weight also4 ,it was    
very dull in me not to understand this  
before1 as it is the natural and        
necessary consequence of attraction2 but
 the idea that bodies were not really   
heavy of themselves1 appeared to me     
quite incomprehensible4 ,but1 ,mrs4 ,b41
 if attraction is a property essential  
to matter1 weight must be so likewise2  
for how can one exist without the other8
  ,mrs4 ,b4 ,suppose there were but one 
body existing in universal space1 what  
would its weight be8  ,caroline4 ,that  
would depend upon its size2 or more     
accurately speaking1 upon the quantity  
of matter it contained4  ,emily4 ,no1   
no2 the body would have no weight1      
whatever were its size2 because nothing 
would attract it4 ,am ,i not right1     
,mrs4 ,b48  ,mrs4 ,b4 ,you are3 you must
 allow1 therefore1 that it would be     
possible for attraction to exist without
 weight2 for each of the particles of   

which the body was composed1 would      
possess the power of attraction2 but    
they could exert it only amongst        
themselves2 the whole mass having       
nothing to attract1 or to be attracted  
by1 would have no weight4  ,caroline4 ,i
 am now well satisfied that weight is   
not essential to the existence of       
bodies2 but what have you to object to  
colours1 ,mrs4 ,b42 you will not1 ,i    
think1 deny that they really exist in   
the bodies themselves4  ,mrs4 ,b4 ,when 
we come to treat of the subject of      
colours1 ,i trust that ,i shall be able 
to convince you1 that colours are       
likewise accidental qualities1 quite    
distinct from the bodies to which they  
appear to belong4  ,caroline4 ,oh do    
pray explain it to us now1 ,i am so very
 curious to know how that is possible4  
,mrs4 ,b4 ,unless we proceed with some  
degree of order and method1 you will in 
the end find yourself but little the    
wiser for all you learn4 ,let us        
therefore go on regularly1 and make     

ourselves well acquainted with the      
general properties of bodies before we  
proceed further4  ,emily4 ,to return1   
then1 to attraction1 7which appears to  
me by far the most interesting of them1 
since it belongs equally to all kinds of
 matter17 it must be mutual between two 
bodies2 and if so1 when a stone falls to
 the earth1 the earth should rise part  
of the way to meet the stone8  ,mrs4 ,b4
 ,certainly2 but you must recollect that
 the force of attraction is proportioned
 to the quantity of matter which bodies 
contain1 and if you consider the        
difference there is in that respect1    
between a stone and the earth1 you will 
not be surprised that you do not        
perceive the earth rise to meet the     
stone2 for though it is true that a     
mutual attraction takes place between   
the earth and the stone1 that of the    
latter is so very small in comparison to
 that of the former1 as to render its   
effect insensible4  ,emily4 ,but since  
attraction is proportioned to the       

quantity of matter which bodies contain1
 why do not the hills attract the houses
 and churches towards them8  ,caroline4 
,what an idea1 ,emily6 ,how can the     
houses and churches be moved1 when they 
are so firmly fixed in the ground6      
,mrs4 ,b4 ,emily's question is not      
absurd1 and your answer1 ,caroline1 is  
perfectly just2 but can you tell us why 
the houses and churches are so firmly   
fixed in the ground8  ,caroline4 ,i am  
afraid ,i have answered right by mere   
chance2 for ,i begin to suspect that    
bricklayers and carpenters could give   
but little stability to their buildings1
 without the aid of attraction4  ,mrs4  
,b4 ,it is certainly the cohesive       
attraction between the bricks and the   
mortar1 which enables them to build     
walls1 and these are so strongly        
attracted by the earth1 as to resist    
every other impulse2 otherwise they     
would necessarily move towards the hills
 and the mountains2 but the lesser force
 must yield to the greater4 ,there are1 

however1 some circumstances in which the
 attraction of a large body has sensibly
 counteracted that of the earth4 ,if    
whilst standing on the declivity of a   
mountain1 you hold a plumb-line in your 
hand1 the weight will not fall          
perpendicular to the earth1 but incline 
a little towards the mountain2 and this 
is owing to the lateral1 or sideways    
attraction of the mountain1 interfering 
with the perpendicular attraction of the
 earth4  ,emily4 ,but the size of a     
mountain is very trifling1 compared to  
the whole earth4  ,mrs4 ,b4 ,attraction1
 you must recollect1 is in proportion to
 the quantity of matter1 and although   
that of the mountain1 is much less than 
that of the earth1 it may yet be        
sufficient to act sensibly upon the     
plumb-line which is so near to it4      
,caroline4 ,pray1 ,mrs4 ,b41 do the two 
scales of a balance hang parallel to    
each other8  ,mrs4 ,b4 ,you mean1 ,i    
suppose1 in other words to inquire      
whether two lines which are             

perpendicular to the earth1 are parallel
 to each other8 ,i believe ,i guess the 
reason of your question2 but ,i wish you
 would endeavour to answer it without my
 assistance4  ,caroline4 ,i was thinking
 that such lines must both tend by      
gravity to the same point1 the centre of
 the earth2 now lines tending to the    
same point cannot be parallel1 as       
parallel lines are always at an equal   
distance from each other1 and would     
never meet4  ,mrs4 ,b4 ,very well       
explained2 you see now the use of your  
knowledge of parallel lines3 had you    
been ignorant of their properties1 you  
could not have drawn such a conclusion4 
,this may enable you to form an idea of 
the great advantage to be derived even  
from a slight knowledge of geometry1 in 
the study of natural philosophy2 and if 
after ,i have made you acquainted with  
the first elements1 you should be       
tempted to pursue the study1 ,i would   
advise you to prepare yourselves by     
acquiring some knowledge of geometry4   

,this science would teach you that lines
 which fall perpendicular to the surface
 of a sphere cannot be parallel1 because
 they would all meet1 if prolonged to   
the centre of the sphere2 while lines   
that fall perpendicular to a plane or   
flat surface1 are always parallel1      
because if prolonged1 they would never  
meet4  ,emily4 ,and yet a pair of       
scales1 hanging perpendicular to the    
earth1 appear parallel8  ,mrs4 ,b4      
,because the sphere is so large1 and the
 scales consequently converge so little1
 that their inclination is not          
perceptible to our senses2 if we could  
construct a pair of scales whose beam   
would extend several degrees1 their     
convergence would be very obvious2 but  
as this cannot be accomplished1 let us  
draw a small figure of the earth1 and   
then we may make a pair of scales of the
 proportion we please4 7fig4 #a4 pl4    
,i47  ,caroline4 ,this figure renders it
 very clear3 then two bodies cannot fall
 to the earth in parallel lines8  ,mrs4 

,b4 ,never4  ,caroline4 ,the reason that
 a heavy body falls quicker than a light
 one1 is1 ,i suppose1 because the earth 
attracts it more strongly4  ,mrs4 ,b4   
,the earth1 it is true1 attracts a heavy
 body more than a light one2 but that   
would not make the one fall quicker than
 the other4  ,caroline4 ,yet1 since it  
is attraction that occasions the fall of
 bodies1 surely the more a body is      
attracted1 the more rapidly it will     
fall4 ,besides1 experience proves it to 
be so4 ,do we not every day see heavy   
bodies fall quickly1 and light bodies   
slowly8  ,emily4 ,it strikes me1 as it  
does ,caroline1 that as attraction is   
proportioned to the quantity of matter1 
the earth must necessarily attract a    
body which contains a great quantity    
more strongly1 and therefore bring it to
 the ground sooner than one consisting  
of a smaller quantity4  ,mrs4 ,b4 ,you  
must consider1 that if heavy bodies are 
attracted more strongly than light ones1
 they require more attraction to make   

them fall4 ,remember that bodies have no
 natural tendency to fall1 any more than
 to rise1 or to move laterally1 and that
 they will not fall unless impelled by  
some force2 now this force must be      
proportioned to the quantity of matter  
it has to move3 a body consisting of    
#ajjj particles of matter1 for instance1
 requires ten times as much attraction  
to bring it to the ground in the same   
space of time as a body consisting of   
only #ajj particles4  ,illustration3    
,,plate ,i4  ,caroline4 ,i do not       
understand that2 for it seems to me1    
that the heavier a body is1 the move    
easily and readily it falls4  ,emily4 ,i
 think ,i now comprehend it2 let me try 
if ,i can explain it to ,caroline4      
,suppose that ,i draw towards me two    
weighty bodies1 the one of #ajj lbs4 the
 other of #ajjj lbs4 must ,i not exert  
ten times as much strength to draw the  
larger one to me1 in the same space of  
time1 as is required for the smaller    
one8 ,and if the earth draws a body of  

#ajjj lbs4 weight to it in the same     
space of time that it draws a body of   
#ajj lbs4 does it not follow that it    
attracts the body of #ajjj lbs4 weight  
with ten times the force that it does   
that of #ajj lbs48  ,caroline4 ,i       
comprehend your reasoning perfectly2 but
 if it were so1 the body of #ajjj lbs4  
weight1 and that of #ajj lbs4 would fall
 with the same rapidity2 and the        
consequence would be1 that all bodies1  
whether light or heavy1 being at an     
equal distance from the ground1 would   
fall to it in the same space of time3   
now it is very evident that this        
conclusion is absurd2 experience every  
instant contradicts it2 observe how much
 sooner this book reaches the floor than
 this sheet of paper1 when ,i let them  
drop together4  ,emily4 ,that is an     
objection ,i cannot answer4 ,i must     
refer it to you1 ,mrs4 ,b4  ,mrs4 ,b4 ,i
 trust that we shall not find it        
insurmountable4 ,it is true that1       
according to the laws of attraction1 all

 bodies at an equal distance from the   
earth1 should fall to it in the same    
space of time2 and this would actually  
take place if no obstacle intervened to 
impede their fall4 ,but bodies fall     
through the air1 and it is the          
resistance of the air which makes bodies
 of different density fall with         
different degrees of velocity4 ,they    
must all force their way through the    
air1 but dense heavy bodies overcome    
this obstacle more easily than rarer or 
lighter ones2 because in the same space 
they contain more gravitating particles4
  ,the resistance which the air opposes 
to the fall of bodies is proportioned to
 their surface1 not to their weight2 the
 air being inert1 cannot exert a greater
 force to support the weight of a cannon
 ball1 than it does to support the      
weight of a ball 7of the same size7 made
 of leather2 but the cannon ball will   
overcome this resistance more easily1   
and fall to the ground1 consequently1   
quicker than the leather ball4          

,caroline4 ,this is very clear and      
solves the difficulty perfectly4 ,the   
air offers the same resistance to a bit 
of lead and a bit of feather of the same
 size2 yet the one seems to meet with no
 obstruction in its fall1 whilst the    
other is evidently resisted and         
supported for some time by the air4     
,emily4 ,the larger the surface of a    
body1 then1 the more air it covers1 and 
the greater is the resistance it meets  
with from it4  ,mrs4 ,b4 ,certainly3    
observe the manner in which this sheet  
of paper falls2 it floats awhile in the 
air1 and then gently descends to the    
ground4 ,i will roll the same piece of  
paper up into a ball3 it offers now but 
a small surface to the air1 and         
encounters therefore but little         
resistance3 see how much more rapidly it
 falls4  ,the heaviest bodies may be    
made to float awhile in the air1 by     
making the extent of their surface      
counterbalance their weight4 ,here is   
some gold1 which is one of the most     

dense bodies we are acquainted with2 but
 it has been beaten into a very thin    
leaf1 and offers so great an extent of  
surface in proportion to its weight1    
that its fall1 you see1 is still more   
retarded by the resistance of the air1  
than that of the sheet of paper4        
,caroline4 ,that is very curious3 and it
 is1 ,i suppose1 upon the same principle
 that a thin slate sinks in water more  
slowly than a round stone4  ,but1 ,mrs4 
,b41 if the air is a real body1 is it   
not also subjected to the laws of       
gravity8  ,mrs4 ,b4 ,undoubtedly4       
,caroline4 ,then why does it not1 like  
all other bodies1 fall to the ground8   
,mrs4 ,b4 ,on account of its spring or  
elasticity4 ,the air is an elastic      
fluid2 and the peculiar property of     
elastic bodies is to resume1 after      
compression1 their original dimensions2 
and you must consider the air of which  
the atmosphere is composed as existing  
in a state of compression1 for its      
particles being drawn towards the earth 

by gravity1 are brought closer together 
than they would otherwise be1 but the   
spring or elasticity of the air by which
 it endeavours to resist compression1   
gives it a constant tendency to expand  
itself1 so as to resume the dimensions  
it would naturally have1 if not under   
the influence of gravity4 ,the air may  
therefore be said constantly to struggle
 with the power of gravity without being
 able to overcome it4 ,gravity thus     
confines the air to the regions of our  
globe1 whilst its elasticity prevents it
 from falling1 like other bodies1 to the
 ground4  ,emily4 ,the air then is1 ,i  
suppose1 thicker1 or ,i should rather   
say more dense1 near the surface of the 
earth1 than in the higher regions of the
 atmosphere2 for that part of the air   
which is nearer the surface of the earth
 must be most strongly attracted4  ,mrs4
 ,b4 ,the diminution of the force of    
gravity1 at so small a distance as that 
to which the atmosphere extends         
7compared with the size of the earth7 is

 so inconsiderable as to be scarcely    
sensible2 but the pressure of the upper 
parts of the atmosphere on those        
beneath1 renders the air near the       
surface of the earth much more dense    
than in the upper regions4 ,the pressure
 of the atmosphere has been compared to 
that of a pile of fleeces of wool1 in   
which the lower fleeces are pressed     
together by the weight of those above2  
these lie light and loose1 in proportion
 as they approach the uppermost fleece1 
which receives no external pressure1 and
 is confined merely by the force of its 
own gravity4  ,emily4 ,i do not         
understand how it is that the air can be
 springy or elastic1 as the particles of
 which it is composed must1 according to
 the general law1 attract each other2   
yet their elasticity1 must arise from a 
tendency to recede from each other4     
,mrs4 ,b4 ,have you forgotten what ,i   
told you respecting the effects of heat1
 a fluid so subtile that it readily     
pervades all substances1 and even in    

solid bodies1 counteracts the attraction
 of cohesion8 ,in air the quantity of   
heat interposed is so great1 as to cause
 its particles actually to repel each   
other1 and it is to this that we must   
ascribe its elasticity2 this1 however1  
does not prevent the earth from exerting
 its attraction upon the individual     
particles of which it consists4         
,caroline4 ,it has just occurred to me  
that there are some bodies which do not 
gravitate towards the earth4 ,smoke and 
steam1 for instance1 rise instead of    
falling4  ,mrs4 ,b4 ,it is still gravity
 which produces their ascent2 at least1 
were that power destroyed1 these bodies 
would not rise4  ,caroline4 ,i shall be 
out of conceit with gravity1 if it is so
 inconsistent in its operations4  ,mrs4 
,b4 ,there is no difficulty in          
reconciling this apparent inconsistency 
of effect4 ,the air near the earth is   
heavier than smoke1 steam1 or other     
vapours2 it consequently not only       
supports these light bodies1 but forces 

them to rise1 till they reach a part of 
the atmosphere1 the weight of which is  
not greater than their own1 and then    
they remain stationary4 ,look at this   
bason of water2 why does the piece of   
paper which ,i throw into it float on   
the surface8  ,emily4 ,because1 being   
lighter than the water1 it is supported 
by it4  ,mrs4 ,b4 ,and now that ,i pour 
more water into the bason1 why does the 
paper rise8  ,emily4 ,the water being   
heavier than the paper1 gets beneath it1
 and obliges it to rise4  ,mrs4 ,b4 ,in 
a similar manner are smoke and vapour   
forced upwards by the air2 but these    
bodies do not1 like the paper1 ascend to
 the surface of the fluid1 because1 as  
we observed before1 the air being less  
dense1 and consequently lighter as it is
 more distant from the earth1 vapours   
rise only till they attain a region of  
air of their own density4 ,smoke1 indeed
 ascends but a very little way2 it      
consists of minute particles of fuel1   
carried up by a current of heated air1  

from the fire below3 heat1 you          
recollect1 expands all bodies2 it       
consequently rarefies air1 and renders  
it lighter than the colder air of the   
atmosphere2 the heated air from the fire
 carries up with it vapour and small    
particles of the combustible materials  
which are burning in the fire4 ,when    
this current of hot air is cooled by    
mixing with the atmosphere1 the minute  
particles of coal1 or other combustible1
 fall2 it is this which produces the    
small black flakes which render the air1
 and every thing in contact with it1 in 
,london1 so dirty4  ,caroline4 ,you     
must1 however1 allow me to make one more
 objection to the universal gravity of  
bodies2 which is the ascent of air      
balloons1 the materials of which are    
undoubtedly heavier than air3 how1      
therefore1 can they be supported by it8 
,mrs4 ,b4 ,i admit that the materials of
 which balloons are made are heavier    
than the air2 but the air with which    
they are filled is an elastic fluid1 of 

a different nature from atmospheric air1
 and considerably lighter2 so that on   
the whole the balloon is lighter than   
the air which it displaces1 and         
consequently will rise1 on the same     
principle as smoke and vapour4 ,now1    
,emily1 let me hear if you can explain  
how the gravity of bodies is modified by
 the effect of the air8  ,emily4 ,the   
air forces bodies which are lighter than
 itself to ascend2 those that are of an 
equal weight will remain stationary in  
it2 and those that are heavier will     
descend through it3 but the air will    
have some effect on these last2 for if  
they are not much heavier1 they will    
with difficulty overcome the resistance 
they meet with in passing through it1   
they will be borne up by it1 and their  
fall will be more or less retarded4     
,mrs4 ,b4 ,very well4 ,observe how      
slowly this light feather falls to the  
ground1 while a heavier body1 like this 
marble1 overcomes the resistance which  
the air makes to its descent much more  

easily1 and its fall is proportionally  
more rapid4 ,i now throw a pebble into  
this tub of water2 it does not reach the
 bottom near so soon as if there were no
 water in the tub1 because it meets with
 resistance from the water4 ,suppose    
that we could empty the tub1 not only of
 water1 but of air also1 the pebble     
would then fall quicker still1 as it    
would in that case meet with no         
resistance at all to counteract its     
gravity4  ,thus you see that it is not  
the different degrees of gravity1 but   
the resistance of the air1 which        
prevents bodies of different weight from
 falling with equal velocities2 if the  
air did not bear up the feather1 it     
would reach the ground as soon as the   
marble4  ,caroline4 ,i make no doubt    
that it is so2 and yet ,i do not feel   
quite satisfied4 ,i wish there was any  
place void of air1 in which the         
experiment could be made4  ,mrs4 ,b4 ,if
 that proof will satisfy your doubts1 ,i
 can give it you4 ,here is a machine    

called an air pump1 7fig4 #b4 pl4 #a47  
by means of which the air may be        
expelled from any close vessel which is 
placed over this opening1 through which 
the air is pumped out4 ,glasses of      
various shapes1 usually called          
receivers1 are employed for this        
purpose4 ,we shall now exhaust the air  
from this tall receiver which is placed 
over the opening1 and we shall find that
 bodies within it1 whatever their weight
 or size1 will fall from the top to the 
bottom in the same space of time4       
,caroline4 ,oh1 ,i shall be delighted   
with this experiment2 what a curious    
machine6 how can you put the two bodies 
of different weight within the glass1   
without admitting the air8  ,mrs4 ,b4 ,a
 guinea and a feather are already placed
 there for the purpose of the           
experiment3 here is1 you see1 a         
contrivance to fasten them in the upper 
part of the glass2 as soon as the air is
 pumped out1 ,i shall turn this little  
screw1 by which means the brass plates  

which support them will be removed1 and 
the two bodies will fall4--,now ,i      
believe ,i have pretty well exhausted   
the air4  ,caroline4 ,pray let me turn  
the screw4--,i declare1 they both       
reached the bottom at the same instant6 
,did you see1 ,emily1 the feather       
appeared as heavy as the guinea8        
,emily4 ,exactly2 and fell just as      
quickly4 ,how wonderful this is6 what a 
number of entertaining experiments might
 be made with this machine6  ,mrs4 ,b4  
,no doubt there are a great many2 but we
 shall reserve them to elucidate the    
subjects to which they relate3 if ,i had
 not explained to you why the guinea1   
and the feather fell with equal         
velocity1 you would not have been so    
well pleased with the experiment4       
,emily4 ,i should have been as much     
surprised1 but not so much interested2  
besides1 experiments help to imprint on 
the memory the facts they are intended  
to illustrate2 it will be better        
therefore for us to restrain our        

curiosity1 and wait for other           
experiments in their proper places4     
,caroline4 ,pray by what means is this  
receiver exhausted of its air8  ,mrs4   
,b4 ,you must learn something of        
mechanics in order to understand the    
construction of a pump4 ,at our next    
meeting1 therefore1 ,i shall endeavour  
to make you acquainted with the laws of 
motion1 as an introduction to that      
subject4   ,questions  #a4 7,pg4 #bb7   
,what are those properties of bodies    
called1 which are not common to all8    
#b4 7,pg4 #bc7 ,why are they so called8 
#c4 7,pg4 #bc7 ,what is the cause of    
weight in bodies8  #d4 7,pg4 #bc7 ,what 
is the reason that all bodies near to   
the surface of the earth1 are drawn     
towards it8  #e4 7,pg4 #bd7 ,if         
attraction is the cause of weight1 could
 you suppose it possible for a body to  
possess the former and not the latter   
property8  #f4 7,pg4 #bd7 ,when a stone 
falls to the ground1 in which of the two
 bodies does the power of attraction    

exist8  #g4 7,pg4 #bd7 ,if the          
attraction be mutual1 why does not the  
earth approach the stone1 as much as the
 stone approaches the earth8  #h4 7,pg4 
#bd7 ,if attraction be in proportion to 
the mass1 why does not a hill1 draw     
towards itself1 a house placed near it8 
#i4 7,pg4 #be7 ,how can the attraction  
of a mountain be rendered sensible8     
#aj4 7,pg4 #be7 ,why cannot two lines   
which are perpendicular to the surface  
of the earth be parallel to each other8 
#aa4 7,pg4 #bf7 ,draw a small figure of 
the earth to exemplify this1 as in fig4 
#a4 plate #a4  #ab4 7,pg4 #bg7 ,if      
bodies were not resisted by the air1    
those which are light1 would fall as    
quickly as those which are heavy1 how   
can you account for this8  #ac4 7,pg4   
#bg7 ,what then is the reason that a    
book1 and a sheet of paper1 let fall    
from the same height1 will not reach the
 ground in the same time8  #ad4 7,pg4   
#bh7 ,what then will be the effect of   
increasing the surface of a body8  #ae4 

7,pg4 #bh7 ,what could you do to a sheet
 of paper1 to make it fall quickly1 and 
why8  #af4 7,pg4 #bh7 ,inform me how a  
very dense body may be made to float in 
the air8  #ag4 7,pg4 #bh7 ,the air is a 
real body1 why does it not fall to the  
ground8  #ah4 7,pg4 #bi7 ,the air is    
more dense near the surface of the      
earth1 and decreases in density as you  
ascend1 how is this accounted for1 and  
to what is it compared8  #ai4 7,pg4 #bi7
 ,what is it which causes the particles 
of air to recede from each other1 and   
seems to destroy their mutual           
attraction8  #bj4 7,pg4 #bi7 ,smoke and 
vapour ascend in the atmosphere1 how can
 you reconcile this with gravitation8   
#ba4 7,pg4 #cj7 ,how would you          
illustrate this by the floating of a    
piece of paper on water8  #bb4 7,pg4    
#cj7 ,does smoke rise to a great height 
in the air1 and if not1 what prevents   
its so doing8  #bc4 7,pg4 #cj7 ,what    
limits the height to which vapours rise8
  #bd4 7,pg4 #cj7 ,of what does smoke   

consist8  #be4 7,pg4 #cj7 ,air balloons 
are formed of heavy materials1 how will 
you account for their rising in the air8
  #bf4 7,pg4 #cj7 ,what influence does  
the air exert1 on bodies less dense than
 itself1 on those of equal1 and on those
 of greater density8  #bg4 7,pg4 #ca7   
,if the air could be entirely removed1  
what influence would this have upon the 
falling of heavy and light bodies8  #bh4
 7,pg4 #ca7 ,how could this be          
exemplified by means of the air pump8   
,,conversation ,,iii4  ,,on ,,the ,,laws
 ,,of ,,motion4  ,,of ,,motion4 ,,of    
,,the ,,inertia ,,of ,,bodies4 ,,of     
,,force ,,to ,,produce ,,motion4        
,,direction ,,of ,,motion4 ,,velocity1  
,,absolute ,,and ,,relative4 ,,uniform  
,,motion4 ,,retarded ,,motion4          
,,accelerated ,,motion4 ,,velocity ,,of 
,,falling ,,bodies4 ,,momentum4 ,,action
 ,,and ,,reaction ,,equal4 ,,elasticity 
,,of ,,bodies4 ,,porosity ,,of ,,bodies4
 ,,reflected ,,motion4 ,,angles ,,of    
,,incidence ,,and ,,reflection4   ,,mrs4

 ,b4  ,the science of mechanics is      
founded on the laws of motion2 it will  
therefore be necessary to make you      
acquainted with these laws before we    
examine the mechanical powers4 ,tell me1
 ,caroline1 what do you understand by   
the word motion8  ,caroline4 ,i think ,i
 understand it perfectly1 though ,i am  
at a loss to describe it4 ,motion is the
 act of moving about1 of going from one 
place to another1 it is the contrary of 
remaining at rest4  ,mrs4 ,b4 ,very     
well4 ,motion then consists in a change 
of place2 a body is in motion whenever  
it is changing its situation with regard
 to a fixed point4  ,now since we have  
observed that one of the general        
properties of bodies is inertia1 that   
is1 an entire passiveness1 either with  
regard to motion or rest1 it follows    
that a body cannot move without being   
put into motion2 the power which puts a 
body into motion is called force2 thus  
the stroke of the hammer is the force   
which drives the nail2 the pulling of   

the horse that which draws the carriage1
 c4 ,force then is the cause which      
produces motion4  ,emily4 ,and may we   
not say that gravity is the force which 
occasions the fall of bodies8  ,mrs4 ,b4
 ,undoubtedly4 ,i have given you the    
most familiar illustrations in order to 
render the explanation clear2 but since 
you seek for more scientific examples1  
you may say that cohesion is the force  
which binds the particles of bodies     
together1 and heat that which drives    
them asunder4  ,the motion of a body    
acted upon by a single force1 is always 
in a straight line1 and in the direction
 in which it received the impulse4      
,caroline4 ,that is very natural2 for as
 the body is inert1 and can move only   
because it is impelled1 it will move    
only in the direction in which it is    
impelled4 ,the degree of quickness with 
which it moves1 must1 ,i suppose1 also  
depend upon the degree of force with    
which it is impelled4  ,mrs4 ,b4 ,yes2  
the rate at which a body moves1 or the  

shortness of the time which it takes to 
move from one place to another1 is      
called its velocity2 and it is one of   
the laws of motion1 that the velocity of
 the moving body is proportional to the 
force by which it is put in motion4 ,we 
must distinguish between absolute and   
relative velocity4  ,the velocity of a  
body is called absolute1 if we consider 
the motion of the body in space1 without
 any reference to that of other bodies4 
,when1 for instance1 a horse goes fifty 
miles in ten hours1 his velocity is five
 miles an hour4  ,the velocity of a body
 is termed relative1 when compared with 
that of another body which is itself in 
motion4 ,for instance1 if one man walks 
at the rate of a mile an hour1 and      
another at the rate of two miles an     
hour1 the relative velocity of the      
latter is double that of the former2 but
 the absolute velocity of the one is one
 mile1 and that of the other two miles  
an hour4  ,emily4 ,let me see if ,i     
understand it--,the relative velocity of

 a body is the degree of rapidity of its
 motion compared with that of another   
body2 thus if one ship sail three times 
as far as another ship in the same space
 of time1 the velocity of the former is 
equal to three times that of the latter4
  ,mrs4 ,b4 ,the general rule may be    
expressed thus3 the velocity of a body  
is measured by the space over which it  
moves1 divided by the time which it     
employs in that motion3 thus if you     
travel one hundred miles in twenty      
hours1 what is your velocity in each    
hour8  ,emily4 ,i must divide the space1
 which is one hundred miles1 by the     
time1 which is twenty hours1 and the    
answer will be five miles an hour4      
,then1 ,mrs4 ,b41 may we not reverse    
this rule1 and say that the time is     
equal to the space divided by the       
velocity2 since the space1 one hundred  
miles1 divided by the velocity1 five    
miles per hour1 gives twenty hours for  
the time8  ,mrs4 ,b4 ,certainly2 and we 
may say also that the space is equal to 

the velocity multiplied by the time4    
,can you tell me1 ,caroline1 how many   
miles you will have travelled1 if your  
velocity is three miles an hour1 and you
 travel six hours8  ,caroline4 ,eighteen
 miles2 for the product of #c multiplied
 by #f1 is #ah4  ,mrs4 ,b4 ,i suppose   
that you understand what is meant by the
 terms uniform1 accelerated and retarded
 motion4  ,emily4 ,i conceive uniform   
motion to be that of a body whose motion
 is regular1 and at an equal rate       
throughout2 for instance a horse that   
goes an equal number of miles every     
hour4 ,but the hand of a watch is a much
 better example1 as its motion is so    
regular as to indicate the time4  ,mrs4 
,b4 ,you have a right idea of uniform   
motion2 but it would be more correctly  
expressed by saying1 that the motion of 
a body is uniform when it passes over   
equal spaces in equal times4 ,uniform   
motion is produced by a force having    
acted on a body once and having ceased  
to act2 as1 for instance1 the stroke of 

a bat on a ball4  ,caroline4 ,but the   
motion of a ball is not uniform2 its    
velocity gradually diminishes till it   
falls to the ground4  ,mrs4 ,b4         
,recollect that the ball is inert1 and  
has no more power to stop1 than to put  
itself in motion2 if it falls1          
therefore1 it must be stopped by some   
force superior to that by which it was  
projected1 and which destroys its       
motion4  ,caroline4 ,and it is no doubt 
the force of gravity which counteracts  
and destroys that of projection2 but if 
there were no such power as gravity1    
would the ball never stop8  ,mrs4 ,b4   
,if neither gravity nor any other force1
 such as the resistance of the air1     
opposed its motion1 the ball1 or even a 
stone thrown by the hand1 would proceed 
onwards in a right line1 and with a     
uniform velocity for ever4  ,caroline4  
,you astonish me6 ,i thought that it was
 impossible to produce perpetual motion8
  ,mrs4 ,b4 ,perpetual motion cannot be 
produced by art1 because gravity        

ultimately destroys all motion that     
human power can produce4  ,emily4 ,but  
independently of the force of gravity1  
,i cannot conceive that the little      
motion ,i am capable of giving to a     
stone would put it in motion for ever4  
,mrs4 ,b4 ,the quantity of motion you   
communicate to the stone would not      
influence its duration2 if you threw it 
with little force it would move slowly1 
for its velocity you must remember1 will
 be proportional to the force with which
 it is projected2 but if there is       
nothing to obstruct its passage1 it will
 continue to move with the same         
velocity1 and in the same direction as  
when you first projected it4  ,caroline4
 ,this appears to me quite              
incomprehensible2 we do not meet with a 
single instance of it in nature4  ,mrs4 
,b4 ,i beg your pardon4 ,when you come  
to study the motion of the celestial    
bodies1 you will find that nature       
abounds with examples of perpetual      
motion2 and that it conduces as much to 

the harmony of the system of the        
universe1 as the prevalence of it on the
 surface of the earth1 would to the     
destruction of all our comforts4 ,the   
wisdom of ,providence has therefore     
ordained insurmountable obstacles to    
perpetual motion here below2 and though 
these obstacles often compel us to      
contend with great difficulties1 yet    
these appear necessary to that order1   
regularity and repose1 so essential to  
the preservation of all the various     
beings of which this world is composed4 
,now can you tell me what is retarded   
motion8  ,caroline4 ,retarded motion is 
that of a body which moves every moment 
slower and slower3 thus when ,i am tired
 with walking fast1 ,i slacken my pace2 
or when a stone is thrown upwards1 its  
velocity is gradually diminished by the 
power of gravity4  ,mrs4 ,b4 ,retarded  
motion is produced by some force acting 
upon the body in a direction opposite to
 that which first put it in motion3 you 
who are an animated being1 endowed with 

power and will1 may slacken your pace1  
or stop to rest when you are tired2 but 
inert matter is incapable of any feeling
 of fatigue1 can never slacken its pace1
 and never stop1 unless retarded or     
arrested in its course by some opposing 
force2 and as it is the laws of inert   
bodies of which mechanical philosophy   
treats1 ,i prefer your illustration of  
the stone retarded in its ascent4 ,now  
,emily1 it is your turn2 what is        
accelerated motion8  ,emily4            
,accelerated motion1 ,i suppose1 takes  
place when the velocity of a body is    
increased2 if you had not objected to   
our giving such active bodies as        
ourselves as examples1 ,i should say    
that my motion is accelerated if ,i     
change my pace from walking to running4 
,i cannot think of any instance of      
accelerated motion in inanimate bodies2 
all motion of inert matter seems to be  
retarded by gravity4  ,mrs4 ,b4 ,not in 
all cases2 for the power of gravitation 
sometimes produces accelerated motion2  

for instance1 a stone falling from a    
height1 moves with a regularly          
accelerated motion4  ,emily4 ,true2     
because the nearer it approaches the    
earth1 the more it is attracted by it4  
,mrs4 ,b4 ,you have mistaken the cause  
of its accelerated motion2 for though it
 is true that the force of gravity      
increases as a body approaches the      
earth1 the difference is so trifling at 
any small distance from its surface1 as 
not to be perceptible4  ,accelerated    
motion is produced when the force which 
put a body in motion1 continues to act  
upon it during its motion1 so that its  
velocity is continually increased4 ,when
 a stone falls from a height1 the       
impulse which it receives from          
gravitation in the first instant of its 
fall1 would be sufficient to bring it to
 the ground with a uniform velocity3    
for1 as we have observed1 a body having 
been once acted upon by a force1 will   
continue to move with a uniform         
velocity2 but the stone is not acted    

upon by gravity merely at the first     
instant of its fall2 this power         
continues to impel it during the whole  
time of its descent1 and it is this     
continued impulse which accelerates its 
motion4  ,emily4 ,i do not quite        
understand that4  ,mrs4 ,b4 ,let us     
suppose that the instant after you have 
let a stone fall from a high tower1 the 
force of gravity were annihilated2 the  
body would nevertheless continue to move
 downwards1 for it would have received a
 first impulse from gravity2 and a body 
once put in motion will not stop unless 
it meets with some obstacle to impede   
its course2 in this case its velocity   
would be uniform1 for though there would
 be no obstacle to obstruct its descent1
 there would be no force to accelerate  
it4  ,emily4 ,that is very clear4  ,mrs4
 ,b4 ,then you have only to add the     
power of gravity constantly acting on   
the stone during its descent1 and it    
will not be difficult to understand that
 its motion will become accelerated1    

since the gravity which acts on the     
stone at the very first instant of its  
descent1 will continue in force every   
instant1 till it reaches the ground4    
,let us suppose that the impulse given  
by gravity to the stone during the first
 instant of its descent1 be equal to    
one2 the next instant we shall find that
 an additional impulse gives the stone  
an additional velocity1 equal to one2 so
 that the accumulated velocity is now   
equal to two2 the following instant     
another impulse increases the velocity  
to three1 and so on till the stone      
reaches the ground4  ,caroline4 ,now ,i 
understand it2 the effects of preceding 
impulses continue1 whilst gravity       
constantly adds new ones1 and thus the  
velocity is perpetually increased4      
,mrs4 ,b4 ,yes2 it has been ascertained1
 both by experiment1 and calculations   
which it would be too difficult for us  
to enter into1 that heavy bodies near   
the surface of the earth1 descending    
from a height by the force of gravity1  

fall sixteen feet the first second of   
time1 three times that distance in the  
next1 five times in the third second1   
seven times in the fourth1 and so on1   
regularly increasing their velocities in
 the proportion of the odd numbers #a1  
#c1 #e1 #g1 #i1 c4 according to the     
number of seconds during which the body 
has been falling4  ,emily4 ,if you throw
 a stone perpendicularly upwards1 is it 
not the same length of time in          
ascending1 that it is in descending8    
,mrs4 ,b4 ,exactly2 in ascending1 the   
velocity is diminished by the force of  
gravity2 in descending1 it is           
accelerated by it4  ,caroline4 ,i should
 then imagine that it would fall1       
quicker than it rose8  ,mrs4 ,b4 ,you   
must recollect that the force with which
 it is projected1 must be taken into the
 account2 and that this force is        
overcome and destroyed by gravity1      
before the body begins to fall4         
,caroline4 ,but the force of projection 
given to a stone in throwing it upwards1

 cannot always be equal to the force of 
gravity in bringing it down again2 for  
the force of gravity is always the same1
 whilst the degree of impulse given to  
the stone is optional2 ,i may throw it  
up gently1 or with violence4  ,mrs4 ,b4 
,if you throw it gently1 it will not    
rise high2 perhaps only sixteen feet1 in
 which case it will fall in one second  
of time4 ,now it is proved by           
experiment1 that an impulse requisite to
 project a body sixteen feet upwards1   
will make it ascend that height in one  
second2 here then the times of the      
ascent and descent are equal4 ,but      
supposing it be required to throw a     
stone twice that height1 the force must 
be proportionally greater4  ,you see    
then1 that the impulse of projection in 
throwing a body upwards1 is always equal
 to the action of the force of gravity  
during its descent2 and that whether the
 body rises to a greater or less        
distance1 these two forces balance each 
other4  ,i must now explain to you what 

is meant by the momentum of bodies4 ,it 
is the force1 or power1 with which a    
body in motion1 strikes against another 
body4 ,the momentum of a body is the    
product of its quantity of matter1      
multiplied by its quantity of motion2 in
 other words1 its weight multiplied by  
its velocity4  ,caroline4 ,the quicker a
 body moves1 the greater1 no doubt1 must
 be the force which it would strike     
against another body4  ,emily4          
,therefore a light body may have a      
greater momentum than a heavier one1    
provided its velocity be sufficiently   
increased2 for instance1 the momentum of
 an arrow shot from a bow1 must be      
greater than that of a stone thrown by  
the hand4  ,caroline4 ,we know also by  
experience1 that the heavier a body is1 
the greater is its force2 it is not     
therefore difficult to understand1 that 
the whole power1 or momentum of a body1 
must be composed of these two           
properties1 its weight and its velocity3
 but ,i do not understand why they      

should be multiplied1 the one by the    
other2 ,i should have supposed that the 
quantity of matter1 should have been    
added to the quantity of motion8  ,mrs4 
,b4 ,it is found by experiment1 that if 
the weight of a body is represented by  
the number #c1 and its velocity also by 
#c1 its momentum will be represented by 
#i1 not by #f1 as would be the case1    
were these figures added1 instead of    
being multiplied together4  ,emily4 ,i  
think that ,i now understand the reason 
of this2 if the quantity of matter is   
increased three-fold1 it must require   
three times the force to move it with   
the same velocity2 and then if we wish  
to give it three times the velocity1 it 
will again require three times the force
 to produce that effect1 which is three 
times three1 or nine2 which number      
therefore1 would represent the momentum4
  ,caroline4 ,i am not quite sure that  
,i fully comprehend what is intended1   
when weight1 and velocity1 are          
represented by numbers alone2 ,i am so  

used to measure space by yards and      
miles1 and weight by pounds and ounces1 
that ,i still want to associate them    
together in my mind4  ,mrs4 ,b4 ,this   
difficulty will be of very short        
duration3 you have only to be careful1  
that when you represent weights and     
velocities by numbers1 the denominations
 or values of the weights and spaces1   
must not be changed4 ,thus1 if we       
estimate the weight of one body in      
ounces1 the weight of others with which 
it is compared1 must be estimated in    
ounces1 and not in pounds2 and in like  
manner1 in comparing velocities1 we must
 throughout1 preserve the same standards
 both of space and of time2 as for      
instance1 the number of feet in one     
second1 or of miles in one hour4        
,caroline4 ,i now understand it         
perfectly1 and think that ,i shall never
 forget a thing which you have rendered 
so clear4  ,mrs4 ,b4 ,i recommend it to 
you to be very careful to remember the  
definition of the momentum of bodies1 as

 it is one of the most important points 
in mechanics3 you will find that it is  
from opposing velocity1 to quantity of  
matter1 that machines derive their      
powers4  ,the reaction of bodies1 is the
 next law of motion which ,i must       
explain to you4 ,when a body in motion  
strikes against another body1 it meets  
with resistance from it2 the resistance 
of the body at rest will be equal to the
 blow struck by the body in motion2 or  
to express myself in philosophical      
language1 action and reaction will be   
equal1 and in opposite directions4      
,caroline4 ,do you mean to say1 that the
 action of the body which strikes1 is   
returned with equal force by the body   
which receives the blow8  ,mrs4 ,b4     
,exactly4  ,caroline4 ,but if a man     
strike another on the face with his     
fist1 he surely does not receive as much
 pain by the reaction1 as he inflicts by
 the blow8  ,mrs4 ,b4 ,no2 but this is  
simply owing to the knuckles1 having    
much less feeling than the face4  ,here 

are two ivory balls suspended by        
threads1 7plate #a4 fig4 #c47 draw one  
of them1 ,a1 a little on one side1--now 
let it go2--it strikes1 you see1 against
 the other ball ,b1 and drives it off1  
to a distance equal to that through     
which the first ball fell2 but the      
motion of ,a is stopped2 because when it
 struck ,b1 it received in return a blow
 equal to that it gave1 and its motion  
was consequently destroyed4  ,emily4 ,i 
should have supposed1 that the motion of
 the ball ,a was destroyed1 because it  
had communicated all its motion to ,b4  
,mrs4 ,b4 ,it is perfectly true1 that   
when one body strikes against another1  
the quantity of motion communicated to  
the second body1 is lost by the first2  
but this loss proceeds from the reaction
 of the body which is struck4  ,here are
 six ivory balls hanging in a row1 7fig4
 #d47 draw the first out of the         
perpendicular1 and let it fall against  
the second4 ,you see none of the balls  
except the last1 appear to move1 this   

flies off as far as the first ball fell2
 can you explain this8  ,caroline4 ,i   
believe so4 ,when the first ball struck 
the second1 it received a blow in       
return1 which destroyed its motion2 the 
second ball1 though it did not appear to
 move1 must have struck against the     
third2 the reaction of which set it at  
rest2 the action of the third ball must 
have been destroyed by the reaction of  
the fourth1 and so on till motion was   
communicated to the last ball1 which1   
not being reacted upon1 flies off4      
,mrs4 ,b4 ,very well explained4         
,observe1 that it is only when bodies   
are elastic1 as these ivory balls are1  
and when their masses are equal1 that   
the stroke returned is equal to the     
stroke given1 and that the striking body
 loses all its motion4 ,i will show you 
the difference with these two balls of  
clay1 7fig4 #e47 which are not elastic2 
when you raise one of these1 ,d1 out of 
the perpendicular1 and let it fall      
against the other1 ,e1 the reaction of  

the latter1 on account of its not being 
elastic1 is not sufficient to destroy   
the motion of the former2 only part of  
the motion of ,d will be communicated to
 ,e1 and the two balls will move on     
together to d and e1 which is not so    
great a distance as that through which  
,d fell4  ,observe how useful reaction  
is in nature4 ,birds in flying strike   
the air with their wings1 and it is the 
reaction of the air1 which enables them 
to rise1 or advance forwards2 reaction  
being always in a contrary direction to 
action4  ,caroline4 ,i thought that     
birds might be lighter than the air1    
when their wings were expanded1 and were
 by that means enabled to fly4  ,mrs4   
,b4 ,when their wings are spread1 this  
does not alter their weight1 but they   
are better supported by the air1 as they
 cover a greater extent of surface2 yet 
they are still much too heavy to remain 
in that situation1 without continually  
flapping their wings1 as you may have   
noticed when birds hover over their     

nests3 the force with which their wings 
strike against the air1 must equal the  
weight of their bodies1 in order that   
the reaction of the air1 may be able to 
support that weight2 the bird will then 
remain stationary4 ,if the stroke of the
 wings is greater than is required      
merely to support the bird1 the reaction
 of the air will make it rise2 if it be 
less1 it will gently descend2 and you   
may have observed the lark1 sometimes   
remaining with its wings extended1 but  
motionless2 in this state it drops      
quietly into its nest4  ,caroline4 ,this
 is indeed a beautiful effect of the law
 of reaction6 ,but if flying is merely a
 mechanical operation1 ,mrs4 ,b41 why   
should we not construct wings1 adapted  
to the size of our bodies1 fasten them  
to our shoulders1 move them with our    
arms1 and soar into the air8  ,mrs4 ,b4 
,such an experiment has been repeatedly 
attempted1 but never with success2 and  
it is now considered as totally         
impracticable4 ,the muscular power of   

birds1 is incomparably greater in       
proportion to their weight1 than that of
 man2 were we therefore furnished with  
wings sufficiently large to enable us to
 fly1 we should not have strength to put
 them in motion4  ,in swimming1 a       
similar action is produced on the water1
 to that on the air1 in flying2 in      
rowing1 also1 you strike the water with 
the oars1 in a direction opposite to    
that in which the boat is required to   
move1 and it is the reaction of the     
water on the oars which drives the boat 
along4  ,emily4 ,you said1 that it was  
in elastic bodies only1 that the whole  
motion of one body1 would be            
communicated to another2 pray what      
bodies are elastic1 besides the air8    
,mrs4 ,b4 ,in speaking of the air1 ,i   
think we defined elasticity to be a     
property1 by means of which bodies that 
are compressed1 return to their former  
state4 ,if ,i bend this cane1 as soon as
 ,i leave it at liberty1 it recovers its
 former position2 if ,i press my finger 

upon your arm1 as soon as ,i remove it1 
the flesh1 by virtue of its elasticity1 
rises and destroys the impression ,i    
made4 ,of all bodies1 the air is the    
most eminent for this property1 and it  
has thence obtained the name of an      
elastic fluid4 ,hard bodies are in the  
next degree elastic2 if two ivory1 or   
hardened steel balls are struck         
together1 the parts at which they touch1
 will be flattened2 but their elasticity
 will make them instantaneously resume  
their former shape4  ,caroline4 ,but    
when two ivory balls strike against each
 other1 as they constantly do on a      
billiard table1 no mark or impression is
 made by the stroke4  ,mrs4 ,b4 ,i beg  
your pardon2 you cannot1 it is true1    
perceive any mark1 because their        
elasticity instantly destroys all trace 
of it4  ,soft bodies1 which easily      
retain impressions1 such as clay1 wax1  
tallow1 butter1 c4 have very little     
elasticity2 but of all descriptions of  
bodies1 liquids are the least elastic4  

,emily4 ,if sealing-wax were elastic1   
instead of retaining the impression of a
 seal1 it would resume a smooth surface1
 as soon as the weight of the seal was  
removed4 ,but pray what is it that      
produces the elasticity of bodies8      
,mrs4 ,b4 ,there is great diversity of  
opinion upon that point1 and ,i cannot  
pretend to decide which approaches      
nearest to the truth4 ,elasticity       
implies susceptibility of compression1  
and the susceptibility of compression   
depends upon the porosity of bodies2 for
 were there no pores or spaces between  
the particles of matter of which a body 
is composed1 it could not be compressed4
  ,caroline4 ,that is to say1 that if   
the particles of bodies were as close   
together as possible1 they could not be 
squeezed closer4  ,emily4 ,bodies then1 
whose particles are most distant from   
each other1 must be most susceptible of 
compression1 and consequently most      
elastic2 and this you say is the case   
with air1 which is perhaps the least    

dense of all bodies8  ,mrs4 ,b4 ,you    
will not in general find this rule hold 
good2 for liquids have scarcely any     
elasticity1 whilst hard bodies are      
eminent for this property1 though the   
latter are certainly of much greater    
density than the former2 elasticity     
implies1 therefore1 not only a          
susceptibility of compression1 but      
depends upon the power possessed by the 
body1 of resuming its former state after
 compression1 in consequence of the     
peculiar arrangement of its particles4  
,caroline4 ,but surely there can be no  
pores in ivory and metals1 ,mrs4 ,b42   
how then can they be susceptible of     
compression8  ,mrs4 ,b4 ,the pores of   
such bodies are invisible to the naked  
eye1 but you must not thence conclude   
that they have none2 it is1 on the      
contrary1 well ascertained that gold1   
one of the most dense of all bodies1 is 
extremely porous2 and that these pores  
are sufficiently large to admit water   
when strongly compressed1 to pass       

through them4 ,this was shown by a      
celebrated experiment made many years   
ago at ,florence4  ,emily4 ,if water can
 pass through gold1 there must certainly
 be pores or interstices which afford it
 a passage2 and if gold is so porous1   
what must other bodies be1 which are so 
much less dense than gold6  ,mrs4 ,b4   
,the chief difference in this respect1  
is ,i believe1 that the pores in some   
bodies are larger than in others2 in    
cork1 sponge and bread1 they form       
considerable cavities2 in wood and      
stone1 when not polished1 they are      
generally perceptible to the naked eye2 
whilst in ivory1 metals1 and all        
varnished and polished bodies1 they     
cannot be discerned4 ,to give you an    
idea of the extreme porosity of bodies1 
sir ,isaac ,newton conjectured that if  
the earth were so compressed as to be   
absolutely without pores1 its dimensions
 might possibly not be more than a cubic
 inch4  ,caroline4 ,what an idea6 ,were 
we not indebted to sir ,isaac ,newton   

for the theory of attraction1 ,i should 
be tempted to laugh at him for such a   
supposition4 ,what insignificant little 
creatures we should be6  ,mrs4 ,b4 ,if  
our consequence arose from the size of  
our bodies1 we should indeed be but     
pigmies1 but remember that the mind of  
,newton was not circumscribed by the    
dimensions of its envelope4  ,emily4 ,it
 is1 however1 fortunate that heat keeps 
the pores of matter open and distended1 
and prevents the attraction of cohesion 
from squeezing us into a nut-shell4     
,mrs4 ,b4 ,let us now return to the     
subject of reaction1 on which we have   
some further observations to make4 ,it  
is because reaction is in its direction 
opposite to action1 that reflected      
motion is produced4 ,if you throw a ball
 against the wall1 it rebounds2 this    
return of the ball is owing to the      
reaction of the wall against which it   
struck1 and is called reflected motion4 
,emily4 ,and ,i now understand why balls
 filled with air rebound better than    

those stuffed with bran or wool2 air    
being most susceptible of compression   
and most elastic1 the reaction is more  
complete4  ,caroline4 ,i have observed  
that when ,i throw a ball straight      
against the wall1 it returns straight to
 my hand2 but if ,i throw it obliquely  
upwards1 it rebounds still higher1 and  
,i catch it when it falls4  ,mrs4 ,b4   
,you should not say straight1 but       
perpendicularly against the wall2 for   
straight is a general term for lines in 
all directions which are neither curved 
nor bent1 and is therefore equally      
applicable to oblique or perpendicular  
lines4  ,caroline4 ,i thought that      
perpendicularly meant either directly   
upwards or downwards8  ,mrs4 ,b4 ,in    
those directions lines are perpendicular
 to the earth4 ,a perpendicular line has
 always a reference to something towards
 which it is perpendicular2 that is to  
say1 that it inclines neither to the one
 side or the other1 but makes an equal  
angle on every side4 ,do you understand 

what an angle is8  ,caroline4 ,yes1 ,i  
believe so3 it is the space contained   
between two lines meeting in a point4   
,mrs4 ,b4 ,well then1 let the line ,a ,b
 7plate #b4 fig4 #a47 represent the     
floor of the room1 and the line ,c ,d   
that in which you throw a ball against  
it2 the line ,c ,d1 you will observe1   
forms two angles with the line ,a ,b1   
and those two angles are equal4  ,emily4
 ,how can the angles be equal1 while the
 lines which compose them are of unequal
 length8  ,mrs4 ,b4 ,an angle is not    
measured by the length of the lines1 but
 by their opening1 or the space between 
them4  ,emily4 ,yet the longer the lines
 are1 the greater is the opening between
 them4  ,mrs4 ,b4 ,take a pair of       
compasses and draw a circle over these  
spaces1 making the angular point the    
centre4  ,emily4 ,to what extent must ,i
 open the compasses8  ,mrs4 ,b4 ,you may
 draw the circle what size you please1  
provided that it cuts the lines of the  
angles we are to measure4 ,all circles1 

of whatever dimensions1 are supposed to 
be divided into #cfj equal parts1 called
 degrees2 the opening of an angle1 being
 therefore a portion of a circle1 must  
contain a certain number of degrees3 the
 larger the angle the greater is the    
number of degrees1 and two angles are   
said to be equal1 when they contain an  
equal number of degrees4  ,emily4 ,now  
,i understand it4 ,as the dimension of  
an angle depends upon the number of     
degrees contained between its lines1 it 
is the opening1 and not the length of   
its lines1 which determines the size of 
the angle4  ,mrs4 ,b4 ,very well3 now   
that you have a clear idea of the       
dimensions of angles1 can you tell me   
how many degrees are contained in the   
two angles formed by one line falling   
perpendicularly on another1 as in the   
figure ,i have just drawn8  ,emily4 ,you
 must allow me to put one foot of the   
compasses at the point of the angles1   
and draw a circle round them1 and then  
,i think ,i shall be able to answer your

 question3 the two angles are together  
just equal to half a circle1 they       
contain therefore #ij degrees each2 #ij 
degrees being a quarter of #cfj4  ,mrs4 
,b4 ,an angle of #ij degrees or         
one-fourth of a circle is called a right
 angle1 and when one line is            
perpendicular to another1 and distant   
from its ends1 it forms1 you see1 7fig4 
#a47 a right angle on either side4      
,angles containing more than #ij degrees
 are called obtuse angles1 7fig4 #b47   
and those containing less than #ij      
degrees are called acute angles1 7fig4  
#c47  ,caroline4 ,the angles of this    
square table are right angles1 but those
 of the octagon table are obtuse angles2
 and the angles of sharp pointed        
instruments are acute angles4           
,illustration3 ,,plate ,,ii4  ,mrs4 ,b4 
,very well4 ,to return now to your      
observation1 that if a ball is thrown   
obliquely against the wall1 it will not 
rebound in the same direction2 tell me1 
have you ever played at billiards8      

,caroline4 ,yes1 frequently2 and ,i have
 observed that when ,i push the ball    
perpendicularly against the cushion1 it 
returns in the same direction2 but when 
,i send it obliquely to the cushion1 it 
rebounds obliquely1 but on an opposite  
side2 the ball in this latter case      
describes an angle1 the point of which  
is at the cushion4 ,i have observed too1
 that the more obliquely the ball is    
struck against the cushion1 the more    
obliquely it rebounds on the opposite   
side1 so that a billiard player can     
calculate with great accuracy in what   
direction it will return4  ,mrs4 ,b4    
,very well4 ,this figure 7fig4 #d4 plate
 #b47 represents a billiard table2 now  
if you draw a line ,a ,b from the point 
where the ball ,a strikes perpendicular 
to the cushion1 you will find that it   
will divide the angle which the ball    
describes into two parts1 or two angles2
 the one will show the obliquity of the 
direction of the ball in its passage    
towards the cushion1 the other its      

obliquity in its passage back from the  
cushion4 ,the first is called the angle 
of incidence1 the other the angle of    
reflection2 and these angles are always 
equal1 if the bodies are perfectly      
elastic4  ,caroline4 ,this then is the  
reason why1 when ,i throw a ball        
obliquely against the wall1 it rebounds 
in an opposite oblique direction1       
forming equal angles of incidence and of
 reflection4  ,mrs4 ,b4 ,certainly2 and 
you will find that the more obliquely   
you throw the ball1 the more obliquely  
it will rebound4  ,we must now conclude2
 but ,i shall have some further         
observations to make upon the laws of   
motion1 at our next meeting4            
,questions  #a4 7,pg4 #cb7 ,on what is  
the science of mechanics founded8  #b4  
7,pg4 #cb7 ,in what does motion consist8
  #c4 7,pg4 #cc7 ,what is the           
consequence of inertia1 on a body at    
rest8  #d4 7,pg4 #cc7 ,what do we call  
that which produces motion8  #e4 7,pg4  
#cc7 ,give some examples4  #f4 7,pg4    

#cc7 ,what may we say of gravity1 of    
cohesion1 and of heat1 as forces8  #g4  
7,pg4 #cc7 ,how will a body move1 if    
acted on by a single force8  #h4 7,pg4  
#cc7 ,what is the reason of this8  #i4  
7,pg4 #cc7 ,what do we intend by the    
term velocity1 and to what is it        
proportional8  #aj4 7,pg4 #cc7 ,velocity
 is divided into absolute and relative2 
what is meant by absolute velocity8     
#aa4 7,pg4 #cc7 ,how is relative        
velocity distinguished8  #ab4 7,pg4 #cd7
 ,how do we measure the velocity of a   
body8  #ac4 7,pg4 #cd7 ,the time8  #ad4 
7,pg4 #cd7 ,the space8  #ae4 7,pg4 #cd7 
,what is uniform motion8 and give an    
example4  #af4 7,pg4 #cd7 ,how is       
uniform motion produced8  #ag4 7,pg4    
#cd7 ,a ball struck by a bat gradually  
loses its motion2 what causes produce   
this effect8  #ah4 7,pg4 #ce7 ,if       
gravity did not draw a projected body   
towards the earth1 and the resistance of
 the air were removed1 what would be the
 consequence8  #ai4 7,pg4 #ce7 ,in this 

case would not a great degree of force  
be required to produce a continued      
motion8  #bj4 7,pg4 #ce7 ,what is       
retarded motion8  #ba4 7,pg4 #ce7 ,give 
some examples4  #bb4 7,pg4 #cf7 ,what is
 accelerated motion8  #bc4 7,pg4 #cf7   
,give an example4  #bd4 7,pg4 #cf7      
,explain the mode in which gravity      
operates in producing this effect4  #be4
 7,pg4 #cg7 ,what number of feet will a 
heavy body descend in the first second  
of its fall1 and at what rate will its  
velocity increase8  #bf4 7,pg4 #cg7     
,what is the difference in the time of  
the ascent and descent1 of a stone1 or  
other body thrown upwards8  #bg4 7,pg4  
#cg7 ,by what reasoning is it proved    
that there is no difference8  #bh4 7,pg4
 #ch7 ,what is meant by the momentum of 
a body8  #bi4 7,pg4 #ch7 ,how do we     
ascertain the momentum8  #cj4 7,pg4 #ch7
 ,how may a light body have a greater   
momentum than one which is heavier8     
#ca4 7,pg4 #ch7 ,why must we multiply   
the weight and velocity together in     

order to find the momentum8  #cb4 7,pg4 
#ci7 ,when we represent weight and      
velocity by numbers1 what must we       
carefully observe8  #cc4 7,pg4 #ci7 ,why
 is it particularly important1 to       
understand the nature of momentum8  #cd4
 7,pg4 #ci7 ,what is meant by reaction1 
and what is the rule respecting it8     
#ce4 7,pg4 #ci7 ,how is this exemplified
 by the ivory balls represented in plate
 #a4 fig4 #c8  #cf4 7,pg4 #dj7 ,explain 
the manner in which the six balls       
represented in fig4 #d1 illustrate this 
fact4  #cg4 7,pg4 #dj7 ,what must be the
 nature of bodies1 in which the whole   
motion is communicated from one to the  
other8  #ch4 7,pg4 #dj7 ,what is the    
result if the balls are not elastic1 and
 how is this explained by fig4 #e8  #ci4
 7,pg4 #dj7 ,how will reaction assist us
 in explaining the flight of a bird8    
#dj4 7,pg4 #dj7 ,how must their wings   
operate in enabling them to remain      
stationary1 to rise1 and to descend8    
#da4 7,pg4 #da7 ,why cannot a man fly by

 the aid of wings8  #db4 7,pg4 #da7 ,how
 does reaction operate in enabling us to
 swim1 or to row a boat8  #dc4 7,pg4    
#da7 ,what constitutes elasticity8  #dd4
 7,pg4 #da7 ,give some examples4  #de4  
7,pg4 #da7 ,what name is given to air1  
and for what reason8  #df4 7,pg4 #da7   
,what hard bodies are mentioned as      
elastic8  #dg4 7,pg4 #da7 ,do elastic   
bodies exhibit any indentation after a  
blow8 and why not8  #dh4 7,pg4 #db7     
,what do we conclude from elasticity    
respecting the contact of the particles 
of a body8  #di4 7,pg4 #db7 ,are those  
bodies always the most elastic1 which   
are the least dense8  #ej4 7,pg4 #db7   
,give examples to prove that this is not
 the case4  #ea4 7,pg4 #db7 ,all bodies 
are believed to be porous1 what is said 
on this subject respecting gold8  #eb4  
7,pg4 #dc7 ,what conjecture was made by 
sir ,isaac ,newton1 respecting the      
porosity of bodies in general8  #ec4    
7,pg4 #dc7 ,if you throw an elastic body
 against a wall1 it will rebound2 what  

is this occasioned by1 and what is this 
return motion called8  #ed4 7,pg4 #dc7  
,what do we mean by a perpendicular     
line8  #ee4 7,pg4 #dc7 ,what is an      
angle8  #ef4 7,pg4 #dc7 ,what is        
represented by fig4 #a4 plate #b8  #eg4 
7,pg4 #dd7 ,have the length of the lines
 which meet in a point1 any thing to do 
with the measurement of an angle8  #eh4 
7,pg4 #dd7 ,what use can we make of     
compasses in measuring an angle8  #ei4  
7,pg4 #dd7 ,into what number of parts do
 we suppose a whole circle divided1 and 
what are these parts called8  #fj4 7,pg4
 #dd7 ,when are two angles said to be   
equal8  #fa4 7,pg4 #dd7 ,upon what does 
the dimension of an angle depend8  #fb4 
7,pg4 #dd7 ,what number of degrees1 and 
what portion of a circle is there in a  
right angle8  #fc4 7,pg4 #dd7 ,how must 
one line be situated on another to form 
two right angles8 7fig4 #a47  #fd4 7,pg4
 #dd7 ,figure #b represents an angle of 
more than #ij degrees1 what is that     
called8  #fe4 7,pg4 #dd7 ,what are those

 of less than #ij degrees called as in  
fig4 #c8  #ff4 7,pg4 #de7 ,if you make  
an elastic ball strike a body at right  
angles1 how will it return8  #fg4 7,pg4 
#de7 ,how if it strikes obliquely8  #fh4
 7,pg4 #de7 ,explain by fig4 #d what is 
meant by the angles of incidence and of 
reflection4     ,,conversation ,,iv4    
,,on ,,compound ,,motion4  ,,compound   
,,motion1 ,,the ,,result ,,of ,,two     
,,opposite ,,forces4 ,,of ,,curvilinear 
,,motion1 ,,the ,,result ,,of ,,two     
,,forces4 ,,centre ,,of ,,motion1 ,,the 
,,point ,,at ,,rest ,,while ,,the       
,,other ,,parts ,,of ,,the ,,body ,,move
 ,,round ,,it4 ,,centre ,,of            
,,magnitude1 ,,the ,,middle ,,of ,a     
,,body4 ,,centripetal ,,force1 ,,that   
,,which ,,impels ,a ,,body ,,towards ,a 
,,fixed ,,central ,,point4 ,,centrifugal
 ,,force1 ,,that ,,which ,,impels ,a    
,,body ,,to ,,fly ,,from ,,the ,,centre4
 ,,fall ,,of ,,bodies ,,in ,a           
,,parabola4 ,,centre ,,of ,,gravity1    
,,the ,,point ,,about ,,which ,,the     

