,,transcriber',s ,,note ,italic text is denoted by underscores4 ,bold text is denoted by equal signs4 ,obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources4 ,more detail can be found at the end of the book4 ,,science ,,for ,,the ,,school ,,and ,,family4 ,,part ,i4 ,,natural ,,philosophy4 ,,by ,,worthington ,,hooker1 ,m4,d41 ,,professor ,,of ,,the ,,theory ,,and ,,practice ,,of ,,medicine ,,in ,,yale ,,college1 ,,author ,,of ,,human ,,physiology1 ,,child',s ,,book ,,of ,,nature1 ,,natural ,,history1 ,c4 ,illustrated by nearly #cjj ,engravings4 ,,new ,,york3 ,,harper ,,brothers1 ,,publishers1 ,,franklin ,,square4 #ahgc4 ,by ,dr4 ,worthington ,hooker4 ,the ,child's ,book of ,nature4 ,for the ,use of ,families and ,schools2 intended to aid ,mothers and ,teachers in training ,children in the ,observation of ,nature4 ,in three ,parts4 ,illustrated by ,engravings4 ,the ,three ,parts complete in one vol4 ,small #dto1 ,cloth1 #b #jj2 ,separately1 ,cloth1 #ij cents each4 ,,part ,i4 ,,plants4 ,,part ,,ii4 ,,animals4 ,,part ,,iii4 ,,air1 ,,water1 ,,heat1 ,,light1 c4 ,first ,book in ,chemistry4 ,for the ,use of ,schools and ,families4 ,illustrated by ,engravings4 ,square #dto1 ,cloth1 #ij cents4 ,natural ,history4 ,for the ,use of ,schools and ,families4 ,illustrated by nearly #cjj ,engravings4 #abmo1 ,cloth1 #a #ej4 ,science for the ,school and ,family4 ,,part ,i4 ,,natural ,,philosophy4 ,illustrated by nearly #cjj ,engravings4 #abmo1 ,cloth1 #a #ej4 ,,part ,,ii4 ,,chemistry4 ,illustrated by numerous ,engravings1 #abmo1 ,cloth1 #a #ej4 ,,part ,,iii4 ,,mineralogy ,,and ,,geology4 ,illustrated by numerous ,engravings4 #abmo4 ,cloth1 #a #ej4 ,published by ,,harper ,,brothers1 ,franklin ,square1 ,n4 ,y4 ,any of the above ,works sent to any part of the ,united ,states1 postage pre-paid1 upon receipt of the ,price4 ,entered1 according to ,act of ,congress1 in the year one thousand eight hundred and sixty-three1 by ,,harper ,,brothers1 in the ,clerk's ,office of the ,district ,court of the ,southern ,district ,court of ,new ,york4 ,,preface4 ,daniel ,webster1 in his ,autobiography1 speaks thus of his entering upon the study of law3 ,i was put to study in the old way--that is1 the hardest books first1 and lost much time4 ,i read ,coke on ,littleton through without understanding a quarter part of it4 ,happening to take up ,espinasse's ,law of ,nisi ,prius1 ,i found ,i could understand it2 and arguing that the object of reading was to understand what was written1 ,i laid down the venerable ,coke et alios similes reverendos1 and kept company for a time with ,mr4 ,espinasse and others1 the most plain1 easy1 and intelligible writers4 ,a boy of twenty1 with no previous knowledge on such subjects1 can not understand ,coke4 ,it is folly to set him on such an author4 ,there are propositions in ,coke so abstract1 and distinctions so nice1 and doctrines embracing so many conditions and qualifications1 that it requires an effort1 not only of a mature mind1 but of a mind both strong and mature1 to understand him4 ,why disgust and discourage a boy by telling him that he must break into his profession through such a wall as this8 ,i really often despaired4 ,i thought that ,i never could make myself a lawyer1 and was almost going back to the business of schoolkeeping4 ,mr4 ,espinasse1 however1 helped me out of this in the way that ,i have mentioned1 and ,i have always felt greatly obliged to him4 ,here is most graphically depicted a defect which is now1 as it was then1 very prominent in all departments of education4 ,it is even more so in early education than in that of the college and the professional school4 ,even in tender childhood pupils are put to studying books of which1 as was true of ,webster with his ,coke on ,littleton1 they do not understand a quarter part4 ,if the rule is not the hardest books first1 there are many things in the books that it is not only hard but impossible for them to understand4 ,and the hardest things are often put first4 ,for example1 in a very popular primary geography which lies before me the pupil is introduced to the world and its grand divisions at the outset1 while he is taught about his own state and country only at the conclusion of the book4 ,and this unnatural mode is the one very commonly pursued4 ,similar criticism can be passed upon most of the books used in teaching young children4 ,some of them are wholly useless4 ,this is true of the grammars for primary schools4 ,the formal statements1 called the rules of grammar1 are beyond the understanding of very young scholars1 and therefore are useless burdens upon their memories4 ,they are as useless to them as the three-fourths of ,coke which ,webster could not understand was to him4 ,if we follow education1 upward from the primary school we find the same defect throughout the whole course4 ,in the books which are used in teaching natural science it is especially prominent4 ,even in the elementary books1 or compendiums1 so called1 formal propositions and technical terms render the study uninviting1 and to a great extent unintelligible4 ,the pupil is apt to be disgusted and discouraged1 as ,webster was with ,coke on ,littleton1 and for the same reason4 ,another defect intimately connected with that of which ,i have spoken is the very sparing and late introduction of the physical sciences4 ,they are generally postponed to the latter part of the course of education1 and then but little time is devoted to them4 ,generally1 when a pupil designs to go through college1 the study of these sciences is wholly neglected in his preparation1 because a knowledge of them is not required for admission4 ,then in the college they are not attended to till the latter part of the course1 and in the short time allotted to them there is so much to be learned that the teaching of them is a failure4 ,especially is this true of ,chemistry and ,geology4 ,this defect is a radical one4 ,a thorough change should be effected in this respect in the whole course of education4 ,the natural sciences should be made prominent from the beginning to the end1 not only because they are of practical value1 but also because they are as useful in their way for mental discipline as the study of mathematics and of language4 ,they can be taught to some little extent to the youngest pupils4 ,there are facts about air1 water1 and the various objects that they see around them1 which they can understand if they be presented in the right manner4 ,and the busy inquiries which they make after the reasons of the facts1 and their appreciation of them if stated simply and without technical terms1 show the appropriateness of such teaching4 ,children are really very good philosophers in their way4 ,they have great activity not only of their perceptive but of their reasoning faculties also1 to which due range should be given in their education4 ,beginning thus1 not a year should pass during the whole course when the pupil shall not be engaged in studying some one of the physical sciences to some extent4 ,this continued attention to such studies in a reasonable amount1 so far from interfering with the due prosecution of the other studies deemed so essential1 will so promote the pupil's advance in them as to more than make up for the time that is taken from them4 ,it will