,,parts ,,balance ,,each ,,other4       
,,mrs4 ,b4  ,i must now explain to you  
the nature of compound motion4 ,let us  
suppose a body to be struck by two equal
 forces in opposite directions1 how will
 it move8  ,emily4 ,if the forces are   
equal1 and their directions are in exact
 opposition to each other1 ,i suppose   
the body would not move at all4  ,mrs4  
,b4 ,you are perfectly right2 but       
suppose the forces instead of acting    
upon the body in direct opposition to   
each other1 were to move in lines       
forming an angle of ninety degrees1 as  
the lines ,y ,a1 ,x ,a1 7fig4 #e4 plate 
#b47 and were to strike the ball ,a1 at 
the same instant2 would it not move8    
,emily4 ,the force ,x alone1 would send 
it towards ,b1 and the force ,y towards 
,c2 and since these forces are equal1 ,i
 do not know how the body can obey one  
impulse rather than the other2 and yet  
,i think the ball would move1 because as
 the two forces do not act in direct    
opposition1 they cannot entirely destroy

 the effect of each other4  ,mrs4 ,b4   
,very true2 the ball therefore will not 
follow the direction of either of the   
forces1 but will move in a line between 
them1 and will reach ,d in the same     
space of time1 that the force ,x would  
have sent it to ,b1 and the force ,y    
would have sent it to ,c4 ,now if you   
draw two lines1 one from ,b1 parallel to
 ,a ,c1 and the other from ,c1 parallel 
to ,a ,b1 they will meet in ,d1 and you 
will form a square2 the oblique line    
which the body describes1 is called the 
diagonal of the square4  ,caroline4     
,that is very clear1 but supposing the  
two forces to be unequal1 that the force
 ,x1 for instance1 be twice as great as 
the force ,y8  ,mrs4 ,b4 ,then the force
 ,x1 would drive the ball twice as far  
as the force ,y1 consequently you must  
draw the line ,a ,b 7fig4 #f47 twice as 
long as the line ,a ,c1 the body will in
 this case move to ,d2 and if you draw  
lines from the points ,b and ,c1 exactly
 as directed in the last example1 they  

will meet in ,d1 and you will find that 
the ball has moved in the diagonal of a 
rectangle4  ,emily4 ,allow me to put    
another case4 ,suppose the two forces   
are unequal1 but do not act on the ball 
in the direction of a right angle1 but  
in that of an acute angle1 what will    
result8  ,mrs4 ,b4 ,prolong the lines in
 the directions of the two forces1 and  
you will soon discover which way the    
ball will be impelled2 it will move from
 ,a to ,d1 in the diagonal of a         
parallelogram1 7fig4 #g47 ,forces acting
 in the direction of lines forming an   
obtuse angle1 will also produce motion  
in the diagonal of a parallelogram4 ,for
 instance1 if the body set out from ,b1 
instead of ,a1 and was impelled by the  
forces ,x and ,y1 it would move in the  
dotted diagonal ,b ,c4  ,we may now     
proceed to curvilinear motion3 this is  
the result of two forces acting on a    
body2 by one of which1 it is projected  
forward in a right line2 whilst by the  
other1 it is drawn or impelled towards a

 fixed point4 ,for instance1 when ,i    
whirl this ball1 which is fastened to my
 hand with a string1 the ball moves in a
 circular direction1 because it is acted
 on by two forces2 that which ,i give   
it1 which represents the force of       
projection1 and that of the string which
 confines it to my hand4 ,if1 during its
 motion you were suddenly to cut the    
string1 the ball would fly off in a     
straight line2 being released from that 
confinement which caused it to move     
round a fixed point1 it would be acted  
on by one force only2 and motion        
produced by one force1 you know1 is     
always in a right line4  ,caroline4     
,this circular motion1 is a little more 
difficult to comprehend than compound   
motion in straight lines4  ,mrs4 ,b4    
,you have seen how the water is thrown  
off from a grindstone1 when turned      
rapidly round2 the particles of the     
stone itself have the same tendency1 and
 would also fly off1 was not their      
attraction of cohesion1 greater than    

that of water4 ,and indeed it sometimes 
happens1 that large grindstones fly to  
pieces from the rapidity of their       
motion4  ,emily4 ,in the same way1 the  
rim and spokes of a wheel1 when in rapid
 motion1 would be driven straight       
forwards in a right line1 were they not 
confined to a fixed point1 round which  
they are compelled to move4  ,mrs4 ,b4  
,very well4 ,you must now learn to      
distinguish between what is called the  
centre of motion1 and the axis of       
motion2 the former being considered as a
 point1 the latter as a line4  ,when a  
body1 like the ball at the end of the   
string1 revolves in a circle1 the centre
 of the circle is called the centre of  
its motion1 and the body is said to     
revolve in a plane2 because a line      
extended from the revolving body1 to the
 centre of motion1 would describe a     
plane1 or flat surface4  ,when a body   
revolves round itself1 as a ball        
suspended by a string1 and made to spin 
round1 or a top spinning on the floor1  

whilst it remains on the same spot2 this
 revolution is round an imaginary line  
passing through the body1 and this line 
is called its axis of motion4           
,caroline4 ,the axle of a grindstone1 is
 then the axis of its motion2 but is the
 centre of motion always in the middle  
of a body8  ,mrs4 ,b4 ,no1 not always4  
,the middle point of a body1 is called  
its centre of magnitude1 or position1   
that is1 the centre of its mass or bulk4
 ,bodies have also another centre1      
called the centre of gravity1 which ,i  
shall explain to you2 but at present we 
must confine ourselves to the axis of   
motion4 ,this line you must observe     
remains at rest1 whilst all the other   
parts of the body move around it2 when  
you spin a top1 the axis is stationary1 
whilst every other part is in motion    
round it4  ,caroline4 ,but a top        
generally has a motion forwards besides 
its spinning motion2 and then no point  
within it can be at rest8  ,mrs4 ,b4    
,what ,i say of the axis of motion1     

relates only to circular motion2 that is
 to say1 motion round a line1 and not to
 that which a body may have at the same 
time in any other direction4 ,there is  
one circumstance to which you must      
carefully attend2 namely1 that the      
further any part of a body is from the  
axis of motion1 the greater is its      
velocity3 as you approach that line1 the
 velocity of the parts gradually        
diminish till you reach the axis of     
motion1 which is perfectly at rest4     
,caroline4 ,but1 if every part of the   
same body did not move with the same    
velocity1 that part which moved         
quickest1 must be separated from the    
rest of the body1 and leave it behind8  
,mrs4 ,b4 ,you perplex yourself by      
confounding the idea of circular motion1
 with that of motion in a right line2   
you must think only of the motion of a  
body round a fixed line1 and you will   
find1 that if the parts farthest from   
the centre had not the greatest         
velocity1 those parts would not be able 

to keep up with the rest of the body1   
and would be left behind4 ,do not the   
extremities of the vanes of a windmill  
move over a much greater space1 than the
 parts nearest the axis of motion8      
7plate #c4 fig4 #a47 ,the three dotted  
circles represent the paths in which    
three different parts of the vanes move1
 and though the circles are of different
 dimensions1 each of them is described  
in the same space of time4  ,caroline4  
,certainly they are2 and ,i now only    
wonder1 that we neither of us ever made 
the observation before3 and the same    
effect must take place in a solid body1 
like the top in spinning2 the most      
bulging part of the surface must move   
with the greatest rapidity4  ,mrs4 ,b4  
,the force which draws a body towards a 
centre1 round which it moves1 is called 
the centripetal force2 and that force1  
which impels a body to fly from the     
centre1 is called the centrifugal force2
 when a body revolves round a centre1   
these two forces constantly balance each

 other2 otherwise the revolving body    
would either approach the centre or     
recede from it1 according as the one or 
the other prevailed4  ,caroline4 ,when  
,i see any body moving in a circle1 ,i  
shall remember1 that it is acted on by  
two forces4  ,mrs4 ,b4 ,motion1 either  
in a circle1 an ellipsis1 or any other  
curve-line1 must be the result of the   
action of two forces2 for you know1 that
 the impulse of one single force1 always
 produces motion in a right line4       
,emily4 ,and if any cause should destroy
 the centripetal force1 the centrifugal 
force would alone impel the body1 and it
 would1 ,i suppose1 fly off in a        
straight line from the centre to which  
it had been confined4  ,mrs4 ,b4 ,it    
would not fly off in a right line from  
the centre2 but in a right line in the  
direction in which it was moving1 at the
 instant of its release2 if a stone1    
whirled round in a sling1 gets loose at 
the point ,a1 7plate #c4 fig4 #b47 it   
flies off in the direction ,a ,b2 this  

line is called a tangent1 it touches the
 circumference of the circle1 and forms 
a right angle with a line drawn from    
that point of the circumference to the  
centre of the circle ,c4  ,emily4 ,you  
say1 that motion in a curve-line1 is    
owing to two forces acting upon a body2 
but when ,i throw this ball in a        
horizontal direction1 it describes a    
curve-line in falling2 and yet it is    
only acted upon by the force of         
projection2 there is no centripetal     
force to confine it1 or produce compound
 motion4  ,mrs4 ,b4 ,a ball thus thrown1
 is acted upon by no less than three    
forces2 the force of projection1 which  
you communicate to it2 the resistance of
 the air through which it passes1 which 
diminishes its velocity1 without        
changing its direction2 and the force of
 gravity1 which finally brings it to the
 ground4 ,the power of gravity1 and the 
resistance of the air1 being always     
greater than any force of projection we 
can give a body1 the latter is gradually

 overcome1 and the body brought to the  
ground2 but the stronger the projectile 
force1 the longer will these powers be  
in subduing it1 and the further the body
 will go before it falls4  ,caroline4 ,a
 shot fired from a cannon1 for instance1
 will go much further1 than a stone     
projected by the hand4  ,mrs4 ,b4       
,bodies thus projected1 you observe1    
describe a curve-line in their descent2 
can you account for that8  ,caroline4   
,no2 ,i do not understand why it should 
not fall in the diagonal of a square4   
,mrs4 ,b4 ,you must consider that the   
force of projection is strongest when   
the ball is first thrown2 this force1 as
 it proceeds1 being weakened by the     
continued resistance of the air1 the    
stone1 therefore1 begins by moving in a 
horizontal direction2 but as the        
stronger powers prevail1 the direction  
of the ball will gradually change from a
 horizontal1 to a perpendicular line4   
,projection alone1 would drive the ball 
,a1 to ,b1 7fig4 #c47 gravity would     

bring it to ,c2 therefore1 when acted on
 in different directions1 by these two  
forces1 it moves between1 gradually     
inclining more and more to the force of 
gravity1 in proportion as this          
accumulates2 instead therefore of       
reaching the ground at ,d1 as you       
suppose it would1 it falls somewhere    
about ,e4  ,caroline4 ,it is precisely  
so2 look ,emily1 as ,i throw this ball  
directly upwards1 how gravity and the   
resistance of the air conquer           
projection4 ,now ,i will throw it       
upwards obliquely3 see1 the force of    
projection enables it1 for an instant1  
to act in opposition to that of gravity2
 but it is soon brought down again4     
,mrs4 ,b4 ,the curve-line which the ball
 has described1 is called in geometry a 
parabola2 but when the ball is thrown   
perpendicularly upwards1 it will descend
 perpendicularly2 because the force of  
projection1 and that of gravity1 are in 
the same line of direction4             
,illustration3 ,,plate ,,iii4  ,we have 

noticed the centres of magnitude1 and of
 motion2 but ,i have not yet explained  
to you1 what is meant by the centre of  
gravity2 it is that point in a body1    
about which all the parts exactly       
balance each other2 if therefore that   
point be supported1 the body will not   
fall4 ,do you understand this8  ,emily4 
,i think so2 if the parts round about   
this point have an equal tendency to    
fall1 they will be in equilibrium1 and  
as long as this point is supported1 the 
body cannot fall4  ,mrs4 ,b4 ,caroline1 
what would be the effect1 were the body 
supported in any other single point8    
,caroline4 ,the surrounding parts no    
longer balancing each other1 the body1  
,i suppose1 would fall on the side at   
which the parts are heaviest4  ,mrs4 ,b4
 ,infallibly2 whenever the centre of    
gravity is unsupported1 the body must   
fall4 ,this sometimes happens with an   
overloaded wagon winding up a steep     
hill1 one side of the road being more   
elevated than the other2 let us suppose 

it to slope as is described in this     
figure1 7plate #c4 fig4 #d47 we will    
say1 that the centre of gravity of this 
loaded wagon is at the point ,a4 ,now   
your eye will tell you1 that a wagon    
thus situated1 will overset2 and the    
reason is1 that the centre of gravity   
,a1 is not supported2 for if you draw a 
perpendicular line from it to the ground
 at ,c1 it does not fall under the wagon
 within the wheels1 and is therefore not
 supported by them4  ,caroline4 ,i      
understand that perfectly2 but what is  
the meaning of the other point ,b8      
,mrs4 ,b4 ,let us1 in imagination take  
off the upper part of the load2 the     
centre of gravity will then change its  
situation1 and descend to ,b1 as that   
will now be the point about which the   
parts of the less heavily laden wagon   
will balance each other4 ,will the wagon
 now be upset8  ,caroline4 ,no1 because 
a perpendicular line from that point    
falls within the wheels at ,d1 and is   
supported by them2 and when the centre  

of gravity is supported1 the body will  
not fall4  ,emily4 ,yet ,i should not   
much like to pass a wagon in that       
situation1 for1 as you see1 the point ,d
 is but just within the left wheel2 if  
the right wheel was raised1 by merely   
passing over a stone1 the point ,d would
 be thrown on the outside of the left   
wheel1 and the wagon would upset4       
,caroline4 ,a wagon1 or any carriage    
whatever1 will then be most firmly      
supported1 when the centre of gravity   
falls exactly between the wheels2 and   
that is the case in a level road4  ,mrs4
 ,b4 ,the centre of gravity of the human
 body1 is a point somewhere in a line   
extending perpendicularly through the   
middle of it1 and as long as we stand   
upright1 this point is supported by the 
feet2 if you lean on one side1 you will 
find that you no longer stand firm4 ,a  
rope-dancer performs all his feats of   
agility1 by dexterously supporting his  
centre of gravity2 whenever he finds    
that he is in danger of losing his      

balance1 he shifts the heavy pole which 
he holds in his hands1 in order to throw
 the weight towards the side that is    
deficient2 and thus by changing the     
situation of the centre of gravity1 he  
restores his equilibrium4  ,caroline4   
,when a stick is poised on the tip of   
the finger1 is it not by supporting its 
centre of gravity8  ,mrs4 ,b4 ,yes2 and 
it is because the centre of gravity is  
not supported1 that spherical bodies    
roll down a slope4 ,a sphere being      
perfectly round1 can touch the slope but
 by a single point1 and that point      
cannot be perpendicularly under the     
centre of gravity1 and therefore cannot 
be supported1 as you will perceive by   
examining this figure4 7fig4 #e4 plate  
#c47  ,emily4 ,so it appears3 yet ,i    
have seen a cylinder of wood roll up a  
slope2 how is that contrived8  ,mrs4 ,b4
 ,it is done by plugging or loading one 
side of the cylinder with lead1 as at   
,b1 7fig4 #e4 plate #c47 the body being 
no longer of a uniform density1 the     

centre of gravity is removed from the   
middle of the body to some point in or  
near the lead1 as that substance is much
 heavier than wood2 now you may observe 
that should this cylinder roll down the 
plane1 as it is here situated1 the      
centre of gravity must rise1 which is   
impossible2 the centre of gravity must  
always descend in moving1 and will      
descend by the nearest and readiest     
means1 which will be by forcing the     
cylinder up the slope1 until the centre 
of gravity is supported1 and then it    
stops4  ,caroline4 ,the centre of       
gravity1 therefore1 is not always in the
 middle of a body4  ,mrs4 ,b4 ,no1 that 
point we have called the centre of      
magnitude2 when the body is of an       
uniform density1 and of a regular form1 
as a cube1 or sphere1 the centres of    
gravity and of magnitude are in the same
 point2 but when one part of the body is
 composed of heavier materials than     
another1 the centre of gravity can no   
longer correspond with the centre of    

magnitude4 ,thus you see the centre of  
gravity of this cylinder plugged with   
lead1 cannot be in the same spot as the 
centre of magnitude4  ,emily4 ,bodies1  
therefore1 consisting but of one kind of
 substance1 as wood1 stone1 or lead1 and
 whose densities are consequently       
uniform1 must stand more firmly1 and be 
more difficult to overset1 than bodies  
composed of a variety of substances1 of 
different densities1 which may throw the
 centre of gravity on one side4  ,mrs4  
,b4 ,that depends upon the situation of 
the materials2 if those which are most  
dense1 occupy the lower part1 the       
stability will be increased1 as the     
centre of gravity will be near the base4
 ,but there is another circumstance     
which more materially affects the       
firmness of their position1 and that is 
their form4 ,bodies that have a narrow  
base are easily upset1 for if they are a
 little inclined1 their centre of       
gravity is no longer supported1 as you  
may perceive in fig4 #f4  ,caroline4 ,i 

have often observed with what difficulty
 a person carries a single pail of      
water2 it is owing1 ,i suppose1 to the  
centre of gravity being thrown on one   
side2 and the opposite arm is stretched 
out to endeavour to bring it back to its
 original situation2 but a pail hanging 
to each arm is carried with less        
difficulty1 because they balance each   
other1 and the centre of gravity remains
 supported by the feet4  ,mrs4 ,b4 ,very
 well2 ,i have but one more remark to   
make on the centre of gravity1 which is1
 that when two bodies are fastened      
together by an inflexible rod1 they are 
to be considered as forming but one     
body2 if the two bodies be of equal     
weight1 the centre of gravity will be in
 the middle of the line which unites    
them1 7fig4 #g47 but if one be heavier  
than the other1 the centre of gravity   
will be proportionally nearer the heavy 
body than the light one4 7fig4 #h47 ,if 
you were to carry a rod or pole with an 
equal weight fastened at each end of it1

 you would hold it in the middle of the 
rod1 in order that the weights should   
balance each other2 whilst if the       
weights were unequal1 you would hold it 
nearest the greater weight1 to make them
 balance each other4  ,emily4 ,and in   
both cases we should support the centre 
of gravity2 and if one weight be very   
considerably larger than the other1 the 
centre of gravity will be thrown out of 
the rod into the heaviest weight4 7fig4 
#i47  ,mrs4 ,b4 ,undoubtedly4           
,questions  #a4 7,pg4 #df7 ,if a body be
 struck by two equal forces in opposite 
directions1 what will be the result8    
#b4 7,pg4 #df7 ,what is fig4 #e4 plate  
#b4 intended to represent8  #c4 7,pg4   
#dg7 ,how would the ball move1 and how  
would you represent the direction of its
 motion8  #d4 7,pg4 #dg7 ,what is       
supposed respecting the forces          
represented in fig4 #f8  #e4 7,pg4 #dg7 
,how would the body move if so impelled8
  #f4 7,pg4 #dg7 ,if the forces are     
unequal and not at right angles1 how    

would the body move1 as illustrated by  
fig4 #g8  #g4 7,pg4 #dg7 ,how must a    
body be acted on1 to produce motion in a
 curve1 and what example is given8  #h4 
7,pg4 #dh7 ,when is a body said to      
revolve in a plane1 and what is meant by
 the centre of motion8  #i4 7,pg4 #dh7  
,what is intended by the axis of motion1
 and what are examples8  #aj4 7,pg4 #dh7
 ,what is the middle point of a body    
called8  #aa4 7,pg4 #dh7 ,what is said  
of the axis of motion1 whilst the body  
is revolving8  #ab4 7,pg4 #dh7 ,when a  
body revolves on an axis1 do all its    
parts move with equal velocity8  #ac4   
7,pg4 #di7 ,how is this explained by    
fig4 #a4 plate #c8  #ad4 7,pg4 #di7     
,what are the two forces called which   
cause a body to move in a curve2 and    
what proportion do these two forces bear
 to each other when a body revolves     
round a centre8  #ae4 7,pg4 #di7 ,if the
 centripetal force were destroyed1 how  
would a body be carried by the          
centrifugal8  #af4 7,pg4 #ej7 ,explain  

what is meant by a tangent1 as shown in 
fig4 #b4 plate #c4  #ag4 7,pg4 #ej7     
,what forces impede a body thrown       
horizontally8  #ah4 7,pg4 #ej7 ,give the
 reason why a body so projected1 falls  
in a curve4 7fig4 #c4 plate #c47  #ai4  
7,pg4 #ea7 ,the curve in which it falls1
 is not a part of a true circle3 what is
 it denominated8  #bj4 7,pg4 #ea7 ,what 
is the centre of gravity defined to be8 
#ba4 7,pg4 #ea7 ,what results from      
supporting1 or not supporting the centre
 of gravity8  #bb4 7,pg4 #ea7 ,what is  
intended to be explained by fig4 #d4    
plate #c8  #bc4 7,pg4 #ea7 ,what would  
be the effect of taking off the upper   
portion of the load8  #bd4 7,pg4 #eb7   
,when will a carriage stand most firmly8
  #be4 7,pg4 #eb7 ,what is said of the  
centre of gravity of the human body1 and
 how does a rope dancer preserve his    
equilibrium8  #bf4 7,pg4 #eb7 ,why      
cannot a sphere remain at rest on an    
inclined plane8 7fig4 #e4 plate #c47    
#bg4 7,pg4 #eb7 ,a cylinder of wood1 may

 be made to rise to a small distance up 
an inclined plane4 ,how may this be     
effected8 7fig4 #e4 plate #c47  #bh4    
7,pg4 #ec7 ,when do we find the centres 
of gravity1 and of magnitude in         
different points8  #bi4 7,pg4 #ec7 ,what
 influence will the density of the parts
 of a body exert upon its stability8    
#cj4 7,pg4 #ec7 ,what other circumstance
 materially affects the firmness of     
position8 7fig4 #f4 plate #c47  #ca4    
7,pg4 #ec7 ,why is it more easy to carry
 a weight in each hand1 than in one     
only8  #cb4 7,pg4 #ec7 ,what is said    
respecting two bodies united by an      
inflexible rod8  #cc4 7,pg4 #ec7 ,what  
is fig4 #g1 plate #c1 intended to       
illustrate8 ,what fig4 #h2 what fig4 #i8
     ,,conversation ,v4  ,,on ,,the     
,,mechanical ,,powers4  ,,of ,,the      
,,power ,,of ,,machines4 ,,of ,,the     
,,lever ,,in ,,general4 ,,of ,,the      
,,lever ,,of ,,the ,,first ,,kind1      
,,having ,,the ,,fulcrum ,,between ,,the
 ,,power ,,and ,,the ,,weight4 ,,of     

,,the ,,lever ,,of ,,the ,,second       
,,kind1 ,,having ,,the ,,weight         
,,between ,,the ,,power ,,and ,,the     
,,fulcrum4 ,,of ,,the ,,lever ,,of ,,the
 ,,third ,,kind1 ,,having ,,the ,,power 
,,between ,,the ,,fulcrum ,,and ,,the   
,,weight4   ,,mrs4 ,b4  ,we may now     
proceed to examine the mechanical       
powers2 they are six in number3 ,the    
lever1 the pulley1 the wheel and axle1  
the inclined plane1 the wedge and the   
screw2 one or more of which enters into 
the composition of every machine4  ,a   
mechanical power is an instrument by    
which the effect of a given force is    
increased1 whilst the force remains the 
same4  ,in order to understand the power
 of a machine1 there are four things to 
be considered4 #ast4 ,the power that    
acts3 this consists in the effort of men
 or horses1 of weights1 springs1 steam1 
c4  #b;dly4 ,the resistance which is to 
be overcome by the power3 this is       
generally a weight to be moved4 ,the    
power must always be superior to the    

resistance1 otherwise the machine could 
not be put in motion4  ,caroline4 ,if   
for instance the resistance of a        
carriage was greater than the strength  
of the horses employed to draw it1 they 
would not be able to make it move4      
,mrs4 ,b4 #c;dly4 ,we are to consider   
the support or prop1 or as it is termed 
in mechanics1 the fulcrum2 this you may 
recollect is the point upon which the   
body turns when in motion2 and lastly1  
the respective velocities of the power1 
and of the resistance4  ,emily4 ,that   
must in general depend upon their       
respective distances from the fulcrum1  
or from the axis of motion2 as we       
observed in the motion of the vanes of  
the windmill4  ,mrs4 ,b4 ,we shall now  
examine the power of the lever4 ,the    
lever is an inflexible rod or bar1      
moveable about a fulcrum1 and having    
forces applied to two or more points on 
it4 ,for instance1 the steel rod to     
which these scales are suspended is a   
lever1 and the point in which it is     

supported1 the fulcrum1 or centre of    
motion2 now1 can you tell me why the two
 scales are in equilibrium8  ,caroline4 
,being both empty1 and of the same      
weight1 they balance each other4        
,emily4 ,or1 more correctly speaking1   
because the centre of gravity common to 
both1 is supported4  ,mrs4 ,b4 ,very    
well2 and where is the centre of gravity
 of this pair of scales8 7fig4 #a4 plate
 #d47  ,emily4 ,you have told us that   
when two bodies of equal weight were    
fastened together1 the centre of gravity
 was in the middle of the line that     
connected them2 the centre of gravity of
 the scales must therefore be supported 
by the fulcrum ,f of the lever which    
unites the two scales1 and which is the 
centre of motion4  ,caroline4 ,but if   
the scales contained different weights1 
the centre of gravity would no longer be
 in the fulcrum of the lever1 but remove
 towards that scale which contained the 
heaviest weight2 and since that point   
would no longer be supported1 the heavy 

scale would descend1 and out-weigh the  
other4  ,mrs4 ,b4 ,true2 but tell me1   
can you imagine any mode by which bodies
 of different weights can be made to    
balance each other1 either in a pair of 
scales1 or simply suspended to the      
extremities of the lever8 for the scales
 are not an essential part of the       
machine2 they have no mechanical power1 
and are used merely for the convenience 
of containing the substance to be       
weighed4  ,caroline4 ,what6 make a light
 body balance a heavy one8 ,i cannot    
conceive that possible4  ,mrs4 ,b4 ,the 
fulcrum of this pair of scales 7fig4    
#b47 is moveable1 you see2 ,i can take  
it off the beam1 and fasten it on again 
in another part2 this part is now become
 the fulcrum1 but it is no longer in the
 centre of the lever4  ,caroline4 ,and  
the scales are no longer true2 for that 
which hangs on the longest side of the  
lever descends4  ,mrs4 ,b4 ,the two     
parts of the lever divided by the       
fulcrum1 are called its arms2 you should

 therefore say the longest arm1 not the 
longest side of the lever4  ,your       
observation is true that the balance is 
now destroyed2 but it will answer the   
purpose of enabling you to comprehend   
the power of a lever1 when the fulcrum  
is not in the centre4  ,emily4 ,this    
would be an excellent contrivance for   
those who cheat in the weight of their  
goods2 by making the fulcrum a little on
 one side1 and placing the goods in the 
scale which is suspended to the longest 
arm of the lever1 they would appear to  
weigh more than they do in reality4     
,mrs4 ,b4 ,you do not consider how      
easily the fraud would be detected2 for 
on the scales being emptied they would  
not hang in equilibrium4 ,if indeed the 
scale on the shorter arm was made       
heavier1 so as to balance that on the   
longer1 they would appear to be true1   
whilst they were really false4  ,emily4 
,true2 ,i did not think of that         
circumstance4 ,but ,i do not understand 
why the longest arm of the lever should 

not be in equilibrium with the other8   
,caroline4 ,it is because the momentum  
in the longest1 is greater than in the  
shortest arm2 the centre of gravity1    
therefore1 is no longer supported4      
,mrs4 ,b4 ,you are right1 the fulcrum is
 no longer in the centre of gravity2 but
 if we can contrive to make the fulcrum 
in its present situation become the     
centre of gravity1 the scales will again
 balance each other2 for you recollect  
that the centre of gravity is that point
 about which every part of the body is  
in equilibrium4  ,emily4 ,it has just   
occurred to me how this may be          
accomplished2 put a great weight into   
the scale suspended to the shortest arm 
of the lever1 and a smaller one into    
that suspended to the longest arm4 ,yes1
 ,i have discovered it--look ,mrs4 ,b41 
the scale on the shortest arm will carry
 #c lbs41 and that on the longest arm   
only one1 to restore the balance4 7fig4 
#c47  ,mrs4 ,b4 ,you see1 therefore1    
that it is not so impracticable as you  

imagined1 to make a heavy body balance a
 light one2 and this is in fact the     
means by which you observed that an     
imposition in the weight of goods might 
be effected1 as a weight of ten or      
twelve ounces1 might thus be made to    
balance a pound of goods4 ,if you       
measure both arms of the lever1 you will
 find that the length of the longer arm1
 is three times that of the shorter2 and
 that to produce an equilibrium1 the    
weights must bear the same proportion to
 each other1 and that the greater       
weight1 must be on the shorter arm4 ,let
 us now take off the scales1 that we may
 consider the lever simply2 and in this 
state you see that the fulcrum is no    
longer the centre of gravity1 because it
 has been removed from the middle of the
 lever2 but it is1 and must ever be1 the
 centre of motion1 as it is the only    
point which remains at rest1 while the  
other parts move about it4              
,illustration3 ,,plate ,,iv4  ,caroline4
 ,the arms of the lever being different 

in length1 it now exactly resembles the 
steelyards1 with which articles are so  
frequently weighed4  ,mrs4 ,b4 ,it may  
in fact be considered as a pair of      
steelyards1 by which the same power     
enables us to ascertain the weight of   
different articles1 by simply increasing
 the distance of the power from the     
fulcrum2 you know that the farther a    
body is from the axis of motion1 the    
greater is its velocity4  ,caroline4    
,that ,i remember1 and understand       
perfectly4  ,mrs4 ,b4 ,you comprehend   
then1 that the extremity of the longest 
arm of a lever1 must move with greater  
velocity than that of the shortest arm1 
and that its momentum is greater in     
proportion4  ,emily4 ,no doubt1 because 
it is farthest from the centre of       
motion4 ,and pray1 ,mrs4 ,b41 when my   
brothers play at see-saw1 is not the    
plank on which they ride1 a kind of     
lever8  ,mrs4 ,b4 ,certainly2 the log of
 wood which supports it from the ground 
is the fulcrum1 and those who ride1     

represent the power and the resistance  
at the ends of the lever4 ,and have you 
not observed that when those who ride   
are of equal weight1 the plank must be  
supported in the middle1 to make the two
 arms equal2 whilst if the persons      
differ in weight1 the plank must be     
drawn a little farther over the prop1 to
 make the arms unequal1 and the lightest
 person1 who may be supposed to         
represent the power1 must be placed at  
the extremity of the longest arm4       
,caroline4 ,that is always the case when
 ,i ride on a plank with my youngest    
brother2 ,i have observed also that the 
lightest person has the best ride1 as he
 moves both further and quicker2 and ,i 
now understand that it is because he is 
more distant from the centre of motion4 
,mrs4 ,b4 ,the greater velocity with    
which your little brother moves1 renders
 his momentum equal to yours4           
,caroline4 ,yes2 ,i have the most       
weight1 he the greatest velocity2 so    
that upon the whole our momentums are   

equal4 ,but you said1 ,mrs4 ,b41 that   
the power should be greater than the    
resistance1 to put the machine in       
motion2 how then can the plank move if  
the momentums of the persons who ride   
are equal8  ,mrs4 ,b4 ,because each     
person at his descent touches and pushes
 against the ground with his feet2 the  
reaction of which gives him an impulse  
which produces the motion2 this spring  
is requisite to destroy the equilibrium 
of the power and the resistance1        
otherwise the plank would not move4 ,did
 you ever observe that a lever describes
 the arc of a circle in its motion8     
,emily4 ,no2 it appears to me to rise   
and descend perpendicularly2 at least ,i
 always thought so4  ,mrs4 ,b4 ,i       
believe ,i must make a sketch of you and
 your brother riding on a plank1 in     
order to convince you of your error4    
7fig4 #d4 plate #d47 ,you may now       
observe that a lever can move only round
 the fulcrum1 since that is the centre  
of motion2 it would be impossible for   

you to rise perpendicularly1 to the     
point ,a2 or for your brother to descend
 in a straight line1 to the point ,b2   
you must in rising1 and he in           
descending1 describe arcs of your       
respective circles4 ,this drawing shows 
you also how much superior his velocity 
must be to yours2 for if you could swing
 quite round1 you would each complete   
your respective circles1 in the same    
time4  ,caroline4 ,my brother's circle  
being much the largest1 he must         
undoubtedly move the quickest4  ,mrs4   
,b4 ,now tell me1 do you think that your
 brother could raise you as easily      
without the aid of a lever8  ,caroline4 
,oh no1 he could not lift me off the    
ground4  ,mrs4 ,b4 ,then ,i think you   
require no further proof of the power of
 a lever1 since you see what it enables 
your brother to perform4  ,caroline4 ,i 
now understand what you meant by saying1
 that in mechanics1 velocity is opposed 
to weight1 for it is my brother's       
velocity which overcomes my weight4     

,mrs4 ,b4 ,you may easily imagine1 what 
enormous weights may be raised by levers
 of this description1 for the longer1   
when compared with the other1 that arm  
is to which the power is applied1 the   
greater will be the effect produced by  
it2 because the greater is the velocity 
of the power compared to that of the    
weight4  ,levers are of three kinds2 in 
the first the fulcrum is between the    
power and the weight4  ,caroline4 ,this 
kind then comprehends the several levers
 you have described4  ,mrs4 ,b4 ,yes1   
when in levers of the first kind1 the   
fulcrum is equally distant from the     
power and the weight1 as in the balance1
 there will be an equilibrium1 when the 
power and the weight are equal to each  
other2 it is not then a mechanical      
power1 for nothing can in this case be  
gained by velocity2 the two arms of the 
lever being equal1 the velocity of their
 extremities must be so likewise4 ,the  
balance is therefore of no assistance as
 a mechanical power1 although it is     

extremely useful in estimating the      
respective weights of bodies4  ,but when
 7fig4 #e47 the fulcrum ,f of a lever is
 not equally distant from the power and 
the weight1 and the power ,p acts at the
 extremity of the longest arm1 it may be
 less than the weight ,w2 its deficiency
 being compensated by its superior      
velocity1 as we observed in the see-saw4
  ,emily4 ,then when we want to lift a  
great weight1 we must fasten it to the  
shortest arm of a lever1 and apply our  
strength to the longest arm8  ,mrs4 ,b4 
,if the case will admit of your putting 
the end of the lever under the resisting
 body1 no fastening will be required2 as
 you will perceive1 when a nail is drawn
 by means of a hammer1 which1 though    
bent1 is a lever of the first kind2 the 
handle being the longest arm1 the point 
on which it rests1 the fulcrum1 and the 
distance from that to the part which    
holds the nail1 the short arm4 ,but let 
me hear1 ,caroline1 whether you can     
explain the action of this instrument1  

which is composed of two levers united  
in one common fulcrum4  ,caroline4 ,a   
pair of scissors6  ,mrs4 ,b4 ,you are   
surprised2 but if you examine their     
construction1 you will discover that it 
is the power of the lever1 that assists 
us in cutting with scissors4  ,caroline4
 ,yes2 ,i now perceive that the point at
 which the two levers are screwed       
together1 is the fulcrum2 the power of  
the fingers is applied to the handles1  
and the article to be cut1 is the       
resistance2 therefore1 the longer the   
handles1 and the shorter the points of  
the scissors1 the more easily you cut   
with them4  ,emily4 ,that ,i have often 
observed1 for when ,i cut paste-board or
 any hard substance1 ,i always make use 
of that part of the scissors nearest the
 screw or rivet1 and ,i now understand  
why it increases the power of cutting2  
but ,i confess that ,i never should have
 discovered scissors to have been double
 levers2 and pray are not snuffers      
levers of a similar description8  ,mrs4 

,b4 ,yes1 and most kinds of pincers2 the
 great power of which consists in the   
great relative length of the handles4   
,did you ever notice the swingle-tree of
 a carriage to which the horses are     
attached when drawing8  ,emily4 ,o yes2 
this is a lever of the first kind1 but  
the fulcrum being in the middle1 the    
horses should draw with equal power1    
whatever may be their strength4  ,mrs4  
,b4 ,that is generally the case1 but it 
is evident that by making one arm longer
 than the other1 it might be adapted to 
horses of unequal strength4  ,caroline4 
,and of what nature are the other two   
kinds of levers8  ,mrs4 ,b4 ,in levers  
of the second kind1 the weight1 instead 
of being at one end1 is situated between
 the power and the fulcrum1 7fig4 #f47  
,caroline4 ,the weight and the fulcrum  
have here changed places2 and what      
advantage is gained by this kind of     
lever8  ,mrs4 ,b4 ,in moving it1 the    
velocity of the power must necessarily  
be greater than that of the weight1 as  

it is more distant from the centre of   
the motion4 ,have you ever seen your    
brother move a snow-ball by means of a  
strong stick1 when it became too heavy  
for him to move without assistance8     
,caroline4 ,oh yes2 and this was a lever
 of the second kind1 7fig4 #g47 the end 
of the stick1 which he thrusts under the
 ball1 and which rests on the ground1   
becomes the fulcrum2 the ball is the    
weight to be moved1 and the power his   
hands1 applied to the other end of the  
lever4 ,in this instance there is a     
great difference in the length of the   
arms of the lever2 for the weight is    
almost close to the fulcrum4  ,mrs4 ,b4 
,and the advantage gained is            
proportional to this difference4 ,the   
most common example that we have of     
levers of the second kind1 is in the    
doors of our apartments4  ,emily4 ,the  
hinges represent the fulcrum1 our hands 
the power applied to the other end of   
the lever2 but where is the weight to be
 moved8  ,mrs4 ,b4 ,the door is the     

weight1 which in this example occupies  
the whole of the space between the power
 and the fulcrum4 ,nut crackers are     
double levers of this kind3 the hinge is
 the fulcrum1 the nut the resistance1   
and the hands the power4  ,in levers of 
the third kind 7fig4 #h47 the fulcrum is
 again at one extremity1 the weight or  
resistance at the other1 and the power  
is applied between the fulcrum and the  
resistance4  ,emily4 ,the fulcrum1 the  
weight1 or the power1 then1 each in its 
turn1 occupies some part of the lever   
between its extremities4 ,but in this   
third kind of lever1 the weight being   
farther than the power from the centre  
of motion1 the difficulty of raising it 
seems increased rather than diminished4 
,mrs4 ,b4 ,that is very true2 a lever of
 this kind is therefore never used1     
unless absolutely necessary1 as is the  
case in raising a ladder in order to    
place it against a wall2 the man who    
raises it cannot place his hands on the 
upper part of the ladder1 the power1    

therefore1 is necessarily placed much   
nearer to the fulcrum than to the       
weight4  ,caroline4 ,yes1 the hands are 
the power1 the ground the fulcrum1 and  
the upper part of the ladder the weight4
  ,mrs4 ,b4 ,nature employs this kind of
 lever in the structure of the human    
frame4 ,in lifting a weight with the    
hand1 the lower part of the arm becomes 
a lever of the third kind2 the elbow is 
the fulcrum1 the muscles of the fleshy  
part of the arm1 the power2 and as these
 are nearer to the elbow than to the    
hand1 it is necessary that their power  
should exceed the weight to be raised4  
,emily4 ,is it not surprising that      
nature should have furnished us with    
such disadvantageous levers8  ,mrs4 ,b4 
,the disadvantage1 in respect to power1 
is more than counterbalanced by the     
convenience resulting from this         
structure of the arm2 and it is that no 
doubt which is best adapted to enable it
 to perform its various functions4      
,there is one rule which applies to     

every lever1 which is this3 ,in order to
 produce an equilibrium1 the power must 
bear the same proportion to the weight1 
as the length of the shorter arm does to
 that of the longer2 as was shown by    
,emily with the weights of #a lb4 and of
 #c lb4 ,fig4 #c4 plate #d4  ,we have   
dwelt so long on the lever1 that we must
 reserve the examination of the other   
mechanical powers1 to our next          
interview4   ,questions  #a4 7,pg4 #ed7 
,how many mechanical powers are there1  
and what are they named8  #b4 7,pg4 #ed7
 ,what is a mechanical power defined to 
be8  #c4 7,pg4 #ed7 ,what four          
particulars must be observed8  #d4 7,pg4
 #ed7 ,upon what will the velocities    
depend8  #e4 7,pg4 #ee7 ,what is a      
lever8  #f4 7,pg4 #ee7 ,give a familiar 
example4  #g4 7,pg4 #ee7 ,when and why  
do the scales balance each other1 and   
where is their centre of gravity8 7fig4 
#a4 plate #d47  #h4 7,pg4 #ee7 ,why     
would they not balance with unequal     
weights8  #i4 7,pg4 #ee7 ,were the      

fulcrum removed from the middle of the  
beam what would result8  #aj4 7,pg4 #ee7
 ,what do we mean by the arms of a      
lever8  #aa4 7,pg4 #ef7 ,how may a pair 
of scales be false1 and yet appear to be
 true8  #ab4 7,pg4 #ef7 ,if the fulcrum 
be removed from the centre of gravity1  
how may the equilibrium be restored8    
#ac4 7,pg4 #ef7 ,how is this exemplified
 by fig4 #c4 plate #d8  #ad4 7,pg4 #ef7 
,what proportion must the weights bear  
to the lengths of the arms8  #ae4 7,pg4 
#eg7 ,on what principle do we weigh with
 a pair of steelyards1 and what will be 
the difference in the motion of the     
extremities of such a lever8  #af4 7,pg4
 #eh7 ,how is this exemplified by fig4  
#d4 plate #d8  #ag4 7,pg4 #eh7 ,what    
line is described by the ends of a      
lever8 fig4 #d4 plate #d4  #ah4 7,pg4   
#eh7 ,how many kinds are there2 and in  
the first how is the fulcrum situated8  
#ai4 7,pg4 #eh7 ,when may the fulcrum be
 so situated that this lever is not a   
mechanical power1 and why8  #bj4 7,pg4  

#ei7 ,what is represented by fig4 #e4   
plate #d8  #ba4 7,pg4 #ei7 ,give a      
familiar example of the use of a lever  
of the first kind4  #bb4 7,pg4 #ei7 ,in 
what instruments are two such levers    
combined8  #bc4 7,pg4 #ei7 ,how may two 
horses of unequal strength1 be          
advantageously coupled in a carriage8   
#bd4 7,pg4 #fj7 ,describe a lever of the
 second kind4 7,fig4 #f4 plate #d47     
#be4 7,pg4 #fj7 ,what is represented in 
fig4 #g4 plate #d1 and in what          
proportion does this lever gain power8  
#bf4 7,pg4 #fj7 ,what is said respecting
 a door8  #bg4 7,pg4 #fj7 ,describe a   
lever of the third kind4  #bh4 7,pg4    
#fj7 ,in what instance do we use this8  
#bi4 7,pg4 #fa7 ,what remarks are made  
on its employment in the limbs of       
animals8  #cj4 7,pg4 #fa7 ,what are the 
conditions of equilibrium in every      
lever8     ,,conversation ,v4           
,,continued4  ,,on ,,the ,,mechanical   
,,powers4  ,,of ,,the ,,pulley4 ,,of    
,,the ,,wheel ,,and ,,axle4 ,,of ,,the  

,,inclined ,,plane4 ,,of ,,the ,,wedge4 
,,of ,,the ,,screw4   ,,mrs4 ,b4  ,the  
pulley is the second mechanical power we
 are to examine4 ,you both1 ,i suppose1 
have seen a pulley8  ,caroline4 ,yes1   
frequently3 it is a circular1 and flat  
piece of wood or metal1 with a string   
which runs in a groove round it3 by     
means of which1 a weight may be pulled  
up2 thus pulleys are used for drawing up
 curtains4  ,mrs4 ,b4 ,yes2 but in that 
instance the pulleys are fixed2 that is1
 they retain their places1 and merely   
turn round on their axis2 these do not  
increase the power to raise the weights1
 as you will perceive by this figure4   
7plate #e4 fig4 #a47 ,observe that the  
fixed pulley is on the same principle as
 the lever of a pair of scales1 in which
 the fulcrum ,f being in the centre of  
gravity1 the power ,p and the weight ,w1
 are equally distant from it1 and no    
advantage is gained4  ,emily4           
,certainly2 if ,p represents the power  
employed to raise the weight ,w1 the    

power must be greater than the weight in
 order to move it4 ,but of what use then
 is a fixed pulley in mechanics8  ,mrs4 
,b4 ,although it does not increase the  
power1 it is frequently useful for      
altering its direction4 ,a single fixed 
pulley enables us to draw a curtain up1 
by pulling the string connected with it 
downwards2 and we should be at a loss to
 accomplish this simple operation       
without its assistance4  ,caroline4     
,there would certainly be some          
difficulty in ascending to the head of  
the curtain1 in order to draw it up4    
,indeed ,i now recollect having seen    
workmen raise weights to a considerable 
height by means of a fixed pulley1 which
 saved them the trouble of going up     
themselves4  ,mrs4 ,b4 ,the next figure 
represents a pulley which is not fixed2 
7fig4 #b47 and thus situated1 you will  
perceive that it affords us mechanical  
assistance4  ,a is a moveable pulley2   
that is1 one which is attached to the   
weight to be raised1 and which          

consequently moves up or down with it4  
,there is also a fixed pulley ,d1 which 
is only of use to change the direction  
of the power ,p4 ,now it is evident that
 the velocity of the power1 will be     
double that of the weight ,w2 for if the
 rope be pulled at ,p1 until the pulley 
,a ascends with the weight to the fixed 
pulley ,d1 then both parts of the rope1 
,c and ,b1 must pass over the fixed     
pulley1 and consequently the hand at ,p1
 will have descended through a space    
equal to those two parts2 but the weight
 will have ascended only one half of    
that distance4  ,caroline4 ,that ,i     
understand3 if ,p drew the string but   
one inch1 the weight would be raised    
only half an inch1 because it would     
shorten the strings ,b and ,c half an   
inch each1 and consequently the pulley  
with the weight attached to it1 can be  
raised only half an inch4  ,emily4 ,but 
,i do not yet understand the advantage  
of moveable pulleys2 they seem to me to 
increase rather than diminish the       

difficulty of raising weights1 since you
 must draw the string double the length 
that you raise the weight2 whilst with a
 single pulley1 or without any pulley1  
the weight is raised as much as the     
string is shortened4  ,mrs4 ,b4 ,the    
advantage of a moveable pulley consists 
in dividing the difficulty2 we must1 it 
is true1 draw twice the length of the   
string1 but then only half the strength 
is required that would be necessary to  
raise the weight without the assistance 
of a moveable pulley4  ,emily4 ,so that 
the difficulty is overcome in the same  
manner as it would be1 by dividing the  
weight into two equal parts1 and raising
 them successively4  ,mrs4 ,b4 ,exactly4
 ,you must observe1 that with a moveable
 pulley the velocity of the power1 is   
double that of the weight2 since the    
power ,p 7fig4 #b47 moves two inches    
whilst the weight ,w moves one inch2    
therefore the power need not be more    
than half the weight1 to make their     
momentums equal4  ,caroline4 ,pulleys   

act then on the same principle as the   
lever2 the deficiency of weight in the  
power1 being compensated by its superior
 velocity1 so as to make their momentums
 equal4  ,mrs4 ,b4 ,you will find1 that 
all gain of power in mechanics is       
founded on the same principle4  ,emily4 
,but may it not be objected to pulleys1 
that a longer time is required to raise 
a weight by their aid1 than without it8 
for what you gain in power1 you lose in 
time4  ,mrs4 ,b4 ,that1 my dear1 is the 
fundamental law in mechanics3 it is the 
case with the lever1 as well as the     
pulley2 and you will find it to be so   
with all the other mechanical powers4   
,caroline4 ,i do not see any advantage  
in the mechanical powers then1 if what  
we gain by them in one way1 is lost in  
another4  ,mrs4 ,b4 ,since we are not   
able to increase our natural strength is
 not any instrument of obvious utility1 
by means of which we may reduce the     
resistance or weight of any body1 to the
 level of that strength8 ,this the      

mechanical powers enable us to          
accomplish4 ,it is true1 as you observe1
 that it requires a sacrifice of time to
 attain this end1 but you must be       
sensible how very advantageously it is  
exchanged for power4 ,if one man by his 
natural strength could raise one hundred
 pounds only1 it would require five such
 men to raise five hundred pounds2 and  
if one man performs this by the help of 
a suitable engine1 there is then no     
actual loss of time2 as he does the work
 of five men1 although he is five times 
as long in its accomplishment4  ,you can
 now understand1 that the greater the   
number of moveable pulleys connected by 
a string1 the more easily the weight is 
raised2 as the difficulty is divided    
amongst the number of strings1 or rather
 of parts into which the string is      
divided1 by the pulleys4 ,two1 or more  
pulleys thus connected1 form what is    
called a tackle1 or system of pulleys4  
7fig4 #c47 ,you may have seen them      
suspended from cranes to raise goods    

into warehouses4  ,emily4 ,when there   
are two moveable pulleys1 as in the     
figure you have shown to us1 7fig4 #c47 
there must also be two fixed pulleys1   
for the purpose of changing the         
direction of the string1 and then the   
weight is supported by four strings1 and
 of course1 each must bear only one     
fourth part of the weight4  ,mrs4 ,b4   
,you are perfectly correct1 and the rule
 for estimating the power gained by a   
system of pulleys1 is to count the      
number of strings by which the weight is
 supported2 or1 which amounts to the    
same thing1 to multiply the number of   
moveable pulleys by two4  ,in shipping1 
the advantages of both an increase of   
power1 and a change of direction1 by    
means of pulleys1 are of essential      
importance3 for the sails are raised up 
the masts by the sailors on deck1 from  
the change of direction which the pulley
 effects1 and the labour is facilitated 
by the mechanical power of a combination
 of pulleys4  ,illustration3 ,,plate ,v4

  ,emily4 ,but the pulleys on ship-board
 do not appear to me to be united in the
 manner you have shown us4  ,mrs4 ,b4   
,they are1 ,i believe1 generally        
connected as described in figure #d1    
both for nautical1 and a variety of     
other purposes2 but in whatever manner  
pulleys are connected by a single       
string1 the mechanical power is the     
same4  ,the third mechanical power1 is  
the wheel and axle4 ,let us suppose     
7plate #f4 fig4 #e7 the weight ,w1 to be
 a bucket of water in a well1 which we  
raise by winding round the axle the     
rope1 to which it is attached2 if this  
be done without a wheel to turn the     
axle1 no mechanical assistance is       
received4 ,the axle without a wheel is  
as impotent as a single fixed pulley1 or
 a lever1 whose fulcrum is in the       
centre3 but add the wheel to the axle1  
and you will immediately find the bucket
 is raised with much less difficulty4   
,the velocity of the circumference of   
the wheel is as much greater than that  

of the axle1 as it is further from the  
centre of motion2 for the wheel         
describes a great circle in the same    
space of time that the axle describes a 
small one1 therefore the power is       
increased in the same proportion as the 
circumference of the wheel is greater   
than that of the axle4 ,if the velocity 
of the wheel is twelve times greater    
than that of the axle1 a power twelve   
times less than the weight of the       
bucket1 would balance it2 and a small   
increase would raise it4  ,emily4 ,the  
axle acts the part of the shorter arm of
 the lever1 the wheel that of the longer
 arm4  ,caroline4 ,in raising water1    
there is commonly1 ,i believe1 instead  
of a wheel attached to the axle1 only a 
crooked handle1 which answers the       
purpose of winding the rope round the   
axle1 and thus raising the bucket4      
,mrs4 ,b4 ,in this manner 7fig4 #f27 now
 if you observe the dotted circle which 
the handle describes in winding up the  
rope1 you will perceive that the branch 

of the handle ,a1 which is united to the
 axle1 represents the spoke of a wheel1 
and answers the purpose of an entire    
wheel2 the other branch ,b affords no   
mechanical aid1 merely serving as a     
handle to turn the wheel4  ,wheels are a
 very essential part of most machines2  
they are employed in various ways2 but1 
when fixed to the axle1 their mechanical
 power is always the same3 that is1 as  
the circumference of the wheel exceeds  
that of the axle1 so much will the      
energy of the power be increased4       
,caroline4 ,then the larger the wheel1  
in proportion to the axle1 the greater  
must be its effect8  ,mrs4 ,b4          
,certainly4 ,if you have ever seen any  
considerable mills or manufactures1 you 
must have admired the immense wheel1 the
 revolution of which puts the whole of  
the machinery into motion2 and though so
 great an effect is produced by it1 a   
horse or two has sufficient power to    
turn it2 sometimes a stream of water is 
used for that purpose1 but of late      

years1 a steam-engine has been found    
both the most powerful and the most     
convenient mode of turning the wheel4   
,caroline4 ,do not the vanes of a       
windmill represent a wheel1 ,mrs4 ,b48  
,mrs4 ,b4 ,yes2 and in this instance we 
have the advantage of a gratuitous      
force1 the wind1 to turn the wheel4 ,one
 of the great benefits resulting from   
the use of machinery is1 that it gives  
us a sort of empire over the powers of  
nature1 and enables us to make them     
perform the labour which would otherwise
 fall to the lot of man4 ,when a current
 of wind1 a stream of water1 or the     
expansive force of steam1 performs our  
task1 we have only to superintend and   
regulate their operations4  ,the fourth 
mechanical power is the inclined plane2 
this is generally nothing more than a   
plank placed in a sloping direction1    
which is frequently used to facilitate  
the raising of weights1 to a small      
height1 such as the rolling of hogsheads
 or barrels into a warehouse4 ,it is not

 difficult to understand1 that a weight 
may much more easily be rolled up a     
slope than it can be raised the same    
height perpendicularly4 ,but in this1 as
 well as the other mechanical powers1   
the facility is purchased by a loss of  
time 7fig4 #g27 for the weight1 instead 
of moving directly from ,a to ,c1 must  
move from ,b to ,c1 and as the length of
 the plane is to its height1 so much is 
the resistance of the weight diminished4
  ,emily4 ,yes2 for the resistance1     
instead of being confined to the short  
line ,a ,c1 is spread over the long line
 ,b ,c4  ,mrs4 ,b4 ,the wedge1 which is 
the next mechanical power1 is usually   
viewed as composed of two inclined      
planes 7fig4 #h37 you may have seen     
wood-cutters use it to cleave wood4 ,the
 resistance consists in the cohesive    
attraction of the wood1 or any other    
body which the wedge is employed to     
separate2 the advantage gained by this  
power is differently estimated by       
philosophers2 but one thing is certain1 

its power is increased1 in proportion to
 the decrease of its thickness1 compared
 with its length4 ,the wedge is a very  
powerful instrument1 but it is always   
driven forward by blows from a hammer1  
or some other body having considerable  
momentum4  ,emily4 ,the wedge1 then1 is 
rather a compound than a distinct       
mechanical power1 since it is not       
propelled by simple pressure1 or weight1
 like the other powers4  ,mrs4 ,b4 ,it  
is so4 ,all cutting instruments are     
constructed upon the principle of the   
inclined plane1 or the wedge3 those that
 have but one edge sloped1 like the     
chisel1 may be referred to the inclined 
plane2 whilst the axe1 the hatchet1 and 
the knife1 7when used to split asunder7 
are used as wedges4  ,caroline4 ,but a  
knife cuts best when it is drawn across 
the substance it is to divide4 ,we use  
it thus in cutting meat1 we do not chop 
it to pieces4  ,mrs4 ,b4 ,the reason of 
this is1 that the edge of a knife is    
really a very fine saw1 and therefore   

acts best when used like that           
instrument4  ,the screw1 which is the   
last mechanical power1 is more          
complicated than the others4 ,you will  
see by this figure1 7fig4 #i47 that it  
is composed of two parts1 the screw and 
the nut4 ,the screw ,s is a cylinder1   
with a spiral protuberance coiled round 
it1 called the thread2 the nut ,n is    
perforated to receive the screw1 and the
 inside of the nut has a spiral groove1 
made to fit the spiral thread of the    
screw4  ,caroline4 ,it is just like this
 little box1 the lid of which screws on 
the box as you have described2 but what 
is this handle ,l which projects from   
the nut8  ,mrs4 ,b4 ,it is a lever1     
which is attached to the nut1 without   
which the screw is never used as a      
mechanical power4 ,the power of the     
screw1 complicated as it appears1 is    
referable to one of the most simple of  
the mechanical powers2 which of them do 
you think it is8  ,caroline4 ,in        
appearance1 it most resembles the wheel 

and axle4  ,mrs4 ,b4 ,the lever1 it is  
true1 has the effect of a wheel1 as it  
is the means by which you turn the nut1 
or sometimes the screw1 round2 but the  
lever is not considered as composing a  
part of the screw1 though it is true1   
that it is necessarily attached to it4  
,emily4 ,the spiral thread of the screw 
resembles1 ,i think1 an inclined plane3 
it is a sort of slope1 by means of which
 the nut ascends more easily than it    
would do if raised perpendicularly2 and 
it serves to support it when at rest4   
,mrs4 ,b4 ,very well3 if you cut a slip 
of paper in the form of an inclined     
plane1 and wind it round your pencil1   
which will represent the cylinder1 you  
will find that it makes a spiral line1  
corresponding to the spiral protuberance
 of the screw4 7,fig4 #aj47  ,emily4    
,very true2 the nut then ascends an     
inclined plane1 but ascends it in a     
spiral1 instead of a straight line3 the 
closer the threads of the screw1 the    
more easy the ascent3 it is like having 

shallow1 instead of steep steps to      
ascend4  ,mrs4 ,b4 ,yes2 excepting that 
the nut takes no steps1 as it gradually 
winds up or down2 then observe1 that the
 closer the threads of the screw1 the   
less is its ascent in turning round1 and
 the greater is its power2 so that we   
return to the old principle1--what is   
saved in power is lost in time4  ,emily4
 ,cannot the power of the screw be      
increased also1 by lengthening the lever
 attached to the nut8  ,mrs4 ,b4        
,certainly4 ,the screw1 with the        
addition of the lever1 forms a very     
powerful machine1 employed either for   
compression or to raise heavy weights4  
,it is used by book-binders1 to press   
the leaves of books together2 it is used
 also in cider and wine presses1 in     
coining1 and for a variety of other     
purposes4  ,emily4 ,pray1 ,mrs4 ,b41 by 
what rule do you estimate the power of  
the screw8  ,mrs4 ,b4 ,by measuring the 
circumference of the circle1 which the  
end of the lever would form in one whole

 revolution1 and comparing this with the
 distance from the centre of one thread 
of the screw1 to that of its next       
contiguous turn2 for whilst the lever   
travels that whole distance1 the screw  
rises or falls only through the distance
 from one coil to another4  ,caroline4  
,i think that ,i have sometimes seen the
 lever attached to the screw1 and not to
 the nut1 as it is represented in the   
figure4  ,mrs4 ,b4 ,this is frequently  
done1 but it does not in any degree     
affect the power of the instrument4     
,all machines are composed of one or    
more of these six mechanical powers we  
have examined2 ,i have but one more     
remark to make to you relative to them1 
which is1 that friction in a            
considerable degree diminishes their    
force3 allowance must therefore always  
be made for it1 in the construction of  
machinery4  ,caroline4 ,by friction1 do 
you mean one part of the machine rubbing
 against another part contiguous to it8 
,mrs4 ,b4 ,yes2 friction is the         

resistance which bodies meet with in    
rubbing against each other2 there is no 
such thing as perfect smoothness or     
evenness in nature2 polished metals1    
though they wear that appearance more   
than most other bodies1 are far from    
really possessing it2 and their         
inequalities may frequently be perceived
 through a good magnifying glass4 ,when1
 therefore1 the surfaces of the two     
bodies come in contact1 the prominent   
parts of the one1 will often fall into  
the hollow parts of the other1 and      
occasion more or less resistance to     
motion4  ,caroline4 ,but if a machine is
 made of polished metal1 as a watch for 
instance1 the friction must be very     
trifling8  ,mrs4 ,b4 ,in proportion as  
the surfaces of bodies are well         
polished1 the friction is doubtless     
diminished2 but it is always            
considerable1 and it is usually computed
 to destroy one-third of the power of a 
machine4 ,oil or grease is used to      
lessen friction3 it acts as a polish1 by

 filling up the cavities of the rubbing 
surfaces1 and thus making them slide    
more easily over each other4  ,caroline4
 ,is it for this reason that wheels are 
greased1 and the locks and hinges of    
doors oiled8  ,mrs4 ,b4 ,yes2 in these  
instances the contact of the rubbing    
surfaces is so close1 and they are so   
constantly in use1 that they require to 
be frequently oiled1 or a considerable  
degree of friction is produced4  ,there 
are two kinds of friction2 the first is 
occasioned by the rubbing of the        
surfaces of bodies against each other1  
the second1 by the rolling of a circular
 body2 as that of a carriage wheel upon 
the ground3 the friction resulting from 
the first is much the most considerable1
 for great force is required to enable  
the sliding body to overcome the        
resistance which the asperities of the  
surfaces in contact oppose to its       
motion1 and it must be either lifted    
over1 or break through them2 whilst1 in 
the second kind of friction1 the rough  

parts roll over each other with         
comparative facility2 hence it is1 that 
wheels are often used for the sole      
purpose of diminishing the resistance   
from friction4  ,emily4 ,this is one of 
the advantages of carriage wheels1 is it
 not8  ,mrs4 ,b4 ,yes2 and the larger   
the circumference of the wheel the more 
readily it can overcome any considerable
 obstacles1 such as stones1 or          
inequalities in the road4 ,when1 in     
descending a steep hill1 we fasten one  
of the wheels1 we decrease the velocity 
of the carriage1 by increasing the      
friction4  ,caroline4 ,that is to say1  
by converting the rolling friction into 
the rubbing friction4 ,and when you had 
casters put to the legs of the table1 in
 order to move it more easily1 you      
changed the rubbing into the rolling    
friction4  ,mrs4 ,b4 ,there is another  
circumstance which we have already      
noticed1 as diminishing the motion of   
bodies1 and which greatly affects the   
power of machines4 ,this is the         

resistance of the medium1 in which a    
machine is worked4 ,all fluids1 whether 
elastic like air1 or non-elastic like   
water and other liquids1 are called     
mediums2 and their resistance is        
proportioned to their density2 for the  
more matter a body contains1 the greater
 the resistance it will oppose to the   
motion of another body striking against 
it4  ,emily4 ,it would then be much more
 difficult to work a machine under water
 than in the air8  ,mrs4 ,b4 ,certainly1
 if a machine could be worked in vacuo1 
and without friction1 it would not be   
impeded1 but this is unattainable2 a    
considerable reduction of power must    
therefore be allowed for1 from friction 
and the resistance of the medium4  ,we  
shall here conclude our observations on 
the mechanical powers4 ,at our next     
meeting ,i shall endeavour to give you  
an explanation of the motion of the     
heavenly bodies4   ,questions  #ca4     
7,pg4 #fb7 ,describe a pulley1 and its  
use4  #cb4 7,pg4 #fb7 ,what is meant by 

a fixed pulley and why is not power     
gained by its employment8 7fig4 #a4     
plate #e47  #cc4 7,pg4 #fb7 ,of what use
 is the fixed pulley8  #cd4 7,pg4 #fc7  
,how is the power gained by a moveable  
pulley1 explained by means of fig4 #b4  
plate #e8  #ce4 7,pg4 #fc7 ,what        
proportion must the power bear to the   
weight in fig4 #b1 that their momentums 
may be equal8  #cf4 7,pg4 #fd7 ,what is 
a fundamental law as respects power and 
time8  #cg4 7,pg4 #fd7 ,if to gain power
 we must lose time1 what advantage do we
 derive from the mechanical powers8     
#ch4 7,pg4 #fd7 ,what name is given to  
two or more pulleys connected by one    
string8  #ci4 7,pg4 #fd7 ,how do we     
estimate the power gained by a system of
 pulleys8  #dj4 7,pg4 #fe7 ,what is     
represented by fig4 #e4 plate #e8  #da4 
7,pg4 #fe7 ,how does the wheel operate  
in increasing power8  #db4 7,pg4 #fe7   
,how is this compared with the lever8   
#dc4 7,pg4 #fe7 ,how does a handle fixed
 to an axle1 represent a wheel1 fig4 #f8

  #dd4 7,pg4 #fe7 ,how could we increase
 the power in this instrument8  #de4    
7,pg4 #ff7 ,what other forces besides   
the power of men1 do we employ to move  
machines8  #df4 7,pg4 #ff7 ,what will   
serve as an example of an inclined      
plane8  #dg4 7,pg4 #ff7 ,in what        
proportion does it gain power8 7fig4    
#g47  #dh4 7,pg4 #ff7 ,to what is the   
wedge compared8 7fig4 #h47  #di4 7,pg4  
#ff7 ,how does its power increase8  #ej4
 7,pg4 #fg7 ,why is it rather a compound
 than a simple power8  #ea4 7,pg4 #fg7  
,what common instruments act upon the   
principle of the inclined plane1 or the 
wedge8  #eb4 7,pg4 #fg7 ,why does a     
knife cut best when drawn across8  #ec4 
7,pg4 #fg7 ,the screw has two essential 
parts2 what are they8  #ed4 7,pg4 #fg7  
,what other instrument is used to turn  
the screw8  #ee4 7,pg4 #fg7 ,how can you
 compare the screw with an inclined     
plane8 ,fig4 #aj4  #ef4 7,pg4 #fh7 ,by  
what two means may the power of the     
screw be increased8  #eg4 7,pg4 #fh7    

,how do we estimate the power gained by 
the screw8  #eh4 7,pg4 #fh7 ,is the     
lever always attached to the nut1 as in 
the figure8  #ei4 7,pg4 #fh7 ,what is   
said respecting the composition of all  
machines1 and for what must allowance   
always be made in estimating their      
power8  #fj4 7,pg4 #fi7 ,what is meant  
by friction1 and what causes it8  #fa4  
7,pg4 #fi7 ,how may friction be         
diminished8  #fb4 7,pg4 #fi7 ,friction  
is of two kinds1 what are they8  #fc4   
7,pg4 #fi7 ,for what purpose are wheels 
often used8  #fd4 7,pg4 #fi7 ,when is   
the friction of a carriage wheel changed
 from the rolling to the rubbing        
friction8  #fe4 7,pg4 #gj7 ,what is a   
medium1 and in what proportion does it  
diminish motion8  #ff4 7,pg4 #gj7 ,under
 what circumstances must a body be      
placed1 in order to move without        
impediment8    ,,conversation ,,vi4     
,,causes ,,of ,,the ,,motion ,,of ,,the 
,,heavenly ,,bodies4  ,,of ,,the        
,,earth',s ,,annual ,,motion4 ,,of ,,the

 ,,planets ,,and ,,their ,,motion4 ,,of 
,,the ,,diurnal ,,motion ,,of ,,the     
,,earth ,,and ,,planets4   ,,caroline4  
,i am come to you to-day quite elated   
with the spirit of opposition1 ,mrs4    
,b42 for ,i have discovered such a      
powerful objection to your theory of    
attraction1 that ,i doubt whether even  
your conjuror ,newton1 with his magic   
wand of gravitation1 will be able to    
dispel it4  ,mrs4 ,b4 ,well1 my dear1   
pray what is this weighty objection8    
,illustration3 ,,plate ,,vi4  ,caroline4
 ,you say that the earth revolves in its
 orbit round the sun once in a year1 and
 that bodies attract in proportion to   
the quantity of matter they contain2 now
 we all know the sun to be much larger  
than the earth3 why1 therefore does it  
not draw the earth into itself2 you will
 not1 ,i suppose1 pretend to say that we
 are falling towards the sun8  ,emily4  
,however plausible your objection       
appears1 ,caroline1 ,i think you place  
too much reliance upon it3 when any one 

has given such convincing proofs of     
sagacity and wisdom as ,sir ,isaac      
,newton1 when we find that his opinions 
are universally received and adopted1 is
 it to be expected that any objection we
 can advance should overturn them8      
,caroline4 ,yet ,i confess that ,i am   
not inclined to yield implicit faith    
even to opinions of the great ,newton3  
for what purpose are we endowed with    
reason1 if we are denied the privilege  
of making use of it1 by judging for     
ourselves4  ,mrs4 ,b4 ,it is reason     
itself which teaches us1 that when we1  
novices in science1 start objections to 
theories established by men of knowledge
 and wisdom1 we should be diffident     
rather of our own than of their opinion4
 ,i am far from wishing to lay the least
 restraint on your questions2 you cannot
 be better convinced of the truth of a  
system1 than by finding that it resists 
all your attacks1 but ,i would advise   
you not to advance your objections with 
so much confidence1 in order that the   

discovery of their fallacy may be       
attended with less mortification4 ,in   
answer to that you have just proposed1  
,i can only say1 that the earth really  
is attracted by the sun4  ,caroline4    
,take care1 at least1 that we are not   
consumed by him1 ,mrs4 ,b4  ,mrs4 ,b4   
,we are in no danger2 but ,newton1 our  
magician1 as you are pleased to call    
him1 cannot extricate himself from this 
difficulty without the aid of some      
cabalistical figures1 which ,i must draw
 for him4  ,let us suppose the earth1 at
 its creation1 to have been projected   
forwards into universal space3 we know  
that if no obstacle impeded its course  
it would proceed in the same direction1 
and with a uniform velocity for ever4   
,in fig4 #a4 plate #f1 ,a represents the
 earth1 and ,s the sun4 ,we shall       
suppose the earth to be arrived at the  
point in which it is represented in the 
figure1 having a velocity which would   
carry it on to ,b in the space of one   
month2 whilst the sun's attraction would

 bring it to ,c in the same space of    
time4 ,observe that the two forces of   
projection and attraction do not act in 
opposition1 but perpendicularly1 or at a
 right angle to each other4 ,can you    
tell me now1 how the earth will move8   
,emily4 ,i recollect your teaching us   
that a body acted upon by two forces    
perpendicular to each other1 would move 
in the diagonal of a parallelogram2 if1 
therefore1 ,i complete the              
parallelogram1 by drawing the lines ,c  
,d1 ,b ,d1 the earth will move in the   
diagonal ,a ,d4  ,mrs4 ,b4 ,a ball      
struck by two forces acting             
perpendicularly to each other1 it is    
true1 moves in the diagonal of a        
parallelogram2 but you must observe that
 the force of attraction is continually 
acting upon our terrestrial ball1 and   
producing an incessant deviation from   
its course in a right line1 which       
converts it into that of a curve-line2  
every point of which may be considered  
as constituting the diagonal of an      

infinitely small parallelogram4  ,let us
 retain the earth a moment at the point 
,d1 and consider how it will be affected
 by the combined action of the two      
forces in its new situation4 ,it still  
retains its tendency to fly off in a    
straight line2 but a straight line would
 now carry it away to ,f1 whilst the sun
 would attract it in the direction ,d   
,s2 how then will it proceed8  ,emily4  
,it will go on in a curve-line1 in a    
direction between that of the two       
forces4  ,mrs4 ,b4 ,in order to know    
exactly what course the earth will      
follow1 draw another parallelogram      
similar to the first1 in which the line 
,d ,f describes the force of projection1
 and the line ,d ,s that of attraction2 
and you will find that the earth will   
proceed in the curve-line ,d ,g4        
,caroline4 ,you must now allow me to    
draw a parallelogram1 ,mrs4 ,b4 ,let me 
consider in what direction will the     
force of projection now impel the earth4
  ,mrs4 ,b4 ,first draw a line from the 

earth to the sun representing the force 
of attraction2 then describe the force  
of projection at a right angle to it4   
,caroline4 ,the earth will then move in 
the curve ,g ,i1 of the parallelogram ,g
 ,h ,i ,k4  ,mrs4 ,b4 ,you recollect    
that a body acted upon by two forces1   
moves through a diagonal1 in the same   
time that it would have moved through   
one of the sides of the parallelogram1  
were it acted upon by one force only4   
,the earth has passed through the       
diagonals of these three parallelograms1
 in the space of three months1 and has  
performed one quarter of a circle2 and  
on the same principle it will go on till
 it has completed the whole of the      
circle4 ,it will then recommence a      
course1 which it has pursued ever since 
it first issued from the hand of its    
,creator1 and which there is every      
reason to suppose it will continue to   
follow1 as long as it remains in        
existence4  ,emily4 ,what a grand and   
beautiful effect resulting from so      

simple a cause6  ,caroline4 ,it affords 
an example1 on a magnificent scale1 of  
the curvilinear motion1 which you taught
 us in mechanics4 ,the attraction of the
 sun is the centripetal force1 which    
confines the earth to a centre2 and the 
impulse of projection1 the centrifugal  
force1 which impels the earth to quit   
the sun1 and fly off in a tangent4      
,mrs4 ,b4 ,exactly so4 ,a simple mode of
 illustrating the effect of these       
combined forces on the earth1 is to cut 
a slip of card in the form of a         
carpenter's square1 as ,a1 ,b1 ,c2 7fig4
 #b4 plate #f47 the point ,b will be a  
right angle1 the sides of the square    
being perpendicular to each other2 after
 having done this you are to describe a 
small circle at the angular point ,b1   
representing the earth1 and to fasten   
the extremity of one of the legs of the 
square to a fixed point ,a1 which we    
shall consider as the sun4 ,thus        
situated1 the two sides of the square   
will represent both the centrifugal and 

centripetal forces2 ,a ,b1 representing 
the centripetal1 and ,b ,c1 the         
centrifugal force2 if you now draw it   
round the fixed point1 you will see how 
the direction of the centrifugal force  
varies1 constantly forming a tangent to 
the circle in which the earth moves1 as 
it is constantly at a right angle with  
the centripetal force4  ,emily4 ,the    
earth then1 gravitates towards the sun1 
without the slightest danger either of  
approaching nearer1 or receding further 
from it4 ,how admirably this is         
contrived6 ,if the two forces which     
produce this curved motion1 had not been
 so accurately adjusted1 one would      
ultimately have prevailed over the      
other1 and we should either have        
approached so near the sun as to have   
been burnt1 or have receded so far from 
it as to have been frozen4  ,mrs4 ,b4   
,what will you say1 my dear1 when ,i    
tell you1 that these two forces are not1
 in fact1 so proportioned as to produce 
circular motion in the earth8 ,we       

actually revolve round the sun in an    
elliptical or oval orbit1 the sun being 
situated in one of the foci or centres  
of the oval1 so that the sun is at some 
periods much nearer to the earth1 than  
at others4  ,caroline4 ,you must explain
 to us1 at least1 in what manner we     
avoid the threatened destruction4  ,mrs4
 ,b4 ,let us suppose that when the earth
 is at ,a1 7fig4 #c47 its projectile    
force should not have given it a        
velocity sufficient to counterbalance   
that of gravity1 so as to enable these  
powers conjointly to carry it round the 
sun in a circle2 the earth1 instead of  
describing the line ,a ,c1 as in the    
former figure1 will approach nearer the 
sun in the line ,a ,b4  ,caroline4      
,under these circumstances1 ,i see not  
what is to prevent our approaching      
nearer and nearer the sun1 till we fall 
into it3 for its attraction increases as
 we advance towards it1 and produces an 
accelerated velocity in the earth1 which
 increases the danger4  ,mrs4 ,b4 ,there

 is another seeming danger1 of which you
 are not aware4 ,observe1 that as the   
earth approaches the sun1 the direction 
of its projectile force is no longer    
perpendicular to that of its attraction1
 but inclines more nearly to it4 ,when  
the earth reaches that part of its orbit
 at ,b1 the force of projection would   
carry it to ,d1 which brings it nearer  
the sun instead of bearing it away from 
it4  ,emily4 ,if1 then1 we are driven by
 one power1 and drawn by the other to   
this centre of destruction1 how is it   
possible for us to escape8  ,mrs4 ,b4 ,a
 little patience1 and you will find that
 we are not without resource4 ,the earth
 continues approaching the sun with a   
uniformly increasing accelerated motion1
 till it reaches the point ,e2 in what  
direction will the projectile force now 
impel it8  ,emily4 ,in the direction ,e 
,f4 ,here then the two forces act       
perpendicularly to each other1 the lines
 representing them forming a right      
angle1 and the earth is situated just as

 it was in the preceding figure2        
therefore1 from this point1 it should   
revolve round the sun in a circle4      
,mrs4 ,b4 ,no1 all the circumstances do 
not agree4 ,in motion round a centre1   
you recollect that the centrifugal force
 increases with the velocity of the     
body1 or in other words1 the quicker it 
moves the stronger is its tendency to   
fly off in a right line4 ,when the      
earth1 therefore1 arrives at ,e1 its    
accelerated motion will have so far     
increased its velocity1 and consequently
 its centrifugal force1 that the latter 
will prevail over the force of          
attraction1 and force the earth away    
from the sun till it reaches ,g4        
,caroline4 ,it is thus then that we     
escape from the dangerous vicinity of   
the sun2 and in proportion as we recede 
from it1 the force of its attraction1   
and1 consequently1 the velocity of the  
earth's motion1 are diminished4  ,mrs4  
,b4 ,yes4 ,from ,g the direction of     
projection is towards ,h1 that of       

attraction towards ,s1 and the earth    
proceeds between them with a uniformly  
retarded motion1 till it has completed  
its revolution4 ,thus you see that the  
earth travels round the sun1 not in a   
circle1 but an ellipsis1 of which the   
sun occupies one of the foci2 and that  
in its course1 the earth alternately    
approaches and recedes from it1 without 
any danger of being either swallowed up1
 or being entirely carried away from it4
  ,caroline4 ,and ,i observe1 that what 
,i apprehended to be a dangerous        
irregularity1 is the means by which the 
most perfect order and harmony are      
produced4  ,emily4 ,the earth travels   
then at a very unequal rate1 its        
velocity being accelerated as it        
approaches the sun1 and retarded as it  
recedes from it4  ,mrs4 ,b4 ,it is      
mathematically demonstrable1 that1 in   
moving round a point towards which it is
 attracted1 a body passes over equal    
areas1 in equal times4 ,the whole of the
 space contained within the earth's     

orbit1 is in fig4 #d1 divided into a    
number of areas or surfaces2 #a1 #b1 #c1
 #d1 c4 all of which are of equal       
dimensions1 though of very different    
forms2 some of them1 you see1 are long  
and narrow1 others broad and short3 but 
they each of them contain an equal      
quantity of space4 ,an imaginary line   
drawn from the centre of the earth to   
that of the sun1 and keeping pace with  
the earth in its revolution1 passes over
 equal areas in equal times2 that is to 
say1 if it is a month going from ,a to  
,b1 it will be a month going from ,b to 
,c1 and another from ,c to ,e1 and so   
on2 and the areas ,a ,b ,s1 ,b ,c ,s1 ,c
 ,e ,s1 will be equal to each other1    
although the lines ,a ,b1 ,b ,c1 ,c ,e1 
are unequal4  ,caroline4 ,what long     
journeys the earth has to perform in the
 course of a month1 in one part of her  
orbit1 and how short they are in the    
other part6  ,mrs4 ,b4 ,the inequality  
is not so considerable as appears in    
this figure2 for the earth's orbit is   

not so eccentric as it is there         
described2 and in reality1 differs but  
little from a circle3 that part of the  
earth's orbit nearest the sun is called 
its perihelion1 that part most distant  
from the sun1 its aphelion2 and the     
earth is above three millions of miles  
nearer the sun at its perihelion than at
 its aphelion4  ,emily4 ,i think ,i can 
trace a consequence from these different
 situations of the earth2 are not they  
the cause of summer and winter8  ,mrs4  
,b4 ,on the contrary1 during the height 
of summer1 the earth is in that part of 
its orbit which is most distant from the
 sun1 and it is during the severity of  
winter1 that it approaches nearest to   
it4  ,emily4 ,that is very              
extraordinary2 and how then do you      
account for the heat being greatest1    
when we are most distant from the sun8  
,mrs4 ,b4 ,the difference of the earth's
 distance from the sun in summer and    
winter1 when compared with its total    
distance from the sun1 is but           

inconsiderable4 ,the earth1 it is true1 
is above three millions of miles nearer 
the sun in winter than in summer2 but   
that distance1 however great it at first
 appears1 sinks into insignificance in  
comparison with #ie millions of miles1  
which is our mean distance from the sun4
 ,the change of temperature1 arising    
from this difference1 would scarcely be 
sensible1 even were it not completely   
overpowered by other causes which       
produce the variations of the seasons2  
but these ,i shall defer explaining1    
till we have made some further          
observations on the heavenly bodies4    
,caroline4 ,and should not the sun      
appear smaller in summer1 when it is so 
much further from us8  ,mrs4 ,b4 ,it    
actually does1 when accurately measured2
 but the apparent difference in size1   
is1 ,i believe1 not perceptible to the  
naked eye4  ,emily4 ,then1 since the    
earth moves with the greatest velocity  
in that part of its orbit in which it is
 nearest the sun1 it must have completed

 its journey through that half of its   
orbit1 in a shorter time than through   
the other8  ,mrs4 ,b4 ,yes1 it is about 
seven days longer performing the        
summer-half of its orbit1 than the      
winter-half2 and the summers are        
consequently seven days longer in the   
northern1 than they are in the southern 
hemisphere4  ,the revolution of all the 
planets round the sun1 is the result of 
the same causes1 and is performed in the
 same manner1 as that of the earth4     
,caroline4 ,pray what are the planets8  
,mrs4 ,b4 ,they are those celestial     
bodies1 which revolve like our earth1   
about the sun2 they are supposed to     
resemble the earth also in many other   
respects2 and we are led by analogy1 to 
suppose them to be inhabited worlds4    
,caroline4 ,i have heard so1 but do you 
not think such an opinion too great a   
stretch of the imagination8  ,mrs4 ,b4  
,some of the planets are proved to be   
larger than the earth2 it is only their 
immense distance from us1 which renders 

their apparent dimensions so small4     
,now1 if we consider them as enormous   
globes1 instead of small twinkling      
spots1 we shall be led to suppose that  
the ,almighty would not have created    
them merely for the purpose of giving us
 a little light in the night1 as it was 
formerly imagined2 and we should find it
 more consistent with our ideas of the  
,divine wisdom and beneficence1 to      
suppose that these celestial bodies     
should be created for the habitation of 
beings1 who are1 like us1 blessed by his
 providence4 ,both in a moral1 as well  
as a physical point of view1 it appears 
to me more rational to consider the     
planets as worlds revolving round the   
sun2 and the fixed stars as other suns1 
each of them attended by their          
respective system of planets1 to which  
they impart their influence4 ,we have   
brought our telescopes to such a degree 
of perfection1 that from the appearances
 which the moon exhibits when seen      
through them1 we have very good reason  

to conclude that it is a habitable      
globe3 for though it is true that we    
cannot discern its towns and people1 we 
can plainly perceive its mountains and  
valleys3 and some astronomers have gone 
so far as to imagine that they          
discovered volcanos4  ,emily4 ,if the   
fixed stars are suns1 with planets      
revolving round them1 why should we not 
see those planets as well as their suns8
  ,mrs4 ,b4 ,in the first place1 we     
conclude that the planets of other      
systems 7like those of our own7 are much
 smaller than the suns which give them  
light2 therefore at a distance so great 
as to make the suns appear like fixed   
stars1 the planets would be quite       
invisible4 ,secondly1 the light of the  
planets being only reflected light1 is  
much more feeble than that of the fixed 
stars4 ,there is exactly the same       
difference as between the light of the  
sun and that of the moon2 the first     
being a fixed star1 the second a planet4
  ,emily4 ,but the planets appear to us 

as bright as the fixed stars1 and these 
you tell us are suns like our own2 why  
then do we not see them by daylight1    
when they must be just as luminous as   
they are in the night8  ,mrs4 ,b4 ,both 
are invisible from the same cause3 their
 light is so faint1 compared to that of 
the sun1 that it is entirely effaced by 
it3 the light emitted by the fixed stars
 may probably be as great as that of our
 sun1 at an equal distance2 but they    
being so much more remote1 it is        
diffused over a greater space1 and is in
 consequence proportionally lessened4   
,caroline4 ,true2 ,i can see much better
 by the light of a candle that is near  
me1 than by that of one at a great      
distance4 ,but ,i do not understand what
 makes the planets shine8  ,mrs4 ,b4    
,what is that which makes the gilt      
buttons on your brothers coat shine8    
,caroline4 ,the sun4 ,but if it was the 
sun which made the planets shine1 we    
should see them in the day-time1 when   
the sun shone upon them2 or if the      

faintness of their light prevented our  
seeing them in the day1 we should not   
see them at all1 for the sun cannot     
shine upon them in the night4  ,mrs4 ,b4
 ,there you are in error4 ,but in order 
to explain this to you1 ,i must first   
make you acquainted with the various    
motions of the planets4  ,you know1 that
 according to the laws of attraction1   
the planets belonging to our system all 
gravitate towards the sun2 and that this
 force1 combined with that of           
projection1 will occasion their         
revolution round the sun1 in orbits more
 or less elliptical1 according to the   
proportion which these two forces bear  
to each other4  ,but the planets have   
also another motion3 they revolve upon  
their axis4 ,the axis of a planet is an 
imaginary line which passes through its 
centre1 and on which it turns2 and it is
 this motion which produces day and     
night4 ,it is day on that side of the   
planet which faces the sun2 and on the  
opposite side1 which remains in         

darkness1 it is night4 ,our earth1 which
 we consider as a planet1 is #bd hours  
in performing one revolution on its     
axis2 in that period of time1 therefore1
 we have a day and a night2 hence this  
revolution is called the earth's diurnal
 or daily motion2 and it is this        
revolution of the earth from west to    
east which produces an apparent motion  
of the sun1 moon and stars1 in a        
contrary direction4  ,let us now suppose
 ourselves to be beings independent of  
any planet1 travelling in the skies1 and
 looking upon the earth from a point as 
distant from it as from other planets4  
,caroline4 ,it would not be flattering  
to us1 its inhabitants1 to see it make  
so insignificant an appearance4  ,mrs4  
,b4 ,to those accustomed to contemplate 
it in this light1 it could never appear 
more glorious4 ,we are taught by science
 to distrust appearances2 and instead of
 considering the fixed stars and planets
 as little points1 we look upon them    
either as brilliant suns1 or habitable  

worlds2 and we consider the whole       
together as forming one vast and        
magnificent system1 worthy of the       
,divine hand by which it was created4   
,emily4 ,i can scarcely conceive the    
idea of this immensity of creation2 it  
seems too sublime for our               
imagination2--and to think that the     
goodness of ,providence extends over    
millions of worlds throughout a         
boundless universe--,ah6 ,mrs4 ,b41 it  
is we only who become trifling and      
insignificant beings in so magnificent a
 creation6  ,mrs4 ,b4 ,this idea should 
teach us humility1 but without producing
 despondency4 ,the same ,almighty hand  
which guides these countless worlds in  
their undeviating course1 conducts with 
equal perfection1 the blood as it       
circulates through the veins of a fly1  
and opens the eye of the insect to      
behold ,his wonders4 ,notwithstanding   
this immense scale of creation1         
therefore1 we need not fear that we     
shall be disregarded or forgotten4  ,but

 to return to our station in the skies4 
,we were1 if you recollect1 viewing the 
earth at a great distance1 in appearance
 a little star1 one side illumined by   
the sun1 the other in obscurity4 ,but   
would you believe it1 ,caroline1 many of
 the inhabitants of this little star    
imagine that when that part which they  
inhabit is turned from the sun1 darkness
 prevails throughout the universe1      
merely because it is night with them2   
whilst1 in reality1 the sun never ceases
 to shine upon every planet4 ,when1     
therefore1 these little ignorant beings 
look around them during their night1 and
 behold all the stars shining1 they     
cannot imagine why the planets1 which   
are dark bodies1 should shine2          
concluding1 that since the sun does not 
illumine themselves1 the whole universe 
must be in darkness4  ,caroline4 ,i     
confess that ,i was one of these        
ignorant people2 but ,i am now very     
sensible of the absurdity of such an    
idea4 ,to the inhabitants of the other  

planets1 then1 we must appear as a      
little star8  ,mrs4 ,b4 ,yes1 to those  
which revolve round our sun2 for since  
those which may belong to other systems1
 7and whose existence is only           
hypothetical7 are invisible to us1 it is
 probable that we also are invisible to 
them4  ,emily4 ,but they may see our sun
 as we do theirs1 in appearance a fixed 
star8  ,mrs4 ,b4 ,no doubt2 if the      
beings who inhabit those planets are    
endowed with senses similar to ours4 ,by
 the same rule we must appear as a moon 
to the inhabitants of our moon2 but on a
 larger scale1 as the surface of the    
earth is about thirteen times as large  
as that of the moon4  ,emily4 ,the moon1
 ,mrs4 ,b41 appears to move in a        
different direction1 and in a different 
manner from the stars8  ,mrs4 ,b4 ,i    
shall defer the explanation of the      
motion of the moon till our next        
interview1 as it would prolong our      
present lesson too much4   ,questions   
#a4 7,pg4 #ga7 ,what revolution does the

 earth perform in a year8  #b4 7,pg4    
#ga7 ,had the earth received a          
projectile force only1 at the time of   
its creation1 how would it have moved8  
#c4 7,pg4 #gb7 ,what do the lines ,a ,b1
 and ,a ,c1 represent in fig4 #a4 plate 
#f8  #d4 7,pg4 #gb7 ,what have you been 
taught respecting a body acted upon by  
two forces at right angles with each    
other8  #e4 7,pg4 #gb7 ,how does the    
force of gravity change the diagonal    
into a curved line8  #f4 7,pg4 #gb7     
,describe the operation of the forces of
 projection and of gravity as           
illustrated by the parallelograms in the
 figure8  #g4 7,pg4 #gb7 ,what is the   
law respecting the time required for    
motion in the diagonal8  #h4 7,pg4 #gc7 
,what portion of a year is represented  
by the three diagonals in the figure8   
#i4 7,pg4 #gc7 ,how will what you have  
learned respecting motion in a curve1   
apply to the earth's motion8  #aj4 7,pg4
 #gc7 ,in what form are you directed to 
cut a piece of card to aid in           

illustrating the two forces acting upon 
the earth8  #aa4 7,pg4 #gc7 ,how must   
you apply it to this purpose8 7fig4 #b4 
plate #f47  #ab4 7,pg4 #gc7 ,if these   
two forces did not exactly balance each 
other1 what would result8  #ac4 7,pg4   
#gc7 ,does the earth revolve in a       
circular orbit8  #ad4 7,pg4 #gc7 ,what  
results from its motion in an ellipsis8 
#ae4 7,pg4 #gd7 ,what is represented by 
the lines ,a ,c1 ,a ,b1 in fig4 #c4     
plate #f8  #af4 7,pg4 #gd7 ,were the    
projectile force to carry the earth from
 ,b to ,d1 7fig4 #c47 what would result8
  #ag4 7,pg4 #gd7 ,when it has arrived  
at ,e1 what angle will be formed by the 
lines representing the two forces8  #ah4
 7,pg4 #gd7 ,what effect will the       
accelerated motion then produce8  #ai4  
7,pg4 #ge7 ,what is the form of the     
earth's orbit1 and what circumstances   
produce this form8  #bj4 7,pg4 #ge7     
,what is the consequence as regards the 
regularity of the earth's motion8  #ba4 
7,pg4 #ge7 ,what law governs as regards 

the spaces passed over1 and how is this 
explained by fig4 #d4 plate #f8  #bb4   
7,pg4 #ge7 ,what is meant by perihelion1
 and by aphelion8  #bc4 7,pg4 #ge7 ,what
 is the difference of the distance of   
the earth from the sun1 in these two    
points8  #bd4 7,pg4 #gf7 ,at what season
 of the year is it nearest to1 and at   
what furthest from the sun8  #be4 7,pg4 
#gf7 ,what is the mean distance of the  
earth from the sun8  #bf4 7,pg4 #gf7    
,why is but little effect produced1 as  
regards temperature1 by the change of   
distance8  #bg4 7,pg4 #gf7 ,has it any  
influence on the sun's apparent size8   
#bh4 7,pg4 #gf7 ,are the summer and     
winter1 half years1 of the same length2 
what is their difference1 and what is   
the cause8  #bi4 7,pg4 #gf7 ,what are   
the planets8  #cj4 7,pg4 #gg7 ,what     
circumstances render it probable that   
they are habitable globes8  #ca4 7,pg4  
#gg7 ,what is believed respecting the   
fixed stars8  #cb4 7,pg4 #gg7 ,what     
discoveries have been made in the moon8 

#cc4 7,pg4 #gg7 ,what prevents our      
seeing the planets1 if there are any1   
which revolve round the fixed stars8    
#cd4 7,pg4 #gg7 ,what prevents our      
seeing the stars and planets in the     
day-time8  #ce4 7,pg4 #gh7 ,what other  
motions have the earth and planets1     
besides that in their orbits8  #cf4     
7,pg4 #gh7 ,what is the imaginary line  
called1 round which they revolve8  #cg4 
7,pg4 #gh7 ,how does this occasion night
 and day8  #ch4 7,pg4 #gh7 ,in what     
direction does the earth turn upon its  
axis1 and what apparent motion of the   
sun1 moon1 and stars is thereby         
produced8  #ci4 7,pg4 #gi7 ,what must be
 the appearance of the earth to an      
inhabitant of one of the planets8  #dj4 
7,pg4 #gi7 ,what the appearance of the  
sun to the inhabitants of planets in    
other systems8  #da4 7,pg4 #gi7 ,what   
the appearance of the earth to an       
inhabitant of the moon8                 
,,conversation ,,vii4  ,,of ,,the       
,,planets4  ,,of ,,the ,,satellites ,,or

 ,,moons4 ,,gravity ,,diminishes ,,as   
,,the ,,square ,,of ,,the ,,distance4   
,,of ,,the ,,solar ,,system4 ,,of       
,,comets4 ,,constellations1 ,,signs ,,of
 ,,the ,,zodiac4 ,,of ,,copernicus1     
,,newton1 c4   ,,mrs4 ,b4  ,the planets 
are distinguished into primary and      
secondary4 ,those which revolve         
immediately about the sun are called    
primary4 ,many of these are attended in 
their course by smaller planets1 which1 
revolve round them3 these are called    
secondary planets1 satellites1 or moons4
 ,such is our moon which accompanies the
 earth1 and is carried with it round the
 sun4  ,emily4 ,how then can you        
reconcile the motion of the secondary   
planets to the laws of gravitation2 for 
the sun is much larger than any of the  
primary planets2 and is not the power of
 gravity proportional to the quantity of
 matter8  ,caroline4 ,perhaps the sun1  
though much larger1 may be less dense   
than the planets4 ,fire you know1 is    
very light1 and it may contain but      

little matter1 though of great          
magnitude4  ,mrs4 ,b4 ,we do not know of
 what kind of matter the sun is made2   
but we may be certain1 that since it is 
the general centre of attraction of our 
system of planets1 it must be the body  
which contains the greatest quantity of 
matter in that system4  ,you must       
recollect1 that the force of attraction 
is not only proportional to the quantity
 of matter1 but to the degree of        
proximity of the attractive body3 this  
power is weakened by being diffused1 and
 diminishes as the distance increases4  
,emily4 ,then if a planet was to lose   
one-half of its quantity of matter1 it  
would lose one half of its attractive   
power2 and the same effect would be     
produced by removing it to twice its    
former distance from the sun2 that ,i   
understand4  ,mrs4 ,b4 ,not so perfectly
 as you imagine4 ,you are correct as    
respects the diminution in size1 because
 the attractive force is in the same    
proportion as the quantity of matter2   

but were you to remove a planet to      
double its former distance1 it would    
retain but one-fourth part of its       
gravitating force2 for attraction       
decreases not in proportion to the      
simple increase of the distance1 but as 
the squares of the distances increase4  
,caroline4 ,i do not exactly comprehend 
what is meant by the squares1 in this   
case1 although ,i know very well what is
 in general intended by a square4  ,mrs4
 ,b4 ,by the square of a number we mean 
the product of a number1 multiplied by  
itself2 thus two1 multiplied by two1 is 
four1 which is therefore the square of  
two2 in like manner the square of three1
 is nine1 because three multiplied by   
three1 gives that product4  ,emily4     
,then if one planet is three times more 
distant from the sun than another1 it   
will be attracted with but one-ninth    
part of the force2 and if at four times 
the distance1 with but one-sixteenth1   
sixteen being the square of four8  ,mrs4
 ,b4 ,you are correct2 the rule is1 that

 the attractive force is in the inverse 
proportion of the square of the         
distance4 ,and it is easily demonstrated
 by the mathematics1 that the same is   
the case with every power that emanates 
from a centre2 as for example1 the light
 from the sun1 or from any other        
luminous body1 decreases in its         
intensity at the same rate4  ,caroline4 
,then the more distant planets1 move    
much slower in their orbits2 for their  
projectile force must be proportioned to
 that of attraction8 ,but ,i do not see 
how this accounts for the motion of the 
secondary1 round the primary planets1 in
 preference to moving round the sun8    
,emily4 ,is it not because the vicinity 
of the primary planets1 renders their   
attraction stronger than that of the    
sun8  ,mrs4 ,b4 ,exactly so4 ,but since 
the attraction between bodies is mutual1
 the primary planets are also attracted 
by the satellites which revolve round   
them4 ,the moon attracts the earth1 as  
well as the earth the moon2 but as the  

latter is the smaller body1 her         
attraction is proportionally less2      
therefore1 neither the earth revolves   
round the moon1 nor the moon round the  
earth2 but they both revolve round a    
point1 which is their common centre of  
gravity1 and which is as much nearer to 
the earth than to the moon1 as the      
gravity of the former exceeds that of   
the latter4  ,emily4 ,yes1 ,i recollect 
your saying1 that if two bodies were    
fastened together by a wire or bar1     
their common centre of gravity would be 
in the middle of the bar1 provided the  
bodies were of equal weight2 and if they
 differed in weight1 it would be nearer 
the larger body4 ,if then1 the earth and
 moon had no projectile force which     
prevented their mutual attraction from  
bringing them together1 they would meet 
at their common centre of gravity4      
,caroline4 ,the earth then has a great  
variety of motion1 it revolves round the
 sun1 round its own axis1 and round the 
point towards which the moon attracts   

it4  ,mrs4 ,b4 ,just so2 and this is the
 case with every planet which is        
attended by satellites4 ,the complicated
 effect of this variety of motions1     
produces certain irregularities1 which1 
however1 it is not necessary to notice  
at present1 excepting to observe that   
they eventually correct each other1 so  
that no permanent derangement exists4   
,the planets act on the sun1 in the same
 manner as they are themselves acted on 
by their satellites2 for attraction1 you
 must remember1 is always mutual2 but   
the gravity of the planets 7even when   
taken collectively7 is so trifling      
compared with that of the sun1 that were
 they all placed on the same side of    
that luminary1 they would not cause him 
to move so much as one-half of his      
diameter towards them1 and the common   
centre of gravity1 would still remain   
within the body of the sun4 ,the planets
 do not1 therefore1 revolve round the   
centre of the sun1 but round a point at 
a small distance from its centre1 about 

which the sun also revolves4  ,emily4 ,i
 thought the sun had no motion8  ,mrs4  
,b4 ,you were mistaken2 for besides that
 round the common centre of gravity1    
which ,i have just mentioned1 which is  
indeed very inconsiderable1 he revolves 
on his axis in about #be days2 this     
motion is ascertained by observing      
certain spots which disappear1 and      
reappear regularly at stated times4     
,illustration3 ,,plate ,,vii4           
,caroline4 ,a planet has frequently been
 pointed out to me in the heavens2 but  
,i could not perceive that its motion   
differed from that of the fixed stars1  
which only appear to move4  ,mrs4 ,b4   
,the great distance of the planets1     
renders their apparent motion so slow1  
that the eye is not sensible of their   
progress in their orbits1 unless we     
watch them for some considerable length 
of time3 but if you notice the nearness 
of a planet to any particular fixed     
star1 you may in a few nights perceive  
that it has changed its distance from   

it1 whilst the stars themselves always  
retain their relative situations4 ,the  
most accurate idea ,i can give you of   
the situation and motion of the planets 
in their orbits1 will be by the         
examination of this diagram1 7plate #g4 
fig4 #a47 representing the solar system1
 in which you will find every planet1   
with its orbit delineated4  ,emily4 ,but
 the orbits here are all circular1 and  
you said that they were elliptical4 ,the
 planets appear too1 to be moving round 
the centre of the sun2 whilst you told  
us that they moved round a point at a   
little distance from thence4  ,mrs4 ,b4 
,the orbits of the planets are so nearly
 circular1 and the common centre of     
gravity of the solar system1 so near the
 centre of the sun1 that these          
deviations are too small to be          
represented4 ,the dimensions of the     
planets1 in their proportion to each    
other1 you will find delineated in fig4 
#b4  ,mercury is the planet nearest the 
sun2 his orbit is consequently contained

 within ours2 his vicinity to the sun1  
prevents our frequently seeing him1 so  
that very accurate observations cannot  
be made upon ,mercury4 ,he performs his 
revolution round the sun in about #hg   
days1 which is consequently the length  
of his year4 ,the time of his rotation  
on his axis is not known2 his distance  
from the sun is computed to be #cg      
millions of miles1 and his diameter     
#cahj miles4 ,the heat of this planet is
 supposed to be so great1 that water    
cannot exist there but in a state of    
vapour1 and that even quicksilver would 
be made to boil4  ,caroline4 ,oh1 what a
 dreadful climate6  ,mrs4 ,b4 ,though we
 could not live there1 it may be        
perfectly adapted to other beings1      
destined to inhabit it2 or he who       
created it may have so modified the     
heat1 by provisions of which we are     
ignorant1 as to make it habitable even  
by ourselves4  ,venus1 the next in the  
order of planets1 is #fh millions of    
miles from the sun3 she revolves about  

her axis in #bc hours and #ba minutes1  
and goes round the sun in #bdd days1 #ag
 hours4 ,the orbit of ,venus is also    
within ours2 during nearly one-half of  
her course in it1 we see her before     
sun-rise1 and she is then called the    
morning star2 in the other part of her  
orbit she rises later than the sun4     
,caroline4 ,in that case we cannot see  
her1 for she must rise in the day time8 
,mrs4 ,b4 ,true2 but when she rises     
later than the sun1 she also sets later2
 so that we perceive her approaching the
 horizon after sun-set3 she is then     
called ,hesperus1 or the evening star4  
,do you recollect those beautiful lines 
of ,milton8      ,now came still evening
 on1 and twilight gray     ,had in her  
sober livery all things clad2           
,silence accompanied2 for beast and     
bird1     ,they to their grassy couch1  
these to their nests     ,were slunk1   
all but the wakeful nightingale2        
,she all night long her amorous descant 
sung2     ,silence was pleas'd2 now     

glowed the firmament     ,with living   
sapphires4 ,hesperus that led     ,the  
starry host1 rode brightest1 till the   
moon     ,rising in clouded majesty1 at 
length     ,apparent queen unveil'd her 
peerless light1     ,and o'er the dark  
her silver mantle threw4  ,the planet   
next to ,venus is the ,earth1 of which  
we shall soon speak at full length4 ,at 
present ,i shall only observe that we   
are #ie millions of miles distant from  
the sun1 that we perform our annual     
revolution in #cfe days #e hours and #di
 minutes2 and are attended in our course
 by a single moon4  ,next follows ,mars4
 ,he can never come between us and the  
sun1 like ,mercury and ,venus2 his      
motion is1 however1 very perceptible1 as
 he may be traced to different          
situations in the heavens2 his distance 
from the sun is #add millions of miles2 
he turns round his axis in #bd hours and
 #ci minutes2 and he performs his annual
 revolution1 in about #fhg of our days3 
his diameter is #dabj miles4 ,then      

follow four very small planets1 ,juno1  
,ceres1 ,pallas and ,vesta1 which have  
been recently discovered1 but whose     
dimensions1 and distances from the sun1 
have not been very accurately           
ascertained4 ,they are generally called 
asteroids4  ,jupiter is next in order3  
this is the largest of all the planets4 
,he is about #dij millions of miles from
 the sun1 and completes his annual      
period in nearly #ab of our years4 ,he  
turns round his axis in about ten hours4
 ,he is above #abjj times as big as our 
earth2 his diameter is #hf1jjj miles4   
,the respective proportions of the      
planets cannot1 therefore1 you see1 be  
conveniently delineated in a diagram4   
,he is attended by four moons4  ,the    
next planet is ,saturn1 whose distance  
from the sun1 is about #ijj millions of 
miles2 his diurnal rotation is performed
 in #aj hours and a quarter3 his annual 
revolution is nearly #cj of our years4  
,his diameter is #gi1jjj miles4 ,this   
planet is surrounded by a luminous ring1

 the nature of which1 astronomers are   
much at a loss to conjecture3 he has    
seven moons4 ,lastly1 we observe the    
planet ,herschel1 discovered by ,dr4    
,herschel1 by whom it was named the     
,georgium ,sidus1 and which is attended 
by six moons4  ,caroline4 ,how charming 
it must be in the distant planets1 to   
see several moons shining at the same   
time2 ,i think ,i should like to be an  
inhabitant of ,jupiter or ,saturn4      
,mrs4 ,b4 ,not long ,i believe4         
,consider what extreme cold must prevail
 in a planet1 situated as ,saturn is1 at
 nearly ten times the distance at which 
we are from the sun4 ,then his numerous 
moons are far from making so splendid an
 appearance as ours2 for they can       
reflect only the light which they       
receive from the sun2 and both light1   
and heat1 decrease in the same ratio or 
proportion to the distances1 as gravity4
 ,can you tell me now how much more     
light we enjoy than ,saturn8  ,caroline4
 ,the square of ten is a hundred2       

therefore1 ,saturn has a hundred times  
less--or to answer your question        
exactly1 we have a hundred times more   
light and heat1 than ,saturn--this      
certainly does not increase my wish to  
become one of the poor wretches who     
inhabit that planet4  ,mrs4 ,b4 ,may not
 the inhabitants of ,mercury1 with equal
 plausibility1 pity us for the          
insupportable coldness of our situation2
 and those of ,jupiter and ,saturn for  
our intolerable heat8 ,the ,almighty    
power which created these planets1 and  
placed them in their several orbits1 has
 no doubt peopled them with beings1     
whose bodies are adapted to the various 
temperatures and elements1 in which they
 are situated4 ,if we judge from the    
analogy of our own earth1 or from that  
of the great and universal beneficence  
of ,providence1 we must conclude this to
 be the case4  ,caroline4 ,are not      
comets1 in some respects similar to     
planets8  ,mrs4 ,b4 ,yes1 they are2 for 
by the reappearance of some of them1 at 

stated times1 they are known to revolve 
round the sun2 but in orbits so         
extremely eccentric1 that they disappear
 for a great number of years4 ,if they  
are inhabited1 it must be by a species  
of beings very different1 not only from 
the inhabitants of this1 but from those 
of any of the other planets1 as they    
must experience the greatest            
vicissitudes of heat and cold2 one part 
of their orbit being so near the sun1   
that their heat1 when there1 is computed
 to be greater than that of red-hot     
iron2 in this part of its orbit1 the    
comet emits a luminous vapour1 called   
the tail1 which it gradually loses as it
 recedes from the sun2 and the comet    
itself totally disappears from our      
sight1 in the more distant parts of its 
orbit1 which extends considerably beyond
 that of the furthest planet4  ,the     
number of comets belonging to our system
 cannot be ascertained1 as some of them 
are several centuries before they make  
their reappearance4 ,the number that are

 known by their regular reappearance is1
 ,i believe1 only three1 although their 
whole number is very considerable4      
,emily4 ,pray1 ,mrs4 ,b41 what are the  
constellations8  ,mrs4 ,b4 ,they are the
 fixed stars2 which the ancients1 in    
order to recognise them1 formed into    
groups1 and gave the names of the       
figures1 which you find delineated on   
the celestial globe4 ,in order to show  
their proper situations in the heavens1 
they should be painted on the internal  
surface of a hollow sphere1 from the    
centre of which you should view them2   
you would then behold them as they      
appear to be situated in the heavens4   
,the twelve constellations1 called the  
signs of the zodiac1 are those which are
 so situated1 that the earth1 in its    
annual revolution1 passes directly      
between them1 and the sun4 ,their names 
are ,aries1 ,taurus1 ,gemini1 ,cancer1  
,leo1 ,virgo1 ,libra1 ,scorpio1         
,sagittarius1 ,capricornus1 ,aquarius1  
,pisces2 the whole occupying a complete 

circle1 or broad belt1 in the heavens1  
called the zodiac4 7plate #h4 fig4 #a47 
,hence1 a right line drawn from the     
earth1 and passing through the sun1     
would reach one of these constellations1
 and the sun is said to be in that      
constellation at which the line         
terminates3 thus1 when the earth is at  
,a1 the sun would appear to be in the   
constellation or sign ,aries2 when the  
earth is at ,b1 the sun would appear in 
,cancer2 when the earth was at ,c1 the  
sun would be in ,libra2 and when the    
earth was at ,d1 the sun would be in    
,capricorn4 ,you are aware that it is   
the real motion of the earth in its     
orbit1 which gives to the sun this      
apparent motion through the signs4 ,this
 circle1 in which the sun thus appears  
to move1 and which passes through the   
middle of the zodiac1 is called the     
ecliptic4  ,caroline4 ,but many of the  
stars in these constellations appear    
beyond the zodiac4  ,illustration3      
,,plate ,,viii4  ,mrs4 ,b4 ,we have no  

means of ascertaining the distance of   
the fixed stars4 ,when1 therefore1 they 
are said to be in the zodiac1 it is     
merely implied that they are situated in
 that direction1 and that they shine    
upon us through that portion of the     
heavens1 which we call the zodiac4      
,emily4 ,but are not those large bright 
stars1 which are called stars of the    
first magnitude1 nearer to us1 than     
those small ones which we can scarcely  
discern8  ,mrs4 ,b4 ,it may be so2 or   
the difference of size and brilliancy of
 the stars may proceed from their       
difference of dimensions2 this is a     
point which astronomers are not enabled 
to determine4 ,considering them as suns1
 ,i see no reason why different suns    
should not vary in dimensions1 as well  
as the planets belonging to them4       
,emily4 ,what a wonderful and beautiful 
system this is1 and how astonishing to  
think that every fixed star may probably
 be attended by a similar train of      
planets6  ,caroline4 ,you will accuse me

 of being very incredulous1 but ,i      
cannot help still entertaining some     
doubts1 and fearing that there is more  
beauty than truth in this system4 ,it   
certainly may be so2 but there does not 
appear to me to be sufficient evidence  
to prove it4 ,it seems so plain and     
obvious that the earth is motionless1   
and that the sun and stars revolve round
 it2--your solar system1 you must allow1
 is directly in opposition to the       
evidence of our senses4  ,mrs4 ,b4 ,our 
senses so often mislead us1 that we     
should not place implicit reliance upon 
them4  ,caroline4 ,on what then can we  
rely1 for do we not receive all our     
ideas through the medium of our senses8 
,mrs4 ,b4 ,it is true that they are our 
primary source of knowledge2 but the    
mind has the power of reflecting1       
judging1 and deciding upon the ideas    
received by the organs of sense4 ,this  
faculty1 which we call reason1 has      
frequently proved to us1 that our senses
 are liable to err4 ,if you have ever   

sailed on the water1 with a very steady 
breeze1 you must have seen the houses1  
trees1 and every object on the shore    
move1 while you were sailing4           
,caroline4 ,i remember thinking so1 when
 ,i was very young2 but ,i now know that
 their motion is only apparent4 ,it is  
true that my reason1 in this case1      
corrects the error of my sight4  ,mrs4  
,b4 ,it teaches you1 that the apparent  
motion of the objects on shore1 proceeds
 from your being yourself moving1 and   
that you are not sensible of your own   
motion1 because you meet with no        
resistance4 ,it is only when some       
obstacle impedes our motion1 that we are
 conscious of moving2 and if you were to
 close your eyes when you were sailing  
on calm water1 with a steady wind1 you  
would not perceive that you moved1 for  
you could not feel it1 and you could see
 it only by observing the change of     
place of the objects on shore4 ,so it is
 with the motion of the earth3 every    
thing on its surface1 and the air that  

surrounds it1 accompanies it in its     
revolution2 it meets with no resistance3
 therefore1 like the crew of a vessel   
sailing with a fair wind1 in a calm sea1
 we are insensible of our motion4       
,caroline4 ,but the principal reason why
 the crew of a vessel in a calm sea do  
not perceive their motion1 is1 because  
they move exceedingly slow1 while the   
earth1 you say1 revolves with great     
velocity4  ,mrs4 ,b4 ,it is not because 
they move slowly1 but because they move 
steadily1 and meet with no irregular    
resistances1 that the crew of a vessel  
do not perceive their motion2 for they  
would be equally insensible to it1 with 
the strongest wind1 provided it were    
steady1 that they sailed with it1 and   
that it did not agitate the water2 but  
this last condition1 you know1 is not   
possible1 for the wind will always      
produce waves which offer more or less  
resistance to the vessel1 and then the  
motion becomes sensible1 because it is  
unequal4  ,caroline4 ,but1 granting     

this1 the crew of a vessel have a proof 
of their motion1 which the inhabitants  
of the earth cannot have1--the apparent 
motion of the objects on shore1 or their
 having passed from one place to        
another4  ,mrs4 ,b4 ,have we not a      
similar proof of the earth's motion1 in 
the apparent motion of the sun and      
stars8 ,imagine the earth to be sailing 
round its axis1 and successively passing
 by every star1 which1 like the objects 
on land1 we suppose to be moving instead
 of ourselves4 ,i have heard it observed
 by an aerial traveller in a balloon1   
that the earth appears to sink beneath  
the balloon1 instead of the balloon     
rising above the earth4  ,it is a law   
which we discover throughout nature1 and
 worthy of its great ,author1 that all  
its purposes are accomplished by the    
most simple means2 and what reason have 
we to suppose this law infringed1 in    
order that we may remain at rest1 while 
the sun and stars move round us2 their  
regular motions1 which are explained by 

the laws of attraction1 on the first    
supposition1 would be unintelligible on 
the last1 and the order and harmony of  
the universe be destroyed4 ,think what  
an immense circuit the sun and stars    
would make daily1 were their apparent   
motions1 real4 ,we know many of them1 to
 be bodies more considerable than our   
earth2 for our eyes vainly endeavour to 
persuade us1 that they are little       
brilliants sparkling in the heavens2    
while science teaches us that they are  
immense spheres1 whose apparent         
dimensions are diminished by distance4  
,why then should these enormous globes  
daily traverse such a prodigious space1 
merely to prevent the necessity of our  
earth's revolving on its axis8          
,caroline4 ,i think ,i must now be      
convinced4 ,but you will1 ,i hope1 allow
 me a little time to familiarise to     
myself1 an idea so different from that  
which ,i have been accustomed to        
entertain4 ,and pray1 at what rate do we
 move8  ,mrs4 ,b4 ,the motion produced  

by the revolution of the earth on its   
axis1 is about seventeen miles a minute1
 to an inhabitant on the equator4       
,emily4 ,but does not every part of the 
earth move with the same velocity8      
,mrs4 ,b4 ,a moment's reflection would  
convince you of the contrary3 a person  
at the equator must move quicker than   
one situated near the poles1 since they 
both perform a revolution in #bd hours4 
,emily4 ,true1 the equator is farthest  
from the axis of motion4 ,but in the    
earth's revolution round the sun1 every 
part must move with equal velocity8     
,mrs4 ,b4 ,yes1 about a thousand miles a
 minute4  ,caroline4 ,how               
astonishing6--and that it should be     
possible for us to be insensible of such
 a rapid motion4 ,you would not tell me 
this sooner1 ,mrs4 ,b41 for fear of     
increasing my incredulity4  ,before the 
time of ,newton1 was not the earth      
supposed to be in the centre of the     
system1 and the sun1 moon1 and stars to 
revolve round it8  ,mrs4 ,b4 ,this was  

the system of ,ptolemy1 in ancient      
times2 but as long ago as the beginning 
of the sixteenth century it was         
generally discarded1 and the solar      
system1 such as ,i have shown you1 was  
established by the celebrated astronomer
 ,copernicus1 and is hence called the   
,copernican system4 ,but the theory of  
gravitation1 the source from which this 
beautiful and harmonious arrangement    
flows1 we owe to the powerful genius of 
,newton1 who lived at a much later      
period1 and who demonstrated its truth4 
,emily4 ,it appears1 indeed1 far less   
difficult to trace by observation the   
motion of the planets1 than to divine by
 what power they are impelled and       
guided4 ,i wonder how the idea of       
gravitation could first have occurred to
 sir ,isaac ,newton8  ,mrs4 ,b4 ,it is  
said to have been occasioned by a       
circumstance from which one should      
little have expected so grand a theory  
to have arisen4  ,during the prevalence 
of the plague in the year #affe1 ,newton

 retired into the country to avoid the  
contagion3 when sitting one day in an   
orchard1 he observed an apple fall from 
a tree1 and was led to consider what    
could be the cause which brought it to  
the ground4  ,caroline4 ,if ,i dared to 
confess it1 ,mrs4 ,b41 ,i should say    
that such an inquiry indicated rather a 
deficiency than a superiority of        
intellect4 ,i do not understand how any 
one can wonder at what is so natural and
 so common4  ,mrs4 ,b4 ,it is the mark  
of superior genius to find matter for   
wonder1 observation1 and research1 in   
circumstances which1 to the ordinary    
mind1 appear trivial1 because they are  
common2 and with which they are         
satisfied1 because they are natural2    
without reflecting that nature is our   
grand field of observation1 that within 
it1 is contained our whole store of     
knowledge2 in a word1 that to study the 
works of nature1 is to learn to         
appreciate and admire the wisdom of     
,god4 ,thus1 it was the simple          

circumstance of the fall of an apple1   
which led to the discovery of the laws  
upon which the ,copernican system is    
founded2 and whatever credit this system
 had obtained before1 it now rests upon 
a basis from which it cannot be shaken4 
,emily4 ,this was a most fortunate      
apple1 and more worthy to be            
commemorated than all those that have   
been sung by the poets4 ,the apple of   
discord for which the goddesses         
contended2 the golden apples by which   
,atalanta won the race2 nay1 even the   
apple which ,william ,tell shot from the
 head of his son1 cannot be compared to 
this6   ,questions  #a4 7,pg4 #hj7 ,into
 what two classes are the planets       
divided1 and how are they distinguished8
  #b4 7,pg4 #hj7 ,by what reasoning do  
you prove that the sun contains a       
greater quantity of matter than any     
other body in the system8  #c4 7,pg4    
#ha7 ,what two circumstances govern the 
force with which bodies attract each    
other8  #d4 7,pg4 #ha7 ,were a planet   

removed to double its former distance   
from the sun1 what would be the effect  
upon its attractive force8  #e4 7,pg4   
#ha7 ,why would it be reduced to        
one-fourth8  #f4 7,pg4 #ha7 ,what is    
meant by the square of a number1 and    
what examples can you give8  #g4 7,pg4  
#ha7 ,what then would be the effect of  
removing it to three1 or four times its 
former distance8  #h4 7,pg4 #ha7 ,how is
 the rule upon this subject expressed8  
#i4 7,pg4 #ha7 ,does this apply to any  
power excepting gravitation8  #aj4 7,pg4
 #ha7 ,how is it that a secondary planet
 revolves round its primary1 and is not 
drawn off by the sun8  #aa4 7,pg4 #hb7  
,what is said respecting the revolution 
of the moon1 and of the earth1 round a  
common centre of gravity8  #ab4 7,pg4   
#hb7 ,by what law in mechanics is this  
explained8  #ac4 7,pg4 #hb7 ,what       
motions then has the earth1 and are     
these remarks confined to it alone8     
#ad4 7,pg4 #hb7 ,what effect have the   
planets upon the sun1 and what is said  

of the common centre of gravity of the  
system8  #ae4 7,pg4 #hc7 ,what other    
motion has the sun1 and how is it       
proved8  #af4 7,pg4 #hc7 ,how may you   
observe the motion of a planet1 by means
 of a fixed star8  #ag4 7,pg4 #hc7 ,what
 is represented by fig4 #a4 plate #g8   
#ah4 7,pg4 #hc7 ,why are the orbits     
represented as circular8  #ai4 7,pg4    
#hc7 ,in what order do the planets      
increase in size as represented1 fig4   
#b4 plate #g8  #bj4 7,pg4 #hc7 ,what are
 we told respecting ,mercury8  #ba4     
7,pg4 #hd7 ,what respecting ,venus8     
#bb4 7,pg4 #hd7 ,when does ,venus become
 a morning1 and when an evening star8   
#bc4 7,pg4 #hd7 ,what is said of the    
,earth8  #bd4 7,pg4 #hd7 ,what of ,mars8
  #be4 7,pg4 #hd7 ,what four small      
planets follow next8  #bf4 7,pg4 #he7   
,what is said of ,jupiter8  #bg4 7,pg4  
#he7 ,what of ,saturn8  #bh4 7,pg4 #he7 
,what of ,herschel8  #bi4 7,pg4 #he7    
,why do we conclude that the moons of   
,saturn afford less light than ours8    

#cj4 7,pg4 #he7 ,in what proportion will
 the light and heat at ,saturn be       
diminished1 and why8  #ca4 7,pg4 #hf7   
,what do the comets resemble1 and what  
is remarkable in their orbits8  #cb4    
7,pg4 #hf7 ,what is said of the number  
of comets8  #cc4 7,pg4 #hf7 ,what is a  
constellation8  #cd4 7,pg4 #hf7 ,how are
 the twelve constellations1 or signs1   
called the zodiac1 situated8  #ce4 7,pg4
 #hf7 ,name them4  #cf4 7,pg4 #hf7 ,what
 is meant by the sun being in a sign8   
#cg4 7,pg4 #hf7 ,what causes the        
apparent change of the sun's place8     
#ch4 7,pg4 #hg7 ,the stars appear of    
different magnitudes1 by what may this  
be caused8  #ci4 7,pg4 #hg7 ,we are not 
sensible of the motion of the earth2    
what fact is mentioned to illustrate    
this point8  #dj4 7,pg4 #hg7 ,what does 
this teach us8  #da4 7,pg4 #hh7 ,would  
the slowness1 or the rapidity of the    
motion1 if steady1 produce any sensible 
difference8  #db4 7,pg4 #hh7 ,if we do  
not feel the motion of the earth1 how   

may we be convinced of its reality8     
#dc4 7,pg4 #hi7 ,were we to deny the    
motion of the earth upon its axis1 what 
must we admit respecting the heavenly   
bodies8  #dd4 7,pg4 #hi7 ,what distance 
is an inhabitant on the equator carried 
in a minute by the diurnal motion of the
 earth8  #de4 7,pg4 #hi7 ,why is not the
 velocity every where equally great8    
#df4 7,pg4 #hi7 ,what distance does the 
earth travel in a minute1 in its        
revolution round the sun8  #dg4 7,pg4   
#hi7 ,what was formerly supposed        
respecting the motion of all the        
heavenly bodies8  #dh4 7,pg4 #hi7 ,what 
do we mean by the ,copernican system1   
and what is said respecting ,copernicus 
and ,newton8  #di4 7,pg4 #ij7 ,what     
circumstance is said to have given rise 
to the speculations of ,newton1 on the  
subject of gravitation8                 
,,conversation ,,viii4  ,,on ,,the      
,,earth4  ,,of ,,the ,,terrestrial      
,,globe4 ,,of ,,the ,,figure ,,of ,,the 
,,earth4 ,,of ,,the ,,pendulum4 ,,of    

,,the ,,variation ,,of ,,the ,,seasons1 
,,and ,,of ,,the ,,length ,,of ,,days   
,,and ,,nights4 ,,of ,,the ,,causes ,,of
 ,,the ,,heat ,,of ,,summer4 ,,of       
,,solar1 ,,siderial1 ,,and ,,equal ,,or 
,,mean ,,time4   ,,mrs4 ,b4  ,as the    
earth is the planet in which we are the 
most particularly interested1 it is my  
intention this morning1 to explain to   
you the effects resulting from its      
annual1 and diurnal motions2 but for    
this purpose1 it will be necessary to   
make you acquainted with the terrestrial
 globe3 you have not either of you1 ,i  
conclude1 learnt the use of the globes8 
,caroline4 ,no2 ,i once indeed1 learnt  
by heart1 the names of the lines marked 
on the globe1 but as ,i was informed    
they were only imaginary divisions1 they
 did not appear to me worthy of much    
attention1 and were soon forgotten4     
,mrs4 ,b4 ,you supposed1 then1 that     
astronomers had been at the trouble of  
inventing a number of lines1 to little  
purpose4 ,it will be impossible for me  

to explain to you the particular effects
 of the earth's motion1 without your    
having acquired a knowledge of these    
lines3 in plate #h4 fig4 #b4 you will   
find them all delineated3 and you must  
learn them perfectly1 if you wish to    
make any proficiency in astronomy4      
,caroline4 ,i was taught them at so     
early an age1 that ,i could not         
understand their meaning2 and ,i have   
often heard you say1 that the only use  
of words1 was to convey ideas4  ,mrs4   
,b4 ,a knowledge of these lines1 would  
have conveyed some idea of the manner in
 which they were designed to divide the 
globe into parts2 although the use of   
these divisions1 might at that time1    
have been too difficult for you to      
understand4 ,childhood is the season1   
when impressions on the memory are most 
strongly and most easily made3 it is the
 period at which a large stock of terms 
should be treasured up1 the precise     
application of which we may learn when  
the understanding is more developed4 ,it

 is1 ,i think1 a very mistaken notion1  
that children should be taught such     
things only1 as they can perfectly      
understand4 ,had you been early made    
acquainted with the terms which relate  
to figure and motion1 how much it would 
have facilitated your progress in       
natural philosophy4 ,i have been obliged
 to confine myself to the most common   
and familiar expressions1 in explaining 
the laws of nature2 although ,i am      
convinced that appropriate and          
scientific terms1 might have conveyed   
more precise and accurate ideas1 had you
 been prepared to understand them4      
,emily4 ,you may depend upon our        
carefully learning the names of these   
lines1 ,mrs4 ,b42 but before we commit  
them to memory1 will you have the       
goodness to explain them to us8  ,mrs4  
,b4 ,most willingly4 ,this figure of a  
globe1 or sphere1 represents the earth2 
the line which passes through its       
centre1 and on which it turns1 is called
 its axis1 and the two extremities of   

the axis ,a and ,b1 are the poles1      
distinguished by the names of the north 
and the south pole4 ,the circle ,c ,d1  
which divides the globe into two equal  
parts between the poles1 and equally    
distant from them1 is called the        
equator1 or equinoctial line2 that part 
of the globe to the north of the        
equator1 is the northern hemisphere2    
that part to the south of the equator1  
the southern hemisphere4 ,the small     
circle ,e ,f1 which surrounds the north 
pole1 is called the arctic circle2 that 
,g ,h1 which surrounds the south pole1  
the antarctic circle2 these are also    
called polar circles4 ,there are two    
circles1 intermediate between the polar 
circles and the equator2 that to the    
north ,i ,k1 called the tropic of       
,cancer2 that to the south1 ,l ,m1      
called the tropic of ,capricorn4        
,lastly1 this circle1 ,l ,k1 which      
divides the globe into two equal parts1 
crossing the equator and extending      
northward as far as the tropic of       

,cancer1 and southward as far as the    
tropic of ,capricorn1 is called the     
ecliptic4 ,the delineation of the       
ecliptic on the terrestrial globe is not
 without danger of conveying false      
ideas2 for the ecliptic 7as ,i have     
before said7 is an imaginary circle in  
the heavens1 passing through the middle 
of the zodiac1 and situated in the plane
 of the earth's orbit4  ,caroline4 ,i do
 not understand the meaning of the plane
 of the earth's orbit4  ,mrs4 ,b4 ,a    
plane1 is an even flat surface4 ,were   
you to bend a piece of wire1 so as to   
form a hoop1 you might then stretch a   
piece of cloth1 or paper over it1 like  
the head of a drum2 this would form a   
flat surface1 which might be called the 
plane of the hoop4 ,now the orbit of the
 earth1 is an imaginary circle1         
surrounding the sun1 and you can readily
 imagine a plane extending from one side
 of this circle to the other1 filling up
 its whole area3 such a plane would pass
 through the centre of the sun1 dividing

 it into hemispheres4 ,you may then     
imagine this plane extended beyond the  
limits of the earth's orbit1 on every   
side1 until it reached those fixed stars
 which form the signs of the zodiac2    
passing through the middle of these     
signs1 it would give you the place of   
that imaginary circle in the heavens1   
call the ecliptic2 which is the sun's   
apparent path4 ,let fig4 #a4 plate #i1  
represent such a plane1 ,s the sun1 ,e  
the earth with its orbit1 and ,a ,b ,c  
,d the ecliptic passing through the     
middle of the zodiac4  ,illustration3   
,,plate ,,ix4  ,emily4 ,if the ecliptic 
relates only to the heavens1 why is it  
described upon the terrestrial globe8   
,mrs4 ,b4 ,it is convenient for the     
demonstration of a variety of problems  
in the use of the globes2 and besides1  
the obliquity of this circle to the     
equator is rendered more conspicuous by 
its being described on the same globe2  
and the obliquity of the ecliptic shows 
how much the earth's axis is inclined to

 the plane of its orbit4 ,but to return 
to fig4 #b4 plate #h4  ,the spaces      
between the several parallel circles on 
the terrestrial globe are called zones3 
that which is comprehended between the  
tropics is distinguished by the name of 
the torrid zone2 the spaces which extend
 from the tropics to the polar circles1 
the north and south temperate zones2 and
 the spaces contained within the polar  
circles1 the frigid zones4 ,by the term 
zone is meant a belt1 or girdle1 the    
frigid zones1 however1 are not belts1   
but circles1 extending #bc-a_/b degrees 
from their centres1 the poles4  ,the    
several lines which1 you observe to be  
drawn from one pole to the other1       
cutting the equator at right angles1 are
 called meridians2 the number of these  
is unlimited1 as a line passing through 
any place1 directly to the poles1 is    
called the meridian of that place4 ,when
 any one of these meridians is exactly  
opposite to the sun1 it is mid-day1 or  
twelve o'clock in the day1 at all the   

places situated any where on that       
meridian2 and1 at the places situated on
 the opposite meridian1 it is           
consequently midnight4  ,emily4 ,to     
places situated equally distant from    
these two meridians1 it must then be six
 o'clock4  ,mrs4 ,b4 ,yes2 if they are  
to the east of the sun's meridian it is 
six o'clock in the afternoon1 because   
they will have previously passed the    
sun2 if to the west1 it is six o'clock  
in the morning1 and that meridian will  
be proceeding towards the sun4  ,those  
circles which divide the globe into two 
equal parts1 such as the equator and the
 ecliptic1 are called greater circles2  
to distinguish them from those which    
divide it into two unequal parts1 as the
 tropics1 and polar circles1 which are  
called lesser circles4 ,all circles1 you
 know1 are imagined to be divided into  
#cfj equal parts1 called degrees1 and   
degrees are again divided into #fj equal
 parts1 called minutes4 ,the diameter of
 a circle is a right line drawn across  

it1 and passing through its centre2 were
 you1 for instance1 to measure across   
this round table1 that would give you   
its diameter2 but were you to measure   
all round the edge of it1 you would then
 obtain its circumference4  ,now ,emily1
 you may tell me exactly how many       
degrees are contained in a meridian8    
,emily4 ,a meridian1 reaching from one  
pole to the other1 is half a circle1 and
 must therefore contain #ahj degrees4   
,mrs4 ,b4 ,very well2 and what number of
 degrees are there from the equator to  
one of the poles8  ,caroline4 ,the      
equator being equally distant from      
either pole1 that distance must be half 
of a meridian1 or a quarter of the      
circumference of a circle1 and contain  
#ij degrees4  ,mrs4 ,b4 ,besides the    
usual division of circles into degrees1 
the ecliptic is divided into twelve     
equal parts1 called signs1 which bear   
the name of the constellations through  
which this circle passes in the heavens4
 ,the degrees measured on the meridians 

from the equator1 either towards the    
north1 or towards the south1 are called 
degrees of latitude1 of which there may 
be #ij2 those measured from east to     
west1 either on the equator1 or any of  
the lesser circles1 are called degrees  
of longitude1 of which there may be     
#ahj2 these lesser circles are also     
called parallels of latitude4 ,of these 
parallels there may be any number2 a    
circle drawn from east to west1 at any  
distance from the equator1 will always  
be parallel to it1 and is therefore     
called a parallel of latitude4  ,emily4 
,the degrees of longitude must then vary
 in length1 according to the dimensions 
of the circle on which they are         
reckoned2 those1 for instance1 at the   
polar circles1 will be considerably     
smaller than those at the equator8      
,mrs4 ,b4 ,certainly2 since the degrees 
of circles of different dimensions do   
not vary in number1 they must           
necessarily vary in length4 ,the degrees
 of latitude1 you may observe1 never    

vary in length2 for the meridians on    
which they are reckoned are all of the  
same dimensions4  ,emily4 ,and of what  
length is a degree of latitude8  ,mrs4  
,b4 ,sixty geographical miles1 which is 
equal to #fi-a_/b ,english statute      
miles2 or about one-sixth more than a   
common mile4  ,emily4 ,the degrees of   
longitude at the equator1 must then be  
of the same dimensions1 with a degree of
 latitude4  ,mrs4 ,b4 ,they would1 were 
the earth a perfect sphere2 but it is   
not exactly such1 being somewhat        
protuberant about the equator1 and      
flattened towards the poles4 ,this form 
proceeds from the superior action of the
 centrifugal power at the equator1 and  
as this enlarges the circle1 it must1 in
 the same proportion1 increase the      
length of the degrees of longitude      
measured on it4  ,caroline4 ,i thought  
,i had understood the centrifugal force 
perfectly1 but ,i do not comprehend its 
effects in this instance4  ,mrs4 ,b4    
,you know that the revolution of the    

earth on its axis1 must give to every   
particle a tendency to fly off from the 
centre1 that this tendency is stronger1 
or weaker1 in proportion to the velocity
 with which the particle moves2 now a   
particle situated near to one of the    
poles1 makes one rotation in the same   
space of time as a particle at the      
equator2 the latter1 therefore1 having a
 much larger circle to describe1 travels
 proportionally faster1 consequently the
 centrifugal force is much stronger at  
the equator than in the polar regions3  
it gradually decreases as you leave the 
equator and approach the poles1 at which
 points the centrifugal force1 entirely 
ceases4 ,supposing1 therefore1 the earth
 to have been originally in a fluid     
state1 the particles in the torrid zone 
would recede much farther from the      
centre than those in the frigid zones2  
thus the polar regions would become     
flattened1 and those about the equator  
elevated4  ,as a large portion of the   
earth is covered with water1 the        

,creator gave to it the form1           
denominated an oblate spheroid1         
otherwise the polar regions would have  
been without water1 and those about the 
equator1 would have been buried several 
miles below the surface of the ocean4   
,caroline4 ,i did not consider that the 
particles in the neighbourhood of the   
equator1 move with greater velocity than
 those about the poles2 this was the    
reason ,i could not understand you4     
,mrs4 ,b4 ,you must be careful to       
remember1 that those parts of a body    
which are farthest from the centre of   
motion1 must move with the greatest     
velocity3 the axis of the earth is the  
centre of its diurnal motion1 and the   
equatorial regions the parts most       
distant from the axis4  ,caroline4 ,my  
head then moves faster than my feet2 and
 upon the summit of a mountain1 we are  
carried round quicker than in a valley8 
,mrs4 ,b4 ,certainly2 your head is more 
distant from the centre of motion than  
your feet2 the mountain-top than the    

valley2 and the more distant any part of
 a body is from the centre of motion1   
the larger is the circle it will        
describe1 and the greater therefore must
 be its velocity4  ,emily4 ,i have been 
reflecting1 that if the earth is not a  
perfect circle----  ,mrs4 ,b4 ,a sphere 
you mean1 my dear3 a circle is a round  
line1 every part of which is equally    
distant from the centre2 a sphere or    
globe is a round body1 the surface of   
which is every where equally distant    
from the centre4  ,emily4 ,if1 then1 the
 earth is not a perfect sphere1 but     
prominent at the equator1 and depressed 
at the poles1 would not a body weigh    
heavier at the equator than at the      
poles8 ,for the earth being thicker at  
the equator1 the attraction of gravity  
perpendicularly downwards must be       
stronger4  ,mrs4 ,b4 ,your reasoning has
 some plausibility1 but ,i am sorry to  
be obliged to add1 that it is quite     
erroneous2 for the nearer any part of   
the surface of a body is to the centre  

of attraction1 the more strongly it is  
attracted2 because it is then nearest to
 the whole mass of attracting matter4   
,in regard to its effects1 you might    
consider the whole power of gravity1 as 
placed at the centre of attraction4     
,emily4 ,but were you to penetrate deep 
into the earth1 would gravity increase  
as you approached the centre8  ,mrs4 ,b4
 ,certainly not2 ,i am referring only to
 any situation on the surface of the    
earth4 ,were you to penetrate into the  
interior1 the attraction of the parts   
above you1 would counteract that of the 
parts beneath you1 and consequently     
diminish the power of gravity in        
proportion as you approach the centre2  
and if you reached that point1 being    
equally attracted by the parts all      
around you1 the effects of gravity would
 cease1 and you would be without weight4
  ,emily4 ,bodies1 then1 should weigh   
less at the equator than at the poles1  
since they are more distant from the    
centre of gravity in the former than in 

the latter situation8  ,mrs4 ,b4 ,and   
this is really the case2 but the        
difference of weight would be scarcely  
sensible1 were it not augmented by      
another circumstance4  ,caroline4 ,and  
what is this singular circumstance1     
which seems to disturb the laws of      
nature8  ,mrs4 ,b4 ,one that you are    
well acquainted with1 as conducing more 
to the preservation than the destruction
 of order1--the centrifugal force4 ,this
 we have just observed to be strongest  
at the equator2 and as it tends to drive
 bodies from the centre1 it is          
necessarily opposed to1 and must lessen 
the power of gravity1 which attracts    
them towards the centre4 ,we accordingly
 find that bodies weigh lightest at the 
equator1 where the centrifugal force is 
greatest2 and heaviest at the poles1    
where this power is least3 the weight   
being diminished at the equator1 by both
 the causes mentioned4  ,caroline4 ,has 
the experiment been made in these       
different situations8  ,mrs4 ,b4 ,louis 

,,xiv4 of ,france1 sent philosophers    
both to the equator1 and to ,lapland1   
for this purpose3 the severity of the   
climate1 and obstruction from the ice1  
have hitherto rendered every attempt to 
reach the pole abortive2 but the        
difference of gravity at the equator1   
and in ,lapland is very perceptible4    
,caroline4 ,yet ,i do not comprehend how
 the difference of weight could be      
ascertained1 for if the body under trial
 decreased in weight1 the weight which  
was opposed to it in the opposite scale 
must have diminished in the same        
proportion4 ,for instance1 if a pound of
 sugar did not weigh so heavy at the    
equator as at the poles1 the leaden     
pound which served to weigh it1 would   
not be so heavy either2 therefore they  
would still balance each other1 and the 
different force of gravity could not be 
ascertained by this means4  ,mrs4 ,b4   
,your observation is perfectly just3 the
 difference of gravity in bodies        
situated at the poles1 and at the       

equator1 cannot be ascertained by       
weighing them2 a pendulum was therefore 
used for that purpose4  ,caroline4      
,what1 the pendulum of a clock8 how     
could that answer the purpose8  ,mrs4   
,b4 ,a pendulum consists of a line1 or  
rod1 to one end of which a weight is    
attached1 and by the other end it is    
suspended to a fixed point1 about which 
it is made to vibrate4 ,when not in     
motion1 a pendulum1 obeying the general 
law of attraction1 hangs like a plumb   
line1 perpendicular to the surface of   
the earth1 but if you raise the         
pendulum1 gravity will bring it back to 
its perpendicular position4 ,it will1   
however1 not remain stationary there1   
for the momentum it has acquired during 
its descent1 will impel it onwards1 and 
if unobstructed1 it will rise on the    
opposite side to an equal height2 from  
thence it is brought back by gravity1   
and is again forced upwards1 by the     
impulse of its momentum4  ,caroline4 ,if
 so1 the motion of a pendulum would be  

perpetual1 and ,i thought you said1 that
 there was no perpetual motion on the   
earth4  ,mrs4 ,b4 ,the motion of a      
pendulum is opposed by the resistance of
 the air in which it vibrates1 and by   
the friction of the part by which it is 
suspended3 were it possible to remove   
these obstacles1 the motion of a        
pendulum would be perpetual1 and its    
vibrations perfectly regular2 each being
 of equal distance1 and performed in    
equal times4  ,emily4 ,that is the      
natural result of the uniformity of the 
power which produces these vibrations1  
for the force of gravity being always   
the same1 the velocity of the pendulum  
must consequently be uniform4           
,caroline4 ,no1 ,emily1 you are         
mistaken2 the force is not every where  
the same1 and therefore the effect will 
not be so either4 ,i have discovered it1
 ,mrs4 ,b42 since the force of gravity  
is less at the equator than at the      
poles1 the vibrations of the pendulum   
will be slower at the former place than 

at the latter4  ,mrs4 ,b4 ,you are      
perfectly right1 ,caroline2 it was by   
this means that the difference of       
gravity was discovered1 and the true    
figure of the earth ascertained4        
,emily4 ,but how do they contrive to    
regulate their time in the equatorial   
and polar regions8 for1 since in our    
part of the earth the pendulum of a     
clock vibrates exactly once in a second1
 if it vibrates faster at the poles1 and
 slower at the equator1 the inhabitants 
must regulate their clocks in a manner  
different from us4  ,mrs4 ,b4 ,the only 
alteration required is to lengthen the  
pendulum in one case1 and to shorten it 
in the other2 for the velocity of the   
vibrations of a pendulum depends on its 
length2 and when it is said that a      
pendulum vibrates quicker at the pole   
than at the equator1 it is supposed to  
be of the same length4 ,a pendulum which
 vibrates seconds in this latitude is   
about #ci-a_/g inches long4 ,in order to
 vibrate at the equator in the same     

space of time1 it must be somewhat      
shorter2 and at the poles1 it must be   
proportionally lengthened4  ,the        
vibrations of a pendulum1 resemble the  
descent of a body on an inclined plane1 
and are produced by the same cause2 now 
you must recollect1 that the greater the
 perpendicular height of such a plane1  
in proportion to its length1 the more   
rapid will be the descent of the body2 a
 short pendulum ascends to a greater    
height than a larger one1 in vibrating a
 given distance1 and of course its      
descent must be more rapid4  ,i shall   
now1 ,i think1 be able to explain to you
 the cause of the variation of the      
seasons1 and the difference in the      
length of the days and nights in those  
seasons2 both effects resulting from the
 same cause4  ,in moving round the sun1 
the axis of the earth is not            
perpendicular to the plane of its orbit4
 ,supposing this round table to         
represent the plane of the earth's      
orbit1 and this little globe1 the earth2

 through this ,i have passed a wire1    
representing its axis and poles4 ,in    
moving round the table1 ,i do not hold  
the wire perpendicular to it1 but       
obliquely4  ,emily4 ,yes1 ,i understand1
 the earth does not go round the sun in 
an upright position1 its axis is        
slanting or oblique2 and1 it of course1 
forms an angle with a line drawn        
perpendicular to the plane of the       
earth's orbit4  ,mrs4 ,b4 ,all the      
lines1 which you learnt in your last    
lesson1 are delineated on this little   
globe2 you must consider the ecliptic as
 representing the plane of the earth's  
orbit2 and the equator1 which crosses   
the ecliptic in two places1 then shows  
the degree of obliquity of the axis of  
the earth2 which amounts to #bc-a_/b    
degrees1 very nearly4 ,the points in    
which the ecliptic intersects the       
equator1 are called the equinoctial     
points4  ,but ,i believe ,i shall render
 the effects of the obliquity of the    
earth's axis clearer to you1 by the     

revolution of the little globe round a  
candle1 which shall represent the sun4  
7,plate ,,ix4 fig4 #b47  ,as ,i now hold
 it1 at ,a1 you see it in the situation 
in which it is in the midst of summer1  
or what is called the summer solstice1  
which is on the #bast of ,june4  ,emily4
 ,you hold the wire awry1 ,i suppose1 in
 order to show that the axis of the     
earth is not upright8  ,mrs4 ,b4 ,yes2  
in summer1 the north pole is inclined   
towards the sun4 ,in this season1       
therefore1 the northern hemisphere      
enjoys much more of his rays than the   
southern4 ,the sun1 you see1 now shines 
over the whole of the north frigid zone1
 and notwithstanding the earth's diurnal
 revolution1 which ,i imitate by        
twirling the ball on the wire1 it will  
continue to shine upon it as long as it 
remains in this situation1 whilst the   
south frigid zone is at the same time   
completely in darkness4  ,caroline4     
,that is very strange2 ,i never before  
heard that there was constant day or    

night in any part of the world6 ,how    
much happier the inhabitants of the     
north frigid zone must be than those of 
the southern2 the first enjoy           
uninterrupted day1 while the last are   
involved in perpetual darkness4  ,mrs4  
,b4 ,you judge with too much            
precipitation2 examine a little further1
 and you will find1 that the two frigid 
zones share an equal fate4  ,we shall   
now make the earth set off from its     
position in the summer solstice1 and    
carry it round the sun2 observe that the
 pole is always inclined in the same    
direction1 and points to the same spot  
in the heavens4 ,there is a fixed star  
situated near that spot1 which is hence 
called the north polar star4 ,now let us
 stop the earth at ,b1 and examine it in
 its present situation2 it has gone     
through one quarter of its orbit1 and is
 arrived at that point at which the     
ecliptic cuts1 or crosses1 the equator1 
and which is called the autumnal        
equinox4  ,emily4 ,the sun now shines   

from one pole to the other1 just as it  
would constantly do1 if the axis of the 
earth were perpendicular to its orbit4  
,mrs4 ,b4 ,because the inclination of   
the axis is now neither towards the sun1
 nor in the contrary direction2 at this 
period of the year1 the days and nights 
are equal in every part of the earth4   
,but the next step she takes in her     
orbit1 you see1 involves the north pole 
in darkness1 whilst it illumines that of
 the south2 this change was gradually   
preparing as ,i moved the earth from    
summer to autumn2 the arctic circle1    
which was at first entirely illumined1  
began to have short nights1 which       
increased as the earth approached the   
autumnal equinox2 and the instant it    
passed that point1 the long night of the
 north pole commences1 and the south    
pole begins to enjoy the light of the   
sun4 ,we shall now make the earth       
proceed in its orbit1 and you may       
observe that as it advances1 the days   
shorten and the nights lengthen1        

throughout the northern hemisphere1     
until it arrives at the winter solstice1
 on the #bast of ,december1 when the    
north frigid zone is entirely in        
darkness1 and the southern has          
uninterrupted daylight4  ,illustration3 
,,plate ,x4  ,caroline4 ,then1 after    
all1 the sun which ,i thought so        
partial1 confers his favours equally on 
all4  ,mrs4 ,b4 ,not so either3 the     
inhabitants of the torrid zone have much
 more heat than we have1 as the sun's   
rays fall perpendicularly twice in the  
course of a year1 on every place within 
the tropics1 while they shine more or   
less obliquely on the rest of the world1
 and almost horizontally at the poles2  
for during their long day of six months1
 the sun moves round their horizon      
without either rising or setting2 the   
only observable difference1 is that it  
is more elevated by a few degrees at    
mid-day1 than at midnight4  ,emily4 ,to 
a person placed in the temperate zone1  
in the situation in which we are in     