do this not only by the genial influence which such studies exert upon the mind1 but by the contributions which they make to the knowledge of language and mathematics2 for language is largely built up from natural objects and from the acquisitions of science1 and there is an abundance of interesting applications of portions of the mathematics in the facts which the physical sciences develop to us4 ,i have said that the teaching of the natural sciences in our colleges is generally a failure1 and it always will be so as long as the present plan is continued4 ,in order to have it successful there must be the same gradation in teaching them that we have in teaching language and the mathematics4 ,the college student needs to be prepared for the lectures which he hears on natural philosophy1 chemistry1 etc41 and for his study of those branches1 by previous familiarity with the simpler portions of them acquired in the school-room4 ,there is another very important reason for the early introduction of the physical sciences into education4 ,by far the larger portion of pupils in our schools stop short of the college1 or even the academy and high school4 ,that they should go forth into the world with no knowledge of the principles that lie at the basis of the arts in which so many of them are to engage is a shame and a wrong1 if the communication of such knowledge be indeed practicable1 as it undoubtedly is4 ,even those who are not to engage in these arts will be greatly benefited by this knowledge1 because in addition to its constant practical applications in the management of life1 it will contribute to their mental power1 and1 what is no small consideration1 to their enjoyment2 and it is in fact requisite to constitute them well-informed persons4 ,if the views which ,i have presented be correct1 there should be a series of books on the natural sciences carefully adapted to the different periods of the course of study4 ,those intended for the young beginner should be exceedingly simple1 and should not attempt to present any thing like a full view of the subjects treated4 ,they should deal largely with familiar facts or phenomena4 ,the terminology of science and formal statements of principles1 such as we often see in so-called compendiums1 should have no place in them1 but should be gradually introduced as the series advances1 and should be made complete only in the concluding books4 ,it has been the object of the author to supply a part of such a series4 ,the first book in the series is the ,child's ,book of ,common ,things1 intended to teach the observation of familiar facts1 or1 in other words1 the beginnings of philosophy1 to children as soon as they have got well started in reading4 ,next comes the ,child's ,book of ,nature1 which in its three parts 7,part ,i41 ,plants2 ,part ,,ii41 ,animals2 ,part ,,iii41 ,air1 ,water1 ,light1 ,heat1 etc47 extends considerably the knowledge of the philosophy of things which the child has obtained from the first book in the series4 ,then follows the ,first ,book in ,chemistry4 ,on a level with this is my ,first ,book in ,physiology4 ,the next step in the gradation brings us to three books under one title1 ,science for the ,school and the ,family2 ,part ,i41 ,natural ,philosophy2 ,part ,,ii41 ,chemistry2 ,part ,,iii41 ,mineralogy and ,geology4 ,on a level with these is another book1 ,natural ,history1 and another still is to be written1 an ,introduction to ,botany4 ,the three books1 of which the present is one1 are intended for the older scholars in what are commonly called grammar-schools4 ,at the same time they are suited to scholars who are advanced to a higher grade who have not gone through the previous books of the series4 ,the preparation of books especially adapted to high schools and colleges ,i have left to others1 except in one branch of science1 ,physiology1 on which ,i some years ago published a work entitled ,human ,physiology4 ,all of these books are from the press of ,harper and ,brothers except the two works on ,physiology1 published by ,sheldon and ,co41 ,new ,york1 and the ,child's ,book of ,common ,things1 published by ,peck1 ,white1 and ,peck1 ,new ,haven4 ,the general plan and style of these books are very different from what we see in most of the books for schools on the same subjects4 ,the order of the subjects and the mode of developing them differ from the stereotype plan which has so generally been adopted4 ,one prominent feature is the free use of illustrations from familiar phenomena4 ,this leads the pupil to reason or philosophize about common things1 thus giving an eminently practical character to his knowledge4 ,at the same time it makes the books suitable for use in the family as well as the school1 between which there should be more common ground than the present mode of education allows4 ,the style which ,i have chosen for all the books ,i have written for use in teaching is what may be called the lecture style4 ,there are three other kinds of style which are more commonly used in school-books4 ,the most common is what ,i term the formal statement style4 ,in this principles and rules are stated1 and then illustrations are given4 ,this makes a formal and uninviting book4 ,the bare skeleton of the science is generally for the most part presented1 and the young pupil is apt to learn the statements by rote without understanding them4 ,it is a style fitted only for books intended for advanced scholars4 ,another style is the catechetical4 ,this is an unnatural mode of communicating knowledge2 and besides1 it encourages learning by rote as the formal statement style does4 ,in the third style1 the dramatic1 conversations are held between the teacher and some learners4 ,the chief objection to this is that it undertakes to put in permanent shape what should be extemporized in the recitation4 ,what is needed in the book is simply clear and concise statement in an interesting style1 and the living teacher and his scholars can best furnish the conversational element as the recitation goes on4 ,in the lecture style there may be and should be as much precision of statement as in the formal statement style1 while it is more interesting1 because it is the natural mode of communicating knowledge4 ,in this style the facts are ordinarily so stated as to develop principles2 while in the other the order is reversed1 the principles being first stated and the facts given afterward4 ,one of the most successful books ever used in our colleges--,paley's ,natural ,theology--is in the lecture style1 and it is a matter of surprise that this fact has had so little influence with those who have prepared books for instruction4 ,whatever may be true of advanced scholars1 in teaching the young student in science bare1 dry statement should be avoided1 and the subjects should be presented in all their attractive features4 ,i would not be understood as advocating the dressing up of science in adventitious charms4 ,this is not necessary4 ,science possesses in itself an abundance of charms1 which need only to be properly developed to attract the young mind2 and the lecture style furnishes the best vehicle for such a development4 ,one grand essential for giving interest to any study is the presentation of the various points in the natural order in which they should enter the mind4 ,they should be so presented that each portion of a book shall make the following portions more interesting and more easily understood4 ,this principle1 which is so commonly transgressed1 ,i have endeavored to observe strictly in the preparation of these volumes4 ,questions are put at the end of this book for those teachers who desire to use them4 ,there is also an ,index4 ,w4 ,,hooker4 ,january1 #ahfc4 ,,contents4 ,,chapter ,,page ,i4 ,,matter #ac ,,ii4 ,,properties ,,of ,,matter #ai ,,iii4 ,,the ,,essential ,,properties ,,of ,,matter #cc ,,iv4 ,,attraction #ch ,v4 ,,gravitation #ea ,,vi4 ,,centre ,,of ,,gravity #fg ,,vii4 ,,hydrostatics #hj ,,viii4 ,,specific ,,gravity #ajj ,,ix4 ,,pneumatics #aaj ,x4 ,,motion #acc ,,xi4 ,,the ,,mechanical ,,powers #agd ,,xii4 ,,sound #aic ,,xiii4 ,,heat #bjg ,,xiv4 ,,light #beh ,,xv4 ,,electricity #bhg ,,xvi4 ,,magnetism #cjh ,,natural ,,philosophy4 ,,chapter ,i4 ,,matter4 #a4 ,matter and ,spirit4--,the distinction between matter and spirit is almost universally recognized even by those who have given little thought to such subjects4 ,it is a distinction of which we are conscious in our own persons4 ,we know instinctively that there is a something within us that causes the movements of our material bodies1 and that something we call spirit4 #b4 ,bishop ,berkeley's ,ideas4--,some philosophers1 in their speculations1 have denied that there is any such thing in existence as matter4 ,bishop ,berkeley1 for example1 taught that the impressions which we suppose that we receive from material objects do not come from real substances1 but are the effects of the immediate agency of an ever-present ,deity4 ,it is no wonder that the wisdom and learning of a man who could seriously adopt such a belief could not save him from being the dupe of quackery4 ,he believed that tar-water was a sovereign cure for all diseases2 and ,dr4 ,holmes playfully remarks of him1 that he held two very odd opinions3 that tar-water was every thing1 and that the whole material universe was nothing4 #c4 ,hume's ,ideas4--,the infidel ,hume went beyond ,bishop ,berkeley1 denying even the existence of the soul as an individual and responsible agent4 ,he made every thing to consist of ideas and impressions1 and said that these have no necessary connection1 but are a bundle of perceptions that succeed each other with inconceivable rapidity1 and that therefore ,i myself of to-day am no more the ,i myself of yesterday or to-morrow than ,i am ,nebuchadnezzar or ,cleopatra4 ,a wag proposed the following epitaph for his tomb-stone as a suitable illustration of his theory3 ,under this circular idea1 vulgarly called tomb1 ,impressions and ideas rest which constituted ,hume4 #d4 ,origin of the word ,spirit4--,the name spirit came originally from an attenuated form of matter1 the air or breath1 because the air1 like spiritual existence1 is invisible4 ,the formation of language is to a large extent thus based on analogies4 #e4 ,spirit not an ,object of ,sense4--,none of the senses can perceive spirit itself1 though they perceive the effects which spirit produces on material substances4 ,if1 for example1 you move your arm1 it is the spirit within you1 acting upon the muscles through the nerves1 that causes it to move2 and you see here the effects produced by spirit upon matter1 but you do not see the spirit itself4 #f4 ,effects of ,matter on the ,senses4--,some forms of matter can be perceived by all the senses2 others can be perceived by only a part of them2 some by only one4 ,air you can not see1 nor smell1 nor taste2 but you can feel it1 and hear the sound of its motion4 ,sometimes matter affects only the sense of smell1 or that with the sense of taste4 ,sea-air smells salt2 but the salt in the air is so finely divided that we can not see it4 ,and yet it is the salt1 entering the nostrils and coming in contact with the extreme fibres of the nerve of smell1 that produces the effect4 ,so when we smell a flower1 matter comes from it in particles so fine that no microscope can detect them1 but they produce sensation when they strike upon the nerve4 #g4 ,forms of ,matter4--,matter appears in three forms3 solid1 liquid1 and gaseous or aeriform--that is1 like air4 ,sometimes matter is spoken of as having only two forms--solid and fluid4 ,in this case fluids are divided into two classes1 the elastic and non-elastic4 ,the air and the various gases and vapors are the elastic fluids2 while those which are called liquids are the non-elastic fluids4 ,a foot-ball bounds because the air in it is an elastic fluid4 ,if it were filled with a non-elastic fluid2 as water1 it would not bound4 ,when water takes the form of steam it is an elastic fluid4 ,though it is very common to use the expression elastic fluids1 the division of matter into three forms is the one usually recognized4 #h4 ,solids4--,in solid matter the particles can not be moved about among each other2 but each particle generally retains the same position in relation to those particles which are around it--in other words1 it does not change its neighborhood4 ,this is more true of some solids than of others4 ,it is absolutely true of such hard solids as granite and the diamond4 ,in these the particles are always in the same relative position4 ,but it is not so with gold or lead4 ,by hammering these you can change greatly the relative position of their particles4 ,india-rubber is a solid1 but the relative position of its particles can be much altered in various ways4 #i4 ,liquids4--,it is the grand characteristic of a liquid that its particles change their relative position from the slightest causes4 ,it is in strong contrast with solids in this respect4 ,when you move any portion of a solid body you move all the other portions of it1 and generally in the same direction4 ,but a body of liquid can not be moved all together as one body except by confining it2 as1 for example1 in the case of a water-pipe or a syringe4 ,and then1 the moment that the water can escape1 the particles use their liberty of altering their relative position4 ,as wind and other agents act continually upon water1 no particle stays for any length of time in the neighborhood of the same particles4 ,unstable as water is1 then1 an exceedingly significant expression4 ,water is never at rest4 ,a particle of it may at one time be floating on the surface of the ocean1 and at another be in depths beyond the sounding of man4 ,it flies on the wings of the wind1 falls in the rain1 runs in the stream1 is exhaled from a leaf1 trembles in the dew-drop1 flows in the blood of an animal or in the sap of a plant1 and is always ready to be jostled along in its ever-changing course4 #aj4 ,gases4--,the particles of gaseous or aeriform substances move among each other even more freely than those of a liquid4 ,air1 therefore1 is more unstable and restless than water4 ,even when the air seems to be perfectly still its particles are moving about among each other4 ,you can see this to be true if you darken a room1 leaving a single shutter a little open4 ,where the light enters you will see motes flying about in every direction1 which would not be the case if the air were really at rest4 ,the particles of air have a greater range of travel than those of water2 for the sea