,england1 the sun will shine neither so 
obliquely as it does on the poles1 nor  
vertically as at the equator2 but its   
rays will fall upon him more obliquely  
in autumn1 and winter1 than in summer4  
,caroline4 ,and therefore1 the          
inhabitants of the temperate zones1 will
 not have merely one day1 and one night1
 in the year1 as happens at the poles1  
nor will they have equal days1 and equal
 nights1 as at the equator2 but their   
days and nights will vary in length1 at 
different times of the year1 according  
as their respective poles incline       
towards1 or from the sun1 and the       
difference will be greater in proportion
 to their distance from the equator4    
,mrs4 ,b4 ,we shall now follow the earth
 through the other half of her orbit1   
and you will observe1 that now exactly  
the same changes take place in the      
southern hemisphere1 as those we have   
just remarked in the northern4 ,day     
commences at the south pole1 when night 
sets in at the north pole2 and in every 

other part of the southern hemisphere   
the days are longer than the nights1    
while1 on the contrary1 our nights are  
longer than our days4 ,when the earth   
arrives at the vernal equinox1 ,d1 where
 the ecliptic again cuts the equator1 on
 the #bast of ,march1 she is situated1  
with respect to the sun1 exactly in the 
same position1 as in the autumnal       
equinox2 and the only difference with   
respect to the earth1 is1 that it is now
 autumn in the southern hemisphere1     
whilst it is spring with us4  ,caroline4
 ,then the days and nights are again    
every where equal4  ,mrs4 ,b4 ,yes1 for 
the half of the globe which is          
enlightened1 extends exactly from one   
pole to the other1 the sun has just     
risen to the north pole1 and is just    
setting to the south pole2 but in every 
other part of the globe1 the day and    
night is of twelve hours length2 hence  
the word equinox1 which is derived from 
the ,latin1 meaning equal night4  ,as   
our summer advances1 the days lengthen  

in the northern hemisphere1 and shorten 
in the southern1 till the earth reaches 
the summer solstice1 when the north     
frigid zone is entirely illumined1 and  
the southern is in complete darkness2   
and we have now brought the earth again 
to the spot from whence we first        
accompanied her4  ,emily4 ,this is      
indeed a most satisfactory explanation  
of the cause of the different lengths of
 our days and nights1 and of the        
variation of the seasons2 and the more  
,i learn1 the more ,i admire the        
simplicity of means by which such       
wonderful effects are produced4  ,mrs4  
,b4 ,i know not which is most worthy of 
our admiration1 the causes1 or the      
effects of the earth's revolution round 
the sun4 ,the mind can find no object of
 contemplation more sublime1 than the   
course of this magnificent globe1       
impelled by the combined powers of      
projection and attraction1 to roll in   
one invariable course1 around the source
 of light and heat3 and what can be more

 delightful than the beneficent effects 
of this vivifying power on its attendant
 planet4 ,it is at once the grand       
principle which animates and fecundates 
nature4  ,emily4 ,there is one          
circumstance in which this little ivory 
globe appears to me to differ from the  
earth2 it is not quite dark on that side
 of it which is turned from the candle1 
as is the case with the earth when      
neither moon nor stars are visible4     
,mrs4 ,b4 ,this is owing to the light of
 the candle1 being reflected by the     
walls of the room1 on every part of the 
globe1 consequently that side of the    
globe1 on which the candle does not     
directly shine1 is not in total         
darkness4 ,now the skies have no walls  
to reflect the sun's light on that side 
of our earth which is in darkness4      
,caroline4 ,i beg your pardon1 ,mrs4    
,b41 ,i think that the moon1 and stars1 
answer the purpose of walls in          
reflecting the sun's light to us in the 
night4  ,mrs4 ,b4 ,very well1 ,caroline2

 that is to say1 the moon and planets2  
for the fixed stars1 you know1 shine by 
their own light4  ,emily4 ,you say1 that
 the superior heat of the equatorial    
parts of the earth1 arises from the rays
 falling perpendicularly on those       
regions1 whilst they fall obliquely on  
these more northern regions2 now ,i do  
not understand why perpendicular rays   
should afford more heat than oblique    
rays4  ,caroline4 ,you need only hold   
your hand perpendicularly over the      
candle1 and then hold it sideways       
obliquely1 to be sensible of the        
difference4  ,emily4 ,i do not doubt the
 fact1 but ,i wish to have it explained4
  ,mrs4 ,b4 ,you are quite right2 if    
,caroline had not been satisfied with   
ascertaining the fact1 without          
understanding it1 she would not have    
brought forward the candle as an        
illustration2 the reason why you feel so
 much more heat if you hold your hand   
perpendicularly over the candle1 than if
 you hold it sideways1 is because a     

stream of heated vapour constantly      
ascends from the candle1 or any other   
burning body1 which being lighter than  
the air of the room1 does not spread    
laterally but rises perpendicularly1 and
 this led you to suppose that the rays  
were hotter in the latter direction4    
,had you reflected1 you would have      
discovered that rays issuing from the   
candle sideways1 are no less            
perpendicular to your hand when held    
opposite to them1 than the rays which   
ascend when your hand is held over them4
  ,the reason why the sun's rays afford 
less heat when in an oblique direction1 
than when perpendicular1 is because     
fewer of them fall upon an equal portion
 of the earth2 this will be understood  
better by referring to plate #aj4 fig4  
#a1 which represents two equal portions 
of the sun's rays1 shining upon         
different parts of the earth4 ,here it  
is evident1 that the same quantity of   
rays fall on the space ,a ,b1 as fall on
 the space ,b ,c2 and as ,a ,b is less  

than ,b ,c1 the heat and light will be  
much stronger in the former than in the 
latter2 ,a ,b1 you see1 represents the  
equatorial regions1 where the sun shines
 perpendicularly2 and ,b ,c1 the        
temperate and frozen climates1 where his
 rays fall more obliquely4  ,emily4     
,this accounts not only for the greater 
heat of the equatorial regions1 but for 
the greater heat of our summers1 as the 
sun shines less obliquely in summer than
 in winter4  ,mrs4 ,b4 ,this you will   
see exemplified in figure #b1 in which  
the earth is represented1 as it is      
situated on the #bast of ,june1 and     
,england receives less oblique1 and     
consequently a greater number of rays1  
than at any other season2 and figure #c1
 shows the situation of ,england on the 
#bast of ,december1 when the rays of the
 sun fall most obliquely upon her4 ,but 
there is also another reason why oblique
 rays give less heat1 than perpendicular
 rays2 which is1 that they have a       
greater portion of the atmosphere to    

traverse2 and though it is true1 that   
the atmosphere is itself a transparent  
body1 freely admitting the passage of   
the sun's rays1 yet it is always loaded 
more or less with dense and foggy       
vapour1 which the rays of the sun cannot
 easily penetrate2 therefore1 the       
greater the quantity of atmosphere the  
sun's rays have to pass through in their
 way to the earth1 the less heat they   
will retain when they reach it4 ,this   
will be better understood1 by referring 
to fig4 #d4 ,the dotted line round the  
earth1 describes the extent of the      
atmosphere1 and the lines which proceed 
from the sun to the earth1 the passage  
of two equal portions of the sun's rays1
 to the equatorial and polar regions2   
the latter you see1 from its greater    
obliquity1 passes through a greater     
extent of atmosphere4  ,caroline4 ,and  
this1 no doubt1 is the reason why the   
sun1 in the morning and in the evening1 
gives so much less heat1 than at        
mid-day4  ,mrs4 ,b4 ,the diminution of  

heat1 morning and evening1 is certainly 
owing to the greater obliquity of the   
sun's rays2 and they are also affected  
by the other1 both the cause1 which ,i  
have just explained to you2 the         
difficulty of passing through a foggy   
atmosphere is perhaps more particularly 
applicable to them1 as mist and vapours 
are prevalent about the time of sunrise 
and sunset4 ,but the diminished         
obliquity of the sun's rays1 is not the 
sole cause of the heat of summer2 the   
length of the days greatly conduces to  
it2 for the longer the sun is above the 
horizon1 the more heat he will          
communicate to the earth4  ,caroline4   
,both the longest days1 and the most    
perpendicular rays1 are on the #bast of 
,june2 and yet the greatest heat        
prevails in ,july and ,august4  ,mrs4   
,b4 ,those parts of the earth which are 
once heated1 retain the heat for some   
length of time1 and the additional heat 
they receive1 occasions an elevation of 
temperature1 although the days begin to 

shorten1 and the sun's rays to fall more
 obliquely4 ,for the same reason1 we    
have generally more heat at three       
o'clock in the afternoon1 than at       
twelve1 when the sun is on the meridian4
  ,emily4 ,and pray1 have the other     
planets the same vicissitudes of        
seasons1 as the earth8  ,mrs4 ,b4 ,some 
of them more1 some less1 according as   
their axes deviate more or less from the
 perpendicular1 to the plane of their   
orbits4 ,the axis of ,jupiter1 is nearly
 perpendicular to the plane of his      
orbit2 the axes of ,mars1 and of        
,saturn1 are each1 inclined at angles of
 about sixty degrees2 whilst the axis of
 ,venus is believed to be elevated only 
fifteen or twenty degrees above her     
orbit2 the vicissitudes of her seasons  
must therefore be considerably greater  
than ours4 ,for further particulars     
respecting the planets1 ,i shall refer  
you to ,bonnycastle's ,introduction to  
,astronomy4  ,i have but one more       
observation to make to you1 relative to 

the earth's motion2 which is1 that      
although we have but #cfe days and      
nights in the year1 she performs #cff   
complete revolutions on her axis1 during
 that time4  ,caroline4 ,how is that    
possible8 for every complete revolution 
must bring the same place back to the   
sun4 ,it is now just twelve o'clock1 the
 sun is1 therefore1 on our meridian2 in 
twenty-four hours will it not have      
returned to our meridian again1 and will
 not the earth have made a complete     
rotation on its axis8  ,mrs4 ,b4 ,if the
 earth had no progressive motion in its 
orbit whilst it revolves on its axis1   
this would be the case2 but as it       
advances almost a degree westward in its
 orbit1 in the same time that it        
completes a revolution eastward on its  
axis1 it must revolve nearly one degree 
more in order to bring the same meridian
 back to the sun4  ,caroline4 ,oh1 yes6 
it will require as much more of a second
 revolution to bring the same meridian  
back to the sun1 as is equal to the     

space the earth has advanced in her     
orbit2 that is1 nearly a degree2 this   
difference is1 however1 very little4    
,mrs4 ,b4 ,these small daily portions of
 rotation1 are each equal to the three  
hundred and sixty-fifth part of a       
circle1 which at the end of the year    
amounts to one complete rotation4       
,emily4 ,that is extremely curious4 ,if 
the earth then1 had no other than its   
diurnal motion1 we should have #cff days
 in the year4  ,mrs4 ,b4 ,we should have
 #cff days in the same period of time   
that we now have #cfe2 but if we did not
 revolve round the sun1 we should have  
no natural means of computing years4    
,you will be surprised to hear1 that if 
time is calculated by the stars instead 
of the sun1 the irregularity which we   
have just noticed does not occur1 and   
that one complete rotation of the earth 
on its axis1 brings the same meridian   
back to any fixed star4  ,emily4 ,that  
seems quite unaccountable2 for the earth
 advances in her orbit with regard to   

the fixed stars1 the same as with regard
 to the sun4  ,mrs4 ,b4 ,true1 but then 
the distance of the fixed stars is so   
immense1 that our solar system is in    
comparison to it but a spot1 and the    
whole extent of the earth's orbit but a 
point2 therefore1 whether the earth     
remain stationary1 or whether it        
revolved in its orbit during its        
rotation on its axis1 no sensible       
difference would be produced with regard
 to the fixed stars4 ,one complete      
revolution brings the same meridian back
 to the same fixed star2 hence the fixed
 stars appear to go round the earth in a
 shorter time than the sun by three     
minutes fifty-six seconds of time4      
,caroline4 ,these three minutes         
fifty-six seconds is the time which the 
earth takes to perform the additional   
three hundred and sixty-fifth part of   
the circle1 in order to bring the same  
meridian back to the sun4  ,mrs4 ,b4    
,precisely4 ,hence the stars gain every 
day three minutes fifty-six seconds on  

the sun1 which makes them rise that     
portion of time earlier every day4      
,when time is calculated by the stars it
 is called sidereal time2 when by the   
sun1 solar1 or apparent time4           
,caroline4 ,then a sidereal day is three
 minutes fifty-six seconds shorter1 than
 a solar day of twenty-four hours4      
,mrs4 ,b4 ,i must also explain to you   
what is meant by a sidereal year4  ,the 
common year1 called the solar or        
tropical year1 containing #cfe days1    
five hours1 forty-eight minutes and     
fifty-two seconds1 is measured from the 
time the sun sets out from one of the   
equinoxes1 or solstices1 till it returns
 to the same again2 but this year is    
completed1 before the earth has finished
 one entire revolution in its orbit4    
,emily4 ,i thought that the earth       
performed one complete revolution in its
 orbit1 every year2 what is the reason  
of this variation8  ,mrs4 ,b4 ,it is    
owing to the spheroidal figure of the   
earth4 ,the elevation about the equator 

produces much the same effect as if a   
similar mass of matter1 collected in the
 form of a moon1 revolved round the     
equator4 ,when this moon acted on the   
earth1 in conjunction with1 or in       
opposition to the sun1 variations in the
 earth's motion would be occasioned1 and
 these variations produce what is called
 the precession of the equinoxes4       
,illustration3 ,,plate ,,xi4  ,emily4   
,what does that mean8 ,i thought the    
equinoctial points1 were fixed points in
 the heavens1 in which the equator cuts 
the ecliptic4  ,mrs4 ,b4 ,these points  
are not quite fixed1 but have an        
apparently retrograde motion1 among the 
signs of the zodiac2 that is to say1    
instead of being at every revolution in 
the same place1 they move backwards4    
,thus if the vernal equinox is at ,a1   
7fig4 #a4 plate ,,xi47 the autumnal one1
 will be at ,b1 instead of ,c1 and the  
following vernal equinox1 at ,d1 instead
 of at ,a1 as would be the case if the  
equinoxes were stationary1 at opposite  

points of the earth's orbit4  ,caroline4
 ,so that when the earth moves from one 
equinox to the other1 though it takes   
half a year to perform the journey1 it  
has not travelled through half its      
orbit4  ,mrs4 ,b4 ,and1 consequently1   
when it returns again to the first      
equinox1 it has not completed the whole 
of its orbit4 ,in order to ascertain    
when the earth has performed an entire  
revolution in its orbit1 we must observe
 when the sun returns in conjunction    
with any fixed star2 and this is called 
a sidereal year4 ,supposing a fixed star
 situated at ,e1 7fig4 #a4 plate ,,xi47 
the sun would not appear in conjunction 
with it1 till the earth had returned to 
,a1 when it would have completed its    
orbit4  ,emily4 ,and how much longer is 
the sidereal1 than the solar year8      
,mrs4 ,b4 ,only twenty minutes2 so that 
the variation of the equinoctial points 
is very inconsiderable4 ,i have given   
them a greater extent in the figure1 in 
order to render them sensible4  ,in     

regard to time1 ,i must further add1    
that the earth's diurnal motion on an   
inclined axis1 together with its annual 
revolution in an elliptic orbit1        
occasions so much complication in its   
motion1 as to produce many              
irregularities2 therefore the true time 
cannot be measured by the apparent place
 of the sun4 ,a perfectly correct clock1
 would in some parts of the year be     
before the sun1 and in other parts after
 it4 ,there are but four periods in     
which the sun and a perfect clock would 
agree1 which is the #aeth of ,april1 the
 #afth of ,june1 the #bc;d of ,august1  
and the #bdth of ,december4  ,emily4    
,and is there any considerable          
difference between solar time1 and true 
time8  ,mrs4 ,b4 ,the greatest          
difference amounts to between fifteen   
and sixteen minutes4 ,tables of equation
 are constructed for the purpose of     
pointing out1 and correcting these      
differences between solar time and equal
 or mean time1 which is the denomination

 given by astronomers1 to true time4    
,questions  #a4 7,pg4 #ib7 ,what does   
the line ,a ,b1 7fig4 #b plate #h47     
represent1 and what are its extremities 
called8  #b4 7,pg4 #ib7 ,what is meant  
by the equator1 and how is it situated8 
#c4 7,pg4 #ib7 ,there are two           
hemispheres2 how are they named and     
distinguished8  #d4 7,pg4 #ib7 ,what are
 the circles near the poles called8  #e4
 7,pg4 #ib7 ,what do the lines ,i ,k1   
and ,l ,m1 represent8  #f4 7,pg4 #ib7   
,what circle is in part represented by  
the line ,l ,k8  #g4 7,pg4 #ib7 ,against
 what mistake must you guard respecting 
this line8  #h4 7,pg4 #ib7 ,what is     
meant by a plane1 and how could one be  
represented8  #i4 7,pg4 #ic7 ,describe  
what is intended by the plane of the    
earth's orbit4  #aj4 7,pg4 #ic7         
,extending this plane to the fixed      
stars1 what circle would it form1 and   
among what particular stars would it be 
found8  #aa4 7,pg4 #ic7 ,what is fig4   
#a4 plate #i1 designed to represent8    

#ab4 7,pg4 #ic7 ,the ecliptic does not  
properly belong to the earth1 for what  
purpose then is it described on the     
terrestrial globe8  #ac4 7,pg4 #ic7     
,what does the obliquity of the ecliptic
 to the equator serve to show8  #ad4    
7,pg4 #ic7 ,within what limits do you   
find the torrid zone8  #ae4 7,pg4 #ic7  
,what two zones are there between the   
torrid1 and the two frigid zones8  #af4 
7,pg4 #ic7 ,where are the frigid zones  
situated8  #ag4 7,pg4 #ic7 ,what is     
meant by the term zone2 and are the     
frigid zones properly so called8  #ah4  
7,pg4 #ic7 ,how do meridian lines       
extend1 and what is meant by the        
meridian of a place8  #ai4 7,pg4 #ic7   
,what is said of the meridian to which  
the sun is opposite1 and where is it    
then midnight8  #bj4 7,pg4 #id7 ,what   
hour is it then1 at places exactly half 
way between these meridians8  #ba4 7,pg4
 #id7 ,how are greater and lesser       
circles distinguished8  #bb4 7,pg4 #id7 
,what part of a circle is a degree1 and 

how are these further divided8  #bc4    
7,pg4 #id7 ,what is the diameter1 and   
what the circumference of a circle1 and 
what proportion do they bear to each    
other8  #bd4 7,pg4 #id7 ,what part of a 
circle is a meridian8  #be4 7,pg4 #id7  
,how many degrees are there between the 
equator and the poles8  #bf4 7,pg4 #id7 
,into what parts1 besides degrees1 is   
the ecliptic divided8  #bg4 7,pg4 #id7  
,how are degrees of latitude measured1  
and to what number do they extend8  #bh4
 7,pg4 #id7 ,on what circles are degrees
 of longitude measured1 and to what     
number do they extend8  #bi4 7,pg4 #id7 
,what is a parallel of latitude8  #cj4  
7,pg4 #ie7 ,degrees of longitude vary in
 length2 what is the cause of this8     
#ca4 7,pg4 #ie7 ,what is the length of a
 degree of latitude1 and why do not     
these vary8  #cb4 7,pg4 #ie7 ,what      
causes the equator to be somewhat larger
 than a great circle passing through the
 poles1 and what effect has this on     
degrees of longitude measured on the    

equator8  #cc4 7,pg4 #ie7 ,what is the  
cause of this form being given to the   
earth8  #cd4 7,pg4 #if7 ,what would have
 been a consequence of the centrifugal  
force1 had the earth been a perfect     
sphere8  #ce4 7,pg4 #if7 ,a body        
situated at the poles1 is attracted more
 forcibly than if placed at the equator1
 what is the reason8  #cf4 7,pg4 #ig7   
,what effect would be produced upon the 
gravity of a body1 were it placed       
beneath the surface of the earth1 and   
what supposing it at its centre8  #cg4  
7,pg4 #ig7 ,what two circumstances      
combine1 to lessen the weight of a body 
on the equator8  #ch4 7,pg4 #ig7 ,why   
could not this be proved by weighing a  
body at the poles1 and at the equator8  
#ci4 7,pg4 #ih7 ,what is a pendulum8    
#dj4 7,pg4 #ih7 ,what causes it to      
vibrate8  #da4 7,pg4 #ih7 ,why are not  
its vibrations perpetual8  #db4 7,pg4   
#ih7 ,two pendulums of the same length1 
will not1 in different latitudes1       
perform their vibrations in equal times1

 what is the cause of this8  #dc4 7,pg4 
#ih7 ,to what use has this property of  
the pendulum been applied8  #dd4 7,pg4  
#ii7 ,what change must be made in       
pendulums situated at the equator and at
 the poles1 to render their vibrations  
equal8  #de4 7,pg4 #ii7 ,what do the    
vibrations of a pendulum resemble1 and  
why will it vibrate more rapidly if     
shortened8  #df4 7,pg4 #ii7 ,in the     
revolution of the earth round the sun1  
what is the position of its axis8  #dg4 
7,pg4 #ii7 ,how much is the axis of the 
earth inclined1 and with what line does 
it form this angle8  #dh4 7,pg4 #ii7    
,what is represented by fig4 #b1 plate  
#i8  #di4 7,pg4 #ajj7 ,how is the north 
pole inclined in the middle of our      
summer1 and what effect has this on the 
north frigid zone8  #ej4 7,pg4 #ajj7 ,in
 what direction does the north pole     
always point8  #ea4 7,pg4 #ajj7 ,what is
 shown by the position of the earth at  
,b1 in the figure8  #eb4 7,pg4 #ajj7    
,how does the sun then shine at the     

poles1 and what is the effect on the    
days and nights8  #ec4 7,pg4 #aja7 ,when
 the earth has passed the autumnal      
equinox1 what changes take place at the 
poles1 and also in the whole northern   
and southern hemispheres8  #ed4 7,pg4   
#aja7 ,why is the heat greatest within  
the torrid zone8  #ee4 7,pg4 #aja7 ,how 
does the sun appear at the poles1 during
 the period of day there8  #ef4 7,pg4   
#aja7 ,in what will the days and nights 
differ in the temperate zone1 from those
 at the poles1 and at the equator8  #eg4
 7,pg4 #ajb7 ,trace the earth from the  
winter solstice to the vernal equinox1  
and inform me what changes take place4  
#eh4 7,pg4 #ajb7 ,what takes place at   
the time of the vernal equinox1 and what
 is meant by the term8  #ei4 7,pg4 #ajb7
 ,in proceeding from the vernal equinox 
to the summer solstice1 what changes    
take place8  #fj4 7,pg4 #ajc7 ,from what
 cause arises the superior heat of the  
equatorial regions8  #fa4 7,pg4 #ajc7   
,why should oblique rays afford less    

heat than those which are perpendicular8
  #fb4 7,pg4 #ajc7 ,how is this         
explained by fig4 #a4 plate #aj8  #fc4  
7,pg4 #ajc7 ,how do you account for the 
superior heat of summer1 and how is this
 exemplified in fig4 #b and #c1 plate   
#aj8  #fd4 7,pg4 #ajd7 ,what other cause
 lessens the intensity of oblique rays8 
#fe4 7,pg4 #ajd7 ,how is this explained 
by fig4 #d8  #ff4 7,pg4 #ajd7 ,what     
causes conspire to lessen the solar heat
 in the morning and evening8  #fg4 7,pg4
 #ajd7 ,the greatest heat of summer is  
after the solstice1 and the greatest    
heat of the day1 after #ab o'clock1     
although the sun's rays are then most   
direct1 how is this accounted for8  #fh4
 7,pg4 #aje7 ,is there any change of    
seasons in the other planets8  #fi4     
7,pg4 #aje7 ,what is said respecting the
 axes of ,jupiter1 of ,mars1 and of     
,saturn8  #gj4 7,pg4 #aje7 ,in #cfe     
days1 how many times does the earth     
revolve on its axis8  #ga4 7,pg4 #aje7  
,how is this accounted for8  #gb4 7,pg4 

#aje7 ,do the fixed stars require the   
same time as the sun1 to return to the  
same meridian8  #gc4 7,pg4 #ajf7 ,how is
 this accounted for8  #gd4 7,pg4 #ajf7  
,what is meant by the solar and the     
sidereal day8  #ge4 7,pg4 #ajf7 ,what is
 the difference in time between them8   
#gf4 7,pg4 #ajf7 ,what is the length of 
the tropical year8  #gg4 7,pg4 #ajg7    
,the solar year is completed before the 
earth has made a complete revolution in 
its orbit1 by what is this caused8  #gh4
 7,pg4 #ajg7 ,what is this called1 and  
what is represented respecting it by    
fig4 #a1 plate #aa8  #gi4 7,pg4 #ajg7   
,by what means can we ascertain the     
period of a complete revolution of the  
earth in its orbit1 as illustrated by   
the fixed star ,e1 in fig4 #a8  #hj4    
7,pg4 #ajg7 ,what difference is there in
 the length of the solar and sidereal   
year8  #ha4 7,pg4 #ajg7 ,why can we not 
always ascertain the true time by the   
apparent place of the sun8  #hb4 7,pg4  
#ajh7 ,what would be the greatest       

difference between solar1 and true time1
 as indicated by a perfect clock8       
,,conversation ,,ix4  ,,on ,,the ,,moon4
  ,,of ,,the ,,moon',s ,,motion4        
,,phases ,,of ,,the ,,moon4 ,,eclipses  
,,of ,,the ,,moon4 ,,eclipses ,,of      
,,jupiter',s ,,moons4 ,,of ,,latitude   
,,and ,,longitude4 ,,of ,,the ,,transits
 ,,of ,,the ,,inferior ,,planets4 ,,of  
,,the ,,tides4   ,,mrs4 ,b4  ,we shall1 
to-day1 confine our attention to the    
moon1 which offers many interesting     
phenomena4  ,the moon revolves round the
 earth in the space of about twenty-nine
 days and a half2 in an orbit1 the plane
 of which is inclined upwards of five   
degrees to that of the earth2 she       
accompanies us in our revolution round  
the sun4  ,emily4 ,her motion then must 
be of a complicated nature2 for as the  
earth is not stationary1 but advances in
 her orbit1 whilst the moon goes round  
her1 the moon1 in passing round the sun1
 must proceed in a sort of scolloped    
circle4  ,mrs4 ,b4 ,that is true2 and   

there are also other circumstances which
 interfere with the simplicity1 and     
regularity of the moon's motion1 but    
which are too intricate for you to      
understand at present4  ,the moon always
 presents the same face to us1 by which 
it is evident that she turns but once   
upon her axis1 while she performs a     
revolution round the earth2 so that the 
inhabitants of the moon have but one    
day1 and one night1 in the course of a  
lunar month4  ,caroline4 ,we afford     
them1 however1 the advantage of a       
magnificent moon to enlighten their long
 nights4  ,mrs4 ,b4 ,that advantage is  
put partial2 for since we always see the
 same hemisphere of the moon1 the       
inhabitants of that hemisphere alone1   
can perceive us4  ,caroline4 ,one half  
of the moon then enjoys our light1 while
 the other half has constantly nights of
 darkness4 ,if there are any astronomers
 in those regions1 they would doubtless 
be tempted to visit the other           
hemisphere1 in order to behold so grand 

a luminary as we must appear to them4   
,but1 pray1 do they see the earth under 
all the changes1 which the moon exhibits
 to us8  ,mrs4 ,b4 ,exactly so4 ,these  
changes are called the phases of the    
moon1 and require some explanation4 ,in 
fig4 #b1 plate #aa1 let us say1 that ,s 
represents the sun1 ,e the earth1 and ,a
 ,b ,c ,d ,e ,f ,g ,h1 the moon1 in     
different parts of her orbit4 ,when the 
moon is at ,a1 her dark side being      
turned towards the earth1 we shall not  
see her as at a2 but her disappearance  
is of very short duration1 and as she   
advances in her orbit1 we perceive her  
under the form of a new moon3 when she  
has gone through one eighth of her orbit
 at ,b1 one quarter of her enlightened  
hemisphere will be turned towards the   
earth1 and she will then appear horned  
as at b2 when she has performed one     
quarter of her orbit1 she shows us one  
half of her enlightened side1 as at c1  
and this is called her first quarter2 at
 d she is said to be gibbous1 and at e  

the whole of the enlightened side       
appears to us1 and the moon is at full4 
,as she proceeds in her orbit1 she      
becomes again gibbous1 and her          
enlightened hemisphere turns gradually  
away from us1 until she arrives at ,g1  
which is her third quarter2 proceeding  
thence she completes her orbit and      
disappears1 and then again resumes her  
form of a new moon1 and passes          
successively1 through the same changes4 
,when the moon is new1 she is said to be
 in conjunction with the sun1 as they   
are then both in the same direction from
 the earth2 at the time of full moon1   
she is said to be in opposition1 because
 she and the sun1 are at opposite sides 
of the earth2 at the time of her first  
and third quarters1 she is said to be in
 her quadratures1 because she is then   
one-fourth of a circle1 or #ij deg41    
from her conjunction1 or the period of  
new moon4  ,emily4 ,are not the eclipses
 of the sun produced by the moon passing
 between the sun and the earth8  ,mrs4  

,b4 ,yes2 when the moon passes between  
the sun and the earth1 she intercepts   
his rays1 or1 in other words1 casts a   
shadow on the earth1 then the sun is    
eclipsed1 and daylight gives place to   
darkness1 while the moon's shadow is    
passing over us4  ,when1 on the         
contrary1 the earth is between the sun  
and the moon1 it is we who intercept the
 sun's rays1 and cast a shadow on the   
moon2 she is then said to be eclipsed1  
and disappears from our view4  ,emily4  
,but as the moon goes round the earth   
every month1 she must be1 once during   
that time1 between the earth and the    
sun2 and the earth must likewise be once
 between the sun and the moon1 and yet  
we have not a solar and a lunar eclipse 
every month8  ,mrs4 ,b4 ,i have already 
informed you1 that the orbits of the    
earth and moon are not in the same      
plane1 but cross or intersect each      
other2 and the moon generally passes    
either above or below that of the earth1
 when she is in conjunction with the    

sun1 and does not therefore intercept   
its rays1 and produce an eclipse2 for   
this can take place only when the moon  
is in1 or near her nodes1 which is the  
name given to those two points in which 
her orbit crosses that of the earth2    
eclipses cannot happen at any other     
time1 because it is then only1 that they
 are both in a right line with the sun4 
,emily4 ,and a partial eclipse of the   
moon takes place1 ,i suppose1 when1 in  
passing by the earth1 she is not        
sufficiently above or below the shadow1 
to escape it entirely8  ,mrs4 ,b4 ,yes1 
one edge of her disk then dips into the 
shadow1 and is eclipsed2 but as the     
earth is larger than the moon1 when     
eclipses happen precisely at the nodes1 
they are not only total1 but last for   
upwards of three hours4  ,illustration3 
,,plate ,,xii4  ,a total eclipse of the 
sun rarely occurs1 and when it happens1 
the total darkness is confined to one   
particular part of the earth1 the       
diameter of the shadow not exceeding    

#ahj miles2 evidently showing that the  
moon is smaller than the sun1 since she 
cannot entirely hide it from the earth4 
,in fig4 #a1 plate #ab1 you will find a 
solar eclipse described2 ,s is the sun1 
,m the moon1 and ,e the earth2 and the  
moon's shadow1 you see1 is not large    
enough to cover the earth4 ,the lunar   
eclipses1 on the contrary1 are visible  
from every part of the earth1 where the 
moon is above the horizon2 and we       
discover1 by the length of time which   
the moon is passing through the earth's 
shadow1 that it would be sufficient to  
eclipse her totally1 were she many times
 her actual size2 it follows1 therefore1
 that the earth is much larger than the 
moon4  ,in fig4 #b1 ,s represents the   
sun1 which pours forth rays of light in 
straight lines1 in every direction4 ,e  
is the earth1 and ,m the moon4 ,now a   
ray of light coming from one extremity  
of the sun's disk1 in the direction ,a  
,b1 will meet another1 coming from the  
opposite extremity1 in the direction ,c 

,b2 the shadow of the earth cannot      
therefore extend beyond ,b2 as the sun  
is larger than the earth1 the shadow of 
the latter is conical1 or in the figure 
of a sugar loaf2 it gradually           
diminishes1 and is much smaller than the
 earth where the moon passes through it1
 and yet we find the moon to be1 not    
only totally eclipsed1 but to remain for
 a considerable length of time in       
darkness1 and hence we are enabled to   
ascertain its real dimensions4  ,emily4 
,when the moon eclipses the sun to us1  
we must be eclipsed to the moon8  ,mrs4 
,b4 ,certainly2 for if the moon         
intercepts the sun's rays1 and casts a  
shadow on us1 we must necessarily       
disappear to the moon1 but only         
partially1 as in fig4 #a4  ,caroline4   
,there must be a great number of        
eclipses in the distant planets1 which  
have so many moons8  ,mrs4 ,b4 ,yes1 few
 days pass without an eclipse taking    
place2 for among the number of          
satellites1 one or the other of them are

 continually passing either between     
their primary and the sun2 or between   
the planet1 and each other4 ,astronomers
 are so well acquainted with the motion 
of the planets1 and their satellites1   
that they have calculated not only the  
eclipses of our moon1 but those of      
,jupiter1 with such perfect accuracy1   
that it has afforded a means of         
ascertaining the longitude4  ,caroline4 
,but is it not very easy to find both   
the latitude and longitude of any place 
by a map or globe8  ,mrs4 ,b4 ,if you   
know where you are situated1 there is no
 difficulty in ascertaining the latitude
 or longitude of the place1 by referring
 to a map2 but supposing that you had   
been a length of time at sea1           
interrupted in your course by storms1 a 
map would afford you very little        
assistance in discovering where you     
were4  ,caroline4 ,under such           
circumstances1 ,i confess ,i should be  
equally at a loss to discover either    
latitude1 or longitude4  ,mrs4 ,b4 ,the 

latitude is usually found by taking the 
altitude of the sun at mid-day2 that is 
to say1 the number of degrees that it is
 elevated above the horizon1 for the sun
 appears more elevated as we approach   
the equator1 and less as we recede from 
it4  ,caroline4 ,but unless you can see 
the sun1 how can you take its altitude8 
,mrs4 ,b4 ,when it is too cloudy to see 
the sun1 the latitude is sometimes found
 at night1 by the polar star2 the north 
pole of the earth1 points constantly    
towards one particular part of the      
heavens1 in which a star is situated1   
called the ,polar star3 this star is    
visible on clear nights1 from every part
 of the northern hemisphere2 the        
altitude of the polar star1 is therefore
 the same number of degrees1 as that of 
the pole2 the latitude may also be      
determined by observations made on any  
of the fixed stars3 the situation       
therefore of a vessel at sea1 with      
regard to north and south1 is easily    
ascertained4 ,the difficulty is1        

respecting east and west1 that is to    
say1 its longitude4 ,as we have no      
eastern poles from which we can reckon  
our distance1 some particular spot1 or  
line1 must be fixed upon for that       
purpose4 ,the ,english1 reckon from the 
meridian of ,greenwich1 where the royal 
observatory is situated2 in ,french     
maps1 you will find that the longitude  
is reckoned from the meridian of ,paris4
  ,the rotation of the earth on its axis
 in #bd hours from west to east1        
occasions1 you know1 an apparent motion 
of the sun and stars in a contrary      
direction1 and the sun appears to go    
round the earth in the space of #bd     
hours1 passing over fifteen degrees1 or 
a twenty-fourth part of the earth's     
circumference every hour2 therefore1    
when it is twelve o'clock in ,london1 it
 is one o'clock in any place situated   
fifteen degrees to the east of ,london1 
as the sun must have passed the meridian
 of that place1 an hour before he       
reaches that of ,london4 ,for the same  

reason it is eleven o'clock in any place
 situated fifteen degrees to the west of
 ,london1 as the sun will not come to   
that meridian till an hour later4  ,if  
then the captain of a vessel at sea1    
could know precisely what was the hour  
at ,london1 he could1 by looking at his 
watch1 and comparing it with the hour at
 the spot in which he was1 ascertain the
 longitude4  ,emily4 ,but if he had not 
altered his watch1 since he sailed from 
,london1 it would indicate the hour it  
then was in ,london4  ,mrs4 ,b4 ,true2  
but in order to know the hour of the day
 at the spot in which he is1 the captain
 of a vessel regulates his watch by the 
sun when it reaches the meridian4       
,emily4 ,then if he had two watches1 he 
might keep one regulated daily1 and     
leave the other unaltered2 the former   
would indicate the hour of the place in 
which he was situated1 and the latter   
the hour at ,london2 and by comparing   
them together1 he would be able to      
calculate his longitude4  ,mrs4 ,b4 ,you

 have discovered1 ,emily1 a mode of     
finding the longitude1 which ,i have the
 pleasure to tell you1 is universally   
adopted3 watches of a superior          
construction1 called chronometers1 or   
time-keepers1 are used for this purpose1
 and are now made with such accuracy1 as
 not to vary more than four or five     
seconds in a whole year2 but the best   
watches are liable to imperfections1 and
 should the time-keeper go too fast or  
too slow1 there would be no means of    
ascertaining the error2 implicit        
reliance1 cannot consequently be placed 
upon them4  ,recourse1 therefore1 is    
sometimes had to the eclipses of        
,jupiter's satellites4 ,a table is made1
 of the precise time at which the       
several moons are eclipsed to a         
spectator at ,london2 when they appear  
eclipsed to a spectator in any other    
spot1 he may1 by consulting the table1  
know what is the hour at ,london2 for   
the eclipse is visible at the same      
moment1 from whatever place on the earth

 it is seen4 ,he has then only to look  
at his watch1 which he regulates by the 
sun1 and which therefore points out the 
hour of the place in which he is1 and by
 observing the difference of time there1
 and at ,london1 he may immediately     
determine his longitude4  ,let us       
suppose1 that a certain moon of ,jupiter
 is always eclipsed at six o'clock in   
the evening2 and that a man at sea      
consults his watch1 and finds that it is
 ten o'clock at night1 where he is      
situated1 at the moment the eclipse     
takes place1 what will be his longitude8
  ,emily4 ,that is four hours later than
 in ,london3 four times fifteen degrees1
 make #fj2 he would1 therefore1 be sixty
 degrees east of ,london1 for the sun   
must have passed his meridian before it 
reaches that of ,london4  ,mrs4 ,b4 ,for
 this reason the hour is always later   
than in ,london1 when the place is east 
longitude1 and earlier when it is west  
longitude4 ,thus the longitude can be   
ascertained whenever the eclipses of    

,jupiter's moons are visible4           
,caroline4 ,but do not the primary      
planets1 sometimes eclipse the sun from 
each other1 as they pass round in their 
orbits8  ,mrs4 ,b4 ,they must of course 
sometimes pass between each other and   
the sun1 but as their shadows never     
reach each other1 they hide so little of
 his light1 that the term eclipse is not
 in this case used2 this phenomenon is  
called a transit4 ,the primary planets  
do not any of them revolve in the same  
plane1 and the times of their revolution
 round the sun is considerable1 it      
therefore but rarely happens that they  
are at the same time1 in conjunction    
with the sun1 and in their nodes4 ,it is
 evident also1 that a planet must be    
inferior 7that is within the orbit of   
another7 in order to its apparently     
passing over the disk of the sun4       
,mercury1 and ,venus1 have sometimes    
passed in a right line between us1 and  
the sun1 but being at so great a        
distance from us1 their shadows did not 

extend so far as the earth2 no darkness 
was therefore produced on any part of   
our globe2 but the planet appeared like 
a small black spot1 passing across the  
sun's disk4  ,it was by the last transit
 of ,venus1 that astronomers were       
enabled to calculate1 with some degree  
of accuracy1 the distance of the earth  
from the sun1 and the dimensions of the 
latter4  ,emily4 ,i have heard that the 
tides are affected by the moon1 but ,i  
cannot conceive what influence it can   
have on them4  ,mrs4 ,b4 ,they are      
produced by the moon's attraction1 which
 draws up the waters of that part of the
 ocean over which the moon passes1 so as
 to cause it to stand considerably      
higher than the surrounding parts4      
,caroline4 ,does attraction act on water
 more powerfully than on land8 ,i should
 have thought it would have been just   
the contrary1 for land is certainly a   
more dense body than water8  ,mrs ,b4   
,tides do not arise from water being    
more strongly attracted than land1 for  

this certainly is not the case2 but the 
cohesion of fluids1 being much less than
 that of solid bodies1 they more easily 
yield to the power of gravity2 in       
consequence of which1 the waters        
immediately below the moon1 are drawn up
 by it1 producing a full tide1 or what  
is commonly called1 high water1 at the  
spot where it happens4 ,so far1 the     
theory of the tides is not difficult to 
understand4  ,caroline4 ,on the         
contrary1 nothing can be more simple2   
the waters1 in order to rise up under   
the moon1 must draw the waters from the 
opposite side of the globe1 and occasion
 ebb-tide1 or low water1 in those parts4
  ,mrs4 ,b4 ,you draw your conclusion   
rather too hastily1 my dear2 for        
according to your theory1 we should have
 full tide only once in about           
twenty-four hours1 that is1 every time  
that we were below the moon1 while we   
find that in this time we have two      
tides1 and that it is high water with   
us1 and with our antipodes1 at the same 

time4  ,caroline4 ,yet it must be       
impossible for the moon to attract the  
sea in opposite parts of the globe1 and 
in opposite directions1 at the same     
time4  ,mrs4 ,b4 ,this opposite tide1 is
 rather more difficult to explain1 than 
that which is immediately beneath the   
moon2 with a little attention1 however1 
,i hope ,i shall be able to make you    
understand the explanation which has    
been given of it1 by astronomers4 ,it   
must be confessed1 however1 that the    
theory upon this subject1 is attended   
with some difficulties4 ,you recollect  
that the earth and the moon mutually    
attract each other1 but do you suppose  
that every part of the earth is equally 
attracted by the moon8  ,emily4         
,certainly not2 you have taught us that 
the force of attraction decreases1 with 
the increase of distance1 and therefore 
that part of the earth which is farthest
 from the moon1 must be attracted less  
powerfully1 than that to which she is   
nearest4  ,mrs4 ,b4 ,this fact will aid 

us in the explanation which ,i am about 
to give to you4  ,in order to render the
 question more simple1 let us suppose   
the earth to be every where covered by  
the ocean1 as represented in 7fig4 #c4  
pl4 #ab47 ,m is the moon1 ,a ,b ,c ,d   
the earth4 ,now the waters on the       
surface of the earth1 about ,a1 being   
more strongly attracted than any other  
part1 will be elevated3 the attraction  
of the moon at ,b and ,c being less1 and
 at ,d least of all4 ,the high tide at  
,a1 is accounted for from the direct    
attraction of the moon2 to produce this 
the waters are drawn from ,b and ,c1    
where it will consequently be low water4
 ,at ,d1 the attraction of the moon     
being considerably decreased1 the waters
 are left relatively high1 which height 
is increased1 by the centrifugal force  
of the earth being greater at ,d than at
 ,a1 in consequence of its greater      
distance from the common centre of      
gravity ,x1 between the earth and the   
moon4  ,emily4 ,the tide ,a1 then1 is   

produced by the moon's attraction1 and  
the tide ,d1 is produced by the         
centrifugal force1 and increased by the 
feebleness of the moon's attraction1 in 
those parts4  ,caroline4 ,and when it is
 high water at ,a and ,d1 it is low     
water at ,b and ,c3 now ,i think ,i     
comprehend the nature of the tides1     
though ,i confess it is not quite so    
easy as ,i at first thought4  ,but1     
,mrs4 ,b41 why does not the sun produce 
tides1 as well as the moon2 for its     
attraction is greater than that of the  
moon8  ,mrs4 ,b4 ,it would be at an     
equal distance1 but our vicinity to the 
moon1 makes her influence more powerful4
 ,the sun has1 however1 a considerable  
effect on the tides1 and increases or   
diminishes them as it acts in           
conjunction with1 or in opposition to   
the moon4  ,emily4 ,i do not quite      
understand that4  ,mrs4 ,b4 ,the moon is
 a month in going round the earth2 twice
 during that time1 therefore1 at full   
and at change1 she is in the same       

direction as the sun2 both1 then act in 
conjunction on the earth1 and produce   
very great tides1 called spring tides1  
as represented in fig4 #d1 at ,a and ,b2
 but when the moon is at the            
intermediate parts of her orbit1 that is
 in her quadratures1 the sun1 instead of
 affording assistance1 weakens her      
power1 by acting in opposition to it2   
and smaller tides are produced1 called  
neap tides1 as represented at ,m1 in    
fig4 #e4  ,emily4 ,i have often observed
 the difference of these tides1 when ,i 
have been at the sea side4  ,but since  
attraction is mutual between the moon   
and the earth1 we must produce tides in 
the moon2 and these must be more        
considerable in proportion as our planet
 is larger4 ,and yet the moon does not  
appear of an oval form4  ,mrs4 ,b4 ,you 
must recollect1 that in order to render 
the explanation of the tides clearer1 we
 suppose the whole surface of the earth 
to be covered with the ocean2 but that  
is not really the case1 either with the 

earth or the moon1 and the land which   
intersects the water1 destroys the      
regularity of the effect4 ,thus1 in     
flowing up rivers1 in passing round     
points of land1 and into bays and       
inlets1 the water is obstructed1 and    
high water must happen much later1 than 
would otherwise be the case4  ,caroline4
 ,true2 we may1 however1 be certain that
 whenever it is high water1 the moon is 
immediately over our heads4  ,mrs4 ,b4  
,not so either2 for as a similar effect 
is produced on that part of the globe   
immediately beneath the moon1 and on    
that part most distant from it1 it      
cannot be over the heads of the         
inhabitants of both those situations1 at
 the same time4 ,besides1 as the orbit  
of the moon is very nearly parallel to  
that of the earth1 she is never         
vertical1 but to the inhabitants of the 
torrid zone4  ,caroline4 ,in the torrid 
zone1 then1 ,i hope you will grant that 
the moon is immediately over1 or        
opposite the spots where it is high     

water8  ,mrs4 ,b4 ,i cannot even admit  
that2 for the ocean naturally partaking 
of the earth's motion1 in its rotation  
from west to east1 the moon1 in forming 
a tide1 has to contend against the      
eastern motion of the waves4 ,all       
matter1 you know1 by its inertia1 makes 
some resistance to a change of state2   
the waters1 therefore1 do not readily   
yield to the attraction of the moon1 and
 the effect of her influence is not     
complete1 till three hours after she has
 passed the meridian1 where it is full  
tide4  ,when a body is impelled by any  
force1 its motion may continue1 after   
the impelling force ceases to act3 this 
is the case with all projectiles4 ,a    
stone thrown from the hand1 continues   
its motion for a length of time1        
proportioned to the force given to it3  
there is a perfect analogy between this 
effect1 and the continued rise of the   
water1 after the moon has passed the    
meridian at any particular place4       
,emily4 ,pray what is the reason that   

the tide is three-quarters of an hour   
later every day8  ,mrs4 ,b4 ,because it 
is twenty-four hours and three-quarters 
before the same meridian1 on our globe1 
returns beneath the moon4 ,the earth    
revolves on its axis in about           
twenty-four hours2 if the moon were     
stationary1 therefore1 the same part of 
our globe would1 every twenty-four      
hours1 return beneath the moon2 but as  
during our daily revolution1 the moon   
advances in her orbit1 the earth must   
make more than a complete rotation1 in  
order to bring the same meridian        
opposite the moon3 we are three-quarters
 of an hour in overtaking her4 ,the     
tides1 therefore1 are retarded1 for the 
same reason that the moon rises later by
 three-quarters of an hour1 every day4  
,we have now1 ,i think1 concluded the   
observations ,i had to make to you on   
the subject of astronomy2 at our next   
interview1 ,i shall attempt to explain  
to you the elements of hydrostatics4    
,questions  #a4 7,pg4 #ajh7 ,in what    

time does the moon revolve round the    
earth8 what is the inclination of her   
orbit8 and how does she accompany the   
earth8  #b4 7,pg4 #ajh7 ,as the moon    
revolves round the earth1 and also      
accompanies it in its annual revolution1
 in what form would you draw the moon's 
orbit8  #c4 7,pg4 #aji7 ,what causes the
 moon always to present the same face to
 the earth1 and what must be the length 
of a day and night to its inhabitants8  
#d4 7,pg4 #aji7 ,can the earth be seen  
from every part of the moon1 and will it
 always exhibit the same appearance8    
#e4 7,pg4 #aji7 ,what are the changes of
 the moon called8  #f4 7,pg4 #aji7 ,how 
are these changes explained by fig4 #b4 
plate #aa8  #g4 7,pg4 #aji7 ,what is    
meant by her first quarter8  #h4 7,pg4  
#aji7 ,what by her being horned1 and her
 being gibbous8  #i4 7,pg4 #aji7 ,what  
by her being full8  #aj4 7,pg4 #aji7    
,what by her third quarter8  #aa4 7,pg4 
#aaj7 ,what is meant by her             
conjunction8--what by her being in      

opposition8--what by her quadratures8   
#ab4 7,pg4 #aaj7 ,by what are eclipses  
of the sun caused8  #ac4 7,pg4 #aaj7    
,what causes eclipses of the moon8  #ad4
 7,pg4 #aaj7 ,what is meant by the      
moon's nodes8  #ae4 7,pg4 #aaj7 ,why do 
not eclipses happen at every new and    
full moon8  #af4 7,pg4 #aaj7 ,what      
causes partial eclipses of the moon8    
#ag4 7,pg4 #aaj7 ,when the moon is      
exactly in one of her nodes1 what length
 of time will she be eclipsed8  #ah4    
7,pg4 #aaj7 ,are total eclipses of the  
sun frequent1 and when they happen what 
is their extent8  #ai4 7,pg4 #aaa7 ,what
 does this prove respecting the size of 
the moon8  #bj4 7,pg4 #aaa7 ,what is    
shown in fig4 #a1 plate #ab8  #ba4 7,pg4
 #aaa7 ,how are lunar eclipses visible1 
and what is proved by their duration8   
#bb4 7,pg4 #aaa7 ,what is illustrated by
 fig4 #b1 plate #ab8  #bc4 7,pg4 #aaa7  
,what remark is made respecting those   
planets which have several moons8  #bd4 
7,pg4 #aaa7 ,what use is made of the    

eclipses of the satellites of ,jupiter8 
#be4 7,pg4 #aab7 ,how is the latitude of
 a place usually found8  #bf4 7,pg4     
#aab7 ,by what other means may latitude 
be found8  #bg4 7,pg4 #aab7 ,from what  
is longitude reckoned8  #bh4 7,pg4 #aab7
 ,how does the rotation of the earth    
upon its axis1 govern the time at       
different places8  #bi4 7,pg4 #aac7     
,what two circumstances1 if known1 will 
enable you to find your longitude from a
 given place8  #cj4 7,pg4 #aac7 ,by what
 means may a captain find the time at   
,london1 and in the place where his ship
 may be8  #ca4 7,pg4 #aac7 ,how may the 
eclipses of ,jupiter's satellites be    
used to find the longitude8  #cb4 7,pg4 
#aac7 ,give an example4  #cc4 7,pg4     
#aad7 ,how will you know whether the    
longitude is east or west8  #cd4 7,pg4  
#aad7 ,what is meant by the transit of a
 planet8  #ce4 7,pg4 #aad7 ,why can we  
see transits of ,venus and ,mercury     
only8  #cf4 7,pg4 #aad7 ,by what are    
tides caused8  #cg4 7,pg4 #aad7 ,why is 

not a similar effect produced on the    
land8  #ch4 7,pg4 #aae7 ,in what two    
parts of the world is it high water at  
the same time8  #ci4 7,pg4 #aae7 ,what  
circumstances respecting the decrease of
 attraction are taken into account1 in  
explaining the tides8  #dj4 7,pg4 #aae7 
,how are the high tides at ,a and ,d1   
and the low ones at ,b and ,c1 in fig4  
#c4 pl4 #ab1 accounted for8  #da4 7,pg4 
#aaf7 ,has the sun any influence on the 
tides1 and why is it less than that of  
the moon8  #db4 7,pg4 #aaf7 ,what is    
meant by spring tides1 and how are they 
produced8  #dc4 7,pg4 #aaf7 ,what by    
neap tides1 and how are they caused8    
#dd4 7,pg4 #aaf7 ,what circumstances    
affect the time of the tide in rivers1  
bays1 c48  #de4 7,pg4 #aag7 ,why in the 
open ocean1 is it high water1 some hours
 after the moon has passed the meridian8
  #df4 7,pg4 #aag7 ,why are the tides   
three-quarters of an hour later every   
day8     ,,conversation ,x4  ,,on ,,the 
,,mechanical ,,properties ,,of ,,fluids4

  ,,definition ,,of ,a ,,fluid4         
,,distinction ,,between ,,fluids ,,and  
,,liquids4 ,,of ,,non-,,elastic         
,,fluids4 ,,scarcely ,,susceptible ,,of 
,,compression4 ,,of ,,the ,,cohesion    
,,of ,,fluids4 ,,of ,,their             
,,gravitation4 ,,of ,,their             
,,equilibrium4 ,,of ,,their ,,pressure4 
,,of ,,specific ,,gravity4 ,,of ,,the   
,,specific ,,gravity ,,of ,,bodies      
,,heavier ,,than ,,water4 ,,of ,,those  
,,of ,,the ,,same ,,weight ,,as ,,water4
 ,,of ,,those ,,lighter ,,than ,,water4 
,,of ,,the ,,specific ,,gravity ,,of    
,,fluids4   ,,mrs4 ,b4  ,we have        
hitherto confined our attention to the  
mechanical properties of solid bodies1  
which have been illustrated1 and1 ,i    
hope1 thoroughly impressed upon your    
memory1 by the conversations we have    
subsequently had1 on astronomy4 ,it will
 now be necessary for me to give you    
some account of the mechanical          
properties of fluids--a science which1  
when applied to liquids1 is divided into

 two parts1 hydrostatics and hydraulics4
 ,hydrostatics1 treats of the weight and
 pressure of fluids2 and hydraulics1 of 
the motion of fluids1 and the effects   
produced by this motion4 ,a fluid is a  
substance which yields to the slightest 
pressure4 ,if you dip your hand into a  
basin of water1 you are scarcely        
sensible of meeting with any resistance4
  ,emily4 ,the attraction of cohesion is
 then1 ,i suppose1 less powerful in     
fluids1 than in solids8  ,mrs4 ,b4 ,yes2
 fluids1 generally speaking1 are bodies 
of less density than solids4 ,from the  
slight cohesion1 of the particles of    
fluids1 and the facility with which they
 slide over each other1 it is inferred1 
that they have but a slight attraction  
for each other1 and that this attraction
 is equal1 in every position of their   
particles1 and therefore produces no    
resistance to a perfect freedom of      
motion among themselves4  ,caroline4    
,pray what is the distinction between a 
fluid and a liquid8  ,mrs4 ,b4 ,liquids 

comprehend only one class of fluids4    
,there is another class1 distinguished  
by the name of elastic fluids1 or gases1
 which comprehends the air of the       
atmosphere1 and all the various kinds of
 air with which you will become         
acquainted1 when you study chemistry4   
,their mechanical properties we shall   
examine hereafter1 and confine our      
attention this morning1 to those of     
liquids1 or non-elastic fluids4  ,water1
 and liquids in general1 are scarcely   
susceptible of being compressed1 or     
squeezed into a smaller space1 than that
 which they naturally occupy4 ,such1    
however1 is the extreme minuteness of   
their particles1 that by strong         
compression1 they sometimes force their 
way through the pores of the substance  
which confines them4 ,this was shown by 
a celebrated experiment1 made at        
,florence many years ago4 ,a hollow     
globe of gold was filled with water1 and
 on its being submitted to great        
pressure1 the water was seen to exude   

through the pores of the gold1 which it 
covered with a fine dew4 ,many          
philosophers1 however1 think that this  
experiment is too much relied upon1 as  
it does not appear that it has ever been
 repeated2 it is possible1 therefore1   
that there may have been some source of 
error1 which was not discovered by the  
experimenters4 ,fluids1 appear to       
gravitate more freely1 than solid       
bodies2 for the strong cohesive         
attraction of the particles of the      
latter1 in some measure counteracts the 
effect of gravity4 ,in this table1 for  
instance1 the cohesion of the particles 
of wood1 enables four slender legs to   
support a considerable weight4 ,were the
 cohesion destroyed1 or1 in other words1
 the wood converted into a fluid1 no    
support could be afforded by the legs1  
for the particles no longer cohering    
together1 each would press separately   
and independently1 and would be brought 
to a level with the surface of the      
earth4  ,emily4 ,this want of cohesion  

is then the reason why fluids can never 
be formed into figures1 or maintained in
 heaps2 for though it is true the wind  
raises water into waves1 they are       
immediately afterwards destroyed by     
gravity1 and water always finds its     
level4  ,mrs4 ,b4 ,do you understand    
what is meant by the level1 or          
equilibrium of fluids8  ,emily4 ,i      
believe ,i do1 though ,i feel rather at 
a loss to explain it4 ,is not a fluid   
level when its surface is smooth and    
flat1 as is the case with all fluids1   
when in a state of rest8  ,mrs4 ,b4     
,smooth1 if you please1 but not flat2   
for the definition of the equilibrium of
 a fluid is1 that every part of the     
surface is equally distant from the     
point to which they gravitate1 that is  
to say1 from the centre of the earth2   
hence the surface of all fluids must be 
spherical1 not flat1 since they will    
partake of the spherical form of the    
globe4 ,this is very evident in large   
bodies of water1 such as the ocean1 but 

the sphericity of small bodies of water1
 is so trifling1 that their surfaces    
appear flat4  ,this level1 or           
equilibrium of fluids1 is the natural   
result of their particles gravitating   
independently of each other2 for when   
any particle of a fluid1 accidentally   
finds itself elevated above the rest1 it
 is attracted down to the level of the  
surface of the fluid1 and the readiness 
with which fluids yield to the slightest
 impression1 will enable the particle by
 its weight1 to penetrate the surface of
 the fluid1 and mix with it4  ,caroline4
 ,but ,i have seen a drop of oil1 float 
on the surface of water1 without mixing 
with it4  ,mrs4 ,b4 ,they do not mix1   
because their particles repel each      
other1 and the oil rises to the surface1
 because oil is a lighter liquid than   
water4 ,if you were to pour water over  
it1 the oil would still rise1 being     
forced up by the superior gravity of the
 water4 ,here is an instrument called a 
spirit-level1 7fig4 #a1 plate #ac47     

which is constructed upon the principle 
of the equilibrium of fluids4 ,it       
consists of a short tube ,a ,b1 closed  
at both ends1 and containing a little   
water1 or more commonly some spirits3 it
 is so nearly filled1 as to leave only a
 small bubble of air2 when the tube is  
perfectly horizontal1 this bubble will  
occupy the middle of it1 but when not   
perfectly horizontal1 the water runs to 
the lower1 and the bubble of air or     
spirit rises to the upper end2 by this  
instrument1 the level of any situation1 
to which we apply it1 may be            
ascertained4  ,from the strong cohesion 
of their particles1 you may therefore   
consider solid bodies as gravitating in 
masses1 while every particle of a fluid 
may be considered as separate1 and      
gravitating independently of each other4
 ,hence the resistance of a fluid1 is   
considerably less1 than that of a solid 
body2 for the resistance of the         
particles1 acting separately1 is more   
easily overcome4  ,emily4 ,a body of    

water1 in falling1 does certainly less  
injury than a solid body of the same    
weight4  ,mrs4 ,b4 ,the particles of    
fluids1 acting thus independently1 press
 against each other in every direction1 
not only downwards1 but upwards1 and    
laterally or sideways2 and in           
consequence of this equality of         
pressure1 every particle remains at     
rest1 in the fluid4 ,if you agitate the 
fluid1 you disturb this equality of     
pressure1 and the fluid will not rest1  
till its equilibrium is restored4       
,illustration3 ,,plate ,,xiii4          
,caroline4 ,the pressure downwards is   
very natural2 it is the effect of       
gravity2 one particle1 weighing upon    
another1 presses on it2 but the pressure
 sideways1 and particularly the pressure
 upwards1 ,i cannot understand4  ,mrs4  
,b4 ,if there were no lateral pressure1 
water would not run out of an opening on
 the side of a vessel4 ,if you fill a   
vessel with sand1 it will not continue  
to run out of such an opening1 because  

there is scarcely any lateral pressure  
among its particles4  ,emily4 ,when     
water runs out of the side of a vessel1 
is it not owing to the weight of the    
water1 above the opening8  ,mrs4 ,b4 ,if
 the particles of fluids were arranged  
in regular columns1 thus1 7fig4 #b47    
there would be no lateral pressure1 for 
when one particle is perpendicularly    
above the other1 it can only press      
downwards2 but as it must continually   
happen1 that a particle presses between 
two particles beneath1 7fig4 #c47 these 
last1 must suffer a lateral pressure4   
,emily4 ,the same as when a wedge is    
driven into a piece of wood1 and        
separates the parts1 laterally4  ,mrs4  
,b4 ,yes4 ,the lateral pressure         
proceeds1 therefore1 entirely from the  
pressure downwards1 or the weight of the
 liquid above2 and consequently1 the    
lower the orifice is made in the vessel1
 the greater will be the velocity of the
 water rushing out of it4 ,here is a    
vessel of water 7fig4 #e471 with three  

stop cocks at different heights2 we     
shall open them1 and you will see with  
what different degrees of velocity1 the 
water issues from them4 ,do you         
understand this1 ,caroline8  ,caroline4 
,oh yes4 ,the water from the upper      
spout1 receiving but a slight pressure1 
on account of its vicinity to the       
surface1 flows but gently2 the second   
cock1 having a greater weight above it1 
the water is forced out with greater    
velocity1 whilst the lowest cock1 being 
near the bottom of the vessel1 receives 
the pressure of almost the whole body of
 water1 and rushes out with the greatest
 impetuosity4  ,mrs4 ,b4 ,very well2 and
 you must observe1 that as the lateral  
pressure1 is entirely owing to the      
pressure downwards1 it is not affected  
by the horizontal dimensions of the     
vessel1 which contains the water1 but   
merely by its depth2 for as every       
particle acts independently of the rest1
 it is only the column of particles     
immediately above the orifice1 that can 

weigh upon1 and press out the water4    
,emily4 ,the breadth and width of the   
vessel then1 can be of no consequence in
 this respect4 ,the lateral pressure on 
one side1 in a cubical vessel1 is1 ,i   
suppose1 not so great as the pressure   
downwards upon the bottom4  ,mrs4 ,b4   
,no2 in a cubical vessel1 the pressure  
downwards will be double the lateral    
pressure on one side2 for every particle
 at the bottom of the vessel is pressed 
upon1 by a column of the whole depth of 
the fluid1 whilst the lateral pressure  
diminishes from the bottom upwards to   
the surface1 where the particles have no
 pressure4  ,caroline4 ,and from whence 
proceeds the pressure of fluids upwards8
 that seems to me the most              
unaccountable1 as it is in direct       
opposition to gravity4  ,mrs4 ,b4 ,and  
yet it is in consequence of their       
pressure downwards4 ,when1 for example1 
you pour water into a tea-pot1 the water
 rises in the spout1 to a level with the
 water in the pot4 ,the particles of    

water at the bottom of the pot1 are     
pressed upon by the particles above     
them2 to this pressure they will yield1 
if there is any mode of making way for  
the superior particles1 and as they     
cannot descend1 they will change their  
direction1 and rise in the spout4       
,suppose the tea-pot to be filled with  
columns of particles of water1 similar  
to that described in fig4 #d41 the      
particle #a1 at the bottom1 will be     
pressed laterally by the particle #b1   
and by this pressure be forced into the 
spout1 where1 meeting with the particle 
#c1 it presses it upwards1 and this     
pressure will be continued from #c to   
#d1 from #d to #e1 and so on1 till the  
water in the spout1 has risen to a level
 with that in the pot4  ,emily4 ,if it  
were not for this pressure upwards1     
forcing the water to rise in the spout1 
the equilibrium of the fluid would be   
destroyed4  ,caroline4 ,true2 but then a
 tea-pot is wide and large1 and the     
weight of so great a body of water as   

the pot will contain1 may easily force  
up and support so small a quantity1 as  
will fill the spout4 ,but would the same
 effect be produced1 if the spout and   
the pot1 were of equal dimensions8      
,mrs4 ,b4 ,undoubtedly it would4 ,you   
may even reverse the experiment1 by     
pouring water into the spout1 and you   
will find that the water will rise in   
the pot1 to a level with that in the    
spout2 for the pressure of the small    
quantity of water in the spout1 will    
force up and support1 the larger        
quantity in the pot4 ,in the pressure   
upwards1 as well as that laterally1 you 
see that the force of pressure1 depends 
entirely on the height1 and is quite    
independent of the horizontal dimensions
 of the fluid4  ,as a tea-pot is not    
transparent1 let us try the experiment  
by filling this large glass goblet1 by  
means of this narrow tube1 7fig4 #f47   
,caroline4 ,look1 ,emily1 as ,mrs4 ,b4  
fills it1 how the water rises in the    
goblet1 to maintain an equilibrium with 

that in the tube4  ,now1 ,mrs4 ,b41 will
 you let me fill the tube1 by pouring   
water into the goblet8  ,mrs4 ,b4 ,that 
is impossible4 ,however1 you may try the
 experiment1 and ,i doubt not that you  
will be able to account for its failure4
  ,caroline4 ,it is very singular1 that 
if so small a column of water as is     
contained in the tube1 can force up and 
support the whole contents of the       
goblet2 that the weight of all the water
 in the goblet1 should not be able to   
force up the small quantity required to 
fill the tube3--oh1 ,i see now the      
reason1 the water in the goblet1 cannot 
force that in the tube above its level1 
and as the end of the tube1 is          
considerably higher than the goblet1 it 
can never be filled by pouring water    
into the goblet4  ,mrs4 ,b4 ,and if you 
continue to pour water into the goblet  
when it is full1 the water will run     
over1 instead of rising above its level 
in the tube4  ,i shall now explain to   
you the meaning of the specific gravity 

of bodies4  ,caroline4 ,what6 is there  
another species of gravity1 with which  
we are not yet acquainted8  ,mrs4 ,b4   
,no3 the specific gravity of a body1    
means simply its weight1 compared with  
that of another body1 of the same size4 
,when we say1 that substances1 such as  
lead1 and stones1 are heavy1 and that   
others1 such as paper and feathers1 are 
light1 we speak comparatively2 that is  
to say1 that the first are heavy1 and   
the latter light1 in comparison with the
 generality of substances in nature4    
,would you call wood1 and chalk1 light  
or heavy bodies8  ,caroline4 ,some kinds
 of wood are heavy1 certainly1 as oak   
and mahogany2 others are light1 as cedar
 and poplar4  ,emily4 ,i think ,i should
 call wood in general1 a heavy body2 for
 cedar and poplar1 are light1 only in   
comparison to wood of a heavier         
description4 ,i am at a loss to         
determine whether chalk should be ranked
 as a heavy1 or a light body2 ,i should 
be inclined to say the former1 if it was

 not that it is lighter than most other 
minerals4 ,i perceive that we have but  
vague notions of light and heavy4 ,i    
wish there was some standard of         
comparison1 to which we could refer the 
weight of all other bodies4  ,mrs4 ,b4  
,the necessity of such a standard1 has  
been so much felt1 that a body has been 
fixed upon for this purpose4 ,what      
substance do you think would be best    
calculated to answer this end8          
,caroline4 ,it must be one generally    
known1 and easily obtained2 lead or     
iron1 for instance4  ,mrs4 ,b4 ,the     
metals1 would not answer the purpose    
well1 for several reasons2 they are not 
always equally compact1 and they are    
rarely quite pure2 two pieces of iron1  
for instance1 although of the same size1
 might not1 from the causes mentioned1  
weigh exactly alike4  ,caroline4 ,but1  
,mrs4 ,b41 if you compare the weight1 of
 equal quantities of different bodies1  
they will all be alike4 ,you know the   
old saying1 that a pound of feathers1 is

 as heavy as a pound of lead8  ,mrs4 ,b4
 ,when therefore we compare the weight  
of different kinds of bodies1 it would  
be absurd to take quantities of equal   
weight1 we must take quantities of equal
 bulk2 pints or quarts1 not ounces or   
pounds4  ,caroline4 ,very true2 ,i      
perplexed myself by thinking that       
quantity referred to weight1 rather than
 to measure4 ,it is true1 it would be as
 absurd to compare bodies of the same   
size1 in order to ascertain which was   
largest1 as to compare bodies of the    
same weight1 in order to discover which 
was heaviest4  ,mrs4 ,b4 ,in estimating 
the specific gravity of bodies1         
therefore1 we must compare equal bulks1 
and we shall find that their specific   
gravity1 will be proportional to their  
weights4 ,the body which has been       
adopted as a standard of reference1 is  
distilled1 or rain water4  ,emily4 ,i am
 surprised that a fluid should have been
 chosen for this purpose1 as it must    
necessarily be contained in some vessel1

 and the weight of the vessel1 will     
require to be deducted4  ,mrs4 ,b4 ,you 
will find that the comparison will be   
more easily made with a fluid1 than with
 a solid2 and water you know can be     
every where obtained4 ,in order to learn
 the specific gravity of a solid body1  
it is not necessary to put a certain    
measure of it in one scale1 and an equal
 measure of water into the other scale3 
but simply to weigh the body under      
trial1 first in air1 and then in water4 
,if you weigh a piece of gold1 in a     
glass of water1 will not the gold       
displace just as much water1 as is equal
 to its own bulk8  ,caroline4           
,certainly1 where one body is1 another  
cannot be at the same time2 so that a   
sufficient quantity of water must be    
removed1 in order to make way for the   
gold4  ,mrs4 ,b4 ,yes1 a cubic inch of  
water1 to make room for a cubic inch of 
gold2 remember that the bulk1 alone1 is 
to be considered2 the weight1 has       
nothing to do with the quantity of water

 displaced1 for an inch of gold1 does   
not occupy more space1 and therefore    
will not displace more water1 than an   
inch of ivory1 or any other substance1  
that will sink in water4  ,well1 you    
will perhaps be surprised to hear that  
the gold will weigh less in water1 than 
it did out of it8  ,emily4 ,and for what
 reason8  ,mrs4 ,b4 ,on account of the  
upward pressure of the particles of     
water1 which in some measure supports   
the gold1 and by so doing1 diminishes   
its weight4 ,if the body immersed in    
water1 was of the same weight as that   
fluid1 it would be wholly supported by  
it1 just as the water which it          
displaces1 was supported1 previous to   
its making way for the solid body4 ,if  
the body is heavier than the water1 it  
cannot be wholly supported by it2 but   
the water will offer some resistance to 
its descent4  ,caroline4 ,and the       
resistance which water offers to the    
descent of heavy bodies immersed in it1 
7since it proceeds from the upward      

pressure of the particles of the fluid17
 must in all cases1 ,i suppose1 be the  
same8  ,mrs4 ,b4 ,yes3 the resistance of
 the fluid1 is proportioned to the bulk1
 and not to the weight1 of the body     
immersed in it2 all bodies of the same  
size1 therefore1 lose the same quantity 
of their weight in water4 ,can you form 
any idea what this loss will be8        
,emily4 ,i should think it would be     
equal to the weight of the water        
displaced2 for1 since that portion of   
the water was supported before the      
immersion of the solid body1 an equal   
weight of the solid body1 will be       
supported4  ,mrs4 ,b4 ,you are perfectly
 right2 a body weighed in water1 loses  
just as much of its weight1 as is equal 
to that of the water it displaces2 so   
that if you were to put the water       
displaced1 into the scale to which the  
body is suspended1 it would restore the 
balance4  ,you must observe1 that when  
you weigh a body in water1 in order to  
ascertain its specific gravity1 you must

 not sink the dish of the balance in the
 water2 but either suspend the body to a
 hook at the bottom of the dish1 or else
 take off the dish1 and suspend to the  
arm of the balance a weight to          
counterbalance the other dish1 and to   
this attach the solid to be weighed1    
7fig4 #g47 ,now suppose that a cubic    
inch of gold1 weighed #ai ounces out of 
water1 and lost one ounce of its weight 
by being weighed in water1 what would be
 its specific gravity8  ,caroline4 ,the 
cubic inch of water it displaced1 must  
weigh that one ounce2 and as a cubic    
inch of gold1 weighs #ai ounces1 gold is
 #ai times1 as heavy as water4  ,emily4 
,i recollect having seen a table of the 
comparative weights of bodies1 in which 
gold appeared to me to be estimated at  
#ai thousand times1 the weight of water4
  ,mrs4 ,b4 ,you misunderstood the      
meaning of the table4 ,in the estimation
 you allude to1 the weight of water was 
reckoned at #ajjj4 ,you must observe1   
that the weight of a substance when not 

compared to that of any other1 is       
perfectly arbitrary2 and when water is  
adopted as a standard1 we may denominate
 its weight by any number we please2 but
 then the weight of all bodies tried by 
this standard1 must be signified by     
proportional numbers4  ,caroline4 ,we   
may call the weight of water1 for       
example1 one1 and then that of gold1    
would be nineteen2 or if we choose to   
call the weight of water #ajjj1 that of 
gold would be #ai1jjj4 ,in short1       
specific gravity1 means how many times  
more a body weighs1 than an equal bulk  
of water4  ,mrs4 ,b4 ,it is rather the  
weight of a body compared with a portion
 of water equal to it in bulk2 for the  
specific gravity of many substances1 is 
less than that of water4  ,caroline4    
,then you cannot ascertain the specific 
gravity of such substances1 in the same 
manner as that of gold2 for a body that 
is lighter than water1 will float on its
 surface1 without displacing any of it4 
,mrs4 ,b4 ,if a body were absolutely    

without weight1 it is true that it would
 not displace a drop of water1 but the  
bodies we are treating of1 have all some
 weight1 however small2 and will1       
therefore1 displace some quantity4 ,if  
the body be lighter than water1 it will 
not sink to a level with its surface1   
and therefore it will not displace so   
much water as is equal to its bulk2 but 
only so much1 as is equal to its weight4
 ,a ship1 you must have observed1 sinks 
to some depth in water1 and the heavier 
it is laden1 the deeper it sinks1 as it 
always displaces a quantity of water1   
equal to its own weight4  ,caroline4    
,but you said just now1 that in the     
immersion of gold1 the bulk1 and not the
 weight of body1 was to be considered4  
,mrs4 ,b4 ,that is the case with all    
substances which are heavier than water2
 but since those which are lighter1 do  
not displace so much as their own bulk1 
the quantity they displace is not a test
 of their specific gravity4  ,in order  
to obtain the specific gravity of a body

 which is lighter than water1 you must  
attach to it a heavy one1 whose specific
 gravity is known1 and immerse them     
together2 the specific gravity of the   
lighter body1 may then be easily        
calculated from observing the loss of   
weight it produces1 in the heavy body4  
,emily4 ,but are there not some bodies  
which have exactly the same specific    
gravity as water8  ,mrs4 ,b4            
,undoubtedly2 and such bodies will      
remain at rest in whatever situation    
they are placed in water4 ,here is a    
piece of wood which ,i have procured1   
because it is of a kind which is        
precisely the weight of an equal bulk of
 water2 in whatever part of this vessel 
of water you place it1 you will find    
that it will remain stationary4         
,caroline4 ,i shall first put it at the 
bottom2 from thence1 of course1 it      
cannot rise1 because it is not lighter  
than water4 ,now ,i shall place it in   
the middle of the vessel2 it neither    
rises nor sinks1 because it is neither  

lighter nor heavier than the water4 ,now
 ,i will lay it on the surface of the   
water2 but there it sinks a little--what
 is the reason of that1 ,mrs4 ,b48      
,mrs4 ,b4 ,since it is not lighter than 
the water1 it cannot float upon its     
surface2 since it is not heavier than   
water1 it cannot sink below its surface3
 it will sink therefore1 only till the  
upper surface of both bodies are on a   
level1 so that the piece of wood is just
 covered with water4 ,if you poured a   
few drops of water into the vessel1 7so 
gently as not to give them momentum7    
they would mix with the water at the    
surface1 and not sink lower4  ,caroline4
 ,i now understand the reason1 why1 in  
drawing up a bucket of water out of a   
well1 the bucket feels so much heavier  
when it rises above the surface of the  
water in the well2 for whilst you raise 
it in the water1 the water within the   
bucket being of the same specific       
gravity as the water on the outside1    
will be wholly supported by the upward  

pressure of the water beneath the       
bucket1 and consequently very little    
force will be required to raise it2 but 
as soon as the bucket rises to the      
surface of the well1 you immediately    
perceive the increase of weight4        
,emily4 ,and how do you ascertain the   
specific gravity of fluids8  ,mrs4 ,b4  
,by means of an hydrometer2 this        
instrument is made of various materials1
 and in different forms1 one of which ,i
 will show you4 ,it consists of a thin  
brass ball ,a1 7fig4 #h1 plate #ac47    
with a graduated tube ,b1 and the       
specific gravity of the liquid1 is      
estimated by the depth to which the     
instrument sinks in it1 or by the weight
 required to sink it to a given depth4  
,there is a small bucket ,c1 suspended  
at the lower end1 and also a little dish
 on the graduated tube2 into either of  
these1 small weights may be put1 until  
the instrument sinks in the fluid1 to a 
mark on the tube ,b2 the amount of      
weight necessary for this1 will enable  

you to discover the specific gravity of 
the fluid4  ,i must now take leave of   
you2 but there remain yet many          
observations to be made on fluids3 we   
shall1 therefore1 resume this subject at
 our next interview4   ,questions  #a4  
7,pg4 #aah7 ,what are the two divisions 
of the science which treats of the      
mechanical properties of liquids8  #b4  
7,pg4 #aah7 ,of what do hydrostatics and
 hydraulics treat8  #c4 7,pg4 #aah7     
,what is a fluid defined to be8  #d4    
7,pg4 #aah7 ,from what is fluidity      
supposed to arise8  #e4 7,pg4 #aah7     
,into what two classes are fluids       
divided8  #f4 7,pg4 #aai7 ,what is said 
of the incompressibility of liquids1 and
 what experiment is related8  #g4 7,pg4 
#aai7 ,ought this experiment to be      
considered as conclusive8  #h4 7,pg4    
#aai7 ,why do fluids appear to gravitate
 more freely than solids8  #i4 7,pg4    
#abj7 ,when is a fluid said to be in    
equilibrium8  #aj4 7,pg4 #abj7 ,what is 
there in the nature of a fluid1 which   

causes it to seek this level8  #aa4     
7,pg4 #abj7 ,what circumstances occasion
 oil to float upon water8  #ab4 7,pg4   
#abj7 ,what is the nature and use of the
 instrument represented in fig4 #a1     
plate #ac8  #ac4 7,pg4 #abj7 ,what      
difference is there in the gravitation  
of solid masses1 and of fluids8  #ad4   
7,pg4 #aba7 ,what results as regards the
 pressure of fluids8  #ae4 7,pg4 #aba7  
,how is this illustrated by fig4 #b1 #c1
 plate #ac8  #af4 7,pg4 #aba7 ,from what
 does the lateral pressure proceed8 and 
to what is it proportioned1 as          
exemplified in fig4 #e1 plate #ac8  #ag4
 7,pg4 #abb7 ,has the extent of the     
surface of a fluid1 any effect upon its 
pressure downwards8  #ah4 7,pg4 #abb7   
,what will be the difference between the
 pressure upon the bottom1 and upon one 
side of a cubical vessel8  #ai4 7,pg4   
#abb7 ,what occasions the upward        
pressure1 and how is it explained by    
fig4 #d1 plate #ac8  #bj4 7,pg4 #abc7   
,how could the equilibrium of fluids be 

exemplified by pouring water in at the  
spout of a tea-pot8  #ba4 7,pg4 #abc7   
,how by the apparatus represented at    
fig4 #f1 plate #ac8  #bb4 7,pg4 #abc7   
,what is meant by the specific gravity  
of a body8  #bc4 7,pg4 #abc7 ,what do we
 in common mean by calling a body heavy1
 or light8  #bd4 7,pg4 #abd7 ,why would 
not the metals answer to compare other  
bodies with8  #be4 7,pg4 #abd7 ,what    
must be supposed equal in estimating the
 specific gravity of a body8  #bf4 7,pg4
 #abd7 ,what has been adopted as a      
standard for comparison8  #bg4 7,pg4    
#abe7 ,what is the first step in        
ascertaining the specific gravity of a  
solid8  #bh4 7,pg4 #abe7 ,what quantity 
of water will the solid displace8  #bi4 
7,pg4 #abe7 ,why will a solid weigh less
 in water than in air1 and to what will 
the loss of weight be equal8  #cj4 7,pg4
 #abf7 ,what is the arrangement         
represented by fig4 #g1 plate #ac8  #ca4
 7,pg4 #abf7 ,what is stated of gold as 
an example8  #cb4 7,pg4 #abf7 ,in       

comparing a body with water1 this is    
sometimes called #ajjj1 what must be    
observed8  #cc4 7,pg4 #abf7 ,what       
quantity of water is displaced1 by a    
body floating upon its surface8  #cd4   
7,pg4 #abg7 ,how can you find the       
specific gravity of a solid which is    
lighter than water8  #ce4 7,pg4 #abg7   
,what is observed of a body whose       
specific gravity is the same as that of 
water8  #cf4 7,pg4 #abg7 ,what is the   
reason that in drawing a bucket of water
 from a well1 its weight is not         
perceived until it rises above the      
surface8  #cg4 7,pg4 #abh7 ,describe the
 instrument represented by fig4 #h1     
plate #ac1 and also how1 and for what it
 is used8     ,,conversation ,,xi4  ,,of
 ,,springs1 ,,fountains1 c4  ,,of ,,the 
,,ascent ,,of ,,vapour ,,and ,,the      
,,formation ,,of ,,clouds4 ,,of ,,the   
,,formation ,,and ,,fall ,,of ,,rain1 c4
 ,,of ,,the ,,formation ,,of ,,springs4 
,,of ,,rivers ,,and ,,lakes4 ,,of       
,,fountains4   ,,caroline4  ,there is a 

question ,i am very desirous of asking  
you1 respecting fluids1 ,mrs4 ,b41 which
 has often perplexed me4 ,what is the   
reason that the great quantity of rain  
which falls upon the earth and sinks    
into it1 does not1 in the course of     
time1 injure its solidity8 ,the sun and 
the wind1 ,i know1 dry the surface1 but 
they have no effect on the interior     
parts1 where there must be a prodigious 
accumulation of moisture4  ,mrs4 ,b4 ,do
 you not know1 that1 in the course of   
time1 all the water which sinks into the
 ground1 rises out of it again8 ,it is  
the same water which successively forms 
seas1 rivers1 springs1 clouds1 rain1 and
 sometimes hail1 snow and ice4 ,if you  
will take the trouble of following it   
through these various changes1 you will 
understand why the earth is not yet     
drowned1 by the quantity of water which 
has fallen upon it1 since its creation2 
and you will even be convinced1 that it 
does not contain a single drop more     
water now1 than it did at that period4  