of atmosphere which envelops the earth rises to the height of about fifty miles4 ,how far water rises in its evaporation we know not2 but it is not at all probable that it rises to the uppermost regions of the atmosphere4 #aa4 ,filling of ,spaces by ,liquids and ,gases4--,it is the freeness with which the particles of liquids and gases move among each other that enables them to insinuate themselves into spaces every where4 ,they are ever ready to enter into any substances which have interstices or pores of such size as will admit them4 ,there are mingled with the grains of the soil not only water1 but air and gases4 ,these are present also in all living substances1 both vegetable and animal4 ,water is the chief part of sap and of blood1 and air and gases always go with water4 ,part of the air that we breathe in enters the blood in the lungs1 and courses with it through the system4 ,the fishes could not live in water if there were not air mingled with it4 ,this can be proved by experiment4 ,if you put a fish into a close vessel it will soon die1 because it uses up all the air that is in the water4 ,in an open vessel the fish is kept alive by the constant accessions of fresh air to the water4 #ab4 ,solution4--,in solutions of solid substances in water it is the freedom with which the particles of water move about among each other that enables them to take in among them the minute particles of the solid4 ,and when water ascends into air by evaporation it may be said to be a real solution of water in the air2 for the particles of water mingle with those of the air1 just as the particles of a solid mingle with those of water in a solution4 #ac4 ,relation of ,heat to the ,forms of ,matter4--,some kinds of matter are seen in all the three forms4 ,whether these shall assume one form or another depends on the amount of heat present4 ,thus when water is solid1 ice1 it is because a part of its heat is gone4 ,apply heat1 and it becomes a liquid1 water4 ,increase the heat to the boiling point1 and it becomes steam1 or an aeriform substance4 ,alcohol has only two forms--liquid and aeriform4 ,it has never been known to be frozen4 ,iron is usually solid2 but in the foundry1 by the application of great heat1 it is made liquid4 ,mercury is liquid in all ordinary temperatures2 but it often becomes solid in the extreme cold of arctic winters4 ,a mercurial thermometer is of course useless under such circumstances1 and the alcoholic thermometer is relied upon to denote the degree of cold4 ,the difference between mercury1 water1 and iron in regard to the liquid state is this3 ,it takes but little heat1 comparatively1 to make mercury liquid1 while more is required for this condition in water1 and much more for it in the case of iron4 #ad4 ,the ,nature of ,matter ,unknown4--,what now1 let us inquire1 do we know of the nature of matter8 ,can we say that we know any thing of it8 ,we may observe its phenomena1 and learn its properties2 but with our most searching analyses of it we can no more determine what matter is than we can what spirit is4 ,newton supposed that ,god in the beginning formed matter in solid1 massy1 hard1 impenetrable particles4 ,this he believed to be true of liquids1 and even of gases1 as well as solids4 ,in the gas these hard particles are much farther apart than in the solid4 ,the supposition is a very probable one2 but if it be true it does not let us know what matter is1 for it leaves us in the dark as to the nature of the particles4 ,newton farther supposed that these particles have always remained unaltered amidst all the changes that are taking place2 these changes being occasioned by the various separations and new associations and motions of these permanent particles4 ,when1 for example1 any thing is burned up1 as it is expressed1 not one of these particles is either destroyed or altered1 but they merely take on new arrangements4 ,though most of the substance has flown off in the form of gas1 the ultimate particles composing the gas are the same now that they were when making a part of the solid substance2 and they may soon again become a part of some new solids4 ,such changes in the forms of matter are every where going on2 and when you become acquainted with ,chemistry1 in the ,second ,part1 you will be familiar with them4 #ae4 ,atomic ,theory4--,these ultimate particles of matter are so minute that they have never been seen by man4 ,the smallest particle that can be seen with the most powerful microscope is probably made up of very many of them united together4 ,these ultimate particles we term atoms2 and the theory in regard to the composition by them of different substances is called the atomic theory4 ,the atoms of different substances are not supposed to be alike1 but to differ in both size and weight4 ,this theory will be more particularly noticed in the ,second ,part4 #af4 ,imponderable ,agents4--,there are certain agents--light1 heat1 electricity1 etc4--which are supposed by some to be forms of matter4 ,if they are1 they are exceedingly attenuated2 for their presence1 as has been proved by many experiments1 never adds in the least to the weight of any substance4 ,they have therefore been styled imponderable agents4 ,their agency is of great importance and very active1 producing every where constant changes4 ,two of them--heat and light--are obviously and immediately essential to life4 ,what their real nature is remains as yet an entire mystery4 ,,chapter ,,ii4 ,,properties ,,of ,,matter4 #ag4 ,variety in the ,properties of ,matter4--,all matter has properties or qualities4 ,some of these are different in the different kinds of matter4 ,thus its three forms have different properties1 as you saw in ,chapter ,i4 ,there is variety also in the properties of substances of the same class4 ,thus liquids are unlike each other in some respects4 ,some1 for example1 are lighter than others4 ,oil is lighter than water4 ,gaseous substances also differ in this and in other respects4 ,but the variety in the properties of solids is greater than in those of gases or liquids4 ,this will appear as ,i proceed4 #ah4 ,divisibility of ,matter4--,any visible portion of matter can be divided into parts4 ,even if it be so small that you can see it only with a powerful microscope1 it could still be divided if you could have an instrument sufficiently fine for the purpose4 ,divisibility1 then1 is said to be a general property of matter2 that is1 a property belonging to all kinds of matter4 #ai4 ,examples of ,minute ,division of ,matter4--,there are numerous examples in which the division of matter is carried far beyond that which can be effected by any cutting instrument4 ,some of these ,i will notice3 ,a gold-beater can hammer a grain of gold into a leaf covering a space of fifty square inches4 ,so thin is it that it would take #bhb1jjj of such leaves1 laid upon each other1 to make the thickness of an inch4 ,and yet so even and perfect is this thin layer of gold1 that when it is laid upon any surface in gilding it has the appearance of solid gold4 ,a fifty millionth part of this grain of gold thus hammered out can be seen by the aid of a microscope which magnifies the diameter of an object ten times4 ,but the division of gold is made even more minute than this in the manufacture of the wire