,let us consider how the clouds were    
originally formed4 ,when the first rays 
of the sun warmed the surface of the    
earth1 the heat1 by separating the      
particles of water1 rendered them       
lighter than the air4 ,this1 you know1  
is the case with steam or vapour4 ,what 
then ensues8  ,caroline4 ,when lighter  
than the air1 it will naturally rise2   
and now ,i recollect your telling us in 
a preceding lesson1 that the heat of the
 sun transformed the particles of water 
into vapour2 in consequence of which1 it
 ascended into the atmosphere1 where it 
formed clouds4  ,mrs4 ,b4 ,we have then 
already followed water through two of   
its transformations2 from water it      
becomes vapour1 and from vapour clouds4 
,emily4 ,but since this watery vapour is
 lighter than the air1 why does it not  
continue to rise2 and why does it unite 
again1 to form clouds8  ,mrs4 ,b4       
,because the atmosphere diminishes in   
density1 as it is more distant from the 
earth4 ,the vapour1 therefore1 which the

 sun causes to exhale1 not only from    
seas1 rivers1 and lakes1 but likewise   
from the moisture on the land1 rises    
till it reaches a region of air of its  
own specific gravity2 and there1 you    
know1 it will remain stationary4 ,by the
 frequent accession of fresh vapour1 it 
gradually accumulates1 so as to form    
those large bodies of vapour1 which we  
call clouds3 and the particles1 at      
length uniting1 become too heavy for the
 air to support1 and fall to the ground4
  ,caroline4 ,they do fall to the       
ground1 certainly1 when it rains2 but1  
according to your theory1 ,i should have
 imagined1 that when the clouds became  
too heavy1 for the region of air in     
which they were situated1 to support    
them1 they would descend1 till they     
reached a stratum of air of their own   
weight1 and not fall to the earth2 for  
as clouds are formed of vapour1 they    
cannot be so heavy as the lowest regions
 of the atmosphere1 otherwise the vapour
 would not have risen4  ,mrs4 ,b4 ,if   

you examine the manner in which the     
clouds descend1 it will obviate this    
objection4 ,in falling1 several of the  
watery particles come within the sphere 
of each other's attraction1 and unite in
 the form of a drop of water4 ,the      
vapour thus transformed into a shower1  
is heavier than any part of the         
atmosphere1 and consequently descends to
 the earth4  ,caroline4 ,how wonderfully
 curious6  ,mrs4 ,b4 ,it is impossible  
to consider any part of nature          
attentively1 without being struck with  
admiration at the wisdom it displays2   
and ,i hope you will never contemplate  
these wonders1 without feeling your     
heart glow with admiration and          
gratitude1 towards their bounteous      
,author4 ,observe1 that if the waters   
were never drawn out of the earth1 all  
vegetation would be destroyed by the    
excess of moisture2 if1 on the other    
hand1 the plants were not nourished and 
refreshed by occasional showers1 the    
drought would be equally fatal to them4 

,if the clouds constantly remained in a 
state of vapour1 they might1 as you     
remarked1 descend into a heavier stratum
 of the atmosphere1 but could never fall
 to the ground2 or were the power of    
attraction more than sufficient to      
convert the vapour into drops1 it would 
transform the cloud into a mass of      
water1 which1 instead of nourishing1    
would destroy the produce of the earth4 
,water then ascends in the form of      
vapour1 and descends in that of rain1   
snow1 or hail1 all of which ultimately  
become water4 ,some of this falls into  
the various bodies of water on the      
surface of the globe1 the remainder upon
 the land4 ,of the latter1 part         
reascends in the form of vapour1 part is
 absorbed by the roots of vegetables1   
and part descends into the earth1 where 
it forms springs4  ,emily4 ,is there    
then no difference between rain water1  
and spring water8  ,mrs4 ,b4 ,they are  
originally the same2 but that portion of
 rain water which goes to supply        

springs1 dissolves a number of foreign  
particles1 which it meets with in its   
passage through the various soils it    
traverses4  ,caroline4 ,yet spring water
 is more pleasant to the taste1 appears 
more transparent1 and1 ,i should have   
supposed1 would have been more pure than
 rain water4  ,mrs4 ,b4 ,no2 excepting  
distilled water1 rain water is the most 
pure we can obtain2 it is its purity    
which renders it insipid2 whilst the    
various salts and different ingredients1
 dissolved in spring water1 give it a   
species of flavour1 which habit renders 
agreeable2 these salts do not1 in any   
degree1 affect its transparency2 and the
 filtration it undergoes1 through gravel
 and sand1 cleanses it from all foreign 
matter1 which it has not the power of   
dissolving4  ,emily4 ,how is it that the
 rain water does not continue to descend
 by its gravity1 instead of collecting  
together1 and forming springs8  ,mrs4   
,b4 ,when rain falls on the surface of  
the earth1 it continues making its way  

downwards through the pores and crevices
 in the ground4 ,when several drops meet
 in their subterraneous passage1 they   
unite and form a little rivulet2 this1  
in its progress1 meets with other       
rivulets of a similar description1 and  
they pursue their course together within
 the earth1 till they are stopped by    
some substance1 such as rock1 or clay1  
which they cannot penetrate4  ,caroline4
 ,but you say that there is some reason 
to believe that water can penetrate even
 the pores of gold1 and it cannot meet  
with a substance more dense8  ,mrs4 ,b4 
,but if water penetrate the pores of    
gold1 it is only when under a strong    
compressive force1 as in the ,florentine
 experiment2 now in its passage towards 
the centre of the earth1 it is acted    
upon by no other power than gravity1    
which is not sufficient to make it force
 its way1 even through a stratum of     
clay4 ,this species of earth1 though not
 remarkably dense1 being of great       
tenacity1 will not admit the particles  

of water to pass4 ,when water encounters
 any substance of this nature1          
therefore1 its progress is stopped1 and 
it is diffused through the porous earth1
 and sometimes the pressure of the      
accumulating waters1 forms a bed1 or    
reservoir4 ,this will be more clearly   
explained by fig4 #i1 plate #ac1 which  
represents a section1 of the interior of
 a hill or mountain4 ,a1 is a body of   
water1 such as ,i have described1 which1
 when filled up as high as ,b1 7by the  
continual accession of water it receives
 from the ducts or rivulets a1 a1 a1 a17
 finds a passage out of the cavity1 and1
 impelled by gravity1 it runs on1 till  
it makes its way out of the ground at   
the side of the hill1 and there forms a 
spring1 ,c4  ,caroline4 ,gravity impels 
downwards towards the centre of the     
earth2 and the spring in this figure    
runs in an horizontal direction4  ,mrs4 
,b4 ,not entirely4 ,there is some       
declivity from the reservoir1 to the    
spot where the water issues out of the  

ground2 and gravity1 you know1 will     
bring bodies down an inclined plane1 as 
well as in a perpendicular direction4   
,caroline4 ,but though the spring may   
descend1 on first issuing1 it must      
afterwards rise to reach the surface of 
the earth2 and that is in direct        
opposition to gravity4  ,mrs4 ,b4 ,a    
spring can never rise above the level of
 the reservoir whence it issues2 it     
must1 therefore1 find a passage to some 
part of the surface of the earth1 that  
is lower1 or nearer the centre1 than the
 reservoir4 ,it is true that1 in this   
figure1 the spring rises in its passage 
from ,b to ,c2 but this1 ,i think1 with 
a little reflection1 you will be able to
 account for4  ,emily4 ,oh1 yes2 it is  
owing to the pressure of fluids upwards2
 and the water rises in the duct1 upon  
the same principle as it rises in the   
spout of a tea-pot2 that is to say1 in  
order to preserve an equilibrium with   
the water in the reservoir4 ,now ,i     
think ,i understand the nature of       

springs3 the water will flow through a  
duct1 whether ascending or descending1  
provided it never rises higher than the 
reservoir4  ,mrs4 ,b4 ,water may thus be
 conveyed to every part of a town1 and  
to the upper part of the houses1 if it  
is originally brought from a height1    
superior to any to which it is conveyed4
 ,have you never observed1 when the     
pavements of the streets have been      
mending1 the pipes which serve as ducts 
for the conveyance of the water through 
the town8  ,emily4 ,yes1 frequently2 and
 ,i have remarked that when any of these
 pipes have been opened1 the water      
rushes upwards from them1 with great    
velocity2 which1 ,i suppose1 proceeds   
from the pressure of the water in the   
reservoir1 which forces it out4         
,caroline4 ,i recollect having once seen
 a very curious glass1 called           
,tantalus's cup2 it consists of a       
goblet1 containing a small figure of a  
man1 and whatever quantity of water you 
pour into the goblet1 it never rises    

higher than the breast of the figure4   
,do you know how that is contrived8     
,mrs4 ,b4 ,it is by means of a syphon1  
or bent tube1 which is concealed in the 
body of the figure4 ,this tube rises    
through one of the legs1 as high as the 
breast1 and there turning1 descends     
through the other leg1 and from thence  
through the foot of the goblet1 where   
the water runs out4 7fig4 #a1 plate     
#ad47 ,when you pour water into the     
glass ,a1 it must rise in the syphon ,b1
 in proportion as it rises in the glass2
 and when the glass is filled to a level
 with the upper part of the syphon1 the 
water will run out through the other leg
 of the figure1 and will continue       
running out1 as fast as you pour it in2 
therefore the glass can never fill any  
higher4  ,emily4 ,i think the new well  
that has been made at our country-house1
 must be of that nature4 ,we had a great
 scarcity of water1 and my father has   
been at considerable expense to dig a   
well2 after penetrating to a great      

depth1 before water could be found1 a   
spring was at length discovered1 but the
 water rose only a few feet above the   
bottom of the well2 and sometimes it is 
quite dry4  ,illustration3 ,,plate      
,,xiv4  ,mrs4 ,b4 ,this has1 however1 no
 analogy to ,tantalus's cup2 but is     
owing to the very elevated situation of 
your country-house4  ,emily4 ,i believe 
,i guess the reason4 ,there cannot be a 
reservoir of water near the summit of a 
hill2 as in such a situation1 there will
 not be a sufficient number of rivulets 
formed1 to supply one2 and without a    
reservoir1 there can be no spring4 ,in  
such situations1 therefore1 it is       
necessary to dig very deep1 in order to 
meet with a spring2 and when we give it 
vent1 it can rise only as high as the   
reservoir from whence it flows1 which   
will be but little1 as the reservoir    
must be situated at some considerable   
depth below the summit of the hill4     
,caroline4 ,your explanation appears    
very clear and satisfactory2 but ,i can 

contradict it from experience4 ,at the  
very top of a hill1 near our            
country-house1 there is a large pond1   
and1 according to your theory1 it would 
be impossible there should be springs in
 such a situation to supply it with     
water4 ,then you know that ,i have      
crossed the ,alps1 and ,i can assure    
you1 that there is a fine lake on the   
summit of ,mount ,cenis1 the highest    
mountain we passed over4  ,mrs4 ,b4     
,were there a lake on the summit of     
,mount ,blanc1 which is the highest of  
the ,alps1 it would indeed be wonderful4
 ,but that on ,mount ,cenis1 is not at  
all contradictory to our theory of      
springs2 for this mountain is surrounded
 by others1 much more elevated1 and the 
springs which feed the lake must descend
 from reservoirs of water1 formed in    
those mountains4 ,this must also be the 
case with the pond on the top of the    
hill2 there is doubtless some more      
considerable hill in the neighbourhood1 
which supplies it with water4  ,emily4  

,i comprehend perfectly1 why the water  
in our well never rises high3 but ,i do 
not understand why it should            
occasionally be dry4  ,mrs4 ,b4 ,because
 the reservoir from which it flows1     
being in an elevated situation1 is but  
scantily supplied with water2 after a   
long drought1 therefore1 it may be      
drained1 and the spring dry1 till the   
reservoir be replenished by fresh rains4
 ,it is not uncommon to see springs flow
 with great violence in wet seasons1    
which at other times1 are perfectly dry4
  ,caroline4 ,but there is a spring in  
our grounds1 which more frequently flows
 in dry1 than in wet weather2 how is    
that to be accounted for8  ,mrs4 ,b4    
,the spring1 probably1 comes from a     
reservoir at a great distance1 and      
situated very deep in the ground3 it is1
 therefore1 some length of time before  
the rain reaches the reservoir2 and     
another considerable portion must       
elapse1 whilst the water is making its  
way1 from the reservoir1 to the surface 

of the earth2 so that the dry weather   
may probably have succeeded the rains1  
before the spring begins to flow2 and   
the reservoir may be exhausted1 by the  
time the wet weather sets in again4     
,caroline4 ,i doubt not but this is the 
case1 as the spring is in a very low    
situation1 therefore1 the reservoir may 
be at a great distance from it4  ,mrs4  
,b4 ,springs which do not constantly    
flow1 are called intermitting1 and are  
occasioned by the reservoir being       
imperfectly supplied4 ,independently of 
the situation1 this is always the case1 
when the duct1 or ducts1 which convey   
the water into the reservoir1 are       
smaller than those which carry it off4  
,caroline4 ,if it runs out1 faster than 
it runs in1 it will of course sometimes 
be empty4 ,do not rivers also1 derive   
their source from springs8  ,mrs4 ,b4   
,yes1 they generally take their source  
in mountainous countries1 where springs 
are most abundant4  ,caroline4 ,i       
understood you that springs were more   

rare1 in elevated situations4  ,mrs4 ,b4
 ,you do not consider that mountainous  
countries1 abound equally with high1 and
 low situations4 ,reservoirs of water1  
which are formed in the bosoms of       
mountains1 generally find a vent1 either
 on their declivity1 or in the valley   
beneath2 while subterraneous reservoirs1
 formed in a plain1 can seldom find a   
passage to the surface of the earth1 but
 remain concealed1 unless discovered by 
digging a well4 ,when a spring once     
issues at the surface of the earth1 it  
continues its course externally1 seeking
 always a lower ground1 for it can no   
longer rise4  ,emily4 ,then what is the 
consequence1 if the spring1 or1 as ,i   
should now rather call it1 the rivulet1 
runs into a situation1 which is         
surrounded by higher ground8  ,mrs4 ,b4 
,its course is stopped2 the water       
accumulates1 and it forms a pool1 pond1 
or lake1 according to the dimensions of 
the body of water4 ,the lake of ,geneva1
 in all probability1 owes its origin to 

the ,rhone1 which passes through it3 if1
 when the river first entered the       
valley1 which now forms the bed of the  
,lake1 it found itself surrounded by    
higher grounds1 its waters would there  
accumulate1 till they rose to a level   
with that part of the valley1 where the 
,rhone now continues its course beyond  
the ,lake1 and from whence it flows     
through valleys1 occasionally forming   
other small lakes1 till it reaches the  
sea4  ,emily4 ,and are not fountains1 of
 the nature of springs8  ,mrs4 ,b4      
,exactly4 ,a fountain is conducted      
perpendicularly upwards1 by the spout or
 adjutage ,a1 through which it flows2   
and it will rise nearly as high as the  
reservoir ,b1 from whence it proceeds4  
7,plate #ad4 fig4 #b47  ,caroline4 ,why 
not quite as high8  ,mrs4 ,b4 ,because  
it meets with resistance from the air1  
in its ascent2 and its motion is impeded
 by friction against the spout1 where it
 rushes out4  ,emily4 ,but if the tube  
through which the water rises be smooth1

 can there be any friction8 especially  
with a fluid1 whose particles yield to  
the slightest impression4  ,mrs4 ,b4    
,friction1 7as we observed in a former  
lesson17 may be diminished by polishing1
 but can never be entirely destroyed2   
and though fluids1 are less susceptible 
of friction1 than solid bodies1 they are
 still affected by it4 ,another reason  
why a fountain will not rise so high as 
its reservoir1 is1 that as all the water
 which spouts up1 has to descend again1 
it in doing so1 presses1 or strikes     
against the under parts1 and forces them
 sideways1 spreading the column into a  
head1 and rendering it both wider1 and  
shorter1 than it otherwise would be4    
,at our next meeting1 we shall examine  
the mechanical properties of the air1   
which being an elastic fluid1 differs in
 many respects1 from liquids4           
,questions  #a4 7,pg4 #abi7 ,why do not 
the frequent rains1 fill the earth with 
water8  #b4 7,pg4 #abi7 ,why will vapour
 rise8 to what height will it ascend1   

and what will it form8  #c4 7,pg4 #abi7 
,how may drops of rain be formed8  #d4  
7,pg4 #acj7 ,what becomes of the water  
after it has fallen to the earth8  #e4  
7,pg4 #acj7 ,what is the difference     
between rain water1 and that from       
springs8  #f4 7,pg4 #acj7 ,why is rain  
more pure than spring water8  #g4 7,pg4 
#acj7 ,why is spring water more         
agreeable to the palate8  #h4 7,pg4     
#aca7 ,what causes the water to collect 
and form springs8  #i4 7,pg4 #aca7 ,why 
cannot water penetrate through clay8    
#aj4 7,pg4 #aca7 ,what is represented by
 fig4 #i1 plate #ac8  #aa4 7,pg4 #acb7  
,how can you account for its rising     
upwards1 as represented at ,c8  #ab4    
7,pg4 #acb7 ,in conveying water by means
 of pipes1 how must the reservoir be    
situated8  #ac4 7,pg4 #acb7 ,what is the
 instrument called1 which is represented
 in plate #ad1 fig4 #a1--;and how does  
it operate8  #ad4 7,pg4 #acc7 ,why are  
wells rarely well supplied with water1  
in elevated situations8  #ae4 7,pg4     

#acc7 ,when water is found in elevated  
situations1 whence is it supplied8  #af4
 7,pg4 #acc7 ,wells and springs1 at some
 periods well supplied1 fail at others2 
how is this accounted for8  #ag4 7,pg4  
#acd7 ,some springs flow abundantly in  
dry weather1 which occasionally fail in 
wet weather1 how may this be explained8 
#ah4 7,pg4 #acd7 ,what is meant by      
intermitting springs8  #ai4 7,pg4 #acd7 
,whence do rivers1 in general1 derive   
their water8  #bj4 7,pg4 #acd7 ,why do  
springs abound more in mountainous1 than
 in level countries8  #ba4 7,pg4 #ace7  
,how are lakes formed8  #bb4 7,pg4 #ace7
 ,what causes water to rise in          
fountains1 and how is this explained by 
figure #b1 plate #ad8  #bc4 7,pg4 #ace7 
,why will not the fountain rise to the  
height of the water in the reservoir8   
,,conversation ,,xii4  ,,on ,,the       
,,mechanical ,,properties ,,of ,,air4   
,,of ,,the ,,spring ,,or ,,elasticity   
,,of ,,the ,,air4 ,,of ,,the ,,weight   
,,of ,,the ,,air4 ,,experiments ,,with  

,,the ,,air ,,pump4 ,,of ,,the          
,,barometer4 ,,mode ,,of ,,weighing     
,,air4 ,,specific ,,gravity ,,of ,,air4 
,,of ,,pumps4 ,,description ,,of ,,the  
,,sucking ,,pump4 ,,description ,,of    
,,the ,,forcing ,,pump4   ,,mrs4 ,b4    
,at our last meeting we examined the    
properties of fluids in general1 and    
more particularly of such as are called 
non-elastic fluids1 or liquids4  ,there 
is another class of fluids1             
distinguished by the name of aeriform1  
or elastic fluids1 the principal of     
which is the air we breathe1 which      
surrounds the earth1 and is called the  
atmosphere4  ,emily4 ,there are then    
other kinds of air1 besides the         
atmosphere8  ,mrs4 ,b4 ,yes2 a great    
variety2 but they differ only in their  
chemical1 and not in their mechanical   
properties2 and as it is the latter we  
are to examine1 we shall not at present 
inquire into their composition1 but     
confine our attention to the mechanical 
properties of elastic fluids in general4

  ,caroline4 ,and from whence arises    
this difference1 between elastic1 and   
non-elastic fluids8  ,mrs4 ,b4 ,there is
 no attraction of cohesion1 between the 
particles of elastic fluids2 so that the
 expansive power of heat1 has no        
adversary to contend with1 but gravity2 
any increase of temperature1 therefore1 
expands elastic fluids considerably1 and
 a diminution1 proportionally condenses 
them4  ,the most essential point1 in    
which air1 differs from other fluids is 
in its spring or elasticity2 that is to 
say1 its power of increasing1 or        
diminishing in bulk1 accordingly as it  
is more1 or less1 compressed3 a power of
 which ,i have informed you1 liquids are
 almost wholly deprived4  ,emily4 ,i    
think ,i understand the elasticity of   
the air very well from what you formerly
 said of it2 but what perplexes me is1  
its having gravity2 if it is heavy1 and 
we are surrounded by it1 why do we not  
feel its weight8  ,caroline4 ,it must be
 impossible to be sensible of the weight

 of such infinitely small particles1 as 
those of which the air is composed3     
particles which are too small to be     
seen1 must be too light to be felt4     
,mrs4 ,b4 ,you are mistaken1 my dear2   
the air is much heavier than you        
imagine2 it is true1 that the particles 
which compose it1 are small2 but then1  
reflect on their quantity3 the          
atmosphere extends in height1 a great   
number of miles from the earth1 and its 
gravity is such1 that a man of middling 
stature1 is computed 7when the air is   
heaviest7 to sustain the weight of about
 #ad tons4  ,caroline4 ,is it possible6 
,i should have thought such a weight    
would have crushed any one to atoms4    
,mrs4 ,b4 ,that would1 indeed1 be the   
case1 if it were not for the equality of
 the pressure1 on every part of the     
body2 but when thus diffused1 we can    
bear even a much greater weight1 without
 any considerable inconvenience4 ,in    
bathing we support the weight and       
pressure of the water1 in addition to   

that of the atmosphere2 but because this
 pressure is equally distributed over   
the body1 we are scarcely sensible of   
it2 whilst if your shoulders1 your head1
 or any particular part of your frame1  
were loaded with the additional weight  
of a hundred pounds1 you would soon sink
 under the fatigue4 ,besides this1 our  
bodies contain air1 the spring of which1
 counterbalances the weight of the      
external air1 and renders us insensible 
of its pressure4  ,caroline4 ,but if it 
were possible to relieve me from the    
weight of the atmosphere1 should ,i not 
feel more light and agile8  ,mrs4 ,b4   
,on the contrary1 the air within you1   
meeting with no external pressure to    
restrain its elasticity1 would distend  
your body1 and at length bursting some  
of the parts which confined it1 put a   
period to your existence4  ,caroline4   
,this weight of the atmosphere1 then1   
which ,i was so apprehensive would crush
 me1 is1 in reality1 essential to my    
preservation4  ,emily4 ,i once saw a    

person cupped1 and was told that the    
swelling of the part under the cup1 was 
produced by taking away from that part1 
the pressure of the atmosphere2 but ,i  
could not understand how this pressure  
produced such an effect4  ,mrs4 ,b4 ,the
 air pump affords us the means of making
 a great variety of interesting         
experiments1 on the weight1 and pressure
 of the air3 some of them you have      
already seen4 ,do you not recollect1    
that in a vacuum produced within the air
 pump1 substances of various weights1   
fell to the bottom in the same time2 why
 does not this happen in the atmosphere8
  ,caroline4 ,i remember you told us it 
was owing to the resistance which light 
bodies meet with1 from the air1 during  
their fall4  ,mrs4 ,b4 ,or1 in other    
words1 to the support which they        
received from the air1 and which        
prolonged the time of their fall4 ,now1 
if the air were destitute of weight1 how
 could it support other bodies1 or      
retard their fall8  ,i shall now show   

you some other experiments1 which       
illustrate1 in a striking manner1 both  
the weight1 and elasticity of air4 ,i   
shall tie a piece of bladder over this  
glass receiver1 which1 you will observe1
 is open at the top as well as below4   
,caroline4 ,why do you wet the bladder  
first8  ,mrs4 ,b4 ,it expands by        
wetting1 and contracts in drying2 it is 
also more soft and pliable when wet1 so 
that ,i can make it fit better1 and when
 dry1 it will be tighter4 ,we must hold 
it to the fire in order to dry it2 but  
not too near1 lest it should burst by   
sudden contraction4 ,let us now fix it  
on the air pump1 and exhaust the air    
from underneath it--you will not be     
alarmed if you hear a noise8  ,emily4   
,it was as loud as the report of a gun1 
and the bladder is burst6 ,pray explain 
how the air is concerned in this        
experiment4  ,mrs4 ,b4 ,it is the effect
 of the weight of the atmosphere1 on the
 upper surface of the bladder1 when ,i  
had taken away the air from the under   

surface1 so that there was no longer any
 reaction to counterbalance the pressure
 of the atmosphere1 on the receiver4    
,you observed how the bladder was       
pressed inwards1 by the weight of the   
external air1 in proportion as ,i       
exhausted the receiver3 and before a    
complete vacuum was formed1 the bladder1
 unable to sustain the violence of the  
pressure1 burst with the explosion you  
have just heard4  ,i shall now show you 
an experiment1 which proves the         
expansion of the air1 contained within a
 body1 when it is relieved from the     
pressure of the external air4 ,you would
 not imagine that there was any air     
contained within this shrivelled apple1 
by its appearance2 but take notice of it
 when placed within a receiver1 from    
which ,i shall exhaust the air4         
,caroline4 ,how strange6 it grows quite 
plump1 and looks like a fresh-gathered  
apple4  ,mrs4 ,b4 ,but as soon as ,i let
 the air again into the receiver1 the   
apple1 you see1 returns to its          

shrivelled state4 ,when ,i took away the
 pressure of the atmosphere1 the air    
within the apple1 expanded1 and swelled 
it out2 but the instant the atmospheric 
air was restored1 the expansion of the  
internal air1 was checked and repressed1
 and the apple shrunk to its former     
dimensions4  ,you may make a similar    
experiment with this little bladder1    
which you see is perfectly flaccid1 and 
appears to contain no air3 in this state
 ,i shall tie up the neck of the        
bladder1 so that whatever air remains   
within it1 may not escape1 and then     
place it under the receiver4 ,now       
observe1 as ,i exhaust the receiver1 how
 the bladder distends2 this proceeds    
from the great dilatation of the small  
quantity of air1 which was enclosed     
within the bladder1 when ,i tied it up2 
but as soon as ,i let the air into the  
receiver1 that which the bladder        
contains1 condenses and shrinks into its
 small compass1 within the folds of the 
bladder4  ,emily4 ,these experiments are

 extremely amusing1 and they afford     
clear proofs1 both of the weight1 and   
elasticity of the air2 but ,i should    
like to know1 exactly1 how much the air 
weighs4  ,mrs4 ,b4 ,a column of air     
reaching to the top of the atmosphere1  
and whose base is a square inch1 weighs 
about #ae lbs4 therefore1 every square  
inch of our bodies1 sustains a weight of
 #ae lbs43 and if you wish to know the  
weight of the whole of the atmosphere1  
you must reckon how many square inches  
there are on the surface of the globe1  
and multiply them by #ae4  ,emily4 ,but 
can we not ascertain the weight of a    
small quantity of air8  ,mrs4 ,b4 ,with 
perfect ease4 ,i shall exhaust the air  
from this little bottle1 by means of the
 air pump3 and having emptied the bottle
 of air1 or1 in other words1 produced a 
vacuum within it1 ,i secure it by       
turning this screw adapted to its neck3 
we may now find the exact weight of this
 bottle1 by putting it into one of the  
scales of a balance4 ,it weighs1 you    

see1 just two ounces2 but when ,i turn  
the screw1 so as to admit the air into  
the bottle1 the scale which contains it1
 preponderates4  ,caroline4 ,no doubt   
the bottle filled with air1 is heavier  
than the bottle void of air2 and the    
additional weight required to bring the 
scales again to a balance1 must be      
exactly that of the air which the bottle
 now contains4  ,mrs4 ,b4 ,that weight1 
you see1 is almost two grains4 ,the     
dimensions of this bottle1 are six cubic
 inches4 ,six cubic inches of air1      
therefore1 at the temperature of this   
room1 weighs nearly #b grains4          
,caroline4 ,why do you observe the      
temperature of the room1 in estimating  
the weight of the air8  ,mrs4 ,b4       
,because heat rarefies air1 and renders 
it lighter2 therefore the warmer the air
 is1 which you weigh1 the lighter it    
will be4  ,if you should now be desirous
 of knowing the specific gravity of this
 air1 we need only fill the same bottle1
 with water1 and thus obtain the weight 

of an equal quantity of water--which you
 see is #aeae grs42 now by comparing the
 weight of water1 to that of air1 we    
find it to be in the proportion of about
 #hjj to #a4  ,as you are acquainted    
with decimal arithmetic1 you will       
understand what ,i mean1 when ,i tell   
you1 that water being called #ajjj1 the 
specific gravity of air1 will be #a4b4  
,i will show you another instance1 of   
the weight of the atmosphere1 which ,i  
think will please you3 you know what a  
barometer is8  ,caroline4 ,it is an     
instrument which indicates the state of 
the weather1 by means of a tube of      
quicksilver2 but how1 ,i cannot exactly 
say4  ,mrs4 ,b4 ,it is by showing the   
weight of the atmosphere1 which has     
great influence on the weather4 ,the    
barometer1 is an instrument extremely   
simple in its construction4 ,in order   
that you may understand it1 ,i will show
 you how it is made4 ,i first fill with 
mercury1 a glass tube ,a ,b1 7fig4 #c1  
plate #ad47 about three feet in length1 

and open only at one end2 then stopping 
the open end1 with my finger1 ,i immerse
 it in a cup ,c1 containing a little    
mercury4  ,emily4 ,part of the mercury  
which was in the tube1 ,i observe1 runs 
down into the cup2 but why does not the 
whole of it subside1 for it is contrary 
to the law of the equilibrium of fluids1
 that the mercury in the tube1 should   
not descend to a level with that in the 
cup8  ,mrs4 ,b4 ,the mercury that has   
fallen from the tube1 into the cup1 has 
left a vacant space in the upper part of
 the tube1 to which the air cannot gain 
access2 this space is therefore a       
perfect vacuum2 the mercury in the tube1
 is relieved from the pressure of the   
atmosphere1 whilst that in the cup1     
remains exposed to it4  ,caroline4 ,oh1 
now ,i understand it2 the pressure of   
the air on the mercury in the cup1      
forces it to rise in the tube1 where    
there is not any air to counteract the  
external pressure4  ,emily4 ,or rather  
supports the mercury in the tube1 and   

prevents it from falling4  ,mrs4 ,b4    
,that comes to the same thing2 for the  
power that can support mercury in a     
vacuum1 would also make it ascend1 when 
it met with a vacuum4  ,thus you see1   
that the equilibrium of the mercury is  
destroyed1 only to preserve the general 
equilibrium of fluids4  ,caroline4 ,but 
this simple apparatus is1 in appearance1
 very unlike a barometer4  ,mrs4 ,b4 ,it
 is all that is essential to a          
barometer4 ,the tube and the cup1 or a  
cistern of mercury1 are fixed on a      
board1 for the convenience of suspending
 it2 the brass plate on the upper part  
of the board1 is graduated into inches1 
and tenths of inches1 for the purpose of
 ascertaining the height at which the   
mercury stands in the tube2 and the     
small moveable metal plate1 serves to   
show that height1 with greater accuracy4
  ,emily4 ,and at what height1 will the 
weight of the atmosphere sustain the    
mercury8  ,mrs4 ,b4 ,about #bh or #bi   
inches1 as you will see by this         

barometer2 but it depends upon the      
weight of the atmosphere1 which varies  
much1 in different states of the        
weather4 ,the greater the pressure of   
the air on the mercury in the cup1 the  
higher it will ascend in the tube4 ,now 
can you tell me whether the air is      
heavier1 in wet1 or in dry weather8     
,caroline4 ,without a moment's          
reflection1 the air must be heaviest in 
wet weather4 ,it is so depressing1 and  
makes one feel so heavy1 while in fine  
weather1 ,i feel as light as a feather1 
and as brisk as a bee4  ,mrs4 ,b4 ,would
 it not have been better to have        
answered with a moment's reflection1    
,caroline8 ,it would have convinced you1
 that the air must be heaviest in dry   
weather2 for it is then1 that the       
mercury is found to rise in the tube1   
and consequently1 the mercury in the    
cup1 must be most pressed by the air4   
,caroline4 ,why then does the air feel  
so heavy1 in bad weather8  ,mrs4 ,b4    
,because it is less salubrious1 when    

impregnated with damp4 ,the lungs1 under
 these circumstances1 do not play so    
freely1 nor does the blood circulate so 
well2 thus obstructions are frequently  
occasioned in the smaller vessels1 from 
which arise colds1 asthmas1 agues1      
fevers1 c4  ,emily4 ,since the          
atmosphere diminishes in density1 in the
 upper regions1 is not the air more     
rare1 upon a hill1 than in a plain2 and 
does the barometer indicate this        
difference8  ,mrs4 ,b4 ,certainly4 ,this
 instrument1 is so exact in its         
indications1 that it is used for the    
purpose of measuring the height of      
mountains1 and of estimating the        
elevation of balloons2 the mercury      
descending in the tube1 as you ascend to
 a greater height4  ,emily4 ,and is no  
inconvenience experienced1 from the     
thinness of the air1 in such elevated   
situations8  ,mrs4 ,b4 ,oh1 yes2        
frequently4 ,it is sometimes oppressive1
 from being insufficient for            
respiration2 and the expansion which    

takes place1 in the more dense air      
contained within the body1 is often     
painful3 it occasions distention1 and   
sometimes causes the bursting of the    
smaller blood-vessels1 in the nose1 and 
ears4 ,besides in such situations1 you  
are more exposed1 both to heat1 and     
cold2 for though the atmosphere is      
itself transparent1 its lower regions1  
abound with vapours1 and exhalations1   
from the earth1 which float in it1 and  
act in some degree as a covering1 which 
preserves us equally from the intensity 
of the sun's rays1 and from the severity
 of the cold4  ,caroline4 ,pray1 ,mrs4  
,b41 is not the thermometer constructed 
on the same principles as the barometer8
  ,mrs4 ,b4 ,not at all4 ,the rise and  
fall of the fluid in the thermometer1 is
 occasioned by the expansive power of   
heat1 and the condensation produced by  
cold3 the air has no access to it4 ,an  
explanation of it would1 therefore1 be  
irrelevant to our present subject4      
,emily4 ,i have been reflecting1 that   

since it is the weight of the           
atmosphere1 which supports the mercury1 
in the tube of a barometer1 it would    
support a column of any other fluid1 in 
the same manner4  ,mrs4 ,b4 ,certainly2 
but as mercury1 is heavier than all     
other fluids1 it will support a higher  
column1 of any other fluid2 for two     
fluids are in equilibrium1 when their   
height varies1 inversely as their       
densities4 ,we find the weight of the   
atmosphere1 is equal to sustaining a    
column of water1 for instance1 of no    
less than #cb feet above its level4     
,caroline4 ,the weight of the           
atmosphere1 is then1 as great as that of
 a body of water of #cb feet in height4 
,mrs4 ,b4 ,precisely2 for a column of   
air1 of the height of the atmosphere1 is
 equal to a column of water of about #cb
 feet1 or one of mercury1 of from #bh to
 #bi inches4  ,the common pump1 is      
dependent on this principle4 ,by the act
 of pumping1 the pressure of the        
atmosphere is taken off the water1      

which1 in consequence1 rises4  ,the body
 of a pump1 consists of a large tube or 
pipe1 whose lower end is immersed in the
 water which it is designed to raise4 ,a
 kind of stopper1 called a piston1 is   
fitted to this tube1 and is made to     
slide up and down it1 by means of a     
metallic rod1 fastened to the centre of 
the piston4  ,emily4 ,is it not similar 
to the syringe1 or squirt1 with which   
you first draw in1 and then force out   
water8  ,mrs4 ,b4 ,it is2 but you know  
that we do not wish to force the water  
out of the pump1 at the same end of the 
pipe1 at which we draw it in4 ,the      
intention of a pump1 is to raise water  
from a spring1 or well2 the pipe is1    
therefore1 placed perpendicularly over  
the water1 which enters it at the lower 
extremity1 and it issues at a horizontal
 spout1 towards the upper part of the   
pump2 to effect this1 there are1 besides
 the piston1 two contrivances called    
valves4 ,the pump1 therefore1 is rather 
a more complicated piece of machinery1  

than the syringe4  ,caroline4 ,pray1    
,mrs4 ,b41 is not the leather1 which    
covers the opening1 in the lower board  
of a pair of bellows1 a kind of valve8  
,mrs4 ,b4 ,it is1 valves are made in    
various forms2 any contrivance1 which   
allows a fluid to pass in one direction1
 and prevents its return1 is called a   
valve2 that of the bellows1 and of the  
common pump1 resemble each other1       
exactly4 ,you can now1 ,i think1        
understand the structure of the pump4   
,its various parts1 are delineated in   
this figure3 7fig4 #d4 plate #ad47 ,a ,b
 is the pipe1 or body of the pump1 ,p   
the piston1 ,v a valve1 or little door  
in the piston1 which1 opening upwards1  
admits the water to rise through it1 but
 prevents its returning1 and ,y1 is a   
similar valve1 placed lower down in the 
body of the pump2 ,h is the handle1     
which in this model1 serves to work the 
piston4  ,when the pump is in a state of
 inaction1 the two valves are closed by 
their own weight2 but when1 by working  

the handle of the pump1 the piston      
ascends2 it raises a column of air which
 rested upon it1 and produces a vacuum1 
between the piston1 and the lower valve 
,y2 the air beneath this valve1 which is
 immediately over the surface of the    
water1 consequently expands1 and forces 
its way through it2 the water1 then1    
relieved from the pressure of the air1  
ascends into the pump4 ,a few strokes of
 the handle1 totally excludes the air   
from the body of the pump1 and fills it 
with water1 which1 having passed through
 both the valves1 runs out at the spout4
  ,caroline4 ,i understand this         
perfectly4 ,when the piston is elevated1
 the air1 and the water1 successively   
rise in the pump1 for the same reason as
 the mercury1 rises in the barometer4   
,emily4 ,i thought that water was drawn 
up into a pump1 by suction1 in the same 
manner as water may be sucked through a 
straw4  ,mrs4 ,b4 ,it is so1 into the   
body of the pump2 for the power of      
suction1 is no other than that of       

producing a vacuum over one part of the 
liquid1 into which vacuum the liquid is 
forced1 by the pressure of the          
atmosphere1 on another part4 ,the action
 of sucking through a straw1 consists in
 drawing in1 and confining the breath1  
so as to produce a vacuum in the mouth2 
in consequence of which1 the air within 
the straw1 rushes into the mouth1 and is
 followed by the liquid1 into which1 the
 lower end of the straw1 is immersed4   
,the principle1 you see1 is the same1   
and the only difference consists in the 
mode of producing a vacuum4 ,in suction1
 the muscular powers answer the purpose 
of the piston and valve4  ,emily4 ,water
 cannot1 then1 be raised by a pump1     
above #cb feet2 for the pressure of the 
atmosphere will not sustain a column of 
water1 above that height4  ,mrs4 ,b4 ,i 
beg your pardon4 ,it is true that there 
must never be so great a distance as #cb
 feet1 from the level of the water in   
the well1 to the valve in the piston1   
otherwise the water would not rise      

through that valve2 but when once the   
water has passed that opening1 it is no 
longer the pressure of air on the       
reservoir1 which makes it ascend2 it is 
raised by lifting it up1 as you would   
raise it in a bucket1 of which the      
piston formed the bottom4 ,this common  
pump is1 therefore1 called the sucking1 
or lifting pump1 as it is constructed on
 both these principles4 ,the rod to     
which the piston is attached1 must be   
made sufficiently long1 to allow the    
piston to be within #cb feet of the     
surface of the water in the well1       
however deep it may be4 ,there is       
another sort of pump1 called the forcing
 pump3 it consists of a forcing power1  
added to the sucking part of the pump4  
,this additional power1 is exactly on   
the principle of the syringe3 by raising
 the piston1 you draw the water into the
 pump1 and by causing it to descend1 you
 force the water out4  ,caroline4 ,but  
the water must be forced out at the     
upper part of the pump2 and ,i cannot   

conceive how that can be done by the    
descent of the piston4  ,mrs4 ,b4       
,figure #e1 plate #ad1 will explain the 
difficulty4 ,the large pipe1 ,a ,b1     
represents the sucking part of the pump1
 which differs from the lifting pump1   
only in its piston ,p1 being unfurnished
 with a valve1 in consequence of which  
the water cannot rise above it4 ,when1  
therefore1 the piston descends1 it shuts
 the valve ,y1 and forces the water     
7which has no other vent7 into the pipe 
,d3 this is likewise furnished with a   
valve ,v1 which1 opening upwards1 admits
 the water to pass1 but prevents its    
return4  ,the water1 is thus first      
raised in the pump1 and then forced into
 the pipe1 by the alternate ascending1  
and descending motion of the piston1    
after a few strokes of the handle to    
fill the pipe1 from whence the water    
issues at the spout4  ,emily4 ,does not 
the air pump1 which you used in the     
experiments1 on pneumatics1 operate upon
 the same principles as the sucking     

pump8  ,mrs4 ,b4 ,exactly4 ,the air pump
 which ,i used 7plate #a1 fig4 #b17 has 
two hollow1 brass cylinders1 called     
barrels1 which are made perfectly true4 
,in each of those barrels1 there is a   
piston2 these are worked up1 and down1  
by the same handle2 the pistons1 are    
furnished with valves1 opening upwards1 
like those of the common pump3 there are
 valves also1 placed at the lower part  
of each barrel1 which open upwards2     
there are therefore two pumps1 united to
 produce the same effect3 two tubes1    
connect these barrels with the plate1   
upon which ,i placed the receivers1     
which were to be exhausted4  ,emily4 ,i 
now understand how the air pump acts2   
the receiver contains air1 which is     
exhausted1 just as it is by the common  
pump1 before the water begins to rise4  
,mrs4 ,b4 ,having explained the         
mechanical properties of air1 ,i think  
it is now time to conclude our lesson4  
,when next we meet1 ,i shall give you   
some account of wind1 and of sound1     

which will terminate our observations on
 elastic fluids4  ,caroline4 ,and ,i    
shall run into the garden1 to have the  
pleasure of pumping1 now that ,i        
understand the construction of a pump4  
,mrs4 ,b4 ,and1 to-morrow1 ,i hope you  
will be able to tell me1 whether it is a
 forcing1 or a common lifting pump4     
,questions  #a4 7,pg4 #acf7 ,into what  
two kinds are fluids divided8  #b4 7,pg4
 #acf7 ,there are different kinds of    
elastic fluids1 in what properties are  
they alike1 and in what do they differ8 
#c4 7,pg4 #acf7 ,in what particular do  
elastic1 differ from non-elastic1       
fluids8  #d4 7,pg4 #acf7 ,what is meant 
by the elasticity of air8  #e4 7,pg4    
#acg7 ,what is said respecting the      
weight of the atmosphere8  #f4 7,pg4    
#acg7 ,why do we not feel the pressure  
of the air8  #g4 7,pg4 #acg7 ,what would
 be the effect of relieving us from     
atmospheric pressure8  #h4 7,pg4 #ach7  
,how may the weight of the air be shown 
by the aid of the air pump1 and a piece 

of bladder8  #i4 7,pg4 #ach7 ,how is    
this explained8  #aj4 7,pg4 #ach7 ,how  
may its elasticity be exhibited1 by an  
apple1 and by a bladder8  #aa4 7,pg4    
#aci7 ,what is the absolute weight of a 
given column of atmospheric air1 and how
 could its whole pressure upon the earth
 be ascertained8  #ab4 7,pg4 #aci7 ,how 
can the weight of a small bulk of air be
 found8  #ac4 7,pg4 #adj7 ,in           
ascertaining the weight of air1 we take 
account of its temperature--,why8  #ad4 
7,pg4 #adj7 ,how could you ascertain the
 specific gravity of air1 and what would
 it be8  #ae4 7,pg4 #adj7 ,what are the 
essential parts of a barometer1 as      
represented plate #ad1 fig4 #c8  #af4   
7,pg4 #ada7 ,what sustains the mercury  
in the tube8  #ag4 7,pg4 #ada7 ,of what 
use are the divisions in the upper part 
of the instrument8  #ah4 7,pg4 #ada7 ,to
 what height will the mercury rise1 and 
what occasions this height to vary8     
#ai4 7,pg4 #ada7 ,when is the mercury   
highest1 in wet1 or in dry weather8     

#bj4 7,pg4 #ada7 ,what occasions the    
sensation of oppression1 in damp        
weather8  #ba4 7,pg4 #adb7 ,why will the
 barometer indicate the height of       
mountains1 or of balloons8  #bb4 7,pg4  
#adb7 ,is any inconvenience experienced 
by persons ascending to great heights1  
and from what cause8  #bc4 7,pg4 #adb7  
,what occasions the rise and fall of the
 mercury1 in a thermometer8  #bd4 7,pg4 
#adb7 ,to what height will the pressure 
of the atmosphere raise a column of     
water8  #be4 7,pg4 #adb7 ,what governs  
the difference between the height of the
 mercury1 and of the water8  #bf4 7,pg4 
#adc7 ,how does the common pump1 raise  
water from a well8  #bg4 7,pg4 #adc7    
,what is meant by a piston8  #bh4 7,pg4 
#adc7 ,describe the construction1 and   
use1 of a valve4  #bi4 7,pg4 #adc7 ,what
 are the parts of the pump1 as          
represented1 fig4 #d1 plate #ad48  #cj4 
7,pg4 #add7 ,how do these parts act1 in 
raising the water8  #ca4 7,pg4 #add7 ,in
 what does that which is commonly called

 suction1 consist8  #cb4 7,pg4 #add7    
,how must the piston be situated in the 
pump8  #cc4 7,pg4 #add7 ,what other kind
 of pump is described8  #cd4 7,pg4 #ade7
 ,how is the forcing pump constructed1  
as shown in plate #ad1 fig4 #e8  #ce4   
7,pg4 #ade7 ,describe the construction  
and operation of the air pump1 7fig4 #b1
 plate #a47     ,,conversation ,,xiii4  
,,on ,,wind ,,and ,,sound4  ,,of ,,wind 
,,in ,,general4 ,,of ,,the              
,,trade-,,wind4 ,,of ,,the ,,periodical 
,,trade-,,winds4 ,,of ,,the ,,aerial    
,,tides4 ,,of ,,sounds ,,in ,,general4  
,,of ,,sonorous ,,bodies4 ,,of ,,musical
 ,,sounds4 ,,of ,,concord ,,or          
,,harmony1 ,,and ,,melody4   ,,mrs4 ,b4 
,well1 ,caroline1 have you ascertained  
what kind of pump you have in your      
garden8  ,caroline4 ,i think it must be 
merely a lifting pump1 because no more  
force is required to raise the handle   
than is necessary to lift its weight2   
and as in a forcing pump1 by raising the
 handle1 you force the water into the   

smaller pipe1 the resistance the water  
offers1 must require an exertion of     
strength1 to overcome it4  ,mrs4 ,b4 ,i 
make no doubt you are right2 for lifting
 pumps1 being simple in their           
construction1 are by far the most       
common4  ,i have promised to-day to give
 you some account of the nature of wind4
 ,wind is nothing more than the motion  
of a stream1 or current of air1         
generally produced by a partial change  
of temperature in the atmosphere2 for   
when any one part is more heated than   
the rest1 that part is rarefied1 the air
 in consequence rises1 and the          
equilibrium is destroyed4 ,when this    
happens1 there necessarily follows a    
motion of the surrounding air towards   
that part1 in order to restore it2 this 
spot1 therefore1 receives winds from    
every quarter4 ,those who live to the   
north of it1 experience a north wind2   
those to the south1 a south wind3--do   
you comprehend this8  ,caroline4        
,perfectly4 ,but what sort of weather   

must those people have1 who live on the 
spot1 where these winds meet and        
interfere8  ,mrs4 ,b4 ,they have most   
commonly turbulent and boisterous       
weather1 whirlwinds1 hurricanes1 rain1  
lightning1 thunder1 c4 ,this stormy     
weather occurs most frequently in the   
torrid zone1 where the heat is greatest3
 the air being more rarefied there1 than
 in any other part of the globe1 is     
lighter1 and consequently1 ascends2     
whilst the air from the north and south1
 is continually flowing in1 to restore  
the equilibrium4  ,caroline4 ,this      
motion of the air1 would produce a      
regular and constant north wind1 to the 
inhabitants of the northern hemisphere2 
and a south wind1 to those of the       
southern hemisphere1 and continual      
storms at the equator1 where these two  
adverse winds would meet4  ,mrs4 ,b4    
,these winds do not meet1 for they each 
change their direction before they reach
 the equator4 ,the sun1 in moving over  
the equatorial regions from east to     

west1 rarefies the air as it passes1 and
 causes the denser eastern air to flow  
westwards1 in order to restore the      
equilibrium1 thus producing a regular   
east wind1 about the equator4           
,caroline4 ,the air from the west1 then1
 constantly goes to meet the sun1 and   
repair the disturbance which his beams  
have produced in the equilibrium of the 
atmosphere4 ,but ,i wonder how you will 
reconcile these various winds1 ,mrs4    
,b42 you first led me to suppose there  
was a constant struggle between opposite
 winds at the equator1 producing storm  
and tempest2 but now ,i hear of one     
regular invariable wind1 which must     
naturally be attended by calm weather4  
,emily4 ,i think ,i comprehend it3 do   
not these winds from the north and      
south1 combine with the easterly wind   
about the equator1 and form1 what are   
called1 the trade-winds8  ,mrs4 ,b4     
,just so1 my dear4 ,the composition of  
the two winds1 north and east1 produces 
a constant north-east wind2 and that of 

the two winds1 south and east1 produces 
a regular south-east wind2 these winds  
extend to about thirty degrees on each  
side of the equator1 the regions further
 distant from it1 experiencing only     
their respective northerly and southerly
 winds4  ,caroline4 ,but1 ,mrs4 ,b41 if 
the air is constantly flowing from the  
poles1 to the torrid zone1 there must be
 a deficiency of air1 in the polar      
regions8  ,mrs4 ,b4 ,the light air about
 the equator1 which expands1 and rises  
into the upper regions of the           
atmosphere1 ultimately flows from       
thence1 back to the poles1 to restore   
the equilibrium3 if it were not for this
 resource1 the polar1 atmospheric       
regions1 would soon be exhausted by the 
stream of air1 which1 in the lower      
strata of the atmosphere1 they are      
constantly sending towards the equator4 
,caroline4 ,there is then a sort of     
circulation of air in the atmosphere2   
the air in the lower strata1 flowing    
from the poles towards the equator1 and 

in the upper strata1 flowing back from  
the equator1 towards the poles4  ,mrs4  
,b4 ,exactly2 ,i can show you an example
 of this circulation1 on a smaller      
scale4 ,the air of this room1 being more
 rarefied1 than the external air1 a wind
 or current of air is pouring in from   
the crevices of the windows and doors1  
to restore the equilibrium2 but the     
light air1 with which the room is       
filled1 must find some vent1 in order to
 make way for the heavy air that enters4
 ,if you set the door a-jar1 and hold a 
candle near the upper part of it1 you   
will find that the flame will be blown  
outwards1 showing that there is a       
current of air flowing out from the     
upper part of the room4--,now place the 
candle on the floor1 close by the door1 
and you will perceive1 by the           
inclination of the flame1 that there is 
also a current of air1 setting into the 
room4  ,caroline4 ,it is just so2 the   
upper current is the warm light air1    
which is driven out to make way for the 

stream of cold dense air1 which enters  
the room lower down4  ,mrs4 ,b4 ,besides
 the general1 or trade-winds1 there are 
others1 which are called periodical1    
because they blow in contrary           
directions1 at particular periods4      
,emily4 ,i have heard1 ,mrs4 ,b41 that  
the periodical winds1 called1 in the    
torrid zone1 the sea and land breezes1  
blow towards the land1 in the day time1 
and towards the sea1 at night3 what is  
the reason of that8  ,mrs4 ,b4 ,the land
 reflects into the atmosphere1 a much   
greater quantity of the sun's rays1 than
 the water2 therefore1 that part of the 
atmosphere which is over the land1 is   
more heated and rarefied1 than that     
which is over the sea3 this occasions   
the wind to set in upon the land1 as we 
find that it regularly does on the coast
 of ,guinea1 and other countries in the 
torrid zone4 ,there1 they have only the 
sea breeze1 but on the islands1 they    
have1 in general1 both a land and sea   
breeze1 the latter being produced in the

 way described2 whilst at night1 during 
the absence of the sun1 the earth cools1
 and the air is consequently condensed1 
and flows from the land1 towards the    
sea1 occasioning the land breeze4       
,emily4 ,i have heard much of the       
violent tempests1 occasioned by the     
breaking up of the monsoons2 are not    
they also regular trade-winds8  ,mrs4   
,b4 ,they are called periodical         
trade-winds1 as they change their course
 every half year4 ,this variation is    
produced by the earth's annual course   
round the sun2 the north pole being     
inclined towards that luminary one half 
of the year1 the south pole1 the other  
half4 ,during the summer of the northern
 hemisphere1 the countries of ,arabia1  
,persia1 ,india1 and ,china1 are much   
heated1 and reflect great quantities of 
the sun's rays into the atmosphere1 by  
which it becomes extremely rarefied1 and
 the equilibrium consequently destroyed4
 ,in order to restore it1 the air from  
the equatorial southern regions1 where  

it is colder1 7as well as from the      
colder northern parts17 must necessarily
 have a motion towards those parts4 ,the
 current of air from the equatorial     
regions1 produces the trade-winds for   
the first six months1 in all the seas   
between the heated continent of ,asia1  
and the equator4 ,the other six months1 
when it is summer in the southern       
hemisphere1 the ocean and countries     
towards the southern tropic are most    
heated1 and the air over those parts1   
more rarefied3 then the air about the   
equator alters its course1 and flows    
exactly in an opposite direction4       
,caroline4 ,this explanation of the     
monsoons is very curious2 but what does 
their breaking up mean8  ,mrs4 ,b4 ,it  
is the name given by sailors to the     
shifting of the periodical winds2 they  
do not change their course suddenly1 but
 by degrees1 as the sun moves from one  
hemisphere1 to the other3 this change is
 usually attended by storms and         
hurricanes1 very dangerous for shipping2

 so that those seas are seldom navigated
 at the season of the equinoxes4        
,emily4 ,i think ,i understand the winds
 in the torrid zone perfectly well2 but 
what is it that occasions the great     
variety of winds1 which occur in the    
temperate zones8 for1 according to your 
theory1 there should be only north and  
south winds1 in those climates4  ,mrs4  
,b4 ,since so large a portion of the    
atmosphere1 as is over the torrid zone1 
is in continued agitation1 these        
agitations in an elastic fluid1 which   
yields to the slightest impression1 must
 extend every way1 to a great distance2 
the air1 therefore1 in all climates1    
will suffer more or less perturbation1  
according to the situation of the       
country1 the position of mountains1     
valleys1 and a variety of other causes3 
hence it is easy to conceive1 that      
almost every climate1 must be liable to 
variable winds2 this is particularly the
 case in high latitudes1 where the earth
 is less powerfully affected by the     

sun's rays1 than near the equator4      
,caroline4 ,i have observed1 that the   
wind1 whichever way it blows1 almost    
always falls about sun-set4  ,mrs4 ,b4  
,because the rarefaction of air in the  
particular spot which produces the wind1
 diminishes as the sun declines1 and    
consequently the velocity of the wind1  
abates4  ,emily4 ,since the air is a    
gravitating fluid1 is it not affected by
 the attraction of the moon and the sun1
 in the same manner as the waters8      
,mrs4 ,b4 ,undoubtedly2 but the aerial  
tides are as much greater than those of 
water1 as the density of water exceeds  
that of air1 which1 as you may          
recollect1 we found to be about #hjj to 
#a4  ,caroline4 ,what a prodigious      
protuberance that must occasion6 ,how   
much the weight of such a column of air1
 must raise the mercury in the          
barometer6  ,emily4 ,as this enormous   
tide of air is drawn up and supported1  
as it were1 by the moon1 its weight and 
pressure1 ,i should suppose1 would be   

rather diminished than increased8  ,mrs4
 ,b4 ,the weight of the atmosphere is   
neither increased nor diminished by the 
aerial tides4 ,the moon's attraction    
augments the bulk1 as much as it        
diminishes the weight1 of the column of 
air2 these effects1 therefore1          
counterbalancing each other1 the aerial 
tides do not affect the barometer4      
,caroline4 ,i do not quite understand   
that4  ,mrs4 ,b4 ,let us suppose that   
the additional bulk of air at high tide1
 raises the barometer one inch2 and on  
the other hand1 that the support which  
the moon's attraction affords the air1  
diminishes its weight or pressure1 so as
 to occasion the mercury to fall one    
inch2 under these circumstances the     
mercury must remain stationary4 ,thus1  
you see1 that we can never be sensible  
of aerial tides by the barometer1 on    
account of the equality of pressure of  
the atmosphere1 whatever be its height4 
,the existence of aerial tides is not1  
however1 hypothetical2 it is proved by  

the effect they produce on the apparent 
position of the heavenly bodies2 but    
this ,i cannot explain to you1 till you 
understand the properties of light4     
,emily4 ,and when shall we learn them8  
,mrs4 ,b4 ,i shall first explain to you 
the nature of sound1 which is intimately
 connected with that of air2 and ,i     
think at our next meeting1 we may enter 
upon the subject of optics4  ,we have   
now considered the effects produced by  
the wide1 and extended agitation1 of the
 air2 but there is another kind of      
agitation1 of which the air is          
susceptible--a vibratory trembling      
motion1 which1 striking on the drum of  
the ear1 produces sound4  ,caroline4 ,is
 not sound produced by solid bodies8    
,the voice of animals1 the ringing of   
bells1 the music of instruments1 all    
proceed from solid bodies4 ,i know of no
 sound but that of the wind1 which is   
produced by the air4  ,mrs4 ,b4 ,sound1 
,i assure you1 results from a tremulous 
motion of the air2 and the sonorous     

bodies you enumerate1 are merely the    
instruments by which that peculiar      
species of motion1 is communicated to   
the air4  ,caroline4 ,what6 when ,i ring
 this little bell1 is it the air that   
sounds1 and not the bell8  ,mrs4 ,b4    
,both the bell1 and the air1 are        
concerned in the production of sound4   
,but sound1 strictly speaking1 is a     
perception excited in the mind1 by the  
motion of the air1 on the nerves of the 
ear2 the air1 therefore1 as well as the 
sonorous bodies which put it in motion1 
is only the cause of sound1 the         
immediate effect is produced by the     
sense of hearing3 for without this      
sense1 there would be no sound4  ,emily4
 ,i can with difficulty conceive that4  
,a person born deaf1 it is true1 has no 
idea of sound1 because he hears none2   
yet that does not prevent the real      
existence of sound1 as all those who are
 not deaf1 can testify4  ,mrs4 ,b4 ,i do
 not doubt the existence of sound1 to   
all those who possess the sense of      

hearing2 but it exists neither in the   
sonorous body1 nor in the air1 but in   
the mind of the person whose ear is     
struck1 by the vibratory motion of the  
air1 produced by a sonorous body4       
,sound1 therefore1 is a sensation1      
produced in a living body2 life1 is as  
necessary to its existence1 as it is to 
that of feeling or seeing4  ,to convince
 you that sound does not exist in       
sonorous bodies1 but that air or some   
other vehicle1 is necessary to its      
production1 endeavour to ring the little
 bell1 after ,i have suspended it under 
a receiver in the air pump1 from which  
,i shall exhaust the air4444  ,caroline4
 ,this is indeed very strange3 though ,i
 agitate it so violently1 it produces   
but little sound4  ,mrs4 ,b4 ,by        
exhausting the receiver1 ,i have cut off
 the communication between the air and  
the bell2 the latter1 therefore1 cannot 
impart its motion1 to the air4          
,caroline4 ,are you sure that it is not 
the glass1 which covers the bell1 that  

prevents our hearing it8  ,mrs4 ,b4     
,that you may easily ascertain1 by      
letting the air into the receiver1 and  
then ringing the bell4  ,caroline4 ,very
 true2 ,i can hear it now1 almost as    
loud1 as if the glass did not cover it2 
and ,i can no longer doubt but that air 
is necessary to the production of sound4
  ,mrs4 ,b4 ,not absolutely necessary1  
though by far the most common vehicle of
 sound4 ,liquids1 as well as air1 are   
capable of conveying the vibratory      
motion of a sonorous body1 to the organ 
of hearing2 as sound can be heard under 
water4 ,solid bodies also1 convey sound1
 as ,i can soon convince you by a very  
simple experiment4 ,i shall fasten this 
string by the middle1 round the poker2  
now raise the poker from the ground1 by 
the two ends of the string1 and hold one
 to each of your ears3--,i shall now    
strike the poker1 with a key1 and you   
will find that the sound is conveyed to 
the ear by means of the strings1 in a   
much more perfect manner1 than if it had

 no other vehicle than the air4         
,caroline4 ,that it is1 certainly1 for  
,i am almost stunned by the noise4 ,but 
what is a sonorous body1 ,mrs4 ,b48 for 
all bodies are capable of producing some
 kind of sound1 by the motion they      
communicate to the air4  ,mrs4 ,b4      
,those bodies are called sonorous1 which
 produce clear1 distinct1 regular1 and  
durable sounds1 such as a bell1 a drum1 
musical strings1 wind instruments1 c4   
,they owe this property to their        
elasticity2 for an elastic body1 after  
having been struck1 not only returns to 
its former situation1 but having        
acquired momentum by its velocity1 like 
the pendulum1 it springs out on the     
opposite side4 ,if ,i draw the string ,a
 ,b1 7fig4 #f1 plate #ad17 which is made
 fast at both ends1 to ,c1 it will not  
only return to its original position1   
but proceed onwards1 to ,d4  ,this is   
its first vibration2 at the end of      
which1 it will retain sufficient        
velocity to bring it to ,e1 and back    

again to ,f1 which constitutes its      
second vibration2 the third vibration1  
will carry it only to ,g and ,h1 and so 
on1 till the resistance of the air      
destroys its motion4  ,the vibration of 
a sonorous body1 gives a tremulous      
motion to the air around it1 very       
similar to the motion communicated to   
smooth water1 when a stone is thrown    
into it4 ,this1 first produces a small  
circular wave1 around the spot in which 
the stone falls2 the wave spreads1 and  
gradually communicates its motion to the
 adjacent waters1 producing similar     
waves to a considerable extent4 ,the    
same kind of waves are produced in the  
air1 by the motion of a sonorous body1  
but with this difference1 that as air1  
is an elastic fluid1 the motion does not
 consist of regularly extending waves1  
but of vibrations2 and are composed of a
 motion1 forwards and backwards1 similar
 to those of the sonorous body4 ,they   
differ also1 in the one taking place in 
a plane1 the other1 in all directions3  

the aerial undulations1 being spherical4
  ,emily4 ,but if the air moves         
backwards1 as well as forwards1 how can 
its motion extend so as to convey sound 
to a distance8  ,mrs4 ,b4 ,the first    
sphere of undulations1 which are        
produced immediately around the sonorous
 body1 by pressing against the          
contiguous air1 condenses it4 ,the      
condensed air1 though impelled forward  
by the pressure1 reacts on the first set
 of undulations1 driving them back      
again4 ,the second set of undulations   
which have been put in motion1 in their 
turn1 communicate their motion1 and are 
themselves driven back1 by reaction4    
,thus1 there is a succession of waves in
 the air1 corresponding with the        
succession of waves in the water4       
,caroline4 ,the vibrations of sound1    
must extend much further than the       
circular waves in water1 since sound is 
conveyed to a great distance4  ,mrs4 ,b4
 ,the air is a fluid so much less dense 
than water1 that motion is more easily  

communicated to it4 ,the report of a    
cannon produces vibrations of the air1  
which extend to several miles around4   
,emily4 ,distant sound takes some time  
to reach us1 since it is produced at the
 moment the cannon is fired2 and we see 
the light of the flash1 long before we  
hear the report4  ,mrs4 ,b4 ,the air is 
immediately put in motion1 by the firing
 of a cannon2 but it requires time for  
the vibrations to extend to any distant 
spot4 ,the velocity of sound1 is        
computed to be at the rate of #aadb feet
 in a second4  ,caroline4 ,with what    
astonishing rapidity the vibrations must
 be communicated6 ,but the velocity of  
sound varies1 ,i suppose1 with that of  
the air which conveys it4 ,if the wind  
sets towards us from the cannon1 we must
 hear the report sooner than if it set  
the other way4  ,mrs4 ,b4 ,the direction
 of the wind makes less difference in   
the velocity of sound1 than you would   
imagine4 ,if the wind sets from us1 it  
bears most of the aerial waves away1 and

 renders the sound fainter2 but it is   
not very considerably longer in reaching
 the ear1 than if the wind blew towards 
us4 ,this uniform velocity of sound1    
enables us to determine the distance of 
the object1 from which it proceeds2 as  
that of a vessel at sea1 firing a       
cannon1 or that of a thunder cloud4 ,if 
we do not hear the thunder1 till half a 
minute after we see the lightning1 we   
conclude the cloud to be at the distance
 of six miles and a half4  ,emily4      
,pray1 how is the sound of an echo      
produced8  ,mrs4 ,b4 ,when the aerial   
vibrations meet with an obstacle1 having
 a hard and regular surface1 such as a  
wall1 or rock1 they are reflected back  
to the ear1 and produce the same sound a
 second time2 but the sound will then   
appear to proceed1 from the object by   
which it is reflected4 ,if the          
vibrations fall perpendicularly on the  
obstacle1 they are reflected back in the
 same line2 if obliquely1 the sound     
returns obliquely1 in the opposite      

direction1 the angle of reflection being
 equal to the angle of incidence4       
,caroline4 ,oh1 then1 ,emily1 ,i now    
understand why the echo of my voice     
behind our house is heard so much       
plainer by you than it is by me1 when we
 stand at the opposite ends of the      
gravel walk4 ,my voice1 or rather1 ,i   
should say1 the vibrations of air it    
occasions1 fall obliquely on the wall of
 the house1 and are reflected by it1 to 
the opposite end of the gravel walk4    
,emily4 ,very true2 and we have         
observed1 that when we stand in the     
middle of the walk1 opposite the house1 
the echo returns to the person who      
spoke4  ,mrs4 ,b4 ,speaking-trumpets1   
are constructed on the principle1 that  
sound is reflected4 ,the voice1 instead 
of being diffused in the open air1 is   
confined within the trumpet2 and the    
vibrations which would otherwise spread 
laterally1 fall against the sides of the
 instrument1 and are reflected from the 
different points of incidence1 so as to 

combine with those vibrations which     
proceed straight forwards4 ,the         
vibrations are thus forced onwards1 in  
the direction of the trumpet1 so as     
greatly to increase the sound1 to a     
person situated in that direction4      
,figure #g1 plate #ad1 will give you a  
clearer idea1 of the speaking-trumpet2  
in this1 lines are drawn to represent   
the manner1 in which we may imagine the 
sound to be reflected4 ,there is a point
 in front of the trumpet1 ,f1 which is  
denominated its focus1 because the sound
 is there more intense1 than at any     
other spot4 ,the trumpet used by deaf   
persons1 acts on the same principle2    
although it does not equally increase   
the sound4  ,emily4 ,are the trumpets   
used as musical instruments1 also       
constructed on this principle8  ,mrs4   
,b4 ,so far as their form tends to      
increase the sound1 they are2 but1 as a 
musical instrument1 the trumpet becomes 
itself the sonorous body1 which is made 
to vibrate by blowing into it1 and      

communicates its vibrations to the air4 
,i will attempt to give you1 in a few   
words1 some notion of the nature of     
musical sounds1 which1 as you are fond  
of music1 must be interesting to you4   
,if a sonorous body be struck in such a 
manner1 that its vibrations1 are all    
performed in regular times1 the         
vibrations of the air1 will correspond  
with them2 and striking in the same     
regular manner on the drum of the ear1  
will produce the same uniform sensation1
 on the auditory nerve1 and excite the  
same uniform idea1 in the mind2 or1 in  
other words1 we shall hear one musical  
tone4  ,but if the vibrations of the    
sonorous body1 are irregular1 there will
 necessarily follow a confusion of      
aerial vibrations2 for a second         
vibration may commence1 before the first
 is finished1 meet it half way on its   
return1 interrupt it in its course1 and 
produce harsh jarring sounds1 which are 
called discords4  ,emily4 ,but each set 
of these irregular vibrations1 if       

repeated alone1 and at equal intervals1 
would1 ,i suppose1 produce a musical    
tone8 ,it is only their irregular       
interference1 which occasions discord4  
,mrs4 ,b4 ,certainly4 ,the quicker a    
sonorous body vibrates1 the more acute1 
or sharp1 is the sound produced2 and the
 slower the vibrations1 the more grave  
will be the note4  ,caroline4 ,but if ,i
 strike any one note of the piano-forte1
 repeatedly1 whether quickly or slowly1 
it always gives the same tone4  ,mrs4   
,b4 ,because the vibrations of the same 
string1 at the same degree of tension1  
are always of a similar duration4 ,the  
quickness1 or slowness of the           
vibrations1 relate to the single tones1 
not to the various sounds which they may
 compose1 by succeeding each other4     
,striking the note in quick succession1 
produces a more frequent repetition of  
the tone1 but does not increase the     
velocity of the vibrations of the       
string4  ,the duration of the vibrations
 of strings1 or wires1 depends upon     

their length1 their thickness1 or       
weight1 and their degree of tension3    
thus1 you find1 the low bass notes are  
produced by long1 thick1 loose strings2 
and the high treble notes by short1     
small1 and tight strings4  ,caroline4   
,then1 the different length1 and size1  
of the strings of musical instruments1  
serve to vary the duration of the       
vibrations1 and consequently1 the       
acuteness or gravity of the notes8      
,mrs4 ,b4 ,yes4 ,among the variety of   
tones1 there are some which1 sounded    
together1 please the ear1 producing what
 we call harmony1 or concord4 ,this     
arises from the agreement of the        
vibrations of the two sonorous bodies2  
so that some of the vibrations of each1 
strike upon the ear at the same time4   
,thus1 if the vibrations of two strings 
are performed in equal times1 the same  
tone is produced by both1 and they are  
said to be in unison4  ,emily4 ,now1    
then1 ,i understand why1 when ,i tune my
 harp1 in unison with the piano-forte1  

,i draw the strings tighter1 if it is   
too low1 or loosen them1 if it is too   
high a pitch3 it is in order to bring   
them to vibrate1 in equal times1 with   
the strings of the piano-forte4  ,mrs4  
,b4 ,but concord1 you know1 is not      
confined to unison2 for two different   
tones1 harmonize in a variety of cases4 
,when the vibrations of one string 7or  
other sonorous body7 vibrate in double  
the time of another1 the second         
vibration of the latter1 will strike    
upon the ear1 at the same instant1 as   
the first vibration of the former2 and  
this is the concord of an octave4  ,if  
the vibrations of two strings are as two
 to three1 the second vibration of the  
first1 corresponds with the third       
vibration of the latter1 producing the  
harmony called1 a fifth4  ,caroline4    
,so1 then1 when ,i strike the key-note  
with its fifth1 ,i hear every second    
vibration of one1 and every third of the
 other1 at the same time8  ,mrs4 ,b4    
,yes2 and the key-note1 struck with the 

fourth1 is likewise a concord1 because  
the vibrations1 are as three to four4   
,the vibrations of a major third1 with  
the key-note1 are as four to five2 and  
those of a minor third1 as five to six4 
,there are other tones1 which1 though   
they cannot be struck together without  
producing discord1 if struck            
successively1 give us that succession of
 pleasing sounds1 which is called       
melody4 ,harmony1 you perceive1 arises  
from the combined effect of two1 or more
 concordant sounds1 while melody1 is the
 result of certain simple sounds1 which 
succeed each other4 ,upon these general 
principles1 the science of music is     
founded2 but1 ,i am not sufficiently    
acquainted with it1 to enter into it any
 further4  ,we shall now1 therefore1    
take leave of the subject of sound2 and1
 at our next interview1 enter upon that 
of optics1 in which we shall consider   
the nature of light1 vision1 and        
colours4   ,questions  #a4 7,pg4 #adf7  
,what is wind1 and how is it generally  

produced8  #b4 7,pg4 #adf7 ,how do the  
winds blow1 around the place where the  
air becomes rarefied8  #c4 7,pg4 #adf7  
,what effect is likely to be produced   
where the winds meet8  #d4 7,pg4 #adg7  
,in what part of the globe is the air   
most rarefied1 and what is the          
consequence8  #e4 7,pg4 #adg7 ,how do   
these winds change their direction as   
they approach the equator8  #f4 7,pg4   
#adg7 ,how are the trade-winds produced1
 and how far do they extend8  #g4 7,pg4 
#adg7 ,how is the equilibrium in the air
 restored8  #h4 7,pg4 #adh7 ,how can    
contrary currents of air be shown in a  
room8  #i4 7,pg4 #adh7 ,what causes     
this8  #aj4 7,pg4 #adh7 ,what is meant  
by a periodical wind8  #aa4 7,pg4 #adh7 
,what occasions the land and sea        
breezes1 and where do they prevail8     
#ab4 7,pg4 #adi7 ,what are monsoons8    
#ac4 7,pg4 #adi7 ,how do they change1   
and what is the cause8  #ad4 7,pg4 #adi7
 ,what is meant by their breaking up1   
and what effect is in general produced8 

#ae4 7,pg4 #adi7 ,why is the wind most  
variable in high latitudes8  #af4 7,pg4 
#aej7 ,why is the wind apt to lessen    
about sunset8  #ag4 7,pg4 #aej7 ,what   
effect must the sun and moon produce    
upon the atmosphere1 from their         
attraction8  #ah4 7,pg4 #aej7 ,why do   
not the aerial tides affect the         
barometer8  #ai4 7,pg4 #aea7 ,how is    
sound produced8  #bj4 7,pg4 #aea7 ,does 
sound exist in the sonorous body1 if    
not1 what is it8  #ba4 7,pg4 #aea7 ,by  
what experiment might we prove that air 
is the principal vehicle of sound8  #bb4
 7,pg4 #aeb7 ,what other bodies convey  
sound1 and how can it be shown that they
 do so8  #bc4 7,pg4 #aeb7 ,what is meant
 by a sonorous body8  #bd4 7,pg4 #aeb7  
,to what do they owe this property8     
#be4 7,pg4 #aeb7 ,how is this explained 
by fig4 #f1 plate #ad8  #bf4 7,pg4 #aeb7
 ,how is it illustrated by a stone      
thrown into water1 and how far does this
 illustration apply8  #bg4 7,pg4 #aec7  
,how are the vibrations propagated8     

#bh4 7,pg4 #aec7 ,how can we prove that 
sound1 does not travel as rapidly as    
light8  #bi4 7,pg4 #aec7 ,at what rate  
is sound said to travel8  #cj4 7,pg4    
#aec7 ,is the velocity much influenced  
by the direction of the wind8  #ca4     
7,pg4 #aec7 ,how will sound enable us to
 judge of the distance of objects8  #cb4
 7,pg4 #aed7 ,how are echoes produced8  
#cc4 7,pg4 #aed7 ,what is the operation 
and effect of the speaking-trumpet 7fig4
 #g1 plate #ad78  #cd4 7,pg4 #aee7 ,how 
is a musical tone produced8  #ce4 7,pg4 
#aee7 ,what occasions discords8  #cf4   
7,pg4 #aee7 ,upon what does the         
acuteness or gravity of a sound depend8 
#cg4 7,pg4 #aee7 ,does the force1 with  
which a string is struck1 affect the    
rapidity of its vibrations8  #ch4 7,pg4 
#aee7 ,how are the strings made to      
produce the high and low notes8  #ci4   
7,pg4 #aee7 ,what is meant by harmony1  
or concord1 and how is it produced8     
#dj4 7,pg4 #aef7 ,when are strings said 
to be in unison8  #da4 7,pg4 #aef7 ,how 

are octaves produced8  #db4 7,pg4 #aef7 
,how are fifths produced8  #dc4 7,pg4   
#aef7 ,how major and minor thirds8  #dd4
 7,pg4 #aef7 ,what is meant by melody1  
and in what particular does it differ   
from harmony8  ,illustration3 ,,plate   
,,xv4     ,,conversation ,,xiv4  ,,on   
,,optics4  ,,of ,,luminous1             
,,transparent1 ,,and ,,opaque ,,bodies4 
,,of ,,the ,,radiation ,,of ,,light4    
,,of ,,shadows4 ,,of ,,the ,,reflection 
,,of ,,light4 ,,opaque ,,bodies ,,seen  
,,only ,,by ,,reflected ,,light4        
,,vision ,,explained4 ,,camera          
,,obscura4 ,,image ,,of ,,objects ,,on  
,,the ,,retina4   ,,caroline4  ,i long  
to begin our lesson to-day1 ,mrs4 ,b41  
for ,i expect that it will be very      
entertaining4  ,mrs4 ,b4 ,optics is that
 branch of philosophy1 which treats of  
the nature and properties of light4 ,it 
is certainly one of the most interesting
 branches of ,natural ,philosophy1 but  
not one of the easiest to understand2 ,i
 must1 therefore1 beg that you will give

 me your undivided attention4  ,i shall 
first inquire1 whether you comprehend   
the meaning of a luminous body1 an      
opaque body1 and a transparent body4    
,caroline4 ,a luminous body is one that 
shines2 an opaque4444  ,mrs4 ,b4 ,do not
 proceed to the second1 until we have   
agreed upon the definition of the first4
 ,all bodies that shine1 are not        
luminous2 for a luminous body is one    
that shines by its own light2 as the    
sun1 the fire1 a candle1 c4  ,emily4    
,polished metal then1 when it shines    
with so much brilliancy1 is not a       
luminous body8  ,mrs4 ,b4 ,no1 for it   
would be dark1 if it did not receive    
light from a luminous body2 it belongs1 
therefore1 to the class of dark1 as well
 as of opaque bodies1 which comprehends 
all such as are neither luminous1 nor   
will admit the light to pass through    
them4  ,emily4 ,and transparent bodies1 
are those which admit the light to pass 
through them1 such as glass and water4  
,mrs4 ,b4 ,you are right4 ,transparent1 

or pellucid bodies1 are frequently      
called mediums1 because they allow the  
rays of light to pass through them2 and 
the rays which pass through1 are said to
 be transmitted by them4  ,light1 when  
emanated from the sun1 or any other     
luminous body1 is projected forward in  
straight lines1 in every possible       
direction2 so that the luminous body1 is
 not only the general centre1 from      
whence all the rays proceed2 but every  
point of it1 may be considered as a     
centre1 which radiates light in every   
direction4 7,fig4 #a1 plate #ae47       
,emily4 ,but do not the rays which are  
projected in different directions1 and  
cross each other1 interfere1 and impede 
each other's course8  ,mrs4 ,b4 ,not at 
all4 ,the particles of light1 are so    
extremely minute1 that they are never   
known to interfere with each other4 ,a  
ray of light1 is a single line of light1
 projected from a luminous body2 and a  
pencil of rays1 is a collection of rays1
 proceeding from any one point of a     

luminous body1 as fig4 #b4  ,caroline4  
,is light then a substance composed of  
particles1 like other bodies8  ,mrs4 ,b4
 ,that is a disputed point1 upon which  
,i cannot pretend to decide4 ,in some   
respects1 light is obedient to the laws 
which govern bodies2 in others1 it      
appears to be independent of them3 thus1
 though its course is guided by the laws
 of motion1 it does not seem to be      
influenced by those of gravity4 ,it has 
never been discovered to have weight1   
though a variety of interesting         
experiments have been made with a view  
of ascertaining that point2 but we are  
so ignorant of the intimate nature of   
light1 that an attempt to investigate   
it1 would lead us into a labyrinth of   
perplexity1 if not of error2 we shall1  
therefore1 confine our attention to     
those properties of light1 which are    
well ascertained4  ,let us return to the
 examination of the effects of the      
radiation of light1 from a luminous     
body4 ,since the rays of light are      

projected in straight lines1 when they  
meet with an opaque body through which  
they are unable to pass1 they are       
stopped short in their course2 for they 
cannot move in a curve line round the   
body4  ,caroline4 ,no1 certainly2 for it
 would require some other force besides 
that of projection1 to produce motion in
 a curve line4  ,mrs4 ,b4 ,the          
interruption of the rays of light1 by   
the opaque body1 produces1 therefore1   
darkness on the opposite side of it3 and
 if this darkness fall upon a wall1 a   
sheet of paper1 or any object whatever1 
it forms a shadow4  ,emily4 ,a shadow1  
then1 is nothing more than darkness     
produced by the intervention of an      
opaque body1 which prevents the rays of 
light from reaching an object behind it4
  ,caroline4 ,why then are shadows of   
different degrees of darkness2 for ,i   
should have supposed1 from your         
definition of a shadow1 that it would   
have been perfectly black8  ,mrs4 ,b4   
,it frequently happens that a shadow is 

produced by an opaque body1 interrupting
 the course of the rays from one        
luminous body1 while light from another1
 reaches the space where the shadow is  
formed2 in which case1 the shadow is    
proportionally fainter4 ,this happens   
when the opaque body is lighted by two  
candles3 if you extinguish one of them1 
the shadow will be both deeper1 and more
 distinct4  ,caroline4 ,but yet it will 
not be perfectly dark4  ,mrs4 ,b4       
,because it is still slightly           
illuminated by light reflected from the 
walls of the room1 and other surrounding
 objects4  ,you must observe1 also1 that
 when a shadow is produced by the       
interruption of rays from a single      
luminous body1 the darkness is          
proportioned to the intensity of the    
light4  ,emily4 ,i should have supposed 
the contrary2 for as the light reflected
 from surrounding objects on the shadow1
 must be in proportion to the intensity 
of the light1 the stronger the light1   
the more the shadow will be illumined4  

,mrs4 ,b4 ,your remark is perfectly     
just2 but as we have no means of        
estimating the degrees of light1 and of 
darkness1 but by comparison1 the        
strongest light will appear to produce  
the deepest shadow4 ,hence a total      
eclipse of the sun1 occasions a more    
sensible darkness than midnight1 as it  
is immediately contrasted with the      
strong light of noonday4  ,caroline4    
,the reappearance of the sun1 after an  
eclipse1 must1 by the same contrast1    
appear remarkably brilliant4  ,mrs4 ,b4 
,certainly4 ,there are several things to
 be observed1 in regard to the form1 and
 extent1 of shadows4 ,if the luminous   
body ,a 7fig4 #c47 is larger than the   
opaque body ,b1 the shadow will         
gradually diminish in size1 till it     
terminates in a point4  ,caroline4 ,this
 is the case with the shadows of the    
earth1 and the moon2 as the sun1 which  
illumines them1 is larger than either of
 those bodies4 ,and why is it not the   
case with the shadows of terrestrial    

objects8 ,their shadows1 far from       
diminishing1 are always larger than the 
object1 and increase with the distance  
from it4  ,mrs4 ,b4 ,in estimating the  
effect of shadows1 we must consider the 
dimensions of the luminous body2 when   
the luminous body is less1 than the     
opaque body1 the shadow will increase   
with the distance4 ,this will be best   
exemplified1 by observing the shadow of 
an object lighted by a candle4  ,emily4 
,i have often noticed1 that the shadow  
of my figure1 against the wall1 grows   
larger1 as it is more distant from me1  
which is owing1 no doubt1 to the candle 
that shines on me1 being much smaller   
than myself4  ,mrs4 ,b4 ,yes4 ,the      
shadow of a figure as ,a1 7fig4 #d47    
varies in size1 according to the        
distance of the several surfaces ,b ,c  
,d ,e1 on which it is described4        
,caroline4 ,i have observed1 that two   
candles1 produce two shadows from the   
same object2 whilst it would appear1    
from what you said1 that they should    

rather produce only half a shadow1 that 
is to say1 a very faint one4  ,mrs4 ,b4 
,the number of lights 7in different     
directions7 while it decreases the      
intensity of the shadows1 increases     
their number1 which always corresponds  
with that of the lights2 for each light1
 makes the opaque body cast a different 
shadow1 as illustrated by fig4 #e4 which
 represents a ball ,a1 lighted by three 
candles1 ,b1 ,c1 ,d2 and you observe the
 light ,b1 produces the shadow b1 the   
light ,c1 the shadow c1 and the light   
,d1 the shadow d2 but neither of these  
shadows will be very dark1 because the  
light of one candle only1 is intercepted
 by the ball2 and the spot is still     
illuminated by the other two4  ,emily4  
,i think we now understand the nature of
 shadows very well2 but pray1 what      
becomes of the rays of light1 which     
opaque bodies arrest in their course1   
and the interruption of which1 is the   
occasion of shadows8  ,mrs4 ,b4 ,your   
question leads to a very important      

property of light1 ,reflection4 ,when   
rays of light encounter an opaque body1 
they cannot pass through it1 and part of
 them are absorbed by it1 and part are  
reflected1 and rebound2 just as an      
elastic ball rebounds1 when struck      
against a wall4  ,by reflection1 we mean
 that the light is turned back again1   
through the same medium which it had    
traversed in its first course4  ,emily4 
,and is light1 in its reflection1       
governed by the same laws1 as solid1    
elastic bodies8  ,mrs4 ,b4 ,exactly4 ,if
 a ray of light fall perpendicularly on 
an opaque body1 it is reflected back in 
the same line1 towards the point whence 
it proceeded4 ,if it fall obliquely1 it 
is reflected obliquely1 but in the      
opposite direction2 the ray which falls 
upon the reflecting surface1 is called  
the incident ray1 and that which leaves 
it1 the reflected ray2 the angle of     
incidence1 is always equal to the angle 
of reflection4 ,you recollect that law  
in mechanics8  ,emily4 ,oh yes1         

perfectly4  ,mrs4 ,b4 ,if you will shut 
the shutters1 we will admit a ray of the
 sun's light1 through a very small      
aperture1 and ,i can show you how it is 
reflected4 ,i now hold this mirror1 so  
that the ray shall fall perpendicularly 
upon it4  ,caroline4 ,i see the ray     
which falls upon the mirror1 but not    
that which is reflected by it4  ,mrs4   
,b4 ,because it is turned directly back 
again2 and the ray of incidence1 and    
that of reflection1 are confounded      
together1 both being in the same line1  
though in opposite directions4  ,emily4 
,the ray then1 which appears to us      
single1 is really double1 and is        
composed of the incident ray1 proceeding
 to the mirror1 and of the reflected    
ray1 returning from the mirror4  ,mrs4  
,b4 ,exactly so4 ,we will now separate  
them1 by holding the mirror ,m1 7fig4   
#f17 in such a manner1 that the incident
 ray1 ,a ,b1 shall fall obliquely upon  
it--you see the reflected ray1 ,b ,c1 is
 marching off in another direction4 ,if 

we draw a line from the point of        
incidence ,b1 perpendicularly1 to the   
mirror1 it will divide the angle of     
incidence1 from the angle of reflection1
 and you will see that they are equal4  
,emily4 ,exactly2 and now1 that you hold
 the mirror1 so that the ray falls more 
obliquely upon it1 it is also reflected 
more obliquely1 preserving the equality 
of the angles of incidence1 and of      
reflection4  ,mrs4 ,b4 ,it is by        
reflected rays only1 that we see opaque 
objects4 ,luminous bodies1 send rays of 
light immediately to our eyes1 but the  
rays which they send to other bodies1   
are invisible to us1 and are seen1 only 
when they are reflected by those bodies1
 to our eyes4  ,emily4 ,but have we not 
just seen the ray of light1 in its      
passage from the sun to the mirror1 and 
its reflections8 yet1 in neither case1  
were those rays in a direction to enter 
our eyes4  ,mrs4 ,b4 ,what you saw1 was 
the light reflected to your eyes1 by    
small particles of dust floating in the 

air1 and on which the ray shone1 in its 
passage to1 and from1 the mirror4       
,caroline4 ,yet ,i see the sun1 shining 
on that house yonder1 as clearly as     
possible4  ,mrs4 ,b4 ,indeed you cannot 
see a single ray1 which passes from the 
sun to the house2 you see1 by the aid of
 those rays1 which enter your eyes2     
therefore1 it is the rays which are     
reflected by the house1 to you1 and not 
those which proceed directly from the   
sun1 to the house1 that render the      
building visible to you4  ,caroline4    
,why then does one side of the house    
appear to be in sunshine1 and the other 
in shade8 for1 if ,i cannot see the sun 
shine upon it1 the whole of the house   
should appear in the shade4  ,mrs4 ,b4  
,that side of the house1 which the sun  
shines upon1 receives1 and reflects more
 light1 and therefore1 appears more     
luminous and vivid1 than the side which 
is in shadow2 for the latter is         
illumined only1 by rays reflected upon  
it by other objects2 these rays are1    

therefore1 twice reflected before they  
reach your sight2 and as light is more1 
or less1 absorbed by the bodies it      
strikes upon1 every time a ray is       
reflected1 its intensity is diminished4 
,caroline4 ,still ,i cannot reconcile to
 myself1 the idea that we do not see the
 sun's rays shining on objects1 but only
 those which such objects reflect to us4
  ,mrs4 ,b4 ,i do not1 however1 despair 
of convincing you of it4 ,look at that  
large sheet of water2 can you tell why  
the sun appears to shine on one part of 
it only8  ,caroline4 ,no1 indeed2 for   
the whole of it is equally exposed to   
the sun4 ,this partial brilliancy of    
water1 has often excited my wonder2 but 
it has struck me more particularly by   
moonlight4 ,i have frequently observed a
 vivid streak of moonshine on the sea1  
while the rest of the water remained in 
deep obscurity1 and yet there was no    
apparent obstacle to prevent the moon   
from shining equally on every part of   
the water4  ,mrs4 ,b4 ,by moonlight the 

effect is more remarkable1 on account of
 the deep obscurity of the other parts  
of the water2 while by the sun's light1 
the effect is too strong for the eye to 
be able to observe it so distinctly4    
,caroline4 ,but1 if the sun really      
shines on every part of that sheet of   
water1 why does not every part of it1   
reflect rays to my eyes8  ,mrs4 ,b4 ,the
 reflected rays1 are not attracted out  
of their natural course1 by your eyes4  
,the direction of a reflected ray1 you  
know1 depends on that of the incident   
ray2 the sun's rays1 therefore1 which   
fall with various degrees of obliquity  
upon the water1 are reflected in        
directions equally various2 some of     
these will meet your eyes1 and you will 
see them1 but those which fall          
elsewhere1 are invisible to you4        
,caroline4 ,the streak of sunshine1     
then1 which we now see upon the water1  
is composed of those rays which by their
 reflection1 happen to fall upon my     
eyes8  ,mrs4 ,b4 ,precisely4  ,emily4   

,but is that side of the house yonder1  
which appears to be in shadow1 really   
illuminated by the sun1 and its rays    
reflected another way8  ,mrs4 ,b4 ,no2  
that is a different case1 from the sheet
 of water4 ,that side of the house1 is  
really in shadow2 it is the west side1  
which the sun cannot shine upon1 till   
the afternoon4  ,emily4 ,those objects1 
then1 which are illumined by reflected  
rays1 and those which receive direct    
rays from the sun1 but which do not     
reflect those rays towards us1 appear   
equally in shadow8  ,mrs4 ,b4           
,certainly2 for we see them both        
illumined by reflected rays4 ,that part 
of the sheet of water1 over which the   
trees cast a shadow1 by what light do   
you see it8  ,emily4 ,since it is not by
 the sun's direct rays1 it must be by   
those reflected on it from other        
objects1 and which it again reflects to 
us4  ,caroline4 ,but if we see all      
terrestrial objects by reflected light1 
7as we do the moon17 why do they appear 

so bright and luminous8 ,i should have  
supposed that reflected rays1 would have
 been dull and faint1 like those of the 
moon4  ,mrs4 ,b4 ,the moon reflects the 
sun's light1 with as much vividness as  
any terrestrial object4 ,if you look at 
it on a clear night1 it will appear as  
bright as a sheet of water1 the walls of
 a house1 or any object seen by         
daylight1 and on which the sun shines4  
,the rays of the moon are doubtless     
feeble1 when compared with those of the 
sun2 but that would not be a fair       
comparison1 for the former are incident1
 the latter1 reflected rays4  ,caroline4
 ,true2 and when we see terrestrial     
objects by moonlight1 the light has been
 twice reflected1 and is consequently1  
proportionally fainter4  ,mrs4 ,b4 ,in  
traversing the atmosphere1 the rays1    
both of the sun1 and moon1 lose some of 
their light4 ,for though the pure air1  
is a transparent medium1 which transmits
 the rays of light freely1 we have      
observed1 that near the surface of the  

earth1 it is loaded with vapours and    
exhalations1 by which some portion of   
them are absorbed4  ,caroline4 ,i have  
often noticed1 that an object on the    
summit of a hill1 appears more distinct1
 than one at an equal distance in a     
valley1 or a plain2 which is owing1 ,i  
suppose1 to the air being more free from
 vapours in an elevated situation1 and  
the reflected rays1 being consequently  
brighter4  ,mrs4 ,b4 ,that may have some
 sensible effect2 but1 when an object on
 the summit of a hill1 has a back ground
 of light sky1 the contrast with the    
object1 makes its outline more distinct4
  ,caroline4 ,i now feel well satisfied1
 that we see opaque objects1 only by    
reflected rays2 but ,i do not           
understand1 how these rays1 show us the 
objects from which they proceed4  ,mrs4 
,b4 ,i shall hereafter describe the     
structure of the eye1 very particularly1
 but will now observe1 that the small   
round spot1 which is generally called   
the sight of the eye1 is properly       

denominated the pupil2 and that the     
retina1 is an expansion of the optic    
nerve on the back part of the ball of   
the eye1 upon which1 as upon a screen1  
the rays fall1 which enter at the pupil4
 ,the rays of light1 enter at the pupil 
of the eye1 and proceed to the retina2  
and there they describe the figure1     
colour1 and 7excepting size7 form a     
perfect representation of the object1   
from which they proceed4 ,we shall again
 close the shutters1 and admit the      
light1 through the small hole made for  
that purpose1 and you will see a        
picture1 on the wall1 opposite the      
aperture1 similar to that which is      
delineated on the retina of the eye4    
,the picture is somewhat confused1 but  
by using a lens1 to bring the rays to a 
focus1 it will be rendered very         
distinct4  ,caroline4 ,oh1 how          
wonderful6 ,there is an exact picture in
 miniature of the garden1 the gardener  
at work1 the trees blown about by the   
wind4 ,the landscape1 would be perfect1 

if it were not reversed2 the ground1    
being above1 and the sky beneath4  ,mrs4
 ,b4 ,it is not enough to admire1 you   
must understand1 this phenomenon1 which 
is called a camera obscura1 or dark     
chamber2 from the necessity of darkening
 the room1 in order to exhibit it4 ,the 
camera obscura1 sometimes consists of a 
small box1 properly fitted up1 to       
represent external objects4  ,this      
picture1 you now see1 is produced by the
 rays of light1 reflected from the      
various objects in the garden1 and which
 are admitted through the hole in the   
window shutter4  ,illustration3 ,,plate 
,,xvi4  ,the rays from the glittering   
weathercock1 at the top of the alcove1  
,a1 7plate #af47 represent it in this   
spot1 a2 for the weathercock1 being much
 higher than the aperture in the        
shutter1 only a few of the rays1 which  
are reflected by it1 in an obliquely    
descending direction1 can find entrance 
there4 ,the rays of light1 you know1    
always move in straight lines2 those1   

therefore1 which enter the room1 in a   
descending direction1 will continue     
their course in the same direction1 and 
will consequently fall upon the lower   
part of the wall opposite the aperture1 
and represent the weathercock1 reversed 
in that spot1 instead of erect1 in the  
uppermost part of the landscape4        
,emily4 ,and the rays of light1 from the
 steps1 7,b7 of the alcove1 in entering 
the aperture1 ascend1 and will describe 
those steps in the highest1 instead of  
the lowest1 part of the landscape4      
,mrs4 ,b4 ,observe1 too1 that the rays  
coming from the alcove1 which is to our 
left1 describe it on the wall1 to the   
right2 while those1 which are reflected 
by the walnut tree1 ,c ,d1 to our right1
 delineate its figure in the picture1 to
 the left1 c d4 ,thus the rays1 coming  
in different directions1 and proceeding 
always in right lines1 cross each other 
at their entrance through the aperture2 
those which come from above1 proceed    
below1 those from the right1 go to the  

left1 those from the left1 towards the  
right2 thus every object is represented 
in the picture1 as occupying a          
situation1 the very reverse of that     
which it does in nature4  ,caroline4    
,excepting the flower-pot1 ,e ,f1 which1
 though its position is reversed1 has   
not changed its situation in the        
landscape4  ,mrs4 ,b4 ,the flower-pot1  
is directly in front of the aperture2 so
 that its rays1 fall perpendicularly    
upon it1 and consequently proceed       
perpendicularly to the wall1 where they 
delineate the object1 directly behind   
the aperture4  ,emily4 ,and is it thus1 
that the picture of objects1 is painted 
on the retina of the eye8  ,mrs4 ,b4    
,precisely4 ,the pupil of the eye1      
through which the rays of light enter1  
represents the aperture in the          
window-shutter2 and the image1          
delineated on the retina1 is exactly    
similar to the picture on the wall4     
,caroline4 ,you do not mean to say1 that
 we see only the representation of the  

object1 which is painted on the retina1 
and not the object itself8  ,mrs4 ,b4   
,if1 by sight1 you understand that      
sense1 by which the presence of objects 
is perceived by the mind1 through the   
means of the eyes1 we certainly see only
 the image of those objects1 painted on 
the retina4  ,caroline4 ,this appears to
 me quite incredible4  ,mrs4 ,b4 ,the   
nerves1 are the only part of our frame1 
capable of sensation3 they appear1      
therefore1 to be the instruments1 which 
the mind employs in its perceptions2 for
 a sensation1 always conveys an idea1 to
 the mind4 ,now it is known1 that our   
nerves can be affected only by contact2 
and for this reason1 the organs of      
sense1 cannot act at a distance3 for    
instance1 we are capable of smelling    
only particles which are actually in    
contact with the nerves of the nose4 ,we
 have already observed1 that the odour  
of a flower consists in effluvia1       
composed of very minute particles1 which
 penetrate the nostrils1 and strike upon

 the olfactory nerves1 which instantly  
convey the idea of odour to the mind4   
,emily4 ,and sound1 though it is said to
 be heard at a distance1 is1 in fact1   
heard only when the vibrations of the   
air1 which convey it to our ears1 strike
 upon the auditory nerve4  ,caroline4   
,there is no explanation required1 to   
prove that the senses of feeling and of 
tasting1 are excited only by contact4   
,mrs4 ,b4 ,and ,i hope to convince you1 
that the sense of sight1 is so likewise4
 ,the nerves1 which constitute the sense
 of sight1 are not different in their   
nature from those of the other organs2  
they are merely instruments which convey
 ideas to the mind1 and can be affected 
only on contact4 ,now1 since real       
objects cannot be brought to touch the  
optic nerve1 the image of them is       
conveyed thither by the rays of light1  
proceeding from real objects1 which     
actually strike upon the optic nerve1   
and form that image which the mind      
perceives4  ,caroline4 ,while ,i listen 

to your reasoning1 ,i feel convinced2   
but when ,i look upon the objects       
around1 and think that ,i do not see    
them1 but merely their image painted in 
my eyes1 my belief is again staggered4  
,i cannot reconcile to myself1 the idea1
 that ,i do not really see this book    
which ,i hold in my hand1 nor the words 
which ,i read in it4  ,mrs4 ,b4 ,did it 
ever occur to you as extraordinary1 that
 you never beheld your own face8        
,caroline4 ,no2 because ,i so frequently
 see an exact representation of it in   
the looking-glass4  ,mrs4 ,b4 ,you see a
 far more exact representation of       
objects on the retina of your eye3 it is
 a much more perfect mirror1 than any   
made by art4  ,emily4 ,but is it        
possible1 that the extensive landscape1 
which ,i now behold from the window1    
should be represented on so small a     
space1 as the retina of the eye8  ,mrs4 
,b4 ,it would be impossible for art to  
paint so small and distinct a miniature2
 but nature works with a surer hand1 and

 a more delicate pencil4 ,that power    
alone1 which forms the feathers of the  
butterfly1 and the organs of the        
minutest insect1 can pourtray so        
admirable and perfect a miniature1 as   
that which is represented on the retina 
of the eye4  ,caroline4 ,but1 ,mrs4 ,b41
 if we see only the image of objects1   
why do we not see them reversed1 as you 
showed us they were1 in the camera      
obscura8 ,is not that a strong argument 
against your theory8  ,mrs4 ,b4 ,not an 
unanswerable one1 ,i hope4 ,the image on
 the retina1 it is true1 is reversed1   
like that in the camera obscura2 as the 
rays1 from the different parts of the   
landscape1 intersect each other on      
entering the pupil1 in the same manner  
as they do1 on entering the camera      
obscura4 ,the scene1 however1 does not  
excite the idea of being inverted1      
because we always see an object in the  
direction of the rays which it sends to 
us4  ,emily4 ,i confess ,i do not       
understand that4  ,mrs4 ,b4 ,it is1 ,i  

think1 a difficult point to explain     
clearly4 ,a ray which comes from the    
upper part of an object1 describes the  
image on the lower part of the retina2  
but1 experience having taught us1 that  
the direction of that ray is from above1
 we consider that part of the object it 
represents as uppermost4 ,the rays      
proceeding from the lower part of an    
object1 fall upon the upper part of the 
retina2 but as we know their direction  
to be from below1 we see that part of   
the object they describe as the lowest4 
,caroline4 ,when ,i want to see an      
object above me1 ,i look up2 when an    
object below me1 ,i look down4 ,does not
 this prove that ,i see the objects     
themselves8 for if ,i beheld only the   
image1 there would be no necessity for  
looking up or down1 according as the    
object was higher or lower1 than myself4
  ,mrs4 ,b4 ,i beg your pardon4 ,when   
you look up1 to an elevated object1 it  
is in order that the rays reflected from
 it1 should fall upon the retina of your

 eyes2 but the very circumstance of     
directing your eyes upwards1 convinces  
you that the object is elevated1 and    
teaches you to consider as uppermost1   
the image it forms on the retina1 though
 it is1 in fact1 represented in the     
lowest part of it4 ,when you look down  
upon an object1 you draw your conclusion
 from a similar reasoning2 it is thus   
that we see all objects in the direction
 of the rays which reach our eyes4  ,but
 ,i have a further proof in favour of   
what ,i have advanced1 which1 ,i hope1  
will remove your remaining doubts3 ,i   
shall1 however1 defer it till our next  
meeting1 as the lesson has been         
sufficiently long to-day4   ,questions  
#a4 7,pg4 #aeg7 ,what is optics8  #b4   
7,pg4 #aeg7 ,what is meant by a luminous
 body8  #c4 7,pg4 #aeg7 ,what is meant  
by a dark body1 and what by an opaque   
body8  #d4 7,pg4 #aeg7 ,what are        
transparent bodies8  #e4 7,pg4 #aeg7    
,what is a medium8  #f4 7,pg4 #aeh7 ,how
 is light projected from luminous       

bodies1 and how1 from every point of    
such bodies1 7fig4 #a1 plate #ae87  #g4 
7,pg4 #aeh7 ,why do not the rays of     
light from different points1 stop each  
other's progress8  #h4 7,pg4 #aeh7 ,what
 is a ray1 and what a pencil of rays8   
fig4 #b1 plate #ae4  #i4 7,pg4 #aeh7 ,do
 we know whether light is a substance1  
similar to bodies in general8  #aj4     
7,pg4 #aeh7 ,when a ray of light falls  
upon an opaque body1 what is the result8
  #aa4 7,pg4 #aei7 ,in what does shadow 
consist8  #ab4 7,pg4 #aei7 ,why are     
they1 in general1 but partially dark8   
#ac4 7,pg4 #aei7 ,upon what does the    
intensity of a shadow depend8  #ad4     
7,pg4 #aei7 ,how are shadows affected by
 the size of the luminous body1 as      
represented in plate #ae1 fig4 #c8  #ae4
 7,pg4 #aei7 ,when is the shadow larger 
than the intercepting body8  #af4 7,pg4 
#afj7 ,what is explained by fig4 #d1    
plate #ae8  #ag4 7,pg4 #afj7 ,what will 
be the effect of several lights1 as in  
fig4 #e1 plate #ae8  #ah4 7,pg4 #afj7   

,why will neither of these shadows be   
very dark8  #ai4 7,pg4 #afj7 ,what      
becomes of the light which falls upon an
 opaque body8  #bj4 7,pg4 #afj7 ,what is
 meant by reflection8  #ba4 7,pg4 #afa7 
,what is meant by the incident1 and     
reflected rays8  #bb4 7,pg4 #afa7 ,what 
is the result1 when the incident ray    
falls perpendicularly1 and what1 when it
 falls obliquely8  #bc4 7,pg4 #afa7     
,what two angles are always equal in    
this case8  #bd4 7,pg4 #afa7 ,to what   
law in mechanics1 is this analogous1 as 
represented in fig4 #d1 plate #b8  #be4 
7,pg4 #afa7 ,what is represented by fig4
 #f1 plate #ae8  #bf4 7,pg4 #afa7 ,by   
what light are we enabled to see opaque1
 and by what1 luminous bodies8  #bg4    
7,pg4 #afa7 ,what enables us to see a   
ray of light in its passage1 through a  
darkened room8  #bh4 7,pg4 #afb7 ,by    
what reasoning would you prove that an  
object1 such1 for example1 as a house1  
is seen by reflected light8  #bi4 7,pg4 
#afb7 ,why may one side of such object  

appear more bright than another side8   
#cj4 7,pg4 #afb7 ,how is the fact       
exemplified by the sun1 or moon1 shining
 upon water8  #ca4 7,pg4 #afb7 ,why is  
this best evinced by moonlight8  #cb4   
7,pg4 #afc7 ,by what light do we see the
 moon1 and why is it comparatively      
feeble8  #cc4 7,pg4 #afc7 ,what         
circumstance1 renders objects seen by   
moonlight1 still less vivid8  #cd4 7,pg4
 #afd7 ,what is meant by the pupil of   
the eye8  #ce4 7,pg4 #afd7 ,what by the 
retina8  #cf4 7,pg4 #afd7 ,how do the   
rays of light operate on the eye in     
producing vision8  #cg4 7,pg4 #afd7 ,how
 may this be exemplified1 in a darkened 
room8  #ch4 7,pg4 #afd7 ,what is meant  
by a camera obscura8  #ci4 7,pg4 #afd7  
,how is it explained in plate #af8  #dj4
 7,pg4 #afe7 ,why are the objects       
inverted and reversed8  #da4 7,pg4 #afe7
 ,what analogy is there between the     
camera obscura1 and the eye8  #db4 7,pg4
 #afe7 ,is it the object1 or its picture
 on the retina1 which presents to the   

mind an idea of the object seen8  #dc4  
7,pg4 #aff7 ,by what organs is sensation
 produced1 and how must these organs be 
affected8  #dd4 7,pg4 #aff7 ,how will   
the idea of contact1 apply to objects   
not touching the eye8  #de4 7,pg4 #afg7 
,why do not objects appear reversed to  
the eye1 as in the camera obscura8      
,,conversation ,,xv4                    
,,optics--continued4  ,,on ,,the ,,angle
 ,,of ,,vision1 ,,and ,,the ,,reflection
 ,,of ,,mirrors4  ,,angle ,,of ,,vision4
 ,,reflection ,,of ,,plain ,,mirrors4   
,,reflection ,,of ,,convex ,,mirrors4   
,,reflection ,,of ,,concave ,,mirrors4  
,,caroline4  ,well1 ,mrs4 ,b41 ,i am    
very impatient to hear what further     
proofs you have to offer1 in support of 
your theory4 ,you must allow1 that it   
was rather provoking to dismiss us as   
you did at our last meeting4  ,mrs4 ,b4 
,you press so hard upon me with your    
objections1 that you must give me time  
to recruit my forces4  ,can you tell me1
 ,caroline1 why objects at a distance1  

appear smaller than they really are8    
,caroline4 ,i know no other reason than 
their distance4  ,mrs4 ,b4 ,it is a     
fact1 that distance causes objects to   
appear smaller1 but to state the fact1  
is not to give the reason4 ,we must     
refer again to the camera obscura1 to   
account for this circumstance2 and you  
will find1 that the different apparent  
dimensions of objects at different      
distances1 proceed from our seeing1 not 
the objects themselves1 but merely their
 image on the retina4 ,fig4 #a1 plate   
#ag1 represents a row of trees1 as      
viewed in the camera obscura4 ,i have   
expressed the direction of the rays1    
from the objects to the image1 by lines4
 ,now1 observe1 the ray which comes from
 the top of the nearest tree1 and that  
which comes from the foot of the same   
tree1 meet at the aperture1 forming an  
angle of about twenty-five degrees2 the 
angle under which we see any object1 is 
called1 the visual angle1 or1 angle of  
vision4 ,these rays cross each other at 

the aperture1 forming equal angles on   
each side of it1 and represent the tree 
inverted in the camera obscura4 ,the    
degrees of the image1 are considerably  
smaller than those of the object1 but   
the proportions are perfectly preserved4
  ,illustration3 ,,plate ,,xvii4  ,now1 
let us notice the upper and lower ray1  
from the most distant tree2 they form an
 angle of not more than twelve or       
fifteen degrees1 and an image of        
proportional dimensions4 ,thus1 two     
objects of the same size1 as the two    
trees of the avenue1 form figures of    
different sizes in the camera obscura1  
according to their distance2 or1 in     
other words1 according to the angle of  
vision under which they are seen4 ,do   
you understand this8  ,caroline4        
,perfectly4  ,mrs4 ,b4 ,then you have   
only to suppose1 that the representation
 in the camera obscura1 is similar to   
that on the retina4  ,now1 since objects
 of the same magnitudes1 appear to be of
 different dimensions1 when at different

 distances from us1 let me ask you which
 it is1 that you see2 the real objects1 
which1 we know1 do not vary in size1 or 
the images1 which1 we know1 do vary1    
according to the angle of vision under  
which we see them8  ,caroline4 ,i must  
confess1 that reason is in favour of the
 latter4 ,but does that chair1 at the   
further end of the room1 form an image  
on my retina1 much smaller than this    
which is close to me8 they appear       
exactly of the same size4  ,mrs4 ,b4    
,our senses are imperfect1 but the      
experience we acquire by the sense of   
touch1 corrects the illusions of our    
sight1 with regard to objects within our
 reach4 ,you are so perfectly convinced1
 of the real size of objects1 which you 
can handle1 that you do not attend to   
the apparent difference4  ,does that    
house appear to you much smaller1 than  
when you are close to it8  ,caroline4   
,no1 because it is very near us4  ,mrs4 
,b4 ,and yet you can see the whole of   
it1 through one of the windows of this  

room4 ,the image of the house on your   
retina must1 therefore1 be smaller than 
that of the window through which you see
 it4 ,it is your knowledge of the real  
magnitude of the house which prevents   
your attending to its apparent size4 ,if
 you were accustomed to draw from       
nature1 you would be fully aware of this
 difference4  ,emily4 ,and pray1 what is
 the reason that1 when we look up an    
avenue1 the trees not only appear       
smaller as they are more distant1 but   
seem gradually to approach each other1  
till they meet in a point8  ,mrs4 ,b4   
,not only the trees1 but the road which 
separates the two rows1 forms a smaller 
visual angle1 in proportion as it is    
more distant from us2 therefore1 the    
width of the road gradually diminishes1 
as well as the size of the trees1 till  
at length the road apparently terminates
 in a point1 at which the trees seem to 
meet4  ,emily4 ,i am very glad to       
understand this1 for ,i have lately     
begun to learn perspective1 which       

appeared to me a very dry study2 but now
 that ,i am acquainted with some of the 
principles on which it is founded1 ,i   
shall find it much more interesting4    
,caroline4 ,in drawing a view from      
nature1 it seems that we do not copy the
 real objects1 but the image they form  
on the retina of our eyes8  ,mrs4 ,b4   
,certainly4 ,in sculpture1 we copy      
nature as she really exists2 in         
painting1 we represent her1 as she      
appears to us4  ,we must now conclude   
the observations that remain to be made1
 on the angle of vision4  ,if the rays1 
proceeding from the extremities of an   
object1 with an ordinary degree of      
illumination1 do not enter the eye under
 an angle of more than two seconds1     
which is the #a-ahjjth part of a degree1
 it is invisible4 ,there are1           
consequently1 two cases in which objects
 may be invisible2 if they are either so
 small1 or so distant1 as to form an    
angle of less than two seconds of a     
degree4  ,in like manner1 if the        

velocity of a body does not exceed #bj  
degrees in an hour1 its motion is       
imperceptible4  ,caroline4 ,a very rapid
 motion may then be imperceptible1      
provided the distance of the moving     
body1 is sufficiently great4  ,mrs4 ,b4 
,undoubtedly2 for the greater its       
distance1 the smaller will be the angle1
 under which its motion will appear to  
the eye4 ,it is for this reason1 that   
the motion of the celestial bodies is   
invisible1 although inconceivably rapid4
  ,emily4 ,i am surprised1 that so great
 a velocity as #bj degrees an hour1     
should be invisible4  ,mrs4 ,b4 ,the    
real velocity depends upon the space    
comprehended in each degree1 and upon   
the time1 in which the moving body1     
passes over that space4 ,but we can only
 know the extent of this space1 by      
knowing the distance of the moving body1
 from its centre of motion2 for         
supposing two men to set off at the same
 moment from ,a and ,b1 7fig4 #b47 to   
walk each to the end of their respective

 lines1 ,c and ,d2 if they perform their
 walk in the same space of time1 they   
must have proceeded at a very different 
rate2 and yet to an eye situated at ,e1 
they will appear to have moved with     
equal velocity1 because they will both  
have gone through an equal number of    
degrees1 though over a very unequal     
length of ground4 ,the number of degrees
 over which a body moves in a given     
time1 is called its angular velocity2   
two bodies1 you see1 may have the same  
angular1 or apparent velocity1 whilst   
their real velocities may differ almost 
infinitely4 ,sight is an extremely      
useful sense1 no doubt1 but it cannot   
always be relied on1 it deceives us both
 in regard to the size and the distance 
of objects2 indeed1 our senses would be 
very liable to lead us into error1 if   
experience did not set us right4        
,emily4 ,between the two1 ,i think that 
we contrive to acquire a tolerably      
accurate idea of objects4  ,mrs4 ,b4 ,at
 least sufficiently so1 for the general 

purposes of life4 ,to convince you how  
requisite experience is1 to correct the 
errors of sight1 ,i shall relate to you1
 the case of a young man1 who was blind 
from his infancy1 and who recovered his 
sight at the age of fourteen1 by the    
operation of couching4 ,at first1 he had
 no idea1 either of the size1 or        
distance of objects1 but imagined that  
every thing he saw touched his eyes2 and
 it was not1 till after having          
repeatedly felt them1 and walked from   
one object to another1 that he acquired 
an idea of their respective dimensions1 
their relative situations1 and their    
distances4  ,caroline4 ,the idea that   
objects touched his eyes1 is1 however1  
not so absurd1 as it at first appears2  
for if we consider that we see only the 
image of objects1 this image actually   
touches our eyes4  ,mrs4 ,b4 ,that is1  
doubtless1 the reason of the opinion he 
formed1 before the sense of touch had   
corrected his judgment4  ,caroline4 ,but
 since an image must be formed on the   

retina of each of our eyes1 why do we   
not see objects double8  ,mrs4 ,b4 ,the 
action of the rays1 on the optic nerve  
of each eye1 is so perfectly similar1   
that they produce but a single          
sensation2 the mind1 therefore1 receives
 the same idea1 from the retina of both 
eyes1 and conceives the object to be    
single4  ,caroline4 ,this is difficult  
to comprehend1 and ,i should think1 can 
be but conjectural4  ,mrs4 ,b4 ,i can   
easily convince you1 that you have a    
distinct image of an object formed on   
the retina of each eye4 ,look through   
the window1 with both eyes open1 at some
 object exactly opposite to one of the  
upright bars of the sash4  ,caroline4 ,i
 now see a tree1 the body of which1     
appears to be in a line exactly opposite
 to one of the bars4  ,mrs4 ,b4 ,if you 
now shut your right eye1 and look with  
the left1 it will appear to the left of 
the bar2 then by closing the left eye1  
and looking with the other1 it will     
appear to the right of the bar4         

,caroline4 ,that is true1 indeed6  ,mrs4
 ,b4 ,there are1 evidently1 two         
representations of the tree in different
 situations1 which must be owing to an  
image of it being formed on each eye2 if
 the action of the rays1 therefore1 on  
each retina1 were not so perfectly      
similar as to produce but one sensation1
 we should see double2 and we find that 
to be the case with some persons1 who   
are afflicted with a disease in one eye1
 which prevents the rays of light from  
affecting it in the same manner as the  
other4  ,emily4 ,pray1 ,mrs4 ,b41 when  
we see the image of an object in a      
looking-glass1 why is it not inverted1  
as in the camera obscura1 and on the    
retina of the eye8  ,mrs4 ,b4 ,because  
the rays do not enter the mirror by a   
small aperture1 and cross each other1 as
 they do at the orifice of a camera     
obscura1 or the pupil of the eye4  ,when
 you view yourself in a mirror1 the rays
 from your eyes fall perpendicularly    
upon it1 and are reflected in the same  

line2 the image is1 therefore1 described
 behind the glass1 and is situated in   
the same manner as the object before it4
  ,emily4 ,yes1 ,i see that it is2 but  
the looking-glass is not nearly so tall 
as ,i am1 how is it1 therefore1 that ,i 
can see the whole of my figure in it8   
,mrs4 ,b4 ,it is not necessary that the 
mirror should be more than half your    
height1 in order that you may see the   
whole of your person in it1 7fig4 #c47  
,the ray of light ,a ,b1 from your eye1 
which falls perpendicularly on the      
mirror ,b ,d1 will be reflected back1 in
 the same line2 but the ray from your   
feet1 will fall obliquely on the mirror1
 for it must ascend in order to reach   
it2 it will1 therefore1 be reflected in 
the line ,a ,d3 and since we view       
objects in the direction of the         
reflected rays1 which reach the eye1 and
 since the image appears at the same    
distance1 behind the mirror1 that the   
object is before it1 we must continue   
the line ,a ,d to ,e1 and the line ,c ,d

 to ,f1 at the termination of which1 the
 image will be represented4             
,illustration3 ,,plate ,,xviii4  ,emily4
 ,then ,i do not understand why ,i      
should not see the whole of my person in
 a much smaller mirror1 for a ray of    
light from my feet would always reach   
it1 though more obliquely4  ,mrs4 ,b4   
,true2 but the more obliquely the ray   
falls on the mirror1 the more obliquely 
it will be reflected2 the ray would1    
therefore1 be reflected above your head1
 and you could not see it4 ,this is     
shown by the dotted line 7fig4 #c47     
,now stand a little to the right of the 
mirror1 so that the rays of light from  
your figure may fall obliquely on it----
  ,emily4 ,there is no image formed of  
me in the glass now4  ,mrs4 ,b4 ,i beg  
your pardon1 there is2 but you cannot   
see it1 because the incident rays1      
falling obliquely on the mirror1 will be
 reflected obliquely1 in the opposite   
direction2 the angles of incidence1 and 
reflection1 being equal4 ,caroline1     

place yourself in the direction of the  
reflected rays1 and tell me whether you 
do not see ,emily's image in the glass8 
,caroline4 ,let me consider4--,in order 
to look in the direction of the         
reflected rays1 ,i must place myself as 
much to the left of the glass1 as ,emily
 stands to the right of it4--,now ,i see
 her image1 not straight before me1     
however1 but before her2 and it appears 
at the same distance behind the glass1  
that she is in front of it4  ,mrs4 ,b4  
,you must recollect1 that we always see 
objects in the direction of the last    
rays1 which reach our eyes4 ,figure #d  
represents an eye1 looking at the image 
of a vase1 reflected by a mirror2 it    
must see it in the direction of the ray 
,a ,b1 as that is the ray which brings  
the image to the eye2 prolong the ray to
 ,c1 and in that spot will the image    
appear4  ,caroline4 ,i do not understand
 why a looking-glass reflects the rays  
of light2 for glass is a transparent    
body1 which should transmit them6  ,mrs4

 ,b4 ,it is not the glass that reflects 
the rays which form the image you       
behold1 but the silvering behind it2    
this silvering is a compound of mercury 
and tin1 which forms a brilliant        
metallic coating4 ,the glass acts       
chiefly as a transparent case1 through  
which the rays find an easy passage1 to1
 and from1 the quicksilver4  ,caroline4 
,why then should not mirrors be made    
simply of mercury8  ,mrs4 ,b4 ,because  
mercury is a fluid4 ,by amalgamating it 
with tinfoil1 it becomes of the         
consistence of paste1 attaches itself to
 the glass1 and forms1 in fact1 a       
metallic mirror1 which would be much    
more perfect without its glass cover1   
for the purest glass is never perfectly 
transparent2 some of the rays1          
therefore1 are lost during their passage
 through it1 by being either absorbed1  
or irregularly reflected4  ,this        
imperfection of glass mirrors1 has      
introduced the use of metallic mirrors1 
for optical purposes4  ,emily4 ,but     

since all opaque bodies reflect the rays
 of light1 ,i do not understand why they
 are not all mirrors4  ,caroline4 ,a    
curious idea indeed1 sister2 it would be
 very gratifying to see oneself in every
 object at which one looked4  ,mrs4 ,b4 
,it is very true that all opaque objects
 reflect light2 but the surface of      
bodies1 in general1 is so rough and     
uneven1 that the reflection from them is
 extremely irregular1 and prevents the  
rays from forming an image on the       
retina4 ,this1 you will be able to      
understand better1 when ,i shall explain
 to you the nature of vision1 and the   
structure of the eye4  ,you may easily  
conceive the variety of directions in   
which rays would be reflected by a      
nutmeg-grater1 on account of the        
inequality of its surface1 and the      
number of holes with which it is        
pierced4 ,all solid bodies more or less 
resemble the nutmeg-grater1 in these    
respects2 and it is only those which are
 susceptible of receiving a polish1 that

 can be made to reflect the rays with   
regularity4 ,as hard bodies are of the  
closest texture1 the least porous1 and  
capable of taking the highest polish1   
they make the best mirrors2 none1       
therefore1 are so well calculated for   
this purpose1 as metals4  ,caroline4    
,but the property of regular reflection1
 is not confined to this class of       
bodies2 for ,i have often seen myself1  
in a highly polished mahogany table4    
,mrs4 ,b4 ,certainly2 but as that       
substance is less durable1 and its      
reflection less perfect1 than that of   
metals1 ,i believe it would seldom be   
chosen1 for the purpose of a mirror4    
,there are three kinds of mirrors used  
in optics2 the plain1 or flat1 which are
 the common mirrors we have just        
mentioned2 convex mirrors1 and concave  
mirrors4 ,the reflection of the two     
latter1 is very different from that of  
the former4 ,the plain mirror1 we have  
seen1 does not alter the direction of   
the reflected rays1 and forms an image  

behind the glass1 exactly similar to the
 object before it4 ,a convex mirror has 
the peculiar property of making the     
reflected rays diverge1 by which means  
it diminishes the image2 and a concave  
mirror makes the rays converge1 and     
under certain circumstances1 magnifies  
the image4  ,emily4 ,we have a convex   
mirror in the drawing-room1 which forms 
a beautiful miniature picture of the    
objects in the room2 and ,i have often  
amused myself with looking at my        
magnified face in a concave mirror4 ,but
 ,i hope you will explain to us1 why the
 one enlarges1 while the other          
diminishes the objects it reflects4     
,mrs4 ,b4 ,let us begin by examining the
 reflection of a convex mirror4 ,this is
 formed of a portion of the exterior    
surface of a sphere4 ,when several      
parallel rays fall upon it1 that ray    
only which1 if prolonged1 would pass    
through the centre or axis of the       
mirror1 is perpendicular to it4 ,in     
order to avoid confusion1 ,i have1 in   

fig4 #a1 plate #ah1 drawn only three    
parallel lines1 ,a ,b1 ,c ,d1 ,e ,f1 to 
represent rays falling on the convex    
mirror1 ,m ,n2 the middle ray1 you will 
observe1 is perpendicular to the mirror1
 the others fall on it1 obliquely4      
,caroline4 ,as the three rays are       
parallel1 why are they not all          
perpendicular to the mirror8  ,mrs4 ,b4 
,they would be so to a flat mirror2 but 
as this is spherical1 no ray can fall   
perpendicularly upon it which is not    
directed towards the centre of the      
sphere4  ,emily4 ,just as a weight falls
 perpendicularly to the earth1 when     
gravity attracts it towards the centre4 
,mrs4 ,b4 ,in order1 therefore1 that    
rays may fall perpendicularly to the    
mirror at ,b and ,f1 the rays must be in
 the direction of the dotted lines1     
which1 you may observe1 meet at the     
centre ,o of the sphere1 of which the   
mirror forms a portion4  ,now1 can you  
tell me in what direction the three     
rays1 ,a ,b1 ,c ,d1 ,e ,f1 will be      

reflected8  ,emily4 ,yes1 ,i think so3  
the middle ray1 falling perpendicularly 
on the mirror1 will be reflected in the 
same line3 the two outer rays falling   
obliquely1 will be reflected obliquely  
to ,g and ,h2 for the dotted lines you  
have drawn are perpendiculars1 which    
divide the angles of incidence and      
reflection1 of those two rays4  ,mrs4   
,b4 ,extremely well1 ,emily3 and since  
we see objects in the direction of the  
reflected ray1 we shall see the image   
,l1 which is the point at which the     
reflected rays1 if continued through the
 mirror1 would unite and form an image4 
,this point is equally distant1 from the
 surface and centre of the sphere1 and  
is called the imaginary focus of the    
mirror4  ,caroline4 ,pray1 what is the  
meaning of focus8  ,mrs4 ,b4 ,a point at
 which converging rays1 unite4 ,and it  
is in this case1 called an imaginary    
focus2 because the rays do not really   
unite at that point1 but only appear to 
do so3 for the rays do not pass through 

the mirror1 since they are reflected by 
it4  ,emily4 ,i do not yet understand   
why an object appears smaller1 when     
viewed in a convex mirror4  ,mrs4 ,b4   
,it is owing to the divergence of the   
reflected rays4 ,you have seen that a   
convex mirror1 by reflection1 converts  
parallel rays into divergent rays2 rays 
that fall upon the mirror divergent1 are
 rendered still more so by reflection1  
and convergent rays are reflected either
 parallel1 or less convergent4 ,if then1
 an object be placed before any part of 
a convex mirror1 as the vase ,a ,b1 fig4
 #b1 for instance1 the two rays from its
 extremities1 falling convergent on the 
mirror1 will be reflected less          
convergent1 and will not come to a      
focus1 till they arrive at ,c2 then an  
eye placed in the direction of the      
reflected rays1 will see the image      
formed in 7or rather behind7 the mirror1
 at a b4  ,caroline4 ,but the reflected 
rays1 do not appear to me to converge   
less than the incident rays4 ,i should  

have supposed that1 on the contrary1    
they converged more1 since they meet in 
a point4  ,mrs4 ,b4 ,they would unite   
sooner than they actually do1 if they   
were not less convergent than the       
incident rays3 for observe1 that if the 
incident rays1 instead of being         
reflected by the mirror1 continued their
 course in their original direction1    
they would come to a focus at ,d1 which 
is considerably nearer to the mirror    
than at ,c2 the image1 is1 therefore1   
seen under a smaller angle than the     
object2 and the more distant the latter 
is from the mirror1 the smaller is the  
image reflected by it4  ,you will now   
easily understand the nature of the     
reflection of concave mirrors4 ,these   
are formed of a portion of the internal 
surface of a hollow sphere1 and their   
peculiar property is to converge the    
rays of light4  ,can you discover1      
,caroline1 in what direction the three  
parallel rays1 ,a ,b1 ,c ,d1 ,e ,f1 are 
reflected1 which fall on the concave    

mirror1 ,m ,n1 7fig4 #c478  ,caroline4  
,i believe ,i can4 ,the middle ray is   
sent back in the same line1 in which it 
arrives1 that being the direction of the
 axis of the mirror2 and the two others 
will be reflected obliquely1 as they    
fall obliquely on the mirror4 ,i must   
now draw two dotted lines perpendicular 
to their points of incidence1 which will
 divide their angles of incidence and   
reflection2 and in order that those     
angles may be equal1 the two oblique    
rays must be reflected to ,l1 where they
 will unite with the middle ray4  ,mrs4 
,b4 ,very well explained4 ,thus you see1
 that when any number of parallel rays  
fall on a concave mirror1 they are all  
reflected to a focus3 for in proportion 
as the rays are more distant from the   
axis of the mirror1 they fall more      
obliquely upon it1 and are more         
obliquely reflected2 in consequence of  
which they come to a focus in the       
direction of the axis of the mirror1 at 
a point equally distant from the centre1

 and the surface1 of the sphere2 and    
this point is not an imaginary focus1 as
 happens with the convex mirror1 but is 
the true focus at which the rays unite4 
,emily4 ,can a mirror form more than one
 focus1 by reflecting rays8  ,mrs4 ,b4  
,yes4 ,if rays fall convergent on a     
concave mirror1 7fig4 #d17 they are     
sooner brought to a focus1 ,l1 than     
parallel rays2 their focus is1          
therefore1 nearer to the mirror ,m ,n4  
,divergent rays are brought to a more   
distant focus than parallel rays1 as in 
figure #e1 where the focus is at ,l2 but
 what is called the true focus of       
mirrors1 either convex or concave1 is   
that of parallel rays1 and is equally   
distant from the centre1 and the surface
 of the spherical mirror4  ,i shall now 
show you the real reflection of rays of 
light1 by a metallic concave mirror4    
,this is one made of polished tin1 which
 ,i expose to the sun1 and as it shines 
bright1 we shall be able to collect the 
rays into a very brilliant focus4 ,i    

hold a piece of paper where ,i imagine  
the focus to be situated2 you may see by
 the vivid spot of light on the paper1  
how much the rays converge3 but it is   
not yet exactly in the focus2 as ,i     
approach the paper to that point1       
observe how the brightness of the spot  
of light increases1 while its size      
diminishes4  ,caroline4 ,that must be   
occasioned by the rays approaching      
closer together4 ,i think you hold the  
paper just in the focus now1 the light  
is so small and dazzling--,oh1 ,mrs4    
,b41 the paper has taken fire6  ,mrs4   
,b4 ,the rays of light cannot be        
concentrated1 without1 at the same time1
 accumulating a proportional quantity of
 heat3 hence concave mirrors have       
obtained the name of burning mirrors4   
,emily4 ,i have often heard of the      
surprising effects of burning mirrors1  
and ,i am quite delighted to understand 
their nature4  ,caroline4 ,it cannot be 
the true focus of the mirror1 at which  
the rays of the sun unite1 for as they  

proceed from so large a body1 they      
cannot fall upon the mirror parallel to 
each other4  ,mrs4 ,b4 ,strictly        
speaking1 they certainly do not4 ,but   
when rays1 come from such an immense    
distance as the sun1 they may be        
considered as parallel3 their point of  
union is1 therefore1 the true focus of  
the mirror1 and there the image of the  
object is represented4  ,now that ,i    
have removed the mirror out of the      
influence of the sun's rays1 if ,i place
 a burning taper in the focus1 how will 
its light be reflected8 7,fig4 #f47     
,caroline4 ,that1 ,i confess1 ,i cannot 
say4  ,mrs4 ,b4 ,the ray which falls in 
the direction of the axis of the mirror1
 is reflected back in the same line2 but
 let us draw two other rays from the    
focus1 falling on the mirror at ,b and  
,f2 the dotted lines are perpendicular  
to those points1 and the two rays will1 
therefore1 be reflected to ,a and ,e4   
,caroline4 ,oh1 now ,i understand it    
clearly4 ,the rays which proceed from a 

light placed in the focus of a concave  
mirror fall divergent upon it1 and are  
reflected1 parallel4 ,it is exactly the 
reverse of the former experiment1 in    
which the sun's rays fell parallel on   
the mirror1 and were reflected to a     
focus4  ,mrs4 ,b4 ,yes3 when the        
incident rays are parallel1 the         
reflected rays converge to a focus2     
when1 on the contrary1 the incident rays
 proceed from the focus1 they are       
reflected parallel4 ,this is an         
important law of optics1 and since you  
are now acquainted with the principles  
on which it is founded1 ,i hope that you
 will not forget it4  ,caroline4 ,i am  
sure that we shall not4 ,but1 ,mrs4 ,b41
 you said that the image was formed in  
the focus of a concave mirror2 yet ,i   
have frequently seen glass concave      
mirrors1 where the object has been      
represented within the mirror1 in the   
same manner as in a convex mirror4      
,mrs4 ,b4 ,that is the case only1 when  
the object is placed between the mirror 

and its focus2 the image then appears   
magnified behind the mirror1 or1 as you 
would say1 within it4  ,caroline4 ,i do 
not understand why the image should be  
larger than the object4  ,mrs4 ,b4 ,this
 results from the convergent property of
 the concave mirror4 ,if an object1 ,a  
,b1 7fig4 #g47 be placed between the    
mirror and its focus1 the rays from its 
extremities fall divergent on the       
mirror1 and on being reflected1 become  
less divergent1 as if they proceeded    
from ,c3 to an eye placed in that       
situation1 the image will appear        
magnified behind the mirror at a b1     
since it is seen under a larger angle   
than the object4  ,you now1 ,i hope1    
understand the reflection of light by   
opaque bodies4 ,at our next meeting1 we 
shall enter upon another property of    
light1 no less interesting1 and which is
 called refraction4   ,questions  #a4   
7,pg4 #afh7 ,what is meant by the angle 
of vision1 or the visual angle8  #b4    
7,pg4 #afi7 ,why do objects of the same 

size appear smaller when distant1 than  
when near8  #c4 7,pg4 #afi7 ,why do not 
two objects1 known to be equal in size1 
appear to differ1 when at different     
distances from the eye8  #d4 7,pg4 #afi7
 ,how is this exemplified1 by a house   
seen through a window8  #e4 7,pg4 #agj7 
,why do rows of trees1 forming an       
avenue1 appear to approach as they      
recede from the eye1 until they         
eventually seem to meet8  #f4 7,pg4     
#agj7 ,in drawing a view from nature1   
what do we copy8  #g4 7,pg4 #agj7 ,what 
is the difference in sculpture1 in this 
respect8  #h4 7,pg4 #agj7 ,excepting the
 rays from an object enter the eye1     
under a certain angle1 they cannot be   
seen2 what must this angle exceed8  #i4 
7,pg4 #agj7 ,what two circumstances may 
cause the angle to be so small1 as not  
to produce vision8  #aj4 7,pg4 #agj7    
,motion may be so slow as to become     
imperceptible1 what is said on this     
point8  #aa4 7,pg4 #agj7 ,under what    
circumstances may a body1 moving with   

great rapidity1 appear to be at rest8   
#ab4 7,pg4 #agj7 ,upon what does the    
real velocity of a body1 depend8  #ac4  
7,pg4 #aga7 ,what must be known1 to     
enable us to ascertain the real space   
contained in a degree8  #ad4 7,pg4 #aga7
 ,what is explained by fig4 #b1 plate   
#ag8  #ae4 7,pg4 #aga7 ,what is said    
respecting the evidence afforded by our 
senses1 and how do we correct the errors
 into which they would lead us8  #af4   
7,pg4 #aga7 ,an image of a visible      
object is formed upon the retina of each
 eye1 why1 therefore1 are not objects   
seen double8  #ag4 7,pg4 #agb7 ,by what 
experiment can you prove that a separate
 image of an object is formed in each   
eye8  #ah4 7,pg4 #agb7 ,under what      
circumstances are objects seen double8  
#ai4 7,pg4 #agb7 ,why is not the image  
of an object inverted in the common     
mirror8  #bj4 7,pg4 #agb7 ,your whole   
figure may be seen in a looking-glass1  
which is not more than half your height2
 how is this shown in fig4 #c4 plate    