of gold-lace4 ,it is done in this way3 ,a bar of silver weighing #ahj ounces is covered with a layer of gold weighing an ounce4 ,it is then drawn through a series of holes in a steel plate1 diminishing in diameter1 till it at length comes out a very fine wire #djjj feet long4 ,each foot of it then has only the one #djjjth part of the ounce of gold1 and yet the silver is well covered4 ,a soap-bubble is a beautiful example of the minute division of matter4 ,that thin wall which incloses the air which you have blown into it is composed of particles of the soap and of the water mingled together4 ,it is supposed to be less than one millionth of an inch in thickness4 ,the thread of the silk-worm is so minute that the finest sewing-silk is formed of many of these threads twisted together4 ,but the spider spins much more finely than this4 ,the thread by which you see him letting himself down from any height is made up of about #fjjj threads or filaments1 each coming from a separate hole in his spinning machine4 ,a quarter of an ounce of the thread of a spider's web would extend #djj miles4 ,a grain of blue vitriol1 dissolved in a gallon of water1 will make the whole blue4 ,such a diffusion could not be without an exceedingly minute division of the particles4 ,perhaps the most minute division of matter is exemplified in odors4 ,a grain of musk will scent a room for years1 and yet have no perceptible loss of weight4 ,but all this time the air is filled with fine particles coming from the musk4 ,the microscope reveals to us many wonderful examples of the minuteness of the particles of matter1 both in the vegetable and the animal world4 ,if you press a common puff-ball a dust flies off like smoke4 ,examined with a microscope1 each particle of this dust1 which is the seed of the plant1 is a perfectly round orange-colored ball4 ,this ball is of course made up of very many particles1 arranged in this regular form4 ,beautiful examples of various arrangements of the minute particles of matter we have in the pollen of different plants1 as seen with the microscope4 ,each particle of the dust which adheres to your fingers as you catch a moth is a scale with fine lines upon it regularly arranged4 ,and if you look through the microscope at the wing of the moth1 you will see1 where the dust is rubbed off1 the attachments by which the scales were held standing up from the surface of the wing1 like nail-heads on a roof where the shingles have been torn off4 ,the organization of exceedingly small animals1 as revealed by the microscope1 furnishes us with wonderful examples of the minute division of matter4 ,a little of the dust of guano1 examined through a powerful microscope1 is seen to contain multitudes of shells of various shapes4 ,these shells are the remains of animalcules that lived in the water1 their destiny seeming to be in part to furnish food to other animals larger than themselves4 ,in the chalk formations of the earth are seen multitudes of such shells4 ,they have been discovered even in the glazing of a visiting-card2 for they are so small that the fine grinding up of the chalk does not wholly destroy them4 ,there are animals1 both in the air and in the water1 so small that it would take millions of them to equal in bulk a gram of sand1 and a thousand of them could swim side by side through the eye of a common-sized needle4 ,now in all these animals there are organs1 constructed of particles of matter1 which are arranged in them with as much order and symmetry as in the organs of our bodies4 ,how minute then must these particles be6 ,how do such facts extend our views of the power of the ,deity6 ,the same power that moulded the earth1 sun1 moon1 and the whole host of heaven1 gave form1 and life1 and motion to the millions which sport in every sunbeam2 the same eye that watches the immense heavenly bodies as they move on in their course1 looks upon one and all of these legions of animals in earth1 air1 and water1 though they are unseen by human eyes1 seeing that every particle shall take its right position1 so that this part of creation may with all the rest be pronounced very good2 and the same bountiful hand that dispenses the means of life and enjoyment to the millions of the human race1 forgets not to minister to the brief life and enjoyment of each one of these myriads of animalcules1 though they seem to be almost nothingness itself4 #bj4 ,pores and ,spaces in ,matter4--,in all matter there are spaces about the particles4 ,those bodies which are called porous have quite large spaces in them4 ,but even in those which are not commonly considered porous the particles are by no means close together4 ,a celebrated experiment tried in ,florence a long time ago showed that there are spaces among the particles of so dense a substance as gold sufficiently large to let water through them4 ,a hollow golden globe containing water was subjected to great pressure1 and its surface was bedewed with the water that came out through the pores of the gold4 ,in all substances in which there are pores visible to the naked eye1 or by the aid of the microscope1 there are other spaces or interstices among the particles around the pores4 ,indeed1 it is supposed that there is space around every ultimate particle or atom1 and that no two of these atoms are in actual contact4 ,the fact that substances which have no pores can be compressed into a smaller space than they usually occupy shows that there are spaces or interstices in them4 ,solids can be thus compressed1 some more than others4 ,but the most compressible substances are the gases and vapors4 ,the amount of space between their particles must be very large to allow of so great compression4 ,illustration3 ,fig4 #a4 #ba4 ,space in ,gaseous ,substances4--,we can have some idea of the great amount of space in a gaseous or aeriform substance by observing the difference between water in its liquid and in its aeriform state4 ,a cubic inch of water1 when it becomes steam1 occupies #afif times as much room as it did when it was water4 ,the difference in proportion is exhibited in ,fig4 #a1 the inner circle representing the water1 and the outer the steam into which it is converted4 ,now the water is not altered at all in its nature by being changed into steam4 ,the particles are simply put farther apart by the heat1 and as soon as the heat is withdrawn they come together again to form water1 or1 in other words1 the steam is condensed into water4 ,it is plain1 therefore1 that the space between the particles is #afif times as great in steam as it is in the water from which the steam is made4 #bb4 ,solutions4--,when any substance1 as sugar or salt1 is dissolved in water1 its particles are diffused through the spaces that exist between the particles of the water4 ,so also when water evaporates 7 #ab71 the particles of water are diffused through the spaces between the particles of the air4 ,in like manner are the particles from an odorous substance diffused in these spaces1 and thus mingled with the particles of the air they are carried into the nostrils1 and strike upon the minute extremities of the nerve of smell4 #bc4 ,relation of ,heat to the ,spaces of ,matter4--,the variation in the amount of space between the particles of matter in any substance generally