#ag8  #ba4 7,pg4 #agc7 ,why is the image
 invisible to the person1 when not      
standing directly before the glass8     
#bb4 7,pg4 #agc7 ,in what situation may 
a second person see the image reflected8
  #bc4 7,pg4 #agc7 ,in what direction   
will an object always appear to the eye8
  #bd4 7,pg4 #agc7 ,how is this         
explained by fig4 #d1 plate #ag8  #be4  
7,pg4 #agc7 ,what is it that reflects   
the rays in a looking-glass8  #bf4 7,pg4
 #agd7 ,all opaque bodies reflect some  
light1 why do they not all act as       
mirrors8  #bg4 7,pg4 #agd7 ,what        
substances form the most perfect        
mirrors1 and for what reason8  #bh4     
7,pg4 #agd7 ,what are the three kinds of
 mirrors usually employed for optical   
purposes8  #bi4 7,pg4 #agd7 ,how are the
 rays of light affected by them8  #cj4  
7,pg4 #age7 ,what is the form of a      
convex mirror1 and how do parallel rays 
fall upon it1 as represented in fig4 #a1
 plate #ah8  #ca4 7,pg4 #age7 ,what is  
represented by the dotted line in the   

same figure8  #cb4 7,pg4 #age7 ,explain 
by the figure1 how the parallel rays    
will be reflected4  #cc4 7,pg4 #age7 ,at
 what distance behind such a mirror1    
would an image1 produced by parallel    
rays1 be formed8  #cd4 7,pg4 #age7 ,what
 is that point denominated8  #ce4 7,pg4 
#agf7 ,what is meant by a focus8  #cf4  
7,pg4 #agf7 ,why is the point behind the
 mirror1 called the imaginary focus8    
#cg4 7,pg4 #agf7 ,why does an object    
appear to be lessened by a convex       
mirror1 7fig4 #b478  #ch4 7,pg4 #agf7   
,what is a concave mirror1 and what its 
peculiar property8  #ci4 7,pg4 #agf7    
,how are parallel rays reflected by a   
concave mirror1 as explained by fig4 #c1
 plate #ah8  #dj4 7,pg4 #agg7 ,where is 
the focus of parallel rays1 in a concave
 mirror8  #da4 7,pg4 #agg7 ,if rays fall
 on it convergent1 how are they         
reflected8  #db4 7,pg4 #agg7 ,how if    
divergent8  #dc4 7,pg4 #agg7 ,how1 and  
why1 may concave1 become burning        
mirrors8  #dd4 7,pg4 #agh7 ,why may rays

 of light coming from the sun1 be viewed
 as parallel to each other8  #de4 7,pg4 
#agh7 ,if a luminous body1 as a burning 
taper1 be placed in the focus of a      
concave mirror1 how will the rays from  
it1 be reflected8 7fig4 #f47  #df4 7,pg4
 #agh7 ,what fact is explained by fig4  
#g1 plate #ah8     ,,conversation ,,xvi4
  ,,on ,,refraction ,,and ,,colours4    
,,transmission ,,of ,,light ,,by        
,,transparent ,,bodies4 ,,refraction4   
,,refraction ,,by ,,the ,,atmosphere4   
,,refraction ,,by ,a ,,lens4            
,,refraction ,,by ,,the ,,prism4 ,,of   
,,colour ,,from ,,the ,,rays ,,of       
,,light4 ,,of ,,the ,,colours ,,of      
,,bodies4   ,,mrs4 ,b4  ,the refraction 
of light will furnish the subject of    
to-day's lesson4  ,caroline4 ,that is a 
property of which ,i have not the       
faintest idea4  ,mrs4 ,b4 ,it is the    
effect which transparent mediums produce
 on light in its passage through them4  
,opaque bodies1 you know1 reflect the   
rays1 and transparent bodies transmit   

them2 but it is found1 that if a ray1 in
 passing from one medium1 into another  
of different density1 fall obliquely1 it
 is turned out of its course4 ,the ray  
of light is then said to be refracted4  
,caroline4 ,it must then be acted on by 
some new power1 otherwise it would not  
deviate from its first direction4  ,mrs4
 ,b4 ,the power which causes the        
deviation of the ray1 appears to be the 
attraction of the denser medium4 ,let us
 suppose the two mediums to be air1 and 
water2 if a ray of light passes from    
air1 into water1 it is more strongly    
attracted by the latter1 on account of  
its superior density4  ,emily4 ,in what 
direction does the water attract the    
ray8  ,mrs4 ,b4 ,the ray is attracted   
perpendicularly towards the water1 in   
the same manner in which bodies are     
acted upon by gravity4  ,if then a ray1 
,a ,b1 7fig4 #a1 plate #ai47 fall       
perpendicularly on water1 the attraction
 of the water acts in the same direction
 as the course of the ray3 it will not1 

therefore1 cause a deviation1 and the   
ray will proceed straight on1 to ,e4    
,but if it fall obliquely1 as the ray ,c
 ,b1 the water will attract it out of   
its course4 ,let us suppose the ray to  
have approached the surface of a denser 
medium1 and that it there begins to be  
affected by its attraction2 this        
attraction1 if not counteracted by some 
other power1 would draw it              
perpendicularly to the water1 at ,b2 but
 it is also impelled by its projectile  
force1 which the attraction of the      
denser medium cannot overcome2 the ray1 
therefore1 acted on by both these       
powers1 moves in a direction between    
them1 and instead of pursuing its       
original course to ,d1 or being         
implicitly guided by the water to ,e1   
proceeds towards ,f1 so that the ray    
appears bent or broken4  ,caroline4 ,i  
understand that very well2 and is not   
this the reason that oars appear bent in
 the water8  ,mrs4 ,b4 ,it is owing to  
the refraction of the rays1 reflected by

 the oar2 but this is in passing from a 
dense1 to a rare medium1 for you know   
that the rays1 by means of which you see
 the oar1 pass from water into air4     
,emily4 ,but ,i do not understand why   
refraction takes place1 when a ray      
passes from a dense into a rare medium2 
,i should suppose that it would be less1
 attracted by the latter1 than by the   
former4  ,mrs4 ,b4 ,and it is precisely 
on that account that the ray is         
refracted4 ,let the upper half of fig4  
#b1 represent glass1 and the lower half 
water1 let ,c ,b represent a ray1       
passing obliquely from the glass1 into  
water3 glass1 being the denser medium1  
the ray will be more strongly attracted 
by that which it leaves than by that    
which it enters4 ,the attraction of the 
glass acts in the direction ,a ,b1 while
 the impulse of projection would carry  
the ray to ,f2 it moves1 therefore1     
between these directions towards ,d4    
,emily4 ,so that a contrary refraction  
takes place1 when a ray passes from a   

dense1 into a rare medium4              
,illustration3 ,,plate ,,xix4  ,mrs4 ,b4
 ,the rule upon this subject is this2   
when a ray of light passes from a rare  
into a dense medium1 it is refracted    
towards the perpendicular2 when from a  
dense into a rare medium1 it is         
refracted from the perpendicular4 ,by   
the perpendicular is meant a line1 at   
right angle with the refracting surface4
 ,this may be seen in fig4 #a1 and fig4 
#b1 where the lines ,a ,e1 are the      
perpendiculars4  ,caroline4 ,but does   
not the attraction of the denser medium 
affect the ray before it touches it8    
,mrs4 ,b4 ,the distance at which the    
attraction of the denser medium acts    
upon a ray1 is so small1 as to be       
insensible2 it appears1 therefore1 to be
 refracted only at the point at which it
 passes from one medium into the other4 
,now that you understand the principle  
of refraction1 ,i will show you the real
 refraction of a ray of light4 ,do you  
see the flower painted at the bottom of 

the inside of this tea-cup8 7,fig4 #c47 
,emily4 ,yes4--,but now you have moved  
it just out of sight2 the rim of the cup
 hides it4  ,mrs4 ,b4 ,do not stir4 ,i  
will fill the cup with water1 and you   
will see the flower again4  ,emily4 ,i  
do1 indeed6 ,let me try to explain this3
 when you drew the cup from me1 so as to
 conceal the flower1 the rays reflected 
by it1 no longer met my eyes1 but were  
directed above them2 but now that you   
have filled the cup with water1 they are
 refracted1 and bent downwards when     
passing out of the water1 into the air1 
so as again to enter my eyes4  ,mrs4 ,b4
 ,you have explained it perfectly3 fig4 
#c4 will help to imprint it on your     
memory4 ,you must observe that when the 
flower becomes visible by the refraction
 of the ray1 you do not see it in the   
situation which it really occupies1 but 
the image of the flower appears higher  
in the cup2 for as objects always appear
 to be situated in the direction of the 
rays which enter the eye1 the flower    

will be seen at ,b1 in the direction of 
the refracted ray4  ,emily4 ,then1 when 
we see the bottom of a clear stream of  
water1 the rays which it reflects1 being
 refracted in their passage from the    
water into the air1 will make the bottom
 appear higher than it really is4  ,mrs4
 ,b4 ,and the water will consequently   
appear more shallow4 ,accidents have    
frequently been occasioned by this      
circumstance2 and boys1 who are in the  
habit of bathing1 should be cautioned   
not to trust to the apparent shallowness
 of water1 as it will always prove      
deeper than it appears4  ,the refraction
 of light prevents our seeing the       
heavenly bodies in their real situation3
 the light they send to us being        
refracted in passing into the           
atmosphere1 we see the sun and stars in 
the direction of the refracted ray2 as  
described in fig4 #d1 plate #ai41 the   
dotted line represents the extent of the
 atmosphere1 above a portion of the     
earth1 ,e ,b ,e3 a ray of light coming  

from the sun ,s1 falls obliquely on it1 
at ,a1 and is refracted to ,b2 then1    
since we see the object in the direction
 of the refracted ray1 a spectator at   
,b1 will see an image of the sun at ,c1 
instead of its real situation1 at ,s4   
,emily4 ,but if the sun were immediately
 over our heads1 its rays1 falling      
perpendicularly on the atmosphere1 would
 not be refracted1 and we should then   
see the real sun1 in its true situation4
  ,mrs4 ,b4 ,you must recollect that the
 sun1 is vertical only to the           
inhabitants of the torrid zone2 its     
rays1 therefore1 are always refracted1  
in this latitude4 ,there is also another
 obstacle to our seeing the heavenly    
bodies in their real situations3 light1 
though it moves with extreme velocity1  
is about eight minutes and a quarter1 in
 its passage from the sun to the earth2 
therefore1 when the rays reach us1 the  
sun must have quitted the spot he       
occupied on their departure2 yet we see 
him in the direction of those rays1 and 

consequently in a situation which he had
 abandoned eight minutes and a quarter1 
before4  ,emily4 ,when you speak of the 
sun's motion1 you mean1 ,i suppose1 his 
apparent motion1 produced by the diurnal
 motion of the earth8  ,mrs4 ,b4        
,certainly2 the effect being the same1  
whether it is our earth1 or the heavenly
 bodies1 which move3 it is more easy to 
represent things as they appear to be1  
than as they really are4  ,caroline4    
,during the morning1 then1 when the sun 
is rising towards the meridian1 we must 
7from the length of time the light is in
 reaching us7 see an image of the sun   
below that spot which it really         
occupies4  ,emily4 ,but the refraction  
of the atmosphere1 counteracting this   
effect1 we may1 perhaps1 between the    
two1 see the sun in its real situation4 
,caroline4 ,and in the afternoon1 when  
the sun is sinking in the west1         
refraction1 and the length of time which
 the light is in reaching the earth1    
will conspire to render the image of the

 sun1 higher than it really is4  ,mrs4  
,b4 ,the refraction of the sun's rays1  
by the atmosphere1 prolongs our days1 as
 it occasions our seeing an image of the
 sun1 both before he rises1 and after he
 sets2 when below our horizon1 he still 
shines upon the atmosphere1 and his rays
 are thence refracted to the earth3 so  
likewise we see an image of the sun1    
previously to his rising1 the rays that 
fall upon the atmosphere being refracted
 to the earth4  ,caroline4 ,on the other
 hand1 we must recollect that light is  
eight minutes and a quarter on its      
journey2 so that1 by the time it reaches
 the earth1 the sun may1 perhaps1 have  
risen above the horizon4  ,emily4 ,pray1
 do not glass windows1 refract the      
light8  ,mrs4 ,b4 ,they do2 but this    
refraction would not be perceptible1    
were the surfaces of the glass1         
perfectly flat and parallel1 because1 in
 passing through a pane of glass1 the   
rays suffer two refractions1 which1     
being in contrary directions1 produce   

nearly the same effect as if no         
refraction had taken place4  ,emily4 ,i 
do not understand that4  ,mrs4 ,b3 ,fig4
 #e1 plate #ai1 will make it clear to   
you3 ,a ,a represents a thick pane of   
glass1 seen edgeways4 ,when the ray ,b  
approaches the glass1 at ,c1 it is      
refracted by it2 and instead of         
continuing its course in the same       
direction1 as the dotted line describes1
 it passes through the pane1 to ,d2 at  
that point returning into the air1 it is
 again refracted by the glass1 but in a 
contrary direction to the first         
refraction1 and in consequence proceeds 
to ,e4 ,now you must observe that the   
ray ,b ,c and the ray ,d ,e being       
parallel1 the light does not appear to  
have suffered any refraction3 the       
apparent1 differing so little from the  
true place of any object1 when seen     
through glass of ordinary thickness4    
,emily4 ,so that the effect which takes 
place on the ray entering the glass1 is 
undone on its quitting it4 ,or1 to      

express myself more scientifically1 when
 a ray of light passes from one medium  
into another1 and through that into the 
first again1 the two refractions being  
equal1 and in opposite directions1 no   
sensible effect is produced4  ,caroline4
 ,i think the effect is very sensible1  
for1 in looking through the glass of the
 window1 ,i see objects very much       
distorted2 articles which ,i know to be 
straight1 appear bent and broken1 and   
sometimes the parts seem to be separated
 to a distance from each other4  ,mrs4  
,b4 ,that is because common window glass
 is not flat1 its whole surface being   
uneven4 ,rays from any object1 falling  
upon it under different angles1 are1    
consequently1 refracted in various ways1
 and thus produce the distortion you    
have observed4  ,emily4 ,is it not in   
consequence of refraction1 that the     
glasses in common spectacles1 magnify   
objects seen through them8  ,mrs4 ,b4   
,yes4 ,glasses of this description are  
called lenses2 of these1 there are      

several kinds1 the names of which it    
will be necessary for you to learn4     
,every lens is formed of glass1 ground  
so as to form a segment of a sphere1 on 
one1 or both sides4 ,they are all       
represented at fig4 #a1 plate #bj4 ,the 
most common1 is the double convex lens1 
,d4 ,this is thick in the middle1 and   
thin at the edges1 like common          
spectacles1 or reading glasses4 ,a ,b1  
is a plano-convex lens1 being flat on   
one side1 and convex on the other4 ,e is
 a double concave1 being1 in all        
respects1 the reverse of ,d4 ,c is a    
plano-concave1 flat on one side1 and    
concave on the other4 ,f is called a    
meniscus1 or concavo-convex1 being      
concave on one1 and convex on the other 
side4 ,a line passing through the centre
 of a lens1 is called its axis4         
,caroline4 ,i should like to understand 
how the rays of light are refracted1 by 
means of a lens4  ,mrs4 ,b4 ,when       
parallel rays 7fig4 #f7 fall on a double
 convex lens1 that only1 which falls in 

the direction of the axis of the lens1  
is perpendicular to the surface2 the    
other rays1 falling obliquely1 are      
refracted towards the axis1 and will    
meet at a point beyond the lens1 called 
its focus4  ,of the three rays1 ,a ,b   
,c1 which fall on the lens ,d ,e1 the   
rays ,a and ,c are refracted in their   
passage through it1 to a1 and c2 and on 
quitting the lens1 they undergo a second
 refraction in the same direction1 which
 unites them with the ray ,b1 at the    
focus ,f4  ,emily4 ,and what is the     
distance of the focus1 from the surface 
of the lens8  ,mrs4 ,b4 ,the focal      
distance depends both upon the form of  
the lens1 and on the refracting power of
 the substance of which it is made3 in a
 glass lens1 both sides of which are    
equally convex1 the focus is situated   
nearly at the centre of the sphere1 of  
which the surface of the lens forms a   
portion2 it is at the distance1         
therefore1 of the radius of the sphere4 
,the property of those lenses which have

 a convex surface1 is to collect the    
rays of light to a focus2 and of those  
which have a concave surface1 on the    
contrary1 to disperse them4 ,for the    
rays ,a and ,c1 falling on the concave  
lens ,x ,y1 7fig4 #g1 plate #ai47       
instead of converging towards the ray   
,b1 in the axis of the lens1 will each  
be attracted towards the thick edges of 
the lens1 both on entering and quitting 
it1 and will1 therefore1 by the first   
refraction1 be made to diverge to a1 c1 
and by the seconds1 to d1 e4            
,illustration3 ,,plate ,,xx4  ,caroline4
 ,and lenses which have one side flat1  
and the other convex1 or concave1 as ,a 
and ,b1 7fig4 #a1 plate #bj47 are1 ,i   
suppose1 less powerful in their         
refractions8  ,mrs4 ,b4 ,yes2 the focus 
of the plano-convex1 is at the distance 
of the diameter of a sphere1 of which   
the convex surface of the lens1 forms a 
portion2 as represented in figure #b1   
plate #bj4 ,the three parallel rays1 ,a 
,b ,c1 are brought to a focus by the    

plano-convex lens1 ,x ,y1 at ,f4        
,emily4 ,you have not explained to us1  
,mrs4 ,b41 how the lens serves to       
magnify objects4  ,mrs4 ,b4 ,by turning 
again to fig4 #f1 plate #ai4 you will   
readily understand this4 ,let ,a ,c1 be 
an object placed before the lens1 and   
suppose it to be seen by an eye at ,f2  
the ray from the point ,a1 will be seen 
in the direction ,f ,g1 that from ,c1 in
 the direction ,f ,h2 the visual angle1 
therefore1 will be greatly increased1   
and the object must appear larger1 in   
proportion4  ,i must now explain to you 
the refraction of a ray of light1 by a  
triangular piece of glass1 called a     
prism4 7,fig4 #c47  ,emily4 ,the three  
sides of this glass are flat2 it cannot1
 therefore1 bring the rays to a focus2  
nor do ,i suppose that its refraction   
will be similar to that of a flat pane  
of glass1 because it has not two sides  
parallel2 ,i cannot1 therefore1         
conjecture what effect the refraction by
 a prism1 can produce4  ,mrs4 ,b4 ,the  

refractions of the ray1 both on entering
 and on quitting the prism1 are in the  
same direction1 7,fig4 #c47 ,on entering
 the prism ,p1 the ray ,a is refracted  
from ,b to ,c1 and on quitting it from  
,c to ,d4 ,in the first instance it is  
refracted towards1 and in the last1 from
 the perpendicular2 each causing it to  
deviate in the same way1 from its       
original course1 ,a ,b4  ,i will show   
you this by experiment2 but for this    
purpose it will be advisable to close   
the window-shutters1 and admit1 through 
the small aperture1 a ray of light1     
which ,i shall refract1 by means of this
 prism4  ,caroline4 ,oh1 what beautiful 
colours are represented on the opposite 
wall6 ,there are all the colours of the 
rainbow1 and with a brightness1 ,i never
 saw equalled4 7,fig4 #d1 plate #bj47   
,emily4 ,i have seen an effect1 in some 
respects similar to this1 produced by   
the rays of the sun shining upon glass  
lustres2 but how is it possible that a  
piece of white glass can produce such a 

variety of brilliant colours8  ,mrs4 ,b4
 ,the colours are not formed by the     
prism1 but existed in the ray previously
 to its refraction4  ,caroline4 ,yet1   
before its refraction1 it appeared      
perfectly white4  ,mrs4 ,b4 ,the white  
rays of the sun1 are composed of rays1  
which1 when separated1 produce all these
 colours1 although when blended         
together1 they appear colourless or     
white4  ,sir ,isaac ,newton1 to whom we 
are indebted for the most important     
discoveries respecting light and        
colours1 was the first who divided a    
white ray of light1 and found it to     
consist of an assemblage of coloured    
rays1 which formed an image upon the    
wall1 such as you now see exhibited1    
7fig4 #d47 in which are displayed the   
following series of colours3 red1       
orange1 yellow1 green1 blue1 indigo1 and
 violet4  ,emily4 ,but how does a prism 
separate these coloured rays8  ,mrs4 ,b4
 ,by refraction4 ,it appears that the   
coloured rays have different degrees of 

refrangibility2 in passing through the  
prism1 therefore1 they take different   
directions according to their           
susceptibility of refraction4 ,the      
violet rays deviate most from their     
original course2 they appear at one of  
the ends of the spectrum1 ,a ,b3        
contiguous to the violet1 are the blue  
rays1 being those which have somewhat   
less refrangibility2 then follow1 in    
succession1 the green1 yellow1 orange1  
and lastly1 the red1 which are the least
 refrangible of the coloured rays4      
,caroline4 ,i cannot conceive how these 
colours1 mixed together1 can become     
white8  ,mrs4 ,b4 ,that ,i cannot       
pretend to explain3 but it is a fact    
that the union of these colours1 in the 
proportions in which they appear in the 
spectrum1 produce in us the idea of     
whiteness4 ,if you paint a circular     
piece of card1 in compartments1 with    
these seven colours1 as nearly as       
possible in the proportion1 and of the  
shade exhibited in the spectrum1 and    

whirl it rapidly on a pin1 it will      
appear white2 as the velocity of the    
motion1 will have the effect of blending
 the colours1 in the impression which   
they make upon the eye4  ,but a more    
decisive proof of the composition of a  
white ray is afforded1 by reuniting     
these coloured rays1 and forming with   
them1 a ray of white light4  ,caroline4 
,if you can take a ray of white light to
 pieces1 and put it together again1 ,i  
shall be quite satisfied4  ,mrs4 ,b4    
,this can be done by letting the        
coloured rays1 which have been separated
 by a prism1 fall upon a lens1 which    
will converge them to a focus2 and if1  
when thus reunited1 we find that they   
appear white as they did before         
refraction1 ,i hope you will be         
convinced that the white rays1 are a    
compound of the several coloured rays4  
,the prism ,p1 you see1 7fig4 #e47      
separates a ray of white light1 into    
seven coloured rays1 and the lens ,l ,l 
brings them to a focus at ,f1 where they

 again appear white4  ,caroline4 ,you   
succeed to perfection3 this is indeed a 
most interesting and conclusive         
experiment4  ,emily4 ,yet1 ,mrs4 ,b41 ,i
 cannot help thinking1 that there may1  
perhaps1 be but three distinct colours  
in the spectrum1 red1 yellow1 and blue2 
and that the four others may consist of 
two of these colours blended together2  
for1 in painting1 we find1 that by      
mixing red and yellow1 we produce       
orange2 with different proportions of   
red and blue1 we make violet or any     
shade of purple2 and yellow1 and blue1  
form green4 ,now1 it is very natural to 
suppose1 that the refraction of a prism1
 may not be so perfect as to separate   
the coloured rays of light completely1  
and that those which are contiguous1 in 
order of refrangibility1 may encroach on
 each other1 and by mixing1 produce the 
intermediate colours1 orange1 green1    
violet1 and indigo4  ,mrs4 ,b4 ,your    
observation is1 ,i believe1 neither     
quite wrong1 nor quite right4 ,dr4      

,wollaston1 who has performed many      
experiments on the refraction of light1 
in a more accurate manner than had been 
previously done1 by receiving a very    
narrow line of light on a prism1 found  
that it formed a spectrum1 consisting of
 rays of four colours only2 but they    
were not exactly those you have named as
 primitive colours1 for they consisted  
of red1 green1 blue1 and violet4 ,a very
 narrow line of yellow was visible1 at  
the limit of the red and green1 which   
,dr4 ,wollaston attributed to the       
overlapping of the edges of the red and 
green light4  ,caroline4 ,but red and   
green mixed together1 do not produce    
yellow8  ,mrs4 ,b4 ,not in painting2 but
 it may be so in the primitive rays of  
the spectrum4 ,dr4 ,wollaston observed1 
that1 by increasing the breadth of the  
aperture1 by which the line of light was
 admitted1 the space occupied by each   
coloured ray in the spectrum1 was       
augmented1 in proportion as each portion
 encroached on the neighbouring colour1 

and mixed with it2 so that the          
intervention of orange and yellow1      
between the red and green1 is owing1 he 
supposes1 to the mixture of these two   
colours2 and the blue is blended on the 
one side with the green1 and on the     
other with the violet1 forming the      
spectrum1 as it was originally observed 
by ,sir ,isaac ,newton1 and which ,i    
have just shown you4  ,the rainbow1     
which exhibits a series of colours1 so  
analogous to those of the spectrum1 is  
formed by the refraction of the sun's   
rays1 in their passage through a shower 
of rain2 every drop of which acts as a  
prism1 in separating the coloured rays  
as they pass through it2 the combined   
effect of innumerable drops1 produces   
the bow1 which you know can be seen1    
only when there are both rain1 and      
sunshine4  ,emily4 ,pray1 ,mrs4 ,b41    
cannot the sun's rays be collected to a 
focus by a lens1 in the same manner as  
they are by a concave mirror8  ,mrs4 ,b4
 ,the same effect in concentrating the  

rays1 is produced by the refraction with
 a lens1 as by the reflection from a    
concave mirror3 in the first1 the rays  
pass through the glass and converge to a
 focus1 behind it1 in the latter1 they  
are reflected from the mirror1 and      
brought to a focus1 before it4 ,a lens1 
when used for the purpose of collecting 
the sun's rays1 is called a burning     
glass4 ,i have before explained to you1 
the manner in which a convex lens1      
refracts the rays1 and brings them to a 
focus2 7fig4 #f1 plate #ai47 as these   
rays contain both light and heat1 the   
latter1 as well as the former1 is       
refracted2 and intense heat1 as well as 
light1 will be found in the focal point4
 ,the sun now shines very bright2 if we 
let the rays fall on this lens1 you will
 perceive the focus4  ,emily4 ,oh yes3  
the point of union of the rays1 is very 
luminous4 ,i will hold a piece of paper 
in the focus1 and see if it will take   
fire4 ,the spot of light is extremely   
brilliant1 but the paper does not burn8 

,mrs4 ,b4 ,try a piece of brown         
paper2--that1 you see1 takes fire almost
 immediately4  ,caroline4 ,this is      
surprising2 for the light appeared to   
shine more intensely1 on the white1 than
 on the brown paper4  ,mrs4 ,b4 ,the    
lens collects an equal number of rays to
 a focus1 whether you hold the white or 
the brown paper1 there2 but the white   
paper appears more luminous in the      
focus1 because most of the rays1 instead
 of entering into the paper1 are        
reflected by it2 and this is the reason 
that the paper does not readily take    
fire3 whilst1 on the contrary1 the brown
 paper1 which absorbs more light and    
heat than it reflects1 soon becomes     
heated and takes fire4  ,caroline4 ,this
 is extremely curious2 but why should   
brown paper1 absorb more rays1 than     
white paper8  ,mrs4 ,b4 ,i am far from  
being able to give a satisfactory answer
 to that question4 ,we can form but mere
 conjecture on this point2 it is        
supposed that the tendency to absorb1 or

 reflect rays1 depends on the           
arrangement of the minute particles of  
the body1 and that this diversity of    
arrangement renders some bodies         
susceptible of reflecting one coloured  
ray1 and absorbing the others2 whilst   
other bodies1 have a tendency to reflect
 all the colours1 and others again1 to  
absorb them all4  ,emily4 ,and how do   
you know which colours bodies have a    
tendency to reflect1 or which to absorb8
  ,mrs4 ,b4 ,because a body always      
appears to be of the colour which it    
reflects2 for1 as we see only by        
reflected rays1 it can appear of the    
colour of those rays1 only4  ,caroline4 
,but we see all bodies of their own     
natural colour1 ,mrs4 ,b42 the grass and
 trees1 green2 the sky1 blue2 the       
flowers of various hues4  ,mrs4 ,b4     
,true2 but why is the grass             
green8--because it absorbs all1 except  
the green rays2 it is1 therefore1 these 
only which the grass and trees reflect  
to our eyes1 and this makes them appear 

green4 ,the flowers1 in the same manner1
 reflect the various colours of which   
they appear to us2 the rose1 the red    
rays2 the violet1 the blue2 the jonquil1
 the yellow1 c4  ,caroline4 ,but these  
are the permanent colours of the grass  
and flowers1 whether the sun's rays     
shine on them or not4  ,mrs4 ,b4        
,whenever you see those colours1 the    
flowers must be illumined by some light2
 and light1 from whatever source it     
proceeds1 is of the same nature2        
composed of the various coloured rays   
which paint the grass1 the flowers1 and 
every coloured object in nature4        
,caroline4 ,but1 ,mrs4 ,b41 the grass is
 green1 and the flowers are coloured1   
whether in the dark1 or exposed to the  
light8  ,mrs4 ,b4 ,why should you think 
so8  ,caroline4 ,it cannot be otherwise4
  ,mrs4 ,b4 ,a most philosophical reason
 indeed6 ,but1 as ,i never saw them in  
the dark1 you will allow me to dissent  
from your opinion4  ,caroline4 ,what    
colour do you suppose them to be1 then1 

in the dark8  ,mrs4 ,b4 ,none at all2 or
 black1 which is the same thing4 ,you   
can never see objects1 without light4   
,white light is compounded of rays1 from
 which all the colours in nature are    
produced2 there1 therefore1 can be no   
colour without light2 and though a      
substance is black1 or without colour1  
in the dark1 it may become coloured1 as 
soon as it becomes visible4 ,it is      
visible1 indeed1 only by the coloured   
rays which it reflects2 therefore1 we   
can see it only when coloured4          
,caroline4 ,all you say seems very true1
 and ,i know not what to object to it2  
yet it appears at the same time         
incredible6 ,what1 ,mrs4 ,b41 are we all
 as black as negroes in the dark8 you   
make me shudder at the thought4  ,mrs4  
,b4 ,your vanity need not be alarmed at 
the idea1 as you are certain of never   
being seen1 in that state4  ,caroline4  
,that is some consolation1 undoubtedly2 
but what a melancholy reflection it is1 
that all nature which appears so        

beautifully diversified with colours1 is
 really one uniform mass of blackness6  
,mrs4 ,b4 ,is nature less pleasing for  
being coloured1 as well as illumined1 by
 the rays of light8 and are colours less
 beautiful1 for being accidental1 rather
 than essential properties of bodies8   
,providence seems to have decorated     
nature with the enchanting diversity of 
colours1 which we so much admire1 for   
the sole purpose of beautifying the     
scene1 and rendering it a source of     
sensible gratification3 it is an        
ornament which embellishes nature1      
whenever we behold her4 ,what reason is 
there to regret1 that she does not wear 
it when she is invisible8  ,emily4 ,i   
confess1 ,mrs4 ,b41 that ,i have had my 
doubts1 as well as ,caroline1 though she
 has spared me the pains of expressing  
them3 but ,i have just thought of an    
experiment1 which1 if it succeed1 will1 
,i am sure1 satisfy us both4 ,it is     
certain1 that we cannot see bodies in   
the dark1 to know whether they have then

 any colour4 ,but we may place a        
coloured body in a ray of light1 which  
has been refracted by a prism2 and if   
your theory is true1 the body1 of       
whatever colour it naturally is1 must   
appear of the colour of the ray in which
 it is placed2 for since it receives no 
other coloured rays1 it can reflect no  
others4  ,caroline4 ,oh6 that is an     
excellent thought1 ,emily2 will you     
stand the test1 ,mrs4 ,b48  ,mrs4 ,b4 ,i
 consent3 but we must darken the room1  
and admit only the ray which is to be   
refracted2 otherwise1 the white rays    
will be reflected on the body under     
trial1 from various parts of the room4  
,with what do you choose to make the    
experiment8  ,caroline4 ,this rose3 look
 at it1 ,mrs4 ,b41 and tell me whether  
it is possible to deprive it of its     
beautiful colour8  ,mrs4 ,b4 ,we shall  
see4--,i expose it first to the red     
rays1 and the flower appears of a more  
brilliant hue2 but observe the green    
leaves----  ,caroline4 ,they appear     

neither red nor green2 but of a dingy   
brown with a reddish glow8  ,mrs4 ,b4   
,they cannot appear green1 because they 
have no green rays to reflect2 neither  
are they red1 because green bodies      
absorb most of the red rays4 ,but though
 bodies1 from the arrangement of their  
particles1 have a tendency to absorb    
some rays1 and reflect others1 yet it is
 not natural to suppose1 that bodies are
 so perfectly uniform in their          
arrangement1 as to reflect only pure    
rays of one colour1 and perfectly to    
absorb the others2 it is found1 on the  
contrary1 that a body reflects1 in great
 abundance1 the rays which determine its
 colour1 and the others in a greater or 
less degree1 in proportion as they are  
nearer to or further from its own       
colour1 in the order of refrangibility4 
,the green leaves of the rose1          
therefore1 will reflect a few of the red
 rays1 which1 blended with their natural
 blackness1 give them that brown tinge3 
if they reflected none of the red rays1 

they would appear perfectly black4 ,now 
,i shall hold the rose in the blue      
rays----  ,caroline4 ,oh1 ,emily1 ,mrs4 
,b4 is right6 look at the rose3 it is no
 longer red1 but of a dingy blue colour4
  ,emily4 ,this is the most wonderful1  
of any thing we have yet learnt4 ,but1  
,mrs4 ,b41 what is the reason that the  
green leaves1 are of a brighter blue    
than the rose8  ,mrs4 ,b4 ,the green    
leaves reflect both blue and yellow     
rays1 which produce a green colour4     
,they are now in a coloured ray1 which  
they have a tendency to reflect2 they1  
therefore1 reflect more of the blue rays
 than the rose1 7which naturally absorbs
 that colour17 and will1 of course1     
appear of a brighter blue4  ,emily4     
,yet1 in passing the rose through the   
different colours of the spectrum1 the  
flower takes them more readily than the 
leaves4  ,mrs4 ,b4 ,because the flower  
is of a paler hue4 ,bodies which reflect
 all the rays1 are white2 those which   
absorb them all1 are black3 between     

these extremes1 bodies appear lighter or
 darker1 in proportion to the quantity  
of rays they reflect or absorb4 ,this   
rose is of a pale red2 it approaches    
nearer to white than to black1 and      
therefore1 reflects rays1 more          
abundantly than it absorbs them4        
,emily4 ,but if a rose has so strong a  
tendency to reflect rays1 ,i should     
imagine that it would be of a deep red  
colour4  ,mrs4 ,b4 ,i mean to say1 that 
it has a general tendency to reflect    
rays4 ,pale coloured bodies1 reflect all
 the coloured rays to a certain degree1 
their paleness1 being an approach       
towards whiteness3 but they reflect one 
colour more than the rest3 this         
predominates over the white1 and        
determines the colour of the body4      
,since1 then1 bodies of a pale colour1  
in some degree reflect all the rays of  
light1 in passing through the various   
colours of the spectrum1 they will      
reflect them all1 with tolerable        
brilliancy2 but will appear most vivid1 

in the ray of their natural colour4 ,the
 green leaves1 on the contrary1 are of a
 dark colour1 bearing a stronger        
resemblance to black1 than to white2    
they have1 therefore1 a greater tendency
 to absorb1 than to reflect rays2 and   
reflecting very few of any1 but the     
blue1 and yellow rays1 they will appear 
dingy1 in passing through the other     
colours of the spectrum4  ,caroline4    
,they must1 however1 reflect great      
quantities of the green rays1 to produce
 so deep a colour4  ,mrs4 ,b4 ,deepness 
or darkness of colour1 proceeds rather  
from a deficiency1 than an abundance of 
reflected rays4 ,remember1 that if      
bodies reflected none of the rays1 they 
would be black2 and if a body reflects  
only a few green rays1 it will appear of
 a dark green2 it is the brightness1 and
 intensity of the colour1 which show    
that a great quantity of rays are       
reflected4  ,emily4 ,a white body1 then1
 which reflects all the rays1 will      
appear equally bright in all the colours

 of the spectrum4  ,mrs4 ,b4 ,certainly4
 ,and this is easily proved by passing a
 sheet of white paper1 through the rays 
of the spectrum4  ,white1 you perceive1 
results from a body reflecting all the  
rays which fall upon it2 black1 is      
produced1 when they are all absorbed2   
and colour1 arises from a body          
possessing the power to decompose the   
solar ray1 by absorbing some parts1 and 
reflecting others4  ,caroline4 ,what is 
the reason that articles which are blue1
 often appear green1 by candle-light8   
,mrs4 ,b4 ,the light of a candle1 is not
 of so pure a white as that of the sun3 
it has a yellowish tinge1 and when      
refracted by the prism1 the yellow rays 
predominate2 and blue bodies reflect    
some of the yellow rays1 from their     
being next to the blue1 in the order of 
refrangibility2 the superabundance of   
yellow rays1 which is supplied by the   
candle1 gives to blue bodies1 a greenish
 hue4  ,caroline4 ,candle-light must    
then give to all bodies1 a yellowish    

tinge1 from the excess of yellow rays2  
and yet it is a common remark1 that     
people of a sallow complexion1 appear   
fairer1 or whiter1 by candle-light4     
,mrs4 ,b4 ,the yellow cast of their     
complexion is not so striking1 when     
every surrounding object has a yellow   
tinge4  ,emily4 ,pray1 why does the sun 
appear red1 through a fog8              
,illustration3 ,,plate ,,xxi4  ,mrs4 ,b4
 ,it is supposed to be owing to the     
rays1 which are most refrangible1 being 
also the most easily reflected3 in      
passing through an atmosphere1 loaded   
with moisture1 as in foggy weather1 and 
also in the morning and evening1 when   
mists prevail1 the violet1 indigo1 blue1
 and green rays1 are reflected back by  
the particles which load the air2 whilst
 the yellow1 orange1 and red rays1 being
 less susceptible of reflection1 pass   
on1 and reach the eye4  ,caroline4 ,and1
 pray1 why is the sky of a blue colour8 
,mrs4 ,b4 ,you should rather say1 the   
atmosphere2 for the sky is a very vague 

term1 the meaning of which1 it would be 
difficult to define1 philosophically4   
,caroline4 ,but the colour of the       
atmosphere should be white1 since all   
the rays traverse it1 in their passage  
to the earth4  ,mrs4 ,b4 ,do not forget 
that the direct rays of light which pass
 from the sun to the earth1 do not meet 
our eyes1 excepting when we are looking 
at that luminary1 and thus intercept    
them2 in which case1 you know1 that the 
sun appears white4 ,the atmosphere is a 
transparent medium1 through which the   
sun's rays pass freely to the earth2 but
 the particles of which it is composed1 
also reflect the rays of light1 and it  
appears that they possess the property  
of reflecting the blue rays1 the most   
copiously3 the light1 therefore1 which  
is reflected back into the atmosphere1  
from the surface of the earth1 falls    
upon these particles of air1 and the    
blue rays are returned by reflection3   
this reflection is performed in every   
possible direction2 so that whenever we 

look at the atmosphere1 some of these   
rays fall upon our eyes2 hence we see   
the air of a blue colour4 ,if the       
atmosphere did not reflect any rays1    
though the objects1 on the surface of   
the earth1 would be illuminated1 the sky
 would appear perfectly black4          
,caroline4 ,oh1 how melancholy would    
that be2 and how pernicious to the      
sight1 to be constantly viewing bright  
objects against a black sky4 ,but what  
is the reason that bodies often change  
their colour2 as leaves1 which wither in
 autumn1 or a spot of ink1 which        
produces an iron-mould on linen8  ,mrs4 
,b4 ,it arises from some chemical       
change1 which takes place in the        
arrangement of the component parts2 by  
which they lose their tendency to       
reflect certain colours1 and acquire the
 power of reflecting others4 ,a withered
 leaf thus no linger reflects the blue  
rays2 it appears1 therefore1 yellow1 or 
has a slight tendency to reflect several
 rays1 which produce a dingy brown      

colour4  ,an ink spot on linen1 at first
 absorbs all the rays2 but1 from the    
action of soap1 or of some other agent1 
it undergoes a chemical change1 and the 
spot partially regains its tendency to  
reflect colours1 but with a preference  
to reflect the yellow rays1 and such is 
the colour of the iron-mould4  ,emily4  
,bodies1 then1 far from being of the    
colour which they appear to possess1 are
 of that colour to which they have the  
greatest aversion1 with which they will 
not incorporate1 but reject1 and drive  
from them4  ,mrs4 ,b4 ,it certainly is  
so2 though ,i scarcely dare venture to  
advance such an opinion1 whilst         
,caroline is contemplating her beautiful
 rose4  ,caroline4 ,my poor rose6 you   
are not satisfied with depriving it of  
colour1 but even make it have an        
aversion to it2 and ,i am unable to     
contradict you4  ,emily4 ,since dark    
bodies1 absorb more solar rays than     
light ones1 the former should sooner be 
heated if exposed to the sun8  ,mrs4 ,b4

 ,and they are found1 by experience1 to 
be so4 ,have you never observed a black 
dress1 to be warmer than a white one8   
,emily4 ,yes1 and a white one more      
dazzling3 the black is heated by        
absorbing the rays1 the white is        
dazzling1 by reflecting them4           
,caroline4 ,and this was the reason that
 the brown paper was burnt in the focus 
of the lens1 whilst the white paper     
exhibited the most luminous spot1 but   
did not take fire4  ,mrs4 ,b4 ,it was   
so4 ,it is now full time to conclude our
 lesson4 ,at our next meeting1 ,i shall 
give you a description of the eye4      
,questions  #a4 7,pg4 #agi7 ,what is    
meant by the refraction of light8  #b4  
7,pg4 #agi7 ,what is believed to be the 
cause of refraction8  #c4 7,pg4 #ahj7   
,how is a ray refracted in passing      
obliquely from air into water8  #d4     
7,pg4 #ahj7 ,how is this refraction     
explained in fig4 #a1 plate #ai8  #e4   
7,pg4 #ahj7 ,what is fig4 #b intended to
 explain8  #f4 7,pg4 #ahj7 ,what is the 

rule respecting refraction1 by different
 mediums8  #g4 7,pg4 #aha7 ,what is     
meant by the perpendicular8  #h4 7,pg4  
#aha7 ,how does fig4 #c1 plate #ai1     
elucidate the law of refraction8  #i4   
7,pg4 #aha7 ,what will be the effect on 
the apparent situation of the flower8   
#aj4 7,pg4 #aha7 ,what effect has       
refraction upon the apparent depth of a 
stream of water8  #aa4 7,pg4 #ahb7 ,how 
does the atmosphere refract the rays of 
the sun1 as represented1 fig4 #d8  #ab4 
7,pg4 #ahb7 ,why have we the rays of the
 sun always refracted8  #ac4 7,pg4 #ahb7
 ,what length of time is required for   
light to travel from the sun1 to the    
earth8  #ad4 7,pg4 #ahb7 ,what effect   
has this upon his apparent place8  #ae4 
7,pg4 #ahb7 ,how is the length of the   
day affected by refraction8  #af4 7,pg4 
#ahc7 ,how are rays refracted1 which    
fall obliquely upon a flat pane of      
glass1 7fig4 #e1 plate #ai87  #ag4 7,pg4
 #ahc7 ,what is the reason that objects 
are distorted1 when seen through common 

window glass8  #ah4 7,pg4 #ahd7 ,what is
 meant by a lens8  #ai4 7,pg4 #ahd7     
,what are the five kinds called1        
represented at fig4 #a1 plate #bj8  #bj4
 7,pg4 #ahd7 ,what is meant by the axis 
of a lens8  #ba4 7,pg4 #ahd7 ,how are   
parallel rays1 refracted by the double  
convex lens1 fig4 #f1 plate #ai8  #bb4  
7,pg4 #ahd7 ,what is meant by the focus 
of a lens8  #bc4 7,pg4 #ahd7 ,what is   
the focal distance of parallel rays1    
from a double convex lens8  #bd4 7,pg4  
#ahd7 ,how are the rays refracted by a  
concave lens1 fig4 #g1 plate #ai8  #be4 
7,pg4 #ahe7 ,what is the effect of one  
plane side in a lens8  #bf4 7,pg4 #ahe7 
,how is the focus of the plano-convex   
lens situated1 fig4 #b1 plate #bj8  #bg4
 7,pg4 #ahe7 ,how does a convex lens    
magnify objects1 fig4 #f1 plate #ai8    
#bh4 7,pg4 #ahe7 ,what is the article   
denominated which is represented at fig4
 #c1 plate #bj8  #bi4 7,pg4 #ahe7 ,how  
will a ray be refracted1 which enters on
 one side of the prism1 in the direction

 ,a ,b8  #cj4 7,pg4 #ahe7 ,what effect  
is produced by this refraction1 as      
represented in fig4 #d1 plate #bj8  #ca4
 7,pg4 #ahf7 ,of what are the rays of   
white light said to be composed8  #cb4  
7,pg4 #ahf7 ,what colours are produced8 
#cc4 7,pg4 #ahf7 ,by what property1 in  
light1 does refraction enable us to     
separate these different rays8  #cd4    
7,pg4 #ahg7 ,what experiment may be     
performed with a piece of card1 so as to
 exemplify the compound nature of light8
  #ce4 7,pg4 #ahg7 ,how can the same be 
shown by a lens1 fig4 #e4 plate #bj8    
#cf4 7,pg4 #ahg7 ,is it certain that    
there are seven primitive colours in the
 spectrum8  #cg4 7,pg4 #ahh7 ,how is the
 rainbow produced1 and what is necessary
 to its production8  #ch4 7,pg4 #ahh7   
,how are the solar rays affected by a   
convex lens8  #ci4 7,pg4 #ahh7 ,why is  
such a lens1 called a burning glass8    
#dj4 7,pg4 #ahh7 ,why are bodies of a   
dark colour1 more readily inflamed1 than
 those which are white8  #da4 7,pg4     

#ahi7 ,what is believed to be the       
reason1 why some bodies absorb more rays
 than others8  #db4 7,pg4 #ahi7 ,what   
determines the colour of any particular 
body8  #dc4 7,pg4 #ahi7 ,what           
exemplifications are given8  #dd4 7,pg4 
#ahi7 ,by what reasoning is it proved1  
that bodies do not retain their colours 
in the dark8  #de4 7,pg4 #aij7 ,what    
proof of the truth of this theory of    
colours1 may be afforded by the prism8  
#df4 7,pg4 #aia7 ,why will green leaves1
 when exposed to the red ray1 appear of 
a dingy brown8  #dg4 7,pg4 #aia7        
,bodies1 in general1 when placed in a   
ray differing in colour from their own1 
appear of a mixed hue1 what causes this8
  #dh4 7,pg4 #aia7 ,why will bodies of a
 pale1 or light hue1 most perfectly1    
assume the different colours of the     
spectrum8  #di4 7,pg4 #aib7 ,upon what  
property in a body1 does the darkness of
 its colour depend8  #ej4 7,pg4 #aib7   
,why do some bodies appear white1 others
 black1 and others of different colours8

  #ea4 7,pg4 #aib7 ,from what cause do  
blue articles appear green1 by          
candle-light8  #eb4 7,pg4 #aic7 ,what is
 believed to be the cause1 of the red   
appearance of the sun1 through a fog1 or
 misty atmosphere8  #ec4 7,pg4 #aic7    
,from what is the blue colour of the    
sky1 thought to arise8  #ed4 7,pg4 #aic7
 ,what would be the colour of the sky1  
did not the atmosphere reflect light8   
#ee4 7,pg4 #aic7 ,from what cause do    
some bodies change their colour1 as     
leaves formerly green1 become brown1 and
 ink1 yellow8  #ef4 7,pg4 #aid7 ,why is 
a black dress1 warmer in the sunshine1  
than a white one of the same texture8   
,,conversation ,,xvii4  ,,on ,,the      
,,structure ,,of ,,the ,,eye1 ,,and     
,,optical ,,instruments4  ,,description 
,,of ,,the ,,eye4 ,,of ,,the ,,image    
,,on ,,the ,,retina4 ,,refraction ,,by  
,,the ,,humours ,,of ,,the ,,eye4 ,,of  
,,the ,,use ,,of ,,spectacles4 ,,of     
,,the ,,single ,,microscope4 ,,of ,,the 
,,double ,,microscope4 ,,of ,,the       

,,solar ,,microscope4 ,,magic           
,,lanthorn4 ,,refracting ,,telescope4   
,,reflecting ,,telescope4   ,,mrs4 ,b4  
,the body of the eye1 is of a spherical 
form3 7fig4 #a4 plate #ba47 it has two  
membranous coats1 or coverings2 the     
external one1 a a a1 is called the      
sclerotica1 this is commonly known under
 the name of the white of the eye2 it   
has a projection in that part of the eye
 which is exposed to view1 b b1 which is
 called the transparent cornea1 because1
 when dried1 it has nearly the          
consistence of very fine horn1 and is   
sufficiently transparent for the light  
to obtain free passage through it4  ,the
 second membrane which lines the cornea1
 and envelops the eye1 is called the    
choroid1 c c c2 this has an opening in  
front1 just beneath the cornea1 which   
forms the pupil1 or sight of the eye1 d 
d1 through which the rays of light pass 
into the eye4 ,the pupil is surrounded  
by a coloured border called the iris1 e 
e1 which1 by its muscular motion1 always

 preserves the pupil of a circular form1
 whether it is expanded in the dark1 or 
contracted by a strong light4 ,this you 
will understand better by examining fig4
 #b4  ,emily4 ,i did not know that the  
pupil was susceptible of varying its    
dimensions4  ,mrs4 ,b4 ,the construction
 of the eye is so admirable1 that it is 
capable of adapting itself1 more or     
less1 to the circumstances in which it  
is placed4 ,in a faint light1 the pupil 
dilates so as to receive an additional  
quantity of rays1 and in a strong light1
 it contracts1 in order to prevent the  
intensity of the light from injuring the
 optic nerve4 ,observe ,emily's eyes1 as
 she sits looking towards the windows3  
the pupils appear very small1 and the   
iris1 large4 ,now1 ,emily1 turn from the
 light1 and cover your eyes with your   
hand1 so as entirely to exclude it1 for 
a few moments4  ,caroline4 ,how very    
much the pupils of her eyes are now     
enlarged1 and the iris diminished6 ,this
 is1 no doubt1 the reason why the eyes  

suffer pain1 when from darkness1 they   
suddenly come into a strong light2 for  
the pupil being dilated1 a quantity of  
rays must rush in1 before it has time to
 contract4  ,emily4 ,and when we go from
 a strong light1 into obscurity1 we at  
first imagine ourselves in total        
darkness2 for a sufficient number of    
rays cannot gain admittance into the    
contracted pupil1 to enable us to       
distinguish objects3 but in a few       
minutes it dilates1 and we clearly      
perceive objects which were before      
invisible4  ,mrs4 ,b4 ,it is just so4   
,the choroid c c1 is embued with a black
 liquor1 which serves to absorb all the 
rays that are irregularly reflected1 and
 to convert the body of the eye1 into a 
more perfect camera obscura4 ,when the  
pupil is expanded to its utmost extent1 
it is capable of admitting ten times the
 quantity of light1 that it does when   
most contracted4 ,in cats1 and animals  
which are said to see in the dark1 the  
power of dilatation and contraction of  

the pupil1 is still greater2 it is      
computed that the pupils of their eyes  
may admit one hundred times more light  
at one time than at another4  ,within   
these coverings of the eye-ball1 are    
contained1 three transparent substances1
 called humours4 ,the first occupies the
 space immediately behind the cornea1   
and is called the aqueous humour1 f f1  
from its liquidity and its resemblance  
to water4 ,beyond this1 is situated the 
crystalline humour1 g g1 so called from 
its clearness and transparency3 it has  
the form of a lens1 and refracts the    
rays of light in a greater degree of    
perfection1 than any that have been     
constructed by art3 it is attached by   
two muscles1 m m1 to each side of the   
choroid4 ,the back part of the eye1     
between the crystalline humour and the  
retina1 is filled by the vitreous       
humour1 h h1 which derives its name from
 a resemblance it is supposed to bear1  
to glass1 or vitrified substances4      
,illustration3 ,,plate ,,xxii4  ,the    

membranous coverings of the eye are     
intended chiefly for the preservation of
 the retina1 i i1 which is by far the   
most important part of the eye1 as it is
 that which receives the impression of  
the objects of sight1 and conveys it to 
the mind4 ,the retina is formed by the  
expansion of the optic nerve1 and is of 
a most perfect whiteness3 this nerve    
proceeds from the brain1 enters the eye1
 at n1 on the side next the nose1 and is
 finely spread over the interior surface
 of the choroid4  ,the rays of light    
which enter the eye1 by the pupil1 are  
refracted by the several humours in     
their passage through them1 and unite in
 a focus on the retina4  ,caroline4 ,i  
do not understand the use of these      
refracting humours3 the image of objects
 was represented in the camera obscura1 
without any such assistance4  ,mrs4 ,b4 
,that is true2 but the representation   
became much more strong and distinct1   
when we enlarged the opening of the     
camera obscura1 and received the rays   

into it1 through a lens4  ,i have told  
you1 that rays proceed from bodies in   
all possible directions4 ,we must1      
therefore1 consider every part of an    
object which sends rays to our eyes1 as 
points from which the rays diverge1 as  
from a centre4  ,emily4 ,these divergent
 rays1 issuing from a single point1 ,i  
believe you told us1 were called a      
pencil of rays8  ,mrs4 ,b4 ,yes4 ,now1  
divergent rays1 on entering the pupil1  
do not cross each other2 the pupil1     
however1 is sufficiently large to admit 
a small pencil of them2 and these1 if   
not refracted to a focus1 by the        
humours1 would continue diverging after 
they had passed the pupil1 would fall   
dispersed upon the retina1 and thus the 
image of a single point1 would be       
expanded over a large portion of the    
retina4 ,the divergent rays from every  
other point of the object1 would be     
spread over a similar extent of space1  
and would interfere and be confounded   
with the first2 so that no distinct     

image could be formed1 and the          
representation on the retina would be   
confused1 both in figure and colour4    
,fig4 #c4 represents two pencils of     
rays1 issuing from two points of the    
tree1 ,a ,b1 and entering the pupil ,c1 
refracted by the crystalline humour ,d1 
and forming on the retina1 at a b1      
distinct images of the spot they proceed
 from4 ,fig4 #d4 differs from the       
preceding1 merely from not being        
supplied with a lens2 in consequence of 
which1 the pencils of rays are not      
refracted to a focus1 and no distinct   
image is formed on the retina4 ,i have  
delineated only the rays issuing from   
two points of an object1 and            
distinguished the two pencils in fig4   
#d4 by describing one of them with      
dotted lines3 the interference of these 
two pencils of rays on the retina1 will 
enable you to form an idea of the       
confusion which would arise1 from       
thousands and millions of points1 at the
 same instant pouring their divergent   

rays upon the retina4  ,emily4 ,true2   
but ,i do not yet well understand1 how  
the refracting humours1 remedy this     
imperfection4  ,mrs4 ,b4 ,the refraction
 of these several humours1 unites the   
whole of a pencil of rays1 proceeding   
from any one point of an object1 to a   
corresponding point on the retina1 and  
the image is thus rendered distinct and 
strong4 ,if you conceive1 in fig4 #c41  
every point of the tree to send forth a 
pencil of rays1 similar to those from ,a
 ,b1 every part of the tree will be as  
accurately represented on the retina1 as
 the points a b4  ,emily4 ,how          
admirably1 how wonderfully1 is this     
contrived6  ,caroline4 ,but since the   
eye absolutely requires refracting      
humours1 in order to have a distinct    
representation formed on the retina1 why
 is not the same refraction equally     
necessary1 for the images formed in the 
camera obscura8  ,mrs4 ,b4 ,it is2      
excepting the aperture through which we 
receive the rays into the camera        

obscura1 is extremely small2 so that but
 very few of the rays diverging from a  
point1 gain admittance2 but when we     
enlarged the aperture1 and furnished it 
with a lens1 you found the landscape    
more perfectly represented4  ,caroline4 
,i remember how obscure and confused the
 image was1 when you enlarged the       
opening1 without putting in the lens4   
,mrs4 ,b4 ,such1 or very similar1 would 
be the representation on the retina1    
unassisted by the refracting humours4   
,you will now be able to understand the 
nature of that imperfection of sight1   
which arises from the eyes being too    
prominent4 ,in such cases1 the          
crystalline humour1 ,d1 7fig4 #e47 being
 extremely convex1 refracts the rays too
 much1 and collects a pencil1 proceeding
 from the object ,a ,b1 into a focus1   
,f1 before they reach the retina4 ,from 
this focus1 the rays proceed1 diverging1
 and consequently form a very confused  
image on the retina1 at a b4 ,this is   
the defect in short-sighted people4     

,emily4 ,i understand it perfectly4 ,but
 why is this defect remedied by bringing
 the object nearer to the eye1 as we    
find to be the case with short-sighted  
people8  ,mrs4 ,b4 ,the nearer you bring
 an object to your eye1 the more        
divergent the rays fall upon the        
crystalline humour1 and consequently    
they are not so soon converged to a     
focus3 this focus1 therefore1 either    
falls upon the retina1 or at least      
approaches nearer to it1 and the object 
is proportionally distinct1 as in fig4  
#f4  ,emily4 ,the nearer1 then1 you     
bring an object to a lens1 the further  
the image recedes behind it4  ,mrs4 ,b4 
,certainly4 ,but short-sighted persons  
have another resource1 for objects which
 they can not bring near to their eyes2 
this is1 to place a concave lens1 ,c ,d1
 7fig4 #a1 plate #bb47 before the eye1  
in order to increase the divergence of  
the rays4 ,the effect of a concave lens1
 is1 you know1 exactly the reverse of a 
convex one3 it renders parallel rays    

divergent1 and those which are already  
divergent1 still more so4 ,by the       
assistance of such glasses1 therefore1  
the rays from a distant object1 fall on 
the pupil1 as divergent as those from a 
less distant object2 and1 with          
short-sighted people1 they throw the    
image of a distant object1 back1 as far 
as the retina4  ,caroline4 ,this is an  
excellent contrivance1 indeed4  ,mrs4   
,b4 ,and tell me1 what remedy would you 
devise for such persons as have a       
contrary defect in their sight2 that is 
to say1 who are long-sighted1 in whom   
the crystalline humour1 being too flat1 
does not refract the rays sufficiently1 
so that they reach the retina before    
they are converged to a point8          
,caroline4 ,i suppose that a contrary   
remedy must be applied to this defect2  
that is to say1 a convex lens1 ,l ,m1   
fig4 #b1 to make up for the deficiency  
of convexity of the crystalline humour1 
,o ,p4 ,for the convex lens would bring 
the rays nearer together1 so that they  

would fall1 either less divergent1 or   
parallel1 on the crystalline humour2    
and1 by being sooner converged to a     
focus1 would fall on the retina4  ,mrs4 
,b4 ,very well1 ,caroline4 ,this is the 
reason why elderly people1 the humours  
of whose eyes are decayed by age1 are   
under the necessity of using convex     
spectacles4 ,and when deprived of that  
resource1 they hold the object at a     
distance from their eyes1 as in fig4 #c1
 in order to bring the focus more       
forward4  ,caroline4 ,i have often been 
surprised1 when my grandfather reads    
without his spectacles1 to see him hold 
the book at a considerable distance from
 his eyes4 ,but ,i now understand the   
cause2 the more distant the object is   
from the crystalline lens1 the nearer to
 it1 will the image be formed4  ,emily4 
,i comprehend the nature of these two   
opposite defects very well2 but ,i      
cannot now conceive1 how any sight can  
be perfect3 for1 if the crystalline     
humour is of a proper degree of         

convexity1 to bring the image of distant
 objects to a focus on the retina1 it   
will not represent near objects         
distinctly2 and if1 on the contrary1 it 
is adapted to give a clear image of near
 objects1 it will produce a very        
imperfect one1 of distant objects4      
,mrs4 ,b4 ,your observation is very     
good1 ,emily2 and it is true1 that every
 person would be subject to one of these
 two defects1 if we had it not in our   
power to adapt the eye1 to the distance 
of the object2 it is believed that this 
is accomplished1 by our having a command
 over the crystalline lens1 so as to    
project it towards1 or draw it back from
 the object1 as circumstances require1  
by means of the two muscles1 to which   
the crystalline humour is attached2 so  
that the focus of the rays1 constantly  
falls on the retina1 and an image is    
formed equally distinct1 either of      
distant objects1 or of those which are  
near4  ,caroline4 ,in the eyes of       
fishes1 which are the only eyes ,i have 

ever seen separate from the head1 the   
cornea does not protrude1 in that part  
of the eye which is exposed to view4    
,mrs4 ,b4 ,the cornea of the eye of a   
fish is not more convex than the rest of
 the ball of the eye2 but to supply this
 deficiency1 their crystalline humour is
 spherical1 and refracts the rays so    
much1 that it does not require the      
assistance of the cornea to bring them  
to a focus on the retina4  ,emily4      
,pray1 what is the reason that we cannot
 see an object distinctly1 if we place  
it very near to the eye8  ,mrs4 ,b4     
,because the rays fall on the           
crystalline humour1 too divergent to be 
refracted to a focus on the retina2 the 
confusion1 therefore1 arising from      
viewing an object too near the eye1 is  
similar to that which proceeds from a   
flattened crystalline humour2 the rays  
reach the retina before they are        
collected to a focus1 7fig4 #d47 ,if it 
were not for this imperfection1 we      
should be able to see and distinguish   

the parts of objects1 which1 from their 
minuteness1 are now invisible to us2    
for1 could we place them very near the  
eye1 the image on the retina would be so
 much magnified1 as to render them      
visible4  ,emily4 ,and could there be no
 contrivance1 to convey the rays of     
objects viewed1 close to the eye1 so    
that they should be refracted to a focus
 on the retina8  ,mrs4 ,b4 ,the         
microscope is constructed for this      
purpose4 ,the single microscope 7fig4   
#e47 consists simply of a convex lens1  
commonly called a magnifying glass2 in  
the focus of which the object is placed1
 and through which it is viewed3 by this
 means1 you are enabled to place your   
eye very near to the object1 for the    
lens ,a ,b1 by diminishing the          
divergence of the rays1 before they     
enter the pupil ,c1 makes them fall     
parallel on the crystalline humour ,d1  
by which they are refracted to a focus  
on the retina1 at ,r ,r4  ,emily4 ,this 
is a most admirable invention1 and      

nothing can be more simple2 for the lens
 magnifies the object1 merely by        
allowing us to bring it nearer to the   
eye4  ,illustration3 ,,plate ,,xxiii4   
,mrs4 ,b4 ,those lenses1 therefore1     
which have the shortest focus will      
magnify the object most1 because they   
enable us to place it nearest to the    
eye4  ,emily4 ,but a lens1 that has the 
shortest focus1 is most bulging or      
convex2 and the protuberance of the lens
 will prevent the eye from approaching  
very near to the object4  ,mrs4 ,b4     
,this is remedied by making the lens    
extremely small3 it may then be         
spherical without occupying much space1 
and thus unite the advantages of a short
 focus1 and of allowing the eye to      
approach the object4  ,there is a mode  
of magnifying objects1 without the use  
of a lens3 if you look through a hole1  
not larger than a small pin1 you may    
place a minute object near to the eye1  
and it will be distinct1 and greatly    
enlarged4 ,this piece of tin has been   

perforated for the purpose2 place it    
close to your eye1 and this small print 
before it4  ,caroline4 ,astonishing6 the
 letters appear ten times as large as   
they do without it3 ,i cannot conceive  
how this effect is produced4  ,mrs4 ,b4 
,the smallness of the hole1 prevents the
 entrance into the eye1 of those parts  
of every pencil of rays which diverge   
much2 so that1 notwithstanding the      
nearness of the object1 those rays from 
it1 which enter the eye1 are nearly     
parallel1 and are1 therefore1 brought to
 a focus by the humours of the eye4     
,caroline4 ,we have a microscope at     
home1 which is a much more complicated  
instrument than that you have described4
  ,mrs4 ,b4 ,it is a double microscope1 
7fig4 #f47 in which you see1 not the    
object ,a ,b1 but a magnified image of  
it1 a b4 ,in this microscope1 two lenses
 are employed2 the one1 ,l ,m1 for the  
purpose of magnifying the object1 is    
called the object-glass1 the other1 ,n  
,o1 acts on the principle of the single 

microscope1 and is called the eye-glass4
  ,there is another kind of microscope1 
called the solar microscope1 which is   
the most wonderful from its great       
magnifying power3 in this we also view  
an image formed by a lens1 not the      
object itself4 ,as the sun shines1 ,i   
can show you the effect of this         
microscope2 but for this purpose1 we    
must close the shutters1 and admit only 
a small portion of light1 through the   
hole in the window-shutter1 which we    
used for the camera obscura4 ,we shall  
now place the object ,a ,b1 7plate #bc1 
fig4 #a47 which is a small insect1      
before the lens ,c ,d1 and nearly at its
 focus3 the image ,e ,f1 will then be   
represented on the opposite wall1 in the
 same manner1 as the landscape was in   
the camera obscura2 with this           
difference1 that it will be magnified1  
instead of being diminished4 ,i shall   
leave you to account for this1 by       
examining the figure4  ,emily4 ,i see it
 at once4 ,the image ,e ,f is magnified1

 because it is farther from the lens1   
than the object ,a ,b2 while the        
representation of the landscape was     
diminished1 because it was nearer the   
lens1 than the landscape was4 ,a lens1  
then1 answers the purpose equally well1 
either for magnifying or diminishing    
objects8  ,mrs4 ,b4 ,yes3 if you wish to
 magnify the image1 you place the object
 near the focus of the lens2 if you wish
 to produce a diminished image1 you     
place the object at a distance from the 
lens1 in order that the image may be    
formed in1 or near the focus4           
,caroline4 ,the magnifying power of this
 microscope is prodigious3 but the      
indistinctness of the image1 for want of
 light1 is a great imperfection4 ,would 
it not be clearer1 if the opening in the
 shutter were enlarged1 so as to admit  
more light8  ,mrs4 ,b4 ,if the whole of 
the light admitted1 does not fall upon  
the object1 the effect will only be to  
make the room lighter1 and the image    
consequently less distinct4  ,emily4    

,but could you not by means of another  
lens1 bring a large pencil of rays to a 
focus on the object1 and thus           
concentrate upon it the whole of the    
light admitted8  ,mrs4 ,b4 ,very well4  
,we shall enlarge the opening1 and place
 the lens ,x ,y 7fig4 #b47 in it1 to    
converge the rays to a focus on the     
object ,a ,b4 ,there is but one thing   
more wanting to complete the solar      
microscope1 which ,i shall leave to     
,caroline's sagacity to discover4       
,caroline4 ,our microscope has a small  
mirror attached to it1 upon a moveable  
joint1 which can be so adjusted as to   
receive the sun's rays1 and reflect them
 upon the object3 if a similar mirror   
were placed to reflect light upon the   
lens1 would it not be a means of        
illuminating the object more perfectly8 
,mrs4 ,b4 ,you are quite right4 ,p ,q   
7fig4 #b47 is a small mirror1 placed on 
the outside of the window-shutter1 which
 receives the incident rays ,s ,s1 and  
reflects them on the lens ,x ,y4 ,now   

that we have completed the apparatus1   
let us examine the mites on this piece  
of cheese1 which ,i place near the focus
 of the lens4  ,caroline4 ,oh1 how much 
more distinct the image now is1 and how 
wonderfully magnified6 ,the mites on the
 cheese look like a drove of pigs       
scrambling over rocks4  ,emily4 ,i never
 saw any thing so curious4 ,now1 an     
immense piece of cheese has fallen3 one 
might imagine it an earthquake3 some of 
the poor mites must have been crushed2  
how fast they run--they absolutely seem 
to gallop4  ,but this microscope can be 
used only for transparent objects2 as   
the light must pass through them1 to    
form the image on the wall8  ,mrs4 ,b4  
,very minute objects1 such as are viewed
 in a microscope1 are generally         
transparent1 but when opaque objects are
 to be exhibited1 a mirror ,m ,n 7fig4  
#c47 is used to reflect the light on the
 side of the object next the wall3 the  
image is then formed by light reflected 
from the object1 instead of being       

transmitted through it4  ,emily4 ,pray1 
is not a magic lanthorn constructed on  
the same principles8  ,mrs4 ,b4 ,yes1   
with this difference2 the objects to be 
magnified1 are painted upon pieces of   
glass1 and the light is supplied by a   
lamp1 instead of the sun4  ,the         
microscope is an excellent invention to 
enable us to see and distinguish        
objects1 which are too small to be      
visible to the naked eye4 ,but there are
 objects1 which1 though not really      
small1 appear so to us1 from their      
distance2 to these1 we cannot apply the 
same remedy2 for when a house is so far 
distant1 as to be seen under the same   
angle as a mite which is close to us1   
the effect produced on the retina is the
 same3 the angle it subtends is not     
large enough for it to form a distinct  
image on the retina4  ,emily4 ,since it 
is impossible1 in this case1 to make the
 object approach the eye1 cannot we by  
means of a lens bring an image of it1   
nearer to us8  ,mrs4 ,b4 ,yes2 but then 

the object being very distant from the  
focus of the lens1 the image would be   
too small to be visible to the naked    
eye4  ,emily4 ,then1 why not look at the
 image through another lens1 which will 
act as a microscope1 enable us to bring 
the image close to the eye1 and thus    
render it visible8  ,mrs4 ,b4 ,very     
well1 ,emily2 ,i congratulate you on    
having invented a telescope4 ,in figure 
#d1 the lens ,c ,d1 forms an image ,e   
,f1 of the object ,a ,b2 and the lens ,x
 ,y1 serves the purpose of magnifying   
that image2 and this is all that is     
required in a common refracting         
telescope4  ,emily4 ,but in fig4 #d1 the
 image is not inverted on the retina1 as
 objects usually are3 it should         
therefore appear to us inverted2 and    
that is not the case in the telescopes  
,i have looked through4  ,mrs4 ,b4 ,when
 it is necessary to represent the image 
erect1 two other lenses are required2 by
 which means a second image is formed1  
the reverse of the first1 and           

consequently upright4 ,these additional 
glasses are used to view terrestrial    
objects2 for no inconvenience arises    
from seeing the celestial bodies        
inverted4  ,emily4 ,the difference      
between a microscope and a telescope1   
seems to be this3--a microscope produces
 a magnified image1 because the object  
is nearest the lens2 and a telescope    
produces a diminished image1 because the
 object is furthest from the lens4      
,mrs4 ,b4 ,your observation applies only
 to the lens ,c ,d1 or object-glass1    
which serves to bring an image of the   
object nearer the eye2 for the lens ,x  
,y1 or eye-glass1 is1 in fact1 a        
microscope1 as its purpose is to magnify
 the image4  ,when a very great         
magnifying power is required1 telescopes
 are constructed with concave mirrors1  
instead of lenses4 ,these are called    
reflecting telescopes1 because the image
 is reflected by metallic mirrors4      
,concave mirrors1 you know1 produce by  
reflection1 an effect similar to that of

 convex lenses1 by refraction4 ,in      
reflecting telescopes1 therefore1       
mirrors are used in order to bring the  
image nearer the eye2 and a lens1 or    
eye-glass1 the same as in the refracting
 telescope1 to magnify the image4  ,the 
advantage of the reflecting telescope   
is1 that mirrors whose focus is six     
feet1 will magnify as much as lenses of 
a hundred feet3 an instrument of this   
kind may1 therefore1 possess a high     
magnifying power1 and yet be so short1  
as to be readily managed4  ,caroline4   
,but ,i thought it was the eye-glass    
only which magnified the image2 and that
 the other lens1 served to bring a      
diminished image nearer to the eye4     
,mrs4 ,b4 ,the image is diminished in   
comparison with the object1 it is true2 
but it is magnified1 if you compare it  
to the dimensions of which it would     
appear without the intervention of any  
optical instrument2 and this magnifying 
power is greater in reflecting1 than in 
refracting telescopes4  ,we must now    

bring our observations to a conclusion1 
for ,i have communicated to you the     
whole of my very limited stock of       
knowledge of ,natural ,philosophy4 ,if  
it enable you to make further progress  
in that science1 my wishes will be      
satisfied2 but remember1 in order that  
the study of nature may be productive of
 happiness1 it must lead to an entire   
confidence in the wisdom and goodness of
 its bounteous ,author4   ,questions    
#a4 7,pg4 #aie7 ,what is the form of the
 body of the eye8 fig4 #a1 plate #ba4   
#b4 7,pg4 #aie7 ,what is its external   
coat called8  #c4 7,pg4 #aie7 ,what is  
the transparent part of this coat       
denominated8  #d4 7,pg4 #aie7 ,what is  
the second coat named8  #e4 7,pg4 #aie7 
,what opening is there in this8  #f4    
7,pg4 #aie7 ,what is the coloured part  
which surrounds the pupil8  #g4 7,pg4   
#aie7 ,the pupils dilate and contract1  
what purpose does this answer8  #h4     
7,pg4 #aif7 ,how could you observe the  
dilatation and contraction of the       

pupils8  #i4 7,pg4 #aif7 ,what purpose  
is the choroid said to answer8  #aj4    
7,pg4 #aif7 ,in what animals is the     
change in the iris greatest8  #aa4 7,pg4
 #aif7 ,what are the three humours      
denominated1 and how are they situated8 
#ab4 7,pg4 #aig7 ,what is the part      
represented at i i1 and of what does it 
consist8  #ac4 7,pg4 #aig7 ,what are the
 respective uses of the humours1 and of 
the retina8  #ad4 7,pg4 #aig7 ,why is it
 necessary the rays should be refracted8
  #ae4 7,pg4 #aig7 ,how is this         
illustrated by fig4 #c and #d1 plate    
#ba8  #af4 7,pg4 #aih7 ,what causes a   
person to be short-sighted8 fig4 #e1    
plate #ba4  #ag4 7,pg4 #aih7 ,why does  
placing an object near the eye1 enable  
such1 to see distinctly8 fig4 #f4  #ah4 
7,pg4 #aii7 ,a concave lens remedies    
this defect2 how8 fig4 #a1 plate #bb4   
#ai4 7,pg4 #aii7 ,what is the remedy1   
when a person is long-sighted8 fig4 #b4 
#bj4 7,pg4 #aii7 ,why does holding an   
object far from the eye1 help such      

persons8 fig4 #c4  #ba4 7,pg4 #bjj7 ,how
 is the eye said to adapt itself to     
distant1 and to near objects8  #bb4     
7,pg4 #bjj7 ,why are objects rendered   
indistinct1 when placed very near to the
 eye8 fig4 #d1 plate #bb4  #bc4 7,pg4   
#bjj7 ,what is the single microscope1   
fig4 #e1 and how does it magnify        
objects8  #bd4 7,pg4 #bja7 ,how may     
objects be magnified without the aid of 
a lens8  #be4 7,pg4 #bja7 ,why can an   
object1 very near to the eye1 be        
distinctly seen1 when viewed through a  
small hole8  #bf4 7,pg4 #bja7 ,describe 
the double microscope1 as represented in
 fig4 #f1 plate #bb4  #bg4 7,pg4 #bjb7  
,how does the solar microscope1 7fig4 #a
 plate #bc47 operate8  #bh4 7,pg4 #bjb7 
,why may minute objects be greatly      
magnified by this instrument8  #bi4     
7,pg4 #bjb7 ,in its more perfect form it
 has other appendages1 as seen in fig4  
#b1 what are they8 and what their uses8 
#cj4 7,pg4 #bjc7 ,what is added when    
opaque objects are to be viewed8 fig4   

#c4  #ca4 7,pg4 #bjc7 ,in what does the 
magic lanthorn differ from the solar    
microscope8  #cb4 7,pg4 #bjc7 ,what are 
the use and structure of the telescope1 
as shown in fig4 #d8  #cc4 7,pg4 #bjd7  
,when terrestrial objects are to be     
viewed1 why are two additional lenses   
employed8  #cd4 7,pg4 #bjd7 ,what part  
of the telescope performs the part of a 
microscope8  #ce4 7,pg4 #bjd7 ,in what  
does the reflecting1 differ from the    
refracting telescope8  #cf4 7,pg4 #bjd7 
,what advantages1 do reflecting1 possess
 over refracting telescopes8            
,,glossary4   ,,accelerated ,,motion4   
,motion is said to be accelerated1 when 
the velocity is continually increasing4 
,,accidental ,,properties4 ,those       
properties of bodies which are liable to
 change1 as colour1 form1 c4            
,,acute4--,see ,,angle4  ,,air4 ,an     
elastic fluid4 ,the atmosphere which    
surrounds the earth1 is generally       
understood by this term1 but there are  
many kinds of air4 ,the term is         

synonymous with ,gas4  ,,air ,,pump4 ,an
 instrument by which vessels may be     
exhausted of air4  ,,altitude4 ,the     
height in degrees of the sun1 or any    
heavenly body1 above the horizon4       
,,angle4 ,the space contained between   
two lines inclined to each other1 and   
which meet in a point4 ,angles are      
measured in degrees1 upon a segment of a
 circle described by placing one leg of 
a pair of compasses on the angular      
point1 and with the other1 describing   
the segment between the two lines4 ,if  
the segment be exactly #a-dth of a      
circle1 it is called a right angle1 and 
contains #ij deg4 ,if more than #a-dth  
of a circle1 it is an obtuse angle4 ,if 
less1 an acute angle4 ,see plate #b4    
,,angle ,,of ,,incidence1 is the space  
contained between a ray which falls     
obliquely upon a body1 and a line       
perpendicular to the surface of the     
body1 at the point where the ray falls4 
,,angle ,,of ,,reflection4 ,the space   
contained between a reflected ray1 and a

 line perpendicular to the reflecting   
point4  ,,angle ,,of ,,vision1 or visual
 angle4 ,the space contained between    
lines drawn from the extreme parts of   
any object1 and meeting in the eye4     
,,antarctic ,,circle4 ,a circle         
extending round the south pole1 at the  
distance of #bc #a-b degrees from it4   
,the same as the south frigid zone4     
,,aphelion4 ,that part of the orbit of a
 planet1 in which its distance from the 
sun is the greatest4  ,,area4 ,the      
surface enclosed between the lines which
 form the boundary of any figure1       
whether regular or irregular4  ,,aries4 
,see ,,sign4  ,,asteroids4 ,the name    
given to the four small planets1 ,ceres1
 ,juno1 ,pallas1 and ,vesta4            
,,astronomy4 ,the science which treats  
of the motion and other phenomena of the
 sun1 the planets1 the stars1 and the   
other heavenly bodies4  ,,atmosphere4   
,the air which surrounds the earth1     
extending to an unknown height4 ,wind is
 this air in motion4  ,,attraction4 ,a  

tendency in bodies to approach each     
other1 and to exist in contact4         
,,attraction ,,of ,,cohesion4 ,that     
attraction which causes matter to remain
 in masses1 preventing them from falling
 into powder4 ,for this attraction to   
exist1 the particles must be contiguous4
  ,,attraction ,,of ,,gravitation4 ,by  
this attraction1 masses of matter1      
placed at a distance1 have a tendency to
 approach each other4 ,attraction is    
mutual between the sun and the planets4 
,,axis ,,of ,,the ,,earth1 ,,or ,,of    
,,any ,,of ,,the ,,planets4 ,an         
imaginary line passing through their    
centres1 and terminating at their poles2
 round this their diurnal revolutions   
are performed4  ,,axis ,,of ,,motion4   
,the imaginary line1 around which all   
the parts of a body revolve1 when it has
 a spinning motion4  ,,axis ,,of ,a     
,,lens1 ,,or ,,mirror4 ,a line passing  
through the centre of a lens1 or mirror1
 in a direction perpendicular to its    
surface4   ,,balloon4 ,any hollow globe4