depends on the variation of the amount of heat present4 ,thus heat expands iron2 that is1 it increases the spaces between the particles of the iron4 ,so also heat increases the spaces between the particles of mercury1 and thus makes it occupy more room in the thermometer4 ,this effect of heat will be considered more fully hereafter4 ,the general views which ,i have given of the constitution of matter will throw light upon the different qualities of different substances1 some of which ,i will notice4 #bd4 ,density and ,rarity4--,the density of a substance depends upon the quantity of matter it contains in a given space4 ,the more dense1 therefore1 a substance is the greater is its weight4 ,a piece of lead is forty times heavier than a piece of cork of the same size4 ,mercury is nearly fourteen times heavier than an equal bulk of water4 ,you see1 then1 that density must depend on the nearness of the atoms to each other4 ,in so dense a substance as gold the atoms are all very close together2 in wood there are spaces1 some of which are so large that you can see them2 and in air1 steam1 and the gases there is a great deal of space among the particles 7 #ba71 so that we speak of their rarity instead of their density4 #be4 ,tenacity4--,the power of holding together1 termed tenacity1 depends on the degree of attraction between the particles4 ,by attraction ,i mean a disposition in particles to come together1 this disposition being manifested in opposition to any force tending to draw them apart4 ,i shall soon speak of this more particularly4 ,tenacity does not exist at all in gaseous substances4 ,the particles of air and of steam1 for example1 show no disposition to cling together2 that is1 have no tenacity4 ,this property is weak in liquids4 ,it is only strong enough in water to enable its particles to hang together in the shape of a drop4 ,it is strong in solids1 enabling their particles not only to hold together in large quantities1 but to hold up also heavy weights suspended to them4 ,it is stronger in iron than in any other solid4 ,it is stronger in wrought iron than in cast iron2 and strongest of all in steel4 #bf4 ,comparative ,tenacity of ,substances4--,various metals and other substances have been tested in reference to their comparative tenacity4 ,it was done in this way3 ,wires were made of the metals1 all of the same size4 ,weights were suspended to them1 and additions were made to the weights by little and little till the wires broke4 ,the table underneath was made by placing against each metal the greatest weight that its wire would hold3 ,cast steel #acd pounds4 ,best wrought iron #gj pounds4 ,cast iron #ai pounds4 ,copper #ai pounds4 ,silver #aa pounds4 ,gold #i pounds4 ,tin #e pounds4 ,lead #b pounds4 ,oak wood1 tried in the same way1 was found to hold up #ab pounds1 one more pound than silver4 ,some animal substances have great tenacity1 as the thread of the silk-worm1 hair1 wool1 and the ligaments and tendons of our bodies and of other animals4 #bg4 ,value of ,tenacious ,substances4--,the gradual discovery1 says ,dr4 ,arnot1 of substances possessed of strong tenacity1 and which man could yet easily mould and apply to his purposes1 has been of great importance to his progress in the arts of life4 ,the place of the hempen cordage of ,european navies is still held in ,china by twisted canes and strips of bamboo2 and even the hempen cable of ,europe1 so great an improvement on former usage1 is now rapidly giving way to the more complete and commodious security of the iron chain--of which the material to our remote ancestors existed only as useless stone or earth4 ,and what a magnificent spectacle is it1 at the present day1 to behold chains of tenacious iron stretched high across a channel of the ocean1 as at the ,menai ,strait between ,anglesea and ,england1 and supporting an admirable bridge-road of safety1 along which crowded processions may pour1 regardless of the deep below1 or of the storm2 while ships there1 with sails full-spread1 pursue their course unmolesting and unmolested4 #bh4 ,hardness4--,this property seems to depend upon some peculiar arrangement of the particles of matter4 ,we should suppose that the densest substances would be the hardest4 ,but it is not so4 ,iron is the hardest of the metals1 but its particles are not so close together as those of gold1 which is quite a soft metal4 ,and gold is five times as heavy as the diamond1 which is so hard as to cut glass easily4 ,common flint is hard enough to scratch glass1 but will not cut it like the diamond4 ,illustration3 ,fig4 #b4 #bi4 ,flexibility and ,brittleness4--,if you bend a flexible body as a piece of wood1 as represented in ,fig4 #b1 it is obvious that the particles on the upper or convex side must be put a little farther apart1 while those on the under or concave side are brought a little nearer together4 ,but the wood does not break1 because the particles that are thus moved a little apart still retain their hold upon each other4 ,this is the explanation of what we call flexibility4 ,on the other hand1 the particles in a rod of glass can not be put farther apart in this way4 ,they are not actually in contact any more than the particles of the wood are 7 #bj71 but they are in a fixed relative position2 that is1 a position which can not be disturbed without a permanent separation of particles4 ,if you attempt to bend the rod there is no slight separation of many particles1 as in the bent wood1 but a full and permanent separation in some one part of the rod4 ,we call the property on which this result depends brittleness4 ,brittle substances are generally hard4 ,glass1 while the most brittle of all substances1 is hard enough to scratch iron4 ,brittle substances also have much tenacity4 ,a rod of glass can hold up a heavy weight1 although a slight blow suddenly given would break it4 #cj4 ,flexible and ,brittle ,steel4--,there are two kinds of steel1 flexible and brittle4 ,the steel of most cutting instruments is brittle4 ,the steel of a sword-blade is quite flexible1 and that of a watch-spring is so much so that we can wind it up in a coil4 ,this difference is owing to a difference in the mode of cooling the steel4 ,if it be cooled suddenly1 it is brittle2 if slowly1 it is flexible4 ,the process by which it is cooled slowly is called annealing4 ,the explanation of all this is quite plain4 ,the steel being expanded by heat--that is1 its particles being put farther apart than they usually are--when they are suddenly brought together again they have not time to arrange their relative position properly4 ,brittleness is therefore the result4 ,but1 on the other hand1 when the cooling is effected gradually1 time is given for the arrangement4 #ca4 ,tempering of ,steel4--,steel suddenly hardened is too brittle for common use4 ,a process called tempering is therefore resorted to for diminishing the brittleness4 ,the steel is reheated after the hardening1 and is then allowed to cool slowly4 ,the degree in which the brittleness is lessened depends on the degree of heat to which the steel is subjected4 ,it can be entirely removed by a red heat1 for then the particles have a full opportunity to readjust themselves2 and the more the heat comes short of this point the less thorough will be the adjustment1 because