 ,the term is generally applied to those
 which are made to ascend in the air4   
,,barometer4 ,commonly called a         
weather-glass4 ,it has a glass tube1    
containing quicksilver1 which by rising 
and falling1 indicates any change in the
 pressure of the atmosphere1 and thus   
frequently warns us of changes in the   
weather4  ,,body4 ,the same as ,matter4 
,it may exist in the solid1 liquid1 or  
aeriform state2 and includes every thing
 with which we become acquainted by the 
aid of the senses4  ,,burning-,,glass1  
,,or ,,mirror4 ,a lens1 or a mirror1 by 
which the rays of light1 and heat1 are  
brought to a focus1 so as to set bodies 
on fire4   ,,camera ,,obscura1 a        
darkened room2 or more frequently a box1
 admitting light by one opening1 where a
 lens is placed2 which1 bringing the    
rays of light1 from external objects1 to
 a focus1 presents a perfect picture of 
them1 in miniature4  ,,capillary        
,,tubes4 ,tubes1 the bore of which is   
very small4 ,glass tubes are usually    

employed1 to show the phenomenon of     
capillary attraction4 ,fluids in which  
they are immersed1 rise in such tubes   
above the level of that in the          
containing vessel4  ,,centre ,,of ,a    
,,circle4 ,a point1 equally distant from
 every part of its circumference4       
,,centre ,,of ,,gravity4 ,that point    
within a body1 to which all its         
particles tend1 and around which they   
exactly balance each other4 ,a system of
 bodies1 as the planets1 may have a     
common centre of gravity1 around which  
they revolve in their orbits2 whilst    
each1 like the earth1 has its particular
 centre of gravity within itself4       
,,centre ,,of ,,motion4 ,that point     
about which the parts of a revolving    
body move1 which point is1 itself1      
considered as in a state of rest4       
,,centre ,,of ,,magnitude4 ,the middle  
point of any body4 ,suppose a globe1 one
 side of which is formed of lead1 and   
the other of wood1 the centres of       
magnitude and of gravity1 would not be  

in the same points4  ,,central ,,forces4
 ,those which either impel a body       
towards1 or from1 a centre of motion4   
,,centrifugal4 ,that which gives a      
tendency to fly from a centre4          
,,centripetal4 ,that which impels a     
body1 towards a centre4  ,,circle4 ,a   
figure2 the periphery1 or circumference 
of which1 is every where equally        
distant1 from the point1 called its     
centre4  ,,circle1 ,,great4 ,on the     
globe1 or earth1 is one that divides it 
into two equal parts1 or hemispheres4   
,the equator1 and meridian lines1 are   
great circles4  ,,circle1 ,,lesser4     
,those which divide the globe into      
unequal parts4 ,the tropical1 arctic and
 antarctic circles1 and all parallels of
 latitude1 are lesser circles4          
,,circumference4 ,the boundary line of  
any surface1 as that which surrounds the
 centre of a circle2 the four sides of a
 square1 c4  ,,comets4 ,bodies which    
revolve round the sun1 in very long     
ovals1 approaching him very nearly in   

their perihelion1 but in their aphelion1
 passing to a distance immeasurably     
great4  ,,cohesion4 ,see ,,attraction4  
,,compressible4 ,capable of being forced
 into a smaller space4  ,,concave4      
,hollowed out2 the inner surface of a   
watch-glass is concave1 and may         
represent the form of a concave mirror1 
or lens4  ,,convex4 ,projecting1 or     
bulging out1 as the exterior surface of 
a watch-glass1 which may represent the  
form of a convex mirror1 or lens4       
,,cone4 ,a body somewhat resembling a   
sugar-loaf2 that is1 having a round     
base1 and sloping at the sides1 until it
 terminates in a point4  ,,conjunction4 
,when three of the heavenly bodies are  
in a straight or right line1 if you take
 either of the extreme bodies1 the other
 two are in conjunction with it2 because
 a straight line drawn from it1 might   
pass through the centres of both1 and   
join them together4 ,at the time of new 
moon1 the moon and sun are in           
conjunction with the earth2 and the moon

 and earth1 are in conjunction with the 
sun4  ,,constellation1 ,,or ,,sign4 ,a  
collection of stars4 ,astronomers have  
imagined pictures drawn in the heavens1 
so as to embrace a number of contiguous 
stars1 and have named the group after   
the animal1 or other article supposed to
 be drawn2 an individual star is        
generally designated by its fancied     
location2 as upon the ear of ,leo1 the  
,lion1 c4  ,,convergent ,,rays1 are     
those which approach each other1 so as  
eventually to meet in the same point4   
,,crystals4 ,bodies of a regular form1  
having flat surfaces1 and well defined  
angles4 ,nitre1 and other salts1 are    
familiar examples4 ,many masses of      
matter1 are composed of crystals too    
minute to be discerned without glasses4 
,,curvilinear1 consisting of a line     
which is not straight1 as a portion of a
 circle1 of an oval1 or any curved line4
  ,,cylinder4 ,a body in the form of a  
roller1 having flat circular ends1 and  
being of equal diameter throughout4     

,,degree4 ,if a circle of any size be   
divided into #cfj equal parts1 each of  
these parts is called a degree4 ,one    
quarter of a circle contains ninety     
degrees2 one twelfth of a circle1 thirty
 degrees4 ,the actual length of a       
degree1 must depend upon the size of the
 circle4 ,a degree upon the equator1    
upon a meridian1 or any great circle of 
the earth1 is equal to #fi-a_/b miles4  
,straight lines are sometimes divided   
into equal parts1 called degrees2 but   
these divisions are arbitrary1 bearing  
no relationship to the degrees upon a   
circle4  ,,density4 ,closeness of       
texture4 ,when two bodies are equal in  
bulk1 that which weighs the most1 has   
the greatest density4  ,,diagonal4 ,a   
line drawn so as to connect two remote  
angles of a square1 or other four-sided 
figure4  ,,dilatation4 ,the act of      
increasing in size4 ,bodies in general1 
dilate when heated1 and contract by     
cooling4  ,,discord4 ,when the          
vibrations of the air1 produced by two  

musical tones1 do not bear a certain    
ratio to each other1 a jarring sound is 
produced1 which is called discord4      
,,divergent ,,rays4 ,those which proceed
 from the same point1 but are           
continually receding from each other4   
,,divisibility4 ,capability of being    
divided1 or of having the parts         
separated from each other4 ,this is     
called one of the essential properties  
of matter2 because1 however minute the  
particles may be1 they must still       
contain as many halves1 quarters1 c4 as 
the largest mass of matter4   ,,echo4 ,a
 sound reflected back1 by some          
substance1 so situated as to produce    
this effect4  ,,eclipse4 ,the           
interruption of the light of the sun1 or
 of some other heavenly body1 by the    
intervention of an opaque body4 ,the    
moon passing between the earth and the  
sun1 causes an eclipse of the latter4   
,,ecliptic4 ,a circle in the heavens4   
,the apparent path of the sun1 through  
the twelve signs of the zodiac4 ,this is

 caused by the actual revolution of the 
earth1 round the sun4 ,it is called the 
ecliptic1 because eclipses always happen
 in the direction of that line1 from the
 earth4  ,,elasticity4 ,that property of
 bodies1 by which they resume their     
dimensions and form1 when the force     
which changed them is removed4 ,air is  
eminently elastic4 ,two ivory balls1    
struck together1 become flattened at the
 point of contact2 but immediately      
resuming their form1 they react upon    
each other4  ,,ellipsis4 ,an oval4 ,this
 figure differs from a circle1 in being 
unequal in its diameters1 and in having 
two centres1 or points1 called its foci4
 ,the orbits of the planets are all     
elliptical4  ,,equator4 ,that imaginary 
line which divides the earth into       
northern and southern hemispheres1 and  
which is equally distant from each pole4
  ,,equilibrium4 ,when two articles     
exactly balance each other1 they are in 
equilibrium4 ,they may1 notwithstanding1
 be very unequal in weight1 but they    

must be so situated1 that1 if set in    
motion1 their momentums would be equal4 
,,equinox4 ,the two periods of time at  
which the nights and days are every     
where of equal length4 ,the vernal      
equinox is in ,march1 when the sun      
enters the sign ,aries2 the autumnal    
equinox in ,september1 when the sun     
enters ,libra4 ,at these periods1 the   
sun is vertical at the equator4         
,,exhalations4 ,all those articles which
 arise from the earth1 and mixing with  
the atmosphere1 form vapour4            
,,expansion4 ,the same as dilatation1   
which see4  ,,extension4 ,one of the    
essential properties of matter2 that by 
which it occupies some space1 to the    
exclusion of all other matter4          
,,figure4 ,all matter must exist in some
 form1 or shape2 hence figure is deemed 
an essential property of matter4        
,,fluid4 ,a form of matter1 in which its
 particles readily flow1 or slide1 over 
each other4 ,airs1 or gases1 are called 
elastic fluids1 because they are readily

 reduced to a smaller bulk by pressure4 
,liquids1 are denominated non-elastic   
fluids1 because they suffer but little  
diminution of bulk1 by any mechanical   
force4  ,,focus4 ,that point in which   
converging rays unite4  ,,force4 ,that  
power which acts upon a body1 either    
tending to create1 or to stop motion4   
,,fountain4 ,a jet1 or stream of water1 
forced upwards by the weight of other   
water1 by the elasticity of air1 or some
 other mechanical pressure4  ,,friction4
 ,the rubbing of bodies together1 by    
which their motion is retarded4         
,friction may be lessened1 but cannot be
 destroyed4  ,,frigid ,,zones4 ,the     
spaces or areas1 contained within the   
arctic and antarctic circles4           
,,fulcrum4 ,a prop4 ,the point or axis1 
by which a body is supported1 and about 
which it is susceptible of motion4      
,,gas4 ,any kind of air2 of these there 
are several4 ,the atmosphere consists of
 two kinds1 mixed1 or combined with each
 other4  ,,geometry4 ,that branch of the

 mathematics1 which treats of lines1 of 
surfaces1 and of solids2 and            
investigates their properties1 and      
proportions4  ,,globe4 ,a sphere1 or    
ball4 ,it has a point in its centre of  
magnitude1 from which its surface is    
every where equally distant4  ,,gravity4
 ,that species of attraction which      
appears to be common to matter1 existing
 in its particles1 and giving to them1  
and of course to the masses which they  
compose1 a tendency to approach each    
other4 ,by gravity a stone falls to the 
earth1 and by it the heavenly bodies    
tend towards each other4   ,,harmony4 ,a
 combination of musical sounds1 produced
 by vibrations which bear a certain     
ratio to each other2 and which thence   
affect the mind agreeably1 when heard at
 the same time4 ,sounds not so related1 
produce discord4  ,,hemisphere4 ,half a 
sphere or globe4 ,a plane passing       
through the centre of a globe1 will     
divide it into hemispheres4  ,,horizon4 
,this is generally divided into         

sensible1 and rational4 ,the sensible   
horizon is that portion of the surface  
of the earth1 to which our vision       
extends4 ,our rational horizon is that  
circle in the heavens which bounds our  
vision1 when on the ocean1 an extended  
plane1 or any elevated situation4 ,in   
the heavens our sensible1 and our       
rational horizon are the same2 its plane
 would divide the earth into hemispheres
 at #ij degrees from us2 and a person   
standing on that part of the earth which
 is directly opposite to us1 would1 at  
the same moment1 see in his horizon1 the
 same heavenly bodies1 which would be   
seen in ours4  ,,horizontal4 ,level2 not
 inclined1 or sloping4 ,a perfectly     
round ball1 placed upon a flat surface1 
which is placed horizontally1 will      
remain at rest4  ,,hydraulics4 ,that    
science which treats of water in motion1
 and the means of raising1 conducting1  
and using it for moving machinery1 or   
other purposes4  ,,hydrostatics4 ,treats
 of the weight1 pressure1 and           

equilibrium of fluids1 when in a state  
of rest4  ,,hydrometer4 ,an instrument  
used to ascertain the specific gravity  
of different fluids1 which it does1 by  
the depth to which it sinks when        
floating on them4   ,,image4 ,the       
picture of any object which we perceive 
either by reflected or refracted light4 
,all objects which are visible1 become  
so by forming images on the retina4     
,,impenetrability4 ,that property of    
matter1 by which it excludes all other  
matter from occupying the same space    
with itself at the same time4 ,if two   
particles could exist in the same space1
 so also might any greater number1 and  
indeed all the matter in the universe1  
might be collected in a single point4   
,,incidence4 ,the direction in which a  
body1 or a ray of light1 moves in its   
approach towards any substance1 upon    
which it strikes4  ,,inclined ,,plane4  
,one of the six mechanical powers4 ,any 
plane surface inclined to the horizon1  
may be so denominated4  ,,inertia4 ,one 

of the inherent properties of matter4   
,want of power1 or of any active        
principle within itself1 by which it can
 change its own state1 whether of       
motion1 or of rest4  ,,inherent         
,,properties4 ,those properties which   
are absolutely necessary to the         
existence of a body2 called also        
essential properties4 ,all others are   
denominated accidental4 ,colour is an   
accidental--extension1 an essential     
property of matter4   ,,latitude4       
,distance from the equator1 in a direct 
line towards either pole4 ,this distance
 is measured in degrees and minutes4    
,the degree of latitude cannot exceed   
ninety1 or one quarter of a circle4     
,places to the south of the equator1 are
 in south latitude1 and those to the    
north1 in north latitude4  ,,latitude1  
,,parallels ,,of4 ,lines drawn upon the 
globe1 parallel to the equator1 are so  
called2 every place situated on such a  
line1 has the same latitude1 because    
equally distant from the equator4       

,,lens4 ,a glass1 ground so that one or 
both surfaces form segments of a sphere1
 serving either to magnify1 or diminish 
objects seen through them4 ,glasses used
 in spectacles are lenses4  ,,lever4    
,one of the mechanical powers4 ,an      
inflexible bar of wood or metal1        
supported by a fulcrum1 or prop2 and    
employed to increase the effect of a    
given power4  ,,libra4 ,one of the      
twelve signs of the zodiac4 ,that into  
which the sun enters1 at the autumnal   
equinox4  ,,light4 ,that principle1 by  
the aid of which we are able to discern 
all visible objects4 ,it is generally   
believed to be a substance emitted by   
luminous bodies1 and1 exciting vision by
 passing into the eye4  ,,longitude4    
,distance measured in degrees and       
minutes1 either in an eastern1 or a     
western direction1 from any given point 
either on the equator1 or on a parallel 
of latitude4 ,degrees of longitude may  
amount to #ahj1 or half a circle4 ,a    
degree of longitude measured upon the   

equator1 is of the same length with a   
degree of latitude2 but as the poles are
 approached1 the degrees of longitude   
diminish in length1 because the circles 
upon which they are measured1 become    
less4  ,,lunar4 ,relating to ,luna1 the 
moon4  ,,lunation4 ,the time in which   
the moon completes its circuit4 ,a lunar
 month4  ,,luminous ,,bodies4 ,those    
which emit light from their own         
substance2 not shining by borrowed1 or  
reflected light4   ,,machine4 ,any      
instrument1 either simple or compound1  
by which any mechanical effect is       
produced4 ,a needle1 and a clock1 are   
both machines4  ,,magic ,,lanthorn1 ,,or
 ,,lantern4 ,an optical instrument1 by  
which transparent pictures1 painted upon
 glass1 are magnified and exhibited on a
 white wall or screen1 in a darkened    
room4 ,the phantasmagoria1 is a species 
of magic lanthorn4  ,,mathematics4 ,the 
science of numbers and of extension4    
,common arithmetic1 is a lower branch of
 the mathematics4 ,in its higher        

departments1 it extends to every thing  
which is capable of being either        
numbered or measured4  ,,matter4        
,substance4 ,every thing with which we  
become acquainted by the aid of the     
senses2 every thing however large1 or   
however minute1 which has length1       
breadth1 and thickness4  ,,mechanics4   
,that science which investigates the    
principles1 upon which the action of    
every machine depends2 and teaches their
 proper application in overcoming       
resistance1 and in producing motion1 in 
all the useful purposes to which they   
are applied4  ,,medium4 ,in optics1 is  
any body which transmits light4 ,air1   
water1 glass1 and all other transparent 
bodies1 are media4 ,medium also denotes 
that in which any body moves4 ,air is   
the medium which conveys sound1 and     
which enables birds to fly4  ,,melody4  
,a succession of such single musical    
sounds1 as form a simple air or tune4   
,,mercury4 ,that planet which is nearest
 to the sun4 ,quicksilver1 a metal1     

which remains fluid at the common       
temperature of the atmosphere4 ,it is   
capable of being rendered solid1 by     
intense cold4  ,,meridian4 ,midday4 ,a  
meridian line1 is one which extends     
directly from one pole of the earth to  
the other2 crossing the equator at right
 angles4 ,it is therefore half of a     
great circle4 ,the hour of the day is   
the same at every place situated on the 
same meridian4 ,longitude is measured   
from any given meridian1 to the opposite
 meridian4 ,places at the same distance 
in degrees1 to the east or west of any  
meridian1 have the same longitude4      
,,microscope4 ,an optical instrument1 by
 which minute objects1 are magnified1 so
 as to enable us to perceive and examine
 such as could not be seen by the naked 
eye4  ,,mineral4 ,earths1 stones1       
metals1 salts1 and in general all       
substances dug out of the earth1 are    
denominated minerals4  ,,minute4 ,in    
time1 the sixtieth part of an hour4 ,in 
length1 the sixtieth part of a degree4  

,a minute of time1 is an unvarying      
period2 but a minute in length varies in
 extent1 with the degree of which it    
forms a part4 ,the degrees and minutes  
are equal in number1 upon a common ring1
 upon the equator of the earth1 or1 on  
any circle of the heavens4  ,,mirrors4  
,polished surfaces of metal1 or of glass
 coated with metal1 for the purpose of  
reflecting the rays of light1 and the   
images of objects4 ,common              
looking-glasses1 are mirrors4 ,those    
used in reflecting telescopes1 are made 
of metal4  ,,mobility4 ,capable of being
 moved from one place to another4 ,this 
is accounted one of the essential       
properties of matter1 because we cannot 
conceive of its existence without this  
capacity4  ,,momentum4 ,the force1 or   
power1 with which a body in motion acts 
upon any other body1 or tends to        
preserve its own quantity of motion4    
,the momentum of a body1 is compounded  
of its quantity of matter1 and its      
velocity4 ,a body weighing one pound1   

moving with a velocity of two miles in a
 minute1 will possess the same momentum 
with one weighing two pounds1 moving    
with a velocity of one mile in a minute4
  ,,motion4 ,a continued and successive 
change of place1 either of a whole body1
 or of the particles of which a body is 
composed2 the earth in revolving upon   
its axis only1 would not change its     
place as a body1 but all the particles  
of which it is composed1 would revolve  
round a common axis of motion4 ,in      
revolving in its orbit1 its whole mass  
is constantly occupying a new portion of
 space4   ,,natural ,,philosophy4 ,that 
science which enquires into the laws    
which govern all the natural bodies in  
the universe1 in all their changes of   
place1 or of state4  ,,neap ,,tides4    
,those tides which occur when the moon  
is in her quadratures1 or half way      
between new1 and full moon2 at these    
periods the tides are the lowest4       
,,nodes4 ,those points in the orbit of  
the moon1 or of a planet1 where it      

crosses the ecliptic or plane of the    
earth's orbit4 ,when passing to the     
north of the ecliptic1 it is called the 
ascending node2 when to the south of it1
 the descending node4   ,,oblate4 ,see  
,,spheroid4  ,,octagon4 ,a figure with  
eight sides1 and consequently with eight
 angles4  ,,opaque4 ,not transparent2   
refusing a passage to the rays of light4
  ,,optics4 ,that branch of science     
which treats of light1 and vision4 ,it  
is generally divided into two parts4    
,catoptrics1 which treats of the        
reflection of light1 and ,dioptrics1    
which treats of its refraction4         
,,orbit4 ,the line in which a primary   
planet moves in its revolution round the
 sun2 or a secondary planet1 in its     
revolution round its primary4 ,these    
orbits are all elliptical1 or oval4     
,,parabola4 ,a particular kind of curve2
 that which a body describes in rising  
and in falling1 when thrown upwards1 in 
any direction not perpendicular to the  
horizon4  ,,parallelogram4 ,a figure    

with four sides1 having those which are 
opposite1 parallel to each other4 ,a    
square1 an oblong square1 and the figure
 usually called a diamond1 are          
,parallelograms4  ,,parallel ,,lines4   
,all lines1 whether straight or curved1 
which are every where at an equal       
distance from each other1 are parallel  
lines4  ,,parallel ,,of ,,latitude4 ,see
 ,,latitude4  ,,perihelion4 ,that part  
of the orbit of a planet1 in which it   
approaches the sun most nearly4         
,,pendulum4 ,a body suspended by a rod1 
or line1 so that it may vibrate1 or     
oscillate1 backwards and forwards4      
,pendulums of the same length1 perform  
their vibrations in the same time1      
whatever may be their weight1 and       
whether the arc of vibration1 be long or
 short4  ,,percussion4 ,the striking of 
bodies against each other4 ,the force of
 this1 depends upon the momentum of the 
striking body4  ,,period4 ,the time     
required for the revolution of one of   
the heavenly bodies in its orbit4       

,,perpendicular4 ,making an angle of #ij
 degrees with the horizon4 ,when two    
lines which meet1 make an angle of #ij  
degrees1 they are perpendicular to each 
other4  ,,phases4 ,the various          
appearances of the disc1 or face of the 
moon1 and of the planets2 that portion  
of them which we see illuminated by the 
rays of the sun4  ,,phenomenon4 ,any    
natural appearance is properly so       
called2 the term1 however1 is usually   
applied to extraordinary appearances1 as
 eclipses1 transits1 c4  ,,piston4 ,that
 part of a pump1 or other engine which  
is made to fit into a hollow cylinder1  
or barrel2 and to move up and down in   
it1 in order to raise water1 or for any 
other purpose4  ,,plane4 ,a perfectly   
flat surface4 ,the plane of the orbit of
 a planet1 is an imaginary flat surface1
 extending to every part of the orbit4  
,,planet4 ,those bodies which revolve   
round the sun1 in orbits nearly         
circular4 ,they are divided into        
primary1 and secondary2 these latter are

 also called satellites1 or moons2 they 
revolve round the primary planets1 and  
accompany them in their courses round   
the sun4  ,,plumb-,,line4 ,a string1 or 
cord1 by which a weight is suspended2 it
 is used for the purpose of finding a   
line perpendicular to the horizon2 the  
weight being always attracted towards   
the centre of the earth4  ,,pneumatics4 
,that branch of natural philosophy1     
which treats of the mechanical          
properties of the atmosphere1 or of air 
in general4  ,,poles4 ,the extremities  
of the axis of motion either of our     
earth1 or of any other revolving sphere4
 ,the poles of the earth have never been
 visited2 the regions by which they are 
surrounded1 being obstructed by         
impassable barriers of ice4  ,,power4   
,that force which we apply to any       
mechanical instrument1 to effect a given
 purpose1 is denominated power1 from    
whatever source it may be derived4 ,we  
have the power of weights1 of springs1  
of horses1 of men1 of steam1 c4         

,,prism4 ,the instrument usually so     
called1 is employed in optics to        
decompose the solar ray3 it consists of 
a piece of solid glass1 several inches  
in length1 and having three flat sides2 
the ends are equal in size1 and are of  
course triangular4  ,,precession ,,of   
,,the ,,equinoxes4 ,every equinox takes 
place a few seconds of a degree1 before 
the earth arrives at that part of the   
ecliptic in which the preceding equinox 
occurred4 ,this phenomenon is called the
 precession of the equinoxes4 ,there is 
consequently a gradual change of the    
places of the signs of the zodiac3 a    
fact1 the discovery of which has thrown 
much light on ancient chronology4       
,,projection4 ,that force by which      
motion is given to a body1 by some power
 acting upon it1 independently of       
gravity4  ,,pulley4 ,one of the six     
mechanical powers4 ,a wheel turning upon
 an axis1 with a line passing over it4  
,it is the moveable pulley only1 which  
gives any mechanical advantage4  ,,pump4

 ,an hydraulic1 or pneumatic instrument1
 for the purpose of raising water1 or   
exhausting air4   ,,quadrant4 ,a quarter
 of a circle4 ,an instrument used to    
measure the elevation of a body in      
degrees above the horizon4              
,,quadratures ,,of ,,the ,,moon4 ,that  
period in which she appears in the form 
of a semicircle4 ,she is then either in 
her first1 or her last quarter2 and     
exactly half way1 between the places of 
new1 and of full moon4   ,,radiation4   
,the passage of light or heat in rays1  
or straight lines2 these being projected
 from every luminous1 or heated point1  
in all directions4  ,,radius4 ,the      
distance from the centre of a circle1 to
 its circumference2 or one half of its  
diameter4 ,in the plural denominated    
radii4  ,,rainbow4 ,an appearance in the
 atmosphere1 occasioned by the          
decomposition of solar light1 in its    
refraction1 and reflection1 in passing  
through drops of rain4 ,the bow can be  
seen1 only when the sun is near the     

horizon1 when the back is turned towards
 it1 and there is a shower in the       
opposite direction4  ,,ray4 ,a single   
line of light1 emitted in one direction1
 from any luminous point4  ,,reaction4  
,every body1 whether in a state of      
motion1 or at rest1 tends to remain in  
such state1 and resists the action of   
any other body upon it1 with a force    
equal to that action4 ,this resistance1 
is called its reaction4  ,,receiver4    
,this name is applied to glass vessels  
of various kinds1 appertaining to the   
air pump1 and from which the air may be 
exhausted4 ,they are made to contain1 or
 receive1 any article upon which an     
effect is to be produced1 by taking off 
the pressure of the atmosphere4         
,,refraction1 of the rays of light1 is  
the bending of those rays1 when they    
pass obliquely from one medium into     
another of different density4 ,a stick  
held obliquely in water1 appears bent or
 broken at the surface of the fluid4    
,,refrangibility4 ,capacity of being    

refracted4 ,light is decomposed by the  
prism1 because its component parts are  
refrangible in different degrees1 by the
 same refracting medium4  ,,repulsion4  
,the reverse of attraction4 ,a tendency 
in particles1 or in masses of matter1 to
 recede from each other4 ,the matter of 
heat within a body1 appears to          
counteract the attraction of its        
particles1 so as to prevent absolute    
contact4  ,,retina4 ,that part of the   
ball of the eye1 upon which the images  
of visible objects are formed2 and from 
which1 the idea of such forms1 is       
conveyed to the mind4  ,,revolution1 of 
a planet2 is either diurnal1 or annual2 
the former1 is its turning upon its own 
axis2 the latter1 is its passage in its 
orbit4   ,,satellites4 ,moons1 secondary
 planets4  ,,segment ,,of ,a ,,circle4  
,a portion1 or part of a circle2 called 
also1 an arc of a circle4               
,,semi-,,diameter4 ,half the diameter4  
,the semi-diameter of the earth1 is the 
distance from its surface1 to its       

centre4  ,,siderial4 ,belonging to the  
stars4 ,a siderial day1 is the time     
required for a star to reappear on a    
given meridian4 ,a siderial year1 the   
period in which the sun appears to have 
travelled round the ecliptic1 so as to  
have arrived opposite to any particular 
star1 from which his course was         
calculated4  ,,signs1 or                
,,constellations4 ,collections1 or      
groups1 of stars4 ,those of the zodiac  
are twelve1 corresponding with the      
twelve months in the year4 ,in the      
centre of these the ecliptic is         
situated4 ,the sun appears to pass in   
succession through these signs2 entering
 the first degree of ,aries1 which is   
accounted the first sign1 about the     
#bast of ,march4  ,,sky4 ,that vast     
expanse1 or space1 in which the heavenly
 bodies are situated4 ,its blue         
appearance is supposed to arise from the
 particles of which the atmosphere is   
composed1 possessing the property of    
reflecting the blue rays1 in greatest   

abundance4  ,,solar4 ,appertaining to1  
or governed by1 the sun3 as the solar   
system1 the solar year1 solar eclipses4 
,,solid4 ,not fluid4 ,having its parts  
connected so as to form a mass4 ,solid  
bodies1 are not absolutely so1 all      
undoubtedly containing pores1 or spaces 
void of matter4  ,,solstices4 ,the      
middle of summer and the middle of      
winter2 those two points in the orbit of
 the earth1 in which its poles point    
most directly towards the sun4          
,,sonorous ,,bodies4 ,those bodies which
 are capable of being put into a state  
of vibration1 so as to emit sounds4     
,,specific ,,gravity4 ,the relative     
weight of bodies of different species1  
when the same bulk of each is taken4    
,water has been chosen as the standard  
for comparison4 ,if we say that the     
specific gravity of a body is #f1 we    
mean1 that its weight is six times as   
great as that of a portion of water1    
exactly equal to it in bulk4            
,,spectrum4 ,that appearance of         

differently coloured rays1 which is     
produced by the refraction of the solar 
ray1 by means of a prism1 is called the 
prismatic spectrum2 it exhibits most    
distinctly1 and beautifully1 all the    
colours seen in the rainbow4  ,,sphere4 
,a globe1 or ball4  ,,spheroid4         
,spherical2 a body approaching nearly to
 a sphere in its figure4 ,the earth1 is 
denominated an oblate spheroid2 it not  
being an exact sphere1 but flattened at 
the poles1 so as to cause the polar     
diameter to be upwards of thirty miles  
less than the equatorial4 ,oblate1 is   
the reverse of oblong1 and means shorter
 in one direction1 than in another4     
,,spring ,,tides4 ,those tides which    
occur at the time of new1 or of full    
moon4 ,the tides then rise to a greater 
height than at any other period4        
,,square4 ,a figure having four sides of
 equal length1 and its angles all right 
angles4  ,in numbers2 the product of a  
number multiplied into itself2 thus1 the
 square of #c is #i1 and the square of  

#h is #fd4  ,,star4 ,the fixed stars are
 so called1 because they retain their   
relative situations2 while the planets1 
by revolving in their orbits1 appear to 
wander amongst the fixed stars4         
,,subtend4 ,this term is applied to the 
measurement of an angle2 when the lines 
by which it is bounded recede but little
 from each other1 they are said to      
subtend2 that is1 to be contained under1
 a small angle4  ,,superficies4 ,the    
surface of any figure4 ,space extended  
in length and width4  ,,system4 ,the    
mutual connexion1 and dependance of     
things1 upon each other4 ,the solar1 or 
,copernican system1 includes the sun1   
the planets1 with their moons1 and the  
comets4   ,,tangent4 ,a straight line   
touching the circumference of a circle2 
but which would not cut off any portion 
of it1 were it extended beyond the      
touching point1 in both directions4     
,,telescope4 ,an instrument by which    
distant objects may be distinctly seen2 
the images of objects being brought near

 to the eye1 and greatly magnified4     
,,temperate ,,zones4 ,those portions of 
the surface of the earth situated       
between #bc-a_/b and #ff-a_/b degrees of
 latitude4 ,within these boundaries1 the
 sun is never vertical2 nor does he ever
 remain1 during a whole day1 below the  
horizon4  ,,thermometer4 ,an instrument 
for measuring the temperature of the    
atmosphere1 or of other bodies4         
,,torrid ,,zone4 ,that portion of the   
earth which extends #bc-a_/b degrees on 
each side of the equator1 to the        
tropical circles2 within this limit1 the
 sun is vertical1 twice in the year4    
,,transit4 ,mercury or ,venus1 are said 
to transit the sun1 when they pass      
between the earth and that luminary4    
,they then appear like dark spots1 upon 
the face of the sun4  ,,transparent4    
,allowing the rays of light to pass     
freely through4 ,the reverse of opaque4 
,glass1 water1 air1 c4 are transparent  
bodies4  ,,tropics4 ,two circles on the 
globe on either hemisphere1 at the      

distance of #bc-a_/b degrees from the   
equator4 ,beyond these circles1 the sun 
is never vertical3 and the countries    
within them1 are denominated tropical4  
,,twilight4 ,that portion of the morning
 or evening1 in which the light of the  
sun is perceptible1 although he is below
 the horizon4   ,,vacuum4 ,space void of
 matter4 ,such is supposed to be the    
space in which the planets revolve4 ,we 
are said to produce a vacuum1 when we   
exhaust the air from a receiver4        
,,valve4 ,a part of a pump1 and of some 
other instruments1 which opens to admit 
the passage of a fluid in one direction1
 but closes when pressed in the opposite
 direction1 so as to prevent the return 
of the fluid2 a pair of bellows is      
furnished with a valve4  ,,vapour4      
,exhalations from fluid or solid        
substances1 generally mixing with the   
atmosphere4 ,the most abundant1 is that 
from water4  ,,vertical4 ,exactly over  
our heads3 ninety degrees above our     
horizon4  ,,vibration4 ,the alternate   

motion of a body1 forwards and          
backwards2 swinging1 as a pendulum4     
,,visual4 ,belonging to vision2 as the  
visual angle1 or that angle formed by   
the rays of light which enter the eye1  
from the extremities of any object4     
,,undulation4 ,a vibratory1 or wave-like
 motion communicated to fluids4 ,sound1 
is said to be propagated by the         
undulatory1 or vibratory motion of the  
air4   ,,wedge4 ,one of the mechanical  
powers2 the form of the wedge is well   
known4 ,it is of extensive use2 serving 
to rend bodies of great strength1 and to
 raise enormous weights4  ,,wheel ,,and 
,,axle4 ,one of the mechanical powers1  
used under various modifications4       
,cranes for raising weights1 the wheels 
and pinions of clocks and watches1      
windlasses1 c4 are all applications of  
this power4   ,,zodiac4 ,a broad belt in
 the heavens1 extending nearly eight    
degrees on each side of the ecliptic2   
the planes of the orbits of all the     
planets are included within this space4 

,this belt is divided into twelve parts 
or signs1 each containing #cj degrees4  
,these signs are3    ,aries2 the ,ram4  
,taurus2 the ,bull4   ,gemini2 the      
,twins4   ,cancer2 the ,crab4   ,leo2   
the ,lion4   ,virgo2 the ,virgin4       
,libra2 the ,scales4   ,scorpio2 the    
,scorpion4   ,sagittarius2 the ,archer4 
,capricornus2 the ,goat4   ,aquarius2   
the ,waterer4   ,pisces2 the ,fishes4   
,the first six are called northern      
signs2 because the sun is in them1      
during that half of the year1 in which  
he is vertical to the north of the      
equator2 the last six1 are called       
southern signs2 because1 during his     
journey among them1 he is vertical to   
the south of the equator4  ,the sun     
enters ,aries1 at the time of the vernal
 equinox2 ,cancer1 at the summer        
solstice2 ,libra1 at the autumnal       
equinox2 and ,capricornus1 at the winter
 solstice4  ,the sun is said to enter a 
sign1 when the earth in going round in  
its orbit1 enters the opposite sign4    

,thus1 when the sun appears in the first
 degree of ,libra1 it is in consequence 
of the earth having arrived opposite to 
the first degree of ,aries4 ,a line then
 drawn from the earth1 and passing      
through the centre of the sun1 would1 if
 extended to the fixed stars1 touch the 
first degree of ,libra4  ,,zone4 ,the   
earth is divided into zones1 or belts4  
,see ,,frigid1 ,,temperate1 and ,,torrid
 ,,zones4     ,,index4   ,a4  ,air1 #aa1
 #ae1 #bh1 #ej1 #acf4  ,air-pump1 #ca1  
#ade4  ,angle1 #dd4   acute1 #dd4       
obtuse1 #dd4   right1 #dd4   of         
incidence1 #de1 #aed1 #afj1 #agc4   of  
reflection1 #de1 #aed1 #afj1 #agc4      
visual1 #afh1 #afi1 #agj4  ,angular     
velocity1 #aga4  ,antarctic circle1 #ib4
  ,aphelion1 #ge4  ,arctic circle1 #ib4 
,atmosphere1 #bh1 #ajd1 #abi1 #acf1     
#add1 #aej1 #afc4   colour of1 #aic4    
reflection of1 #aic4   refraction of1   
#ahb4  ,attraction1 #aj1 #ad1 #bc1 #be1 
#agi4   of cohesion1 #ae1 #ai1 #aah4    
capillary1 #ah4   of gravitation1 #ah1  

#bc1 #bi1 #gj1 #hj1 #if1 #aaf1 #acf4    
,avenue1 #agj4  ,auditory nerve1 #aea4  
,axis1 #gh4   of motion1 #dh4   of the  
earth1 #bb1 #ii4   of mirrors1 #agf4    
of a lens1 #ahd4   ,b4  ,balloon1 #cj4  
,barometer1 #adj4  ,bass1 #aee4         
,bladder1 #ach4  ,bodies1 #aj4          
elastic1 #dj4   fall of1 #bc1 #bf1 #cj1 
#cf4   luminous1 #aeg4   opaque1 #aeg4  
sonorous1 #aeb1 #aee4   transparent1    
#aeg4  ,bulk1 #af4   ,c4  ,camera       
obscura1 #ahd1 #aig1 #bja4  ,capillary  
tubes1 #ah4  ,centre1 #dh4   of gravity1
 #dh1 #ea1 #eb1 #aae4   of magnitude1   
#dh1 #ec4   of motion1 #dh1 #ee1 #aae4  
,centrifugal force1 #di1 #gb1 #ie1 #aae4
  ,centripetal force1 #di1 #gb4  ,ceres1
 #hd4  ,circle1 #dd1 #id4               
,circumference1 #id4  ,clouds1 #abi4    
,colours1 #bc1 #ahe4  ,comets1 #hf4     
,compression1 #db4  ,concord1 #aee4     
,constellation1 #hf4  ,convergent rays1 
#age1 #agg4  ,crystals1 #ab4            
,curvilinear motion1 #dg1 #gb4          
,cylinder1 #eb4   ,d4  ,day1 #gh1 #aje1 

#ajf4  ,degrees1 #dd1 #id1 #ii1 #afi1   
#agj4   of latitude1 #id1 #aab4   of    
longitude1 #id1 #aab4  ,density1 #af4   
,diagonal1 #dg4  ,diameter1 #id4        
,discords1 #aee4  ,diurnal1 #gh4        
,divergent rays1 #age1 #agg4            
,divisibility1 #aj1 #ab4   ,e4  ,earth1 
#ah1 #gj1 #hd1 #hh1 #ie4  ,echo1 #aed4  
,eclipse1 #aaj1 #aei4  ,ecliptic1 #hf1  
#ib1 #ii4  ,elasticity1 #da4  ,elastic  
bodies1 #bh1 #dj4   fluids1 #bh1 #da1   
#aah1 #acf4  ,ellipsis1 #ge4  ,equinox1 
#ajj1 #ajg4   precession of1 #ajg4      
,equator1 #ib1 #ii4  ,essential         
properties1 #aj4  ,exhalations1 #ac4    
,extension1 #aj1 #aa4  ,eye1 #aff1 #aie4
   ,f4  ,fall of bodies1 #bd1 #bg1 #ca4 
,figure1 #aj1 #ab4  ,fluids1 #aah1 #abh4
   elastic1 #bh1 #da1 #aah1 #acf4       
equilibrium of1 #abj1 #abb1 #acb4       
non-elastic1 #aai4   pressure of1 #aba4 
,flying1 #dj4  ,focus1 #agf4   of       
concave mirrors1 #agg4   of convex      
mirrors1 #age1 #agg4   of a lens1 #ahd4 
imaginary1 #agf4   virtual1 #agf4       

,force1 #cc4   centrifugal1 #di1 #gb1   
#ie1 #aae4   centripetal1 #di1 #gb4     
projectile1 #dg1 #di4   of gravity1 #dg1
 #di4  ,fountains1 #ace4  ,friction1    
#fh1 #fi1 #ace4  ,frigid zone1 #ic4     
,fulcrum1 #ed4   ,g4  ,general          
properties of bodies1 #aj4  ,georgium   
,sidus1 #he4  ,glass1 #ahc4   burning1  
#ahh4   refraction of1 #ahc4  ,gold1    
#aai1 #abf4  ,gravity1 #ah1 #bc1 #gh1   
#ig4   ,h4  ,harmony1 #aee4  ,heat1 #af1
 #bi1 #ajc4  ,hemisphere1 #ib1 #ajj4    
,herschel1 #he4  ,hydraulics1 #aah4     
,hydrometer1 #abh4  ,hydrostatics1 #aah4
   ,i4  ,image on the retina1 #afe1     
#agb4   reversed1 #afg4   in plain      
mirror1 #agb4   in concave do4 #age4    
in convex do4 #age4  ,impenetrability1  
#aj4  ,inclined plane1 #ed1 #ff4        
,inertia1 #aj1 #ad1 #cb4  ,inherent     
properties1 #aj4  ,juno1 #hd4  ,jupiter1
 #he4   ,l4  ,lake1 #acc1 #ace4         
,latitude1 #id1 #aab4  ,lens1 #ahd4     
concave1 #ahd4   convex1 #ahd4          
meniscus1 #ahd4   plano-concave1 #ahd4  

plano-convex1 #ahd4  ,lever1 #ed1 #ee4  
first kind1 #eh4   second kind1 #fj4    
third kind1 #fj4  ,light1 #aeg4   pencil
 of1 #aeh4   of the moon1 #afb1 #afc4   
absorption of1 #ahh4   reflected1 #afj4 
refraction of1 #agi4  ,liquids1 #aah4   
,longitude1 #id1 #aab4  ,luminous       
bodies1 #aeg4  ,lunar month1 #ajh4      
eclipse1 #aaj4   ,m4  ,machine1 #ed1    
#ff4  ,magic lanthorn1 #bjc4  ,mars1    
#hd4  ,matter1 #aj1 #ac4  ,mechanics1   
#cb4  ,mediums1 #aeg1 #ahj4  ,melody1   
#aef4  ,mercury1 7planet7 #hc1 #he1     
#aad4  ,mercury1 or quicksilver1 #af1   
#adj1 #ada4  ,meridians1 #ic4           
,microscope1 #bjj4   single1 #bjj4      
double1 #bjj4   solar1 #bjb1 #bjc4      
,minerals1 #ab4  ,minutes1 #id4         
,momentum1 #ch1 #ef4  ,monsoons1 #adi4  
,month1 lunar1 #ajh4  ,moon1 #gh1 #gi1  
#hj1 #hb1 #he4  ,moonlight1 #afb1 #afc4 
,motion1 #ad1 #cb1 #cf4   accelerated1  
#cf4   axis of1 #dh4   centre of1 #dh1  
#ee4   compound1 #df4   curvilinear1    
#dg1 #di4   diurnal1 #gh4   perpetual1  

#ce4   retarded1 #ce4   reflected1 #dc4 
uniform1 #cd4  ,mirrors1 #agb4   axis   
of1 #agf4   burning1 #agg4   concave1   
#agd1 #agf1 #bji4   convex1 #agd1 #age4 
plane or flat1 #agb4   reflection of1   
#agc4   ,n4  ,neap tides1 #aaf4         
,nerves1 #aff4   auditory1 #aea1 #aff4  
olfactory1 #aff4   optic1 #afd1 #aff4   
,night1 #gh4  ,nodes1 #aaj4   ,o4       
,octave1 #aef4  ,odour1 #ac4  ,opaque   
bodies1 #aeg1 #aeh4  ,optics1 #aeg4     
,orbit1 #hf4   ,p4  ,pallas1 #hd4       
,parabola1 #ea4  ,parallel lines1 #be4  
,parallel of latitude1 #id4  ,pellucid  
bodies1 #aeg4  ,pencil of rays1 #aeh4   
,pendulum1 #ih4  ,perihelion1 #ge4      
,perpendicular lines1 #be4  ,phases1    
#aji4  ,piston1 #adc1 #ade4  ,plane1    
#ib1 #ic4  ,planets1 #gf1 #ha1 #hc4     
,poles1 #ib1 #ii1 #ajj4  ,polar star1   
#ajj1 #aab4  ,porosity1 #db1 #abf4      
,powers1 mechanical1 #ed4  ,projection1 
#di1 #ej1 #ga4  ,precession of          
equinoxes1 #ajg4  ,pulley1 #ed1 #fc4    
,pump1 #ca4   sucking and lifting1 #adc4

   forcing1 #add1 #ade4   air1 #ca1     
#ade4  ,pupil of the eye1 #afd4   ,r4   
,rain1 #ag1 #abi4  ,rainbow1 #ahh4      
,rarity1 #af4  ,ray of light1 #aeh1     
#agi4   reflected1 #afj1 #afa4          
incident1 #afa4  ,rays1 intersecting1   
#afe4  ,reaction1 #ci4  ,receiver1 #ca4 
,reflection of light1 #afj1 #afc4       
angle of1 #de1 #afa1 #agc4   of mirrors1
 #agc4   of plane mirrors1 #agd4   of   
concave do4 #agd4   of convex do4 #agd4 
,reflected motion1 #dc4  ,refraction1   
#agi1 #ahf4   of the atmosphere1 #ahb4  
of glass1 #ahc4   of a lens1 #ahd4   of 
a prism1 #ahe4  ,resistance1 #ed4       
,retina1 #afe4   image on1 #aff4        
,rivers1 #acd4  ,rivulets1 #aca4   ,s4  
,satellites1 #hj1 #aaa1 #aac4  ,saturn1 
#he4  ,scales1 or balance1 #ee4  ,screw1
 #ed1 #fg4  ,shadow1 #aaj1 #aaa4        
,siderial time1 #ajf4  ,sight1 #afe4    
,signs of the zodiac1 #hf1 #ic4  ,smoke1
 #ad1 #bi4  ,solar microscope1 #bjb4    
,solstice1 #ajj1 #ajb4  ,sound1 #aea4   
acute1 #aee4   musical1 #aee4  ,space1  

#cc4  ,specific gravity1 #abc4   of air1
 #adj4  ,spectrum1 #aij4                
,speaking-trumpet1 #aed4  ,sphere1 #bf4 
,springs1 #acj4  ,spring tides1 #aaf4   
,square1 #ha1 #he4  ,stars1 #gg1 #hf1   
#ajb4  ,storms1 #adg4  ,substance1 #aj4 
,summer1 #gf1 #ajj4  ,sun1 #ga1 #ge1    
#gh1 #afb1 #ahb4  ,swimming1 #da4       
,syphon1 #acb4   ,t4  ,tangent1 #di1    
#gc4  ,telescope1 #bjc1 #bjd4           
reflecting1 #bjd4   refracting1 #bjd4   
,temperate zone1 #ib1 #aja4             
,thermometer1 #adb4  ,tides1 #aad1 #aaf4
   neap1 #aaf4   spring1 #aaf4   aerial1
 #aej4  ,time1 #aje1 #ajg4   siderial1  
#ajg4   equal1 #ajg4   solar1 #ajg4     
,tone1 #aee4  ,torrid zone1 #ic1 #adg1  
#ahb4  ,transit1 #aad4  ,transparent    
bodies1 #aeg4  ,treble and bass1 #aee4  
,tropics1 #ib4   ,v4  ,valve1 #adc4     
,vapour1 #ag1 #bi1 #ajd1 #abi4          
,velocity1 #cc1 #eg4  ,venus1 #hd4      
,vesta1 #hd4  ,vibration1 #ih1 #aeb4    
,vision1 #afd1 #afh4  ,vision1 angle of1
 #afh1 #agj4   double1 #aga4   ,u4      

,undulation1 #aec4  ,unison1 #aee4   ,w4
  ,water1 #aah1 #acj4   spring1 #acj4   
rain1 #acj4   level of1 #abj4  ,wedge1  
#ed1 #ff4  ,weight1 #bc4  ,wheel and    
axle1 #ed1 #fe4  ,wind1 #adf4   trade1  
#adg4   periodical1 #adh4  ,winter1 #gf1
 #aja4   ,y4  ,year1 #ajg4   siderial1  
#ajg4   solar1 #ajg4   ,z4  ,zodiac1    
#hf4  ,zone1 #ic4   torrid1 #ic1 #adg1  
#ahb4   temperate1 #ic1 #aja4   frigid1 
#ic1 #ajj4   ,,the ,,end4     ,,to ,,all
 ,,teachers4  ,,school ,,books4         
,,smiley',s ,,geography ,,and ,,atlas1  
and ,,sacred ,,and ,,ancient ,,geography
 ,,for ,,schools4       ,the above works
 will be found useful and very valuable 
as works      of reference1 as well as  
for schools4 ,the ,maps1 composing the  
,atlases1 will be found equal in        
execution and correctness to      those 
on the most extensive scale4 ,the author
 has received      numerous             
recommendations1 among which are the    
following3       ,dear ,sir--,i have    
looked over your ,easy ,introduction to 

the      ,study of ,geography1 together 
with your ,improved ,atlas4 ,i      have
 no hesitation in declaring1 that ,i    
consider them works of      peculiar    
merit4 ,they do honour to your industry1
 research1 and      talent1 and ,i am   
satisfied1 will facilitate the          
improvement of      the student in      
geographical science4       ,with       
sentiments of sincere consideration1 ,i 
am yours truly1                         
,,wm4 ,,staughton1 ,d4 ,d4              
,president of ,columbia ,college1       
,district of ,columbia4       ,,mr4     
,,thomas ,,smiley4      ,philadelphia1  
,sept4 #a1 #ahbc4         99       99   
99       99       99       ,extract from
 the ,minutes of the ,philadelphia      
,academy of      ,teachers4             
,november #a1 #ahbc4       ,resolved    
unanimously1 ,that the ,academy of      
,teachers highly      approve the       
superior merits of ,mr4 ,smiley's ,easy 
,introduction      to the ,study of     
,geography1 and the accompanying ,atlas1

 and      cordially recommend them to   
the patronage of the public4            
,b4 ,,mayo1 ,president4       ,i4 ,i4   
,,hitchcock1 ,secretary4     ,,the ,,new
 ,,federal ,,calculator1 or ,,scholar',s
 ,,assistant4 ,containing the most      
concise and accurate ,rules for         
performing the operations in common     
,arithmetic2 together with numerous     
,examples under each of the ,rules1     
varied so as to make them conformable to
 almost every kind of business4 ,for the
 ,use of ,schools and ,counting ,houses4
       ,by ,thomas ,t4 ,smiley1         
,teacher3 author of ,an ,easy           
,introduction      to the ,study of     
,geography4 ,also1 of ,sacred ,geography
 for the      ,use of ,schools4         
,among the numerous recommendations     
received to the work1 are      the      
following3       ,,mr4 ,,john ,,grigg4  
,phila4 ,march #h1 #ahbe4               
,,sir--,i have examined with as much    
care as my time would admit1      ,the  
,new ,federal ,calculator1 by ,thomas   

,t4 ,smiley4 ,it appears      to me to  
be a treatise on ,arithmetic of         
considerable merit4      ,there are     
parts in ,mr4 ,smiley's work which are  
very valuable2      the rules given by  
him in ,barter1 ,loss and ,gain1 and    
,exchange1      are a great desideratum 
in a new system or treatise on          
,arithmetic1 and renders his book       
superior to any on the subject      now 
in use2 and when it is considered that  
the calculations in      the work are   
made in ,federal ,money1 the only       
currency now known      in the ,united  
,states1 and that appropriate questions 
follow the      different rules1 by     
which the learner can be exercised as to
      his understanding of each part as 
he progresses2 ,i hesitate not      to  
say1 that1 in my opinion1 it is         
eminently calculated to      promote    
instruction in the science on which it  
treats4 ,mr4      ,smiley deserves the  
thanks of the public and the            
encouragement      of teachers1 for his 

attempt to simplify and improve the     
method      of teaching ,arithmetic4 ,i 
am yours respectfully1                  
,,wm4 ,p4 ,,smith1                      
,preceptor of ,mathematics and ,natural 
,philosophy1                     ,no4   
#aeb1 ,south ,tenth ,street4         99 
99       99       99       99           
,,sir--,i have carefully examined ,the  
,new ,federal ,calculator1 or           
,scholar's ,assistant1 by ,thomas ,t4   
,smiley1 on which you      politely     
requested my opinion2 and freely        
acknowledge that ,i      think it better
 calculated for the use of the ,united  
,states      schools and counting-houses
 than any book on the subject that ,i   
have seen4 ,the author's arrangement of 
the four primary rules      is1 in my   
opinion1 a judicious and laudable       
innovation1      claiming the merit of  
improvement2 as it brings together the  
rules nearest related in their nature   
and uses4 ,his questions      upon the  
rules throughout1 appear to me to be    

admirably      calculated to elicit the 
exertions of the learner4 ,but above    
all1 the preference he has given to the 
currency of his own      country1 in its
 numerous examples1 has stamped a value 
upon      this little work1 which ,i    
believe has not fallen to the lot of    
any other book of the kind1 as yet      
offered to the ,american      public4   
,i am1 sir1 yours respectfully1         
,,john ,,mackay4       ,charleston1 7,s4
 ,c47 ,march #bi1 #ahbe4         99     
99       99       99       99           
,from the ,united ,states ,gazette4     
,among the numerous publications of the 
present day1 devoted to      the        
improvement of youth1 we have noticed a 
new edition of      ,smiley's           
,arithmetic1 just published by ,j4      
,grigg4       ,the general arrangement  
of this book is an improvement upon the 
,arithmetics in present use1 being more 
systematic1 and      according to the   
affinities of different rules4 ,the     
chief      advantage of the present over

 the first edition1 is a      correction
 of several typographical errors1 a     
circumstance      which will render it  
peculiarly acceptable to teachers4 ,in  
referring to the merits of this little  
work1 it is proper to      mention that 
a greater portion of its pages are      
devoted to      ,federal calculation1   
than is generally allowed in primary    
works in this branch of study4 ,the     
heavy tax of time and      patience     
which our youth are now compelled to pay
 to the errors      of their ancestors1 
by performing the various operations of 
pounds1 shillings1 and pence1 should be 
remitted1 and we are      glad to notice
 that the ,federal computation is       
becoming the      prominent practice of 
school arithmetic4       ,in            
recommending ,mr4 ,smiley's book to the 
notice of parents and      teachers1 we 
believe that we invite their attention  
to a work      that will really prove an
 assistant to them1 and a guide      to 
their interesting charge4         99    

99       99       99       99       ,the
 ,editors of the ,new ,york ,telegraph1 
speaking of ,smiley's      ,arithmetic1 
observe that they have within a few days
      attentively examined the above    
,arithmetic1 and say1 ,we do not        
hesitate to pronounce it an improvement 
upon every work of the      kind        
previously before the public2 and as    
such1 recommend its      adoption in all
 our ,schools and ,academies4     ,a    
,,key to the above ,arithmetic1 in which
 all the ,examples necessary for a      
,learner are wrought at large1 and also 
,solutions given of all the various     
,rules4 ,designed principally to        
facilitate the labour of ,teachers1 and 
assist such as have not the opportunity 
of a tutor's aid4 ,by ,t4 ,t4 ,smiley1  
author of the ,new ,federal ,calculator1
 c4 c4     ,,torrey',s ,,spelling       
,,book1 or ,first ,book for ,children4  
,i have examined ,mr4 ,jesse ,torrey's  
,familiar ,spelling ,book4 ,i      think
 it a great improvement in the          

primitive1 and not least      important 
branches of education1 and shall        
introduce it into      the seminaries   
under my care1 as one superior to any   
which has      yet appeared4            
,,ira ,,hill1 ,a4 ,m4                   
,boonsborough1 ,feb4 #b1 #ahbe4         
,the increasing demand for this work is 
the best evidence of its      merits4   
,a ,,pleasing ,,companion ,,for ,,little
 ,,girls ,,and ,,boys1 blending         
,instruction with ,amusement2 being a   
,selection of ,interesting ,stories1    
,dialogues1 ,fables1 and ,poetry4       
,designed for the use of ,primary       
,schools and ,domestic ,nurseries4 ,by  
,jesse ,torrey1 ,jr4       ,to secure   
the perpetuation of our republican form 
of government      to future            
generations1 let ,divines and           
,philosophers1 ,statesmen      and      
,patriots1 unite their endeavours to    
renovate the age1 by      impressing the
 minds of the people with the importance
 of      educating their little boys and

 girls4                                 
,s4 ,,adams4       ,report of the       
,committee of the ,philadelphia ,academy
 of      ,teachers3 adopted ,nov4 #f1   
#ahbd4       ,the ,committee1 to whom   
was referred ,mr4 ,jesse ,torrey's      
,pleasing ,companion for ,little ,girls 
and ,boys1 beg leave to      report1    
,that they have perused the ,pleasing   
,companion1 and have much      pleasure 
in pronouncing as their opinion1 that it
 is a      compilation much better      
calculated for the exercise and         
improvement of small children in the art
 of reading1 and      especially in the 
more rare art of understanding what they
      read1 than the books in general   
use4       ,all which is respectfully   
submitted4                              
,i4 ,,irvine ,,hitchcock1               
,,pardon ,,davis1                       
,,charles ,,mead1                       
,committee4       ,a true copy from the 
minutes of the ,academy4                
,c4 ,b4 ,,trego1 ,secretary4       ,nov4

 #bb1 #ahbd4     ,,the ,,moral          
,,instructor ,,and ,,guide ,,to         
,,virtue1 by ,jesse ,torrey1 ,jr4       
,among the numerous recommendations to  
this valuable ,school      ,book1 are   
the following3--       ,extract of a    
note from the ,hon4 ,thomas ,jefferson1 
late      ,president of the ,united     
,states4       ,i thank you1 sir1 for   
the copy of your ',moral ,instructor4'  
,i      have read the first edition with
 great satisfaction1 and      encouraged
 its reading in my family4         99   
99       99       99       99           
,extracts of a ,letter from the ,hon4   
,james ,madison1 late      ,president of
 the ,united ,states4       ,sir--,i    
have received your letter of the #aeth1 
with a copy of      the ,moral          
,instructor4       ,i have looked enough
 into your little volume to be          
satisfied1      that both the original  
and selected parts contain information  
and instruction which may be useful1 not
 only to juvenile but      most other   

readers4                                
,with friendly respects1                
,,james ,,madison4       ,,dr4 ,,torrey4
         99       99       99       99  
99       ,from ,roberts ,vaux1          
,president of the ,controllers of the   
,public      ,schools in ,philadelphia4 
,the ,moral ,instructor is a valuable   
compilation4 ,it appears to      be well
 adapted for elementary schools1 and it 
will give me      pleasure to learn that
 the lessons which it contains are      
furnished for the improvement of our    
youth generally4                        
,respectfully1                          
,,roberts ,,vaux4       ,philadelphia1  
#eth month1 #h #ahbc4     ,,history ,,of
 ,,england1 from the ,first ,invasion by
 ,julius ,caesar1 to the ,accession of  
,george the ,fourth1 in eighteen hundred
 and twenty3 comprising every ,political
 ,event worthy of remembrance2 a        
,progressive ,view of ,religion1        
,language1 and ,manners2 of ,men eminent
 for their ,virtue or their ,learning2  

their ,patriotism1 ,eloquence1 or       
,philosophical ,research2 of the        
,introduction of ,manufactures1 and of  
,colonial ,establishments4 ,with an     
interrogative ,index1 for the use of    
,schools4 ,by ,william ,grimshaw1 author
 of a ,history of the ,united ,states1  
c4  ,,history ,,of ,,the ,,united       
,,states1 from their first settlement as
 ,colonies1 to the cession of ,florida1 
in #ahba3 comprising every ,important   
,political ,event2 with a ,progressive  
,view of the ,aborigines2 ,population1  
,religion1 ,agriculture1 and ,commerce2 
of the ,arts1 ,sciences1 and            
,literature2 occasional ,biographies of 
the most remarkable ,colonists1         
,writers1 and ,philosophers1 ,warriors1 
and ,statesmen2 and a ,copious          
,alphabetical ,index4 ,by ,william      
,grimshaw1 author of a ,history of      
,england1 c4  ,also1 ,,questions adapted
 to the above ,history1 and a ,,key1    
adapted to the ,questions1 for the use  
of ,teachers4                           

,university of ,georgia1 ,athens1 ,june 
#d1 #ahbe4       ,,dear ,,sir1          
,with grateful pleasure1 ,i have read   
the two small volumes of      ,mr4      
,grimshaw1 7a ,history of ,england1 and 
a ,history of the      ,united ,states7 
which you some time since placed in my  
hands4 ,on      a careful perusal of    
them1 ,i feel no difficulty in giving my
      opinion1 that they are both1 as to
 style and sentiment1 works      of     
uncommon merit in their kind2 and       
admirably adapted to      excite1 in    
youthful minds1 the love of historical  
research4       ,with sincere wishes for
 the success of his literary labours1   
,i am very respectfully1 your friend1   
,m4 ,,waddel1 ,president4       ,e4     
,,jackson1 ,,esq4         99       99   
99       99       99       ,d4 ,,jaudon 
presents his respectful compliments to  
,mr4 ,grimshaw1      and is much obliged
 by his polite attention1 and the       
handsome      compliment of his ,history
 of the ,united ,states with the        

,questions and ,key4       ,mr4 ,j4 has 
been in the use of this book for some   
time2 but      anticipates still more   
pleasure to himself1 and profit to his  
pupils1 in future1 from the help and    
facility which the      questions and   
key will afford in the study of these   
interesting      pages4       ,october  
#ajth1 #ahbb4         99       99       
99       99       99                    
,golgotha1 ,p4 ,edwd4 ,va4 ,sep4 #bf1   
#ahbj4       ,,dear ,,sir1       ,,mr4  
,,grimshaw',s ',history of the ,united  
,states1' c4 was some      time ago put 
into my hands by ,mr4 ,b----1 who       
requested me to      give you my opinion
 as to the merits of the work4 ,the     
history      of the late war is well    
managed by your author3 it has more of  
detail and interest than the former     
part2 and ,i consider it      much      
superior to any of the many compilations
 on that subject1      with which the   
public has been favoured4 ,it may be    
said of the      entire performance1    

that it is decidedly the best           
chronological      series1 and the      
chastest historical narrative1 suited to
 the      capacity of the juvenile mind1
 that has yet appeared4 ,its            
arrangement is judicious2 its style     
neat1 always perspicuous1      and often
 elegant2 and its principles sound4     
,american writings on men and things    
connected with ,america1      have been 
long needed for the young2 and ,i am    
happy to find1      that ,mr4 ,grimshaw 
has not only undertaken to supply this  
want1      but also to ,americanise     
foreign history for the use of our      
schools4 ,in a word1 sir1 ,i am so fond 
of ,american fabrics1 and      so       
anxious to show myself humbly           
instrumental in giving our      youth   
,american feeling and character whilst  
at school2 that ,i      shall without   
hesitation recommend ,mr4 ,grimshaw's   
works to my      young pupils1 as       
introductory to more extensive          
historical      reading4 ,in fine1 the  

work is so unobjectionable1 and puts so 
great a mass of necessary information   
within the reach of      school-boys1 at
 so cheap a rate1 that ,i feel the      
highest      pleasure in recommending it
 to the public1 and wish you            
extensive sales4                        
,yours respectfully1                    
,,william ,,branch1 ,,jr4       ,,mr4   
,,benjamin ,,warner1      ,philadelphia4
         99       99       99       99  
99       ,history of the ,united        
,states1 from their first settlement as 
,colonies1 to the ,peace of ,ghent1 c4  
,by ,william ,grimshaw1 pp4      #cab1  
#abmo4       ,this is the third time1   
within the space of two years1 that we  
have had occasion to review a volume    
from the hand of ,mr4      ,grimshaw4   
,he writes with great rapidity2 and     
improves as he      advances4 ,this is  
the most correctly written of all his   
productions4 ,we could wish that a      
person so well formed for      close1   
and persevering study1 as he must be1   

might find      encouragement to devote 
himself to the interests of             
literature4       ,mr4 ,g4 has our      
thanks for the best concise and         
comprehensive      history of the       
,united ,states which we have seen4     
,theological ,review1 ,october1 #ahai4  
99       99       99       99       99  
,history of ,england1 from the first    
,invasion by ,julius ,caesar1      to   
the ,peace of ,ghent1 c4 ,for the use of
 ,schools4 ,by      ,william ,grimshaw4 
,philadelphia1 #ahai4 ,benjamin ,warner4
 #abmo4      pp4 #cjj4       ,we have   
copied so much of the title of this     
work1 barely to      express our decided
 approbation of the book1 and to        
recommend      its general introduction 
into schools4 ,it is one of the best    
books of the kind to be found1 and is   
instructive even to an      adult       
reader4 ,we should be pleased that      
teachers would rank it      among their 
class-books2 for it is well calculated  
to give      correct impressions1 to its

 readers1 of the gradual progress of    
science1 religion1 government1 and many 
other institutions1 a      knowledge of 
which is beneficial in the present age4 
,among the      many striking merits of 
this book1 are1 the perspicuity of the  
narrative1 and chasteness of the style4 
,it is with no little      pleasure we  
have learned1 that the author has       
prepared a      similar history of the  
,united ,states2 a work long wanted1 to 
fill up a deplorable chasm in the       
education of ,american youth4           
,analectic ,magazine1 ,october1 #ahai4  
99       99       99       99       99  
,philadelphia1 #bh ,june1 #ahai4        
,,sir--,i have read with pleasure and   
profit your ,history of      ,england4  
,i think it is written with perspicuity1
 chasteness1      and impartiality4     
,well written history is the best       
political      instructor1 and under a  
government in which it is the blessing  
of the country that the people govern1  
its pages should be      constantly in  

the hands of our youth1 and lie open to 
the      humblest citizen in our        
wide-spread territories4 ,your book is  
eminently calculated thus to diffuse    
this important knowledge1      and      
therefore entitled to extensive         
circulation2 which ,i most              
cordially wish4 ,with much respect1     
,your obedient servant1                 
,,langdon ,,cheves4       ,,william     
,,grimshaw1 ,,esq4     ,,grimshaw',s    
,,improved ,,edition ,,of ,,goldsmith',s
 ,,greece4--,among the numerous         
recommendations to this valuable ,school
 ,book1 are the following3--            
,although there are many worthless      
,school ,books1 there are but      few  
which are equally impure and inaccurate 
with the original      editions of      
,goldsmith's ,histories1 for the use of 
,schools4 ,i      congratulate both     
teachers and pupils upon the appearance 
of      ,mr4 ,grimshaw's edition of the 
,history of ,greece1 which has      been
 so completely expurgated1 and otherwise

 corrected1 as to      give it the      
character of a new work1 admirably      
adapted to the      purpose for which it
 is intended4                           
,,thos4 ,p4 ,,jones1                    
,professor of ,mechanics in the         
,franklin ,institute                    
of the ,state of ,pennsylvania1 and late
 ,principal of                     the  
,north ,carolina ,female ,academy4      
,philadelphia1 ,sept4 #e1 #ahbf4        
99       99       99       99       99  
,,mr4 ,,john ,,grigg4       ,,dear      
,,sir--,agreeably to your request ,i    
have examined1 with      attention1     
,goldsmith's ,greece1 revised and       
corrected1 and a      vocabulary of     
proper names appended1 with prosodial   
marks1 to      assist in their          
pronunciation1 by ,william ,grimshaw2   
and ,i feel      a perfect freedom to   
say1 that the correction of numerous    
grammatical and other errors1 by ,mr4   
,grimshaw1 together with      the       
rejection of many obscene and indelicate

 passages improper      for the perusal 
of youth1 gives this edition1 in my     
opinion1 a      decided preference over 
the editions of that work heretofore in 
use4       ,the ,questions and ,key1    
likewise supplied by ,mr4 ,grimshaw to  
accompany this edition1 afford a        
facility for communicating              
instruction1 which will be duly         
appreciated by every judicious          
teacher4                                
,i am1 ,sir1                            
,yours truly1                           
,,thos4 ,t4 ,,smiley4                   
,philadelphia1 ,sept4 #h1 #ahbf4        
99       99       99       99       99  
,the ,editor of the ,united ,states     
,gazette1 in speaking of this      work1
 says--,goldsmith's ,greece1 without a  
revision1 is not      calculated for    
schools2 it abounds in errors1 in       
indelicate      description1 improper   
phrases1 and is1 indeed1 a proof how    
very      badly a good author can write1
 if indeed there is not much room       

to doubt ,goldsmith ever composed the   
histories to which his      name is     
attached4 ,mr4 ,grimshaw has adopted the
 easy descriptive      style of that    
writer1 retained his facts1 connected   
his dates1      and entirely and        
handsomely adapted his work to the      
school      desk4 ,the book of questions
 and the accompanying key1 are          
valuable additions to the work1 and will
 be found most      serviceable to      
teacher and pupil4       ,from a        
knowledge of the book1 and some         
acquaintance with the      wants of     
those for whom it was especially        
prepared1 we      unhesitatingly        
recommend ,grimshaw's ,greece as one of 
the best      7in our opinion1 the very 
best of7 works of the kind that has     
been offered to the public4     ,,the   
,,united ,,states ,,speaker1 compiled by
 ,t4 ,t4 ,smiley--preferred generally to
 the ,columbian ,orator and ,scott's    
,lessons1 and works of that kind1 by    
teachers who have examined it4          

,,goldsmith',s ,,history ,,of ,,greece1 
improved by ,grimshaw1 with a           
,vocabulary of the ,proper ,names       
contained in the work1 and the          
,prosodial ,accents1 in conformity with 
the ,pronunciation of ,lempriere--with  
,questions and a ,key1 as above4        
,,grimshaw',s ,,etymological            
,,dictionary ,,and ,,expositor ,,of     
,,the ,,english ,,language4             
99       99       99       99       99  
,transcriber's note3   ,spelling        
variations where there is no obviously  
preferred choice have been preserved1   
except as noted below4 ,irregularities  
include3 bason and basin2 derivatives of
 enquire and inquire2 learned and       
learnt2 sidereal and siderial2 sun-rise 
and sunrise2 sun-set and sunset4  ,the  
original use of commas was preserved1   
except where explicitly noted below4    
,the original spelling of pourtray was  
preserved4  ,both ,roman and ,arabic    
numerals are used to number the plates2 
the text was left as is4  ,preserved the

 non-standard order in the index1 where 
,u comes after ,v4  ,removed extra comma
 after which on page v3 about which the 
parts4  ,changed ,sideral to ,siderial  
on page vi3 ,solar1 ,siderial1 and      
,equal4  ,added comma after ,mrs4 ,b4 on
 page #i3 your assistance1 my ,dear     
,mrs4 ,b41 in a charge4  ,changed       
errroneous to erroneous on page #aj3 an 
erroneous conception4  ,added comma     
after ,mrs4 ,b4 twice on page #bc3 ,yet 
surely1 ,mrs4 ,b41 there2 and ,but1     
,mrs4 ,b41 if attraction4  ,added commas
 before and after ,mrs4 ,b4 on page #be3
 ,pray1 ,mrs4 ,b41 do4  ,changed pullies
 to pulleys on page #fd3 a system of    
pulleys4  ,changed plate #f4 fig4 #e to 
plate #e4 fig4 #e on page #fe in the    
body of the text and in the associated  
question1 to designate the correct      
figure4  ,changed twelves to twelve on  
page #fe3 twelve times less4  ,changed  
stream to steam on page #ff3 expansive  
force of steam4  ,changed ,pray ,mrs4   
,b1 to ,pray1 ,mrs4 ,b41 on page #fh4   

,changed nonelastic to non-elastic on   
page #gj3 non-elastic like water4       
,removed extra comma after one on page  
#gc3 one would ultimately have          
prevailed4  ,changed eliptical to       
elliptical on page #gc3 elliptical or   
oval orbit4  ,changed eclipse to        
ellipsis on page #gc3 motion in an      
ellipsis4  ,changed elipsis to ellipsis 
on page #ge3 but an ellipsis4  ,changed 
fig4 #d plate #c in the question on page
 #ge to fig4 #d4 plate #f to designate  
the correct figure4  ,changed day-light 
to daylight on page #gg3 see them by    
daylight4  ,changed the second question 
numbered #dj to #da from page #gi4      
,changed eliptical to elliptical on page
 #hc3 they were elliptical4             
,capitalised ,mercury on page #hc3 made 
upon ,mercury4  ,added question mark on 
page #hd after those beautiful lines of 
,milton4  ,removed repeated word1 it1 on
 page #hh3 provided it were steady4     
,changed aeriform to aeriform on page   
#acf 7in versions supporting full       

,latin-#a character set74  ,changed     
atmospherical to atmospheric on page    
#aci3 the atmospheric air4  ,changed    
rarifies to rarefies on page #adj3 heat 
rarefies air4  ,changed to day to to-day
 on page #aeg3 our lesson to-day4       
,changed re-appearance to reappearance  
on page #aei3 reappearance of the sun4  
,changed question #bj to #bi on page    
#agd to maintain proper sequence4       
,changed proportionably to              
proportionally on page #aih3            
proportionally distinct4  ,inserted     
comma after ,circle on page #bjf in the 
glossary entry for ,circle1 ,lesser4    
,inserted period on page #bjg at the end
 of the glossary entry for ,cylinder4   
,changed musisical to musical on page   
#bjh in the glossary entry for ,harmony4
  ,changed perpendidicular to           
perpendicular on page #baa3             
perpendicular to each other4  ,changed  
oppoite to opposite on page #bab3 the   
opposite direction4  ,capitalised ,aries
 on page #bae3 the first degree of      

,aries4  ,change jr4 to ,jr4 in the     
advertisement for ,a ,pleasing          
,companion 4443 ,by ,jesse ,torrey1 ,jr4