the less perfectly are the particles released from their suddenly-taken position4 ,in lessening the brittleness we lessen hardness also1 and therefore the tempering is varied in different cases according to the degree of hardness which is desired4 #cb4 ,annealing of ,glass4--,glass is always annealed4 ,if this were not done our glass vessels and windows would be exceedingly brittle1 and would therefore be constantly breaking4 ,articles made of glass are annealed by being passed very slowly indeed through a long oven which is very hot at one end1 the heat gradually lessening toward the other end4 ,illustration3 ,fig4 #c #cc4 ,prince ,rupert's ,drops4--,we have a striking example of brittleness induced by sudden cooling in what are called ,prince ,rupert's drops4 ,these are made by dropping melted green glass into cold water1 and they are of the shape represented in ,fig4 #c4 ,if you break off ever so small a bit of the point of one of these drops1 the whole will at once shiver to pieces4 ,that is1 the sudden arrangement of the particles is so slight and unnatural that the disturbance of the arrangement in a small part suffices to destroy the arrangement of the whole1 very much as a row of bricks falls over from the fall of the first in the row4 ,mr4 ,farraday says that these drops were not1 as is commonly supposed1 invented by ,prince ,rupert1 but were first brought to ,england by him in #affj4 ,they excited much curiosity at that time1 and were considered a kind of miracle in nature4 ,but you see that this1 like many other wonders1 receives with a little thought an easy explanation4 #cd4 ,malleability and ,ductility4--,those metals which can be hammered into thin plates are called malleable4 ,gold furnishes us with the best illustration of this property4 ,silver1 copper1 and tin are quite malleable4 ,most of the other metals are very little so1 and some of them are not at all1 breaking at the first blow4 ,a substance is said to be ductile when it can be drawn out into wire4 ,the principal metals that have this quality are platinum1 silver1 iron1 copper1 and gold1 and in the order in which ,i have named them4 ,melted glass is very ductile4 ,it can be drawn out in a very fine thread1 and when this thread is cut and arranged in branches it resembles beautiful white hair4 ,in hammering metals into plates1 or drawing them into wire1 there is a considerable change of relative position in the particles1 similar to that which we have in fluids1 though nothing like as free4 ,in this change of position those particles that do remain in close neighborhood have a remarkable tenacity or attraction1 preventing their separation4 ,in welding two pieces of iron1 which is done by the blacksmith by hammering them together when red-hot1 there must be enough movement among the particles to have those of one piece mingle somewhat with those of the other4 #ce4 ,compressibility4--,porous substances can be considerably compressed4 ,force applied to them can bring their particles nearer together1 making them to fill up in part their pores4 ,the most familiar example you have of this is in sponge4 ,the more porous wood is the more can it be compressed4 ,but even such dense substances as the metals can be compressed in some degree2 that is1 the interstices between their particles can be made smaller4 ,medals and coins have their figures and letters stamped upon them by pressure1 just as impressions are made upon melted sealing-wax4 ,the heavy and quick pressure required to do this actually compresses the whole piece of the hard metal1 putting all the particles nearer together1 so that it occupies less space than it did before it was stamped4 #cf4 ,incompressibility of ,liquids4--,we should suppose1 from the freeness with which the particles of liquids move among each other1 and from the spaces 7 #bb7 which exist among them1 that these substances could be easily compressed4 ,but it is not so4 ,the heaviest pressure is required to compress them even in a slight degree4 ,water can be compressed so very little that practically it is regarded as incompressible4 #cg4 ,influence of ,heat on the ,bulk of ,liquids4--,although the interstices between the particles of liquids can not be varied by mechanical pressure1 they can be by variations of temperature4 ,liquids are dilated or expanded by heat2 that is1 their particles are put farther apart4 ,they are contracted or compressed by cold2 that is1 their particles are brought nearer together by the abstraction of heat4 ,the most familiar example that we have is in the thermometer4 ,the mercury rises in the tube when the heat increases the interstices between its particles2 and it falls when the loss of heat allows the particles to come near together4 ,the same effects are seen when alcohol is used in the thermometer1 as is done in the arctic regions1 because mercury may freeze there4 ,a thermometer with water in it would answer if we wished only to measure temperatures between the freezing point and the boiling point of water4 ,the expansive influence of heat will be particularly treated of hereafter4 #ch4 ,compressibility of ,aeriform ,substances4--,aeriform bodies are more compressible than any other substances1 showing that in their ordinary condition there is a great deal of space among their particles4 ,while they are thus unlike liquids in compressibility1 they are affected by heat in the same way that liquids are4 ,illustration3 ,fig4 #d4 ,fig4 #e4 #ci4 ,elasticity4--,closely allied with the compressibility of matter is its elasticity4 ,we see this property strikingly exemplified in ,india-rubber4 ,it occasions the rebounding of a ball of this substance when thrown down4 ,observe flow exactly what occurs in this case4 ,the ball as it meets the resistance of the floor is flattened1 as represented in ,fig4 #d4 ,then1 as it assumes the round shape1 as seen in ,fig4 #e1 it pushes downward upon the floor4 ,it is this sudden pushing downward that makes it rebound4 ,it is as if there were a compressed spring between the ball and floor4 ,it may be likened also to jumping4 ,when one jumps he bends his limbs at the thigh and knee joints1 and then1 in straightening himself up1 gives a sudden push1 like that given by the ball as it assumes its round shape1 and so is thrown forward or upward1 according to the direction in which the pushing force is made4 ,the same flattening occurs in an ivory ball1 though not to the same degree4 ,you can prove that it does occur by experiment4 ,let a marble slab be wet and drop the ball upon it4 ,quite a spot will be made dry by the blow of the ball1 showing that it touched more of the marble than it does when it is merely placed upon it4 #dj4 ,elasticity ,shown in ,other ,ways4--,if a stick be bent1 as in ,fig4 #b1 as soon as the bending force is withdrawn the stick becomes straight again from its elasticity4 ,it is this elastic force of the bow1 straightening it1 that speeds the arrow4 ,observe in this case that while the particles on the concave side of the bent bow are brought nearer together or compressed1 those on the convex side are moved apart4 ,this moving apart of the particles is often shown in ,india-rubber4 ,you can see how very far apart particles that are in near neighborhood may be carried1 if you will stick two pins close together in a strip of ,india-rubber before you stretch it4 #da4 ,degrees of ,elasticity in ,different ,substances4--,some substances have so very little elasticity that they are practically considered as having none4 ,lead is one of these4 ,a rod of lead when bent remains so1 and a leaden ball does not rebound4 ,while aeriform substances are the most compressible of all1 they are also the most elastic4 ,air compressed returns to its usual condition the moment that it is relieved from the pressure1 and with a force proportioned to the amount of the pressure4 ,so it is with steam and the gases4 ,the varied results of this quality of aeriform substances will claim our attention more particularly in some other parts of this book4 #db4 ,definition of ,elasticity4--,you see from the illustrations that have been given that elasticity is that property of matter by which its particles1 when brought nearer together or carried farther apart by any force1 return to their usual condition when the force is withdrawn4 #dc4 ,usefulness of ,variety in ,properties of ,matter4--,the various properties of matter brought to view in this chapter are providential adaptations to the necessities of man4 ,each substance has those properties which best fit it for his use4 ,iron1 for example1 designed by the ,creator to be both the strongest and most extensively useful servant of man among the metals1 is therefore provided in great abundance1 and has those strong1 decided1 and various qualities which fit it for the services it is to perform4 ,gold and silver1 on the other hand1 designed for services less extensive1 lighter1 and in a great measure ornamental1 are provided in very much less quantity1 and have properties admirably adapting them to the services for which they are so manifestly intended4 ,the same can be substantially said of all other substances1 and especially of those very abundant ones1 air and water4 ,and it may be remarked also that the ingenuity of man is continually discovering new modes of bringing the various properties of matter into his service4 ,i will give but a single illustration--the tempering of steel4 ,this discovery1 says ,dr4 ,arnot1 is perhaps second in importance to few discoveries which man has made2 for it has given him all the edge-tools and cutting-instruments by which he now moulds every other substance to his wishes4 ,a savage will work for twelve months with fire and sharp stones to fell a great tree and to give it the shape of a canoe1 where a modern carpenter1 with his tools1 could accomplish the object in a day or two4 ,,chapter ,,iii4 ,,the ,,essential ,,properties ,,of ,,matter4 #dd4 ,extension4--,you can not conceive of any portion of matter1 however small it may be1 that has not shape or figure4 ,it may be so small as to appear only as a point to the naked eye1 but seen through the microscope its shape becomes obvious4 ,even an atom must have length1 breadth1 and thickness1 though it be so small that we can not measure it1 nor see its shape with the most powerful microscopes4 ,extension1 which is the term commonly used to express this idea1 is then an essential property of matter2 that is1 it is a property of which no form or kind of matter can be destitute4 ,the distinction1 in this respect1 between this property and those which ,i have before noticed may be made obvious to you by an example4 ,hardness is not an essential quality of matter1 for some kinds of matter are destitute of it2 but no portion of matter1 hard or soft1 can be destitute of extension or shape4 ,the air is sometimes spoken of in common language as being shapeless4 ,this is partly because it is invisible1 and partly because no portion or body of air assumes any definite shape4 ,but air is continually forced into definite shapes by confinement in rooms1 boxes1 etc42 and then its extension in different directions can be measured as accurately as the extension of a solid can be4 ,and besides1 the atoms of which air is composed are undoubtedly solid1 and we can not conceive of their existence without attaching to them the idea of figure or extension4 #de4 ,impenetrability4--,in common language one substance is said to penetrate another4 ,thus a needle penetrates cloth1 a nail penetrates wood1 etc4 ,but this is not strictly true4 ,the needle does not go into the cloth1 but goes between the fibres of it1 pushing them to the one side and the other4 ,so the nail goes between the fibres of the wood1 and not into them4 ,it does not occupy the same room that the fibres do at the same time4 ,so1 also1 no atom of matter can penetrate or go into any other atom4 ,it can only push it out of the way1 and then occupy its place4 ,impenetrability is therefore said to be one of the essential qualities of matter4 ,this means simply that no portion of matter can occupy the same space with another portion of matter at the same time4 ,illustration3 ,fig4 #f4 ,illustration3 ,fig4 #g4 ,illustration3 ,fig4 #h4 #df4 ,illustrations4--,many illustrations might be given of this property4 ,i will give a few4 ,if you press a tumbler into water with its open end downward1 you can not fill it with water1 for the air confined in the tumbler prevents it from rising4 ,it can not occupy the same space with the air4 ,it fills1 indeed1 a portion of the tumbler1 but this is because the air is compressible4 ,if you introduce a glass funnel1 a1 ,fig4 #f1 into a jar of water1 b1 with your thumb on its mouth1 c1 the water will not rise to fill it4 ,but if you take your thumb off1 the water will rise to the level of the water outside of the funnel1 pushing up the air before it4 ,if you have not a funnel1 a vial or bottle with its bottom broken off will answer for the experiment4 ,the following is a very pretty experiment1 illustrating the same point4 ,place a lighted taper1 a1 ,fig4 #g1 on a large flat cork in a jar of water4 ,put over it an open jar or receiver1 b1 having a stop-cock1 c4 ,closing the stop-cock1 press the receiver down into the water1 and you will see the taper sink with it1 as represented in the figure1 the air preventing the water from entering the receiver4 ,if now you open the stop-cock the water rushes in1 thrusting the air upward1 and making the taper to appear as if rising out of the water4 ,the diving-bell offers a good illustration4 ,it consists of a vessel1 a a1 ,fig4 #h1 shaped like a bell1 made sufficiently heavy to sink in water4 ,it is let down by a chain and cable1 as seen in the figure4 ,the water does not enter the bell any farther than the compressibility of the air permits it4 ,in order that the men in it may remain under water for some time fresh air is supplied by the tube b1 it being forced down by a forcing-pump4 ,at the same time the vitiated air can be let off by a valve provided for that purpose4 ,there are windows in the top of the bell to give the requisite light for work on the sea's bottom4 ,treasures are often recovered by this means which would otherwise be lost4 ,you see a resemblance between the diving-bell and the arrangement in ,fig4 #g1 the receiver representing the bell and the lighted taper the men in it4 #dg4 ,other ,illustrations4--,if you drop a bullet into a tumbler of water it pushes the particles to the one side and the other1 and occupies the room thus made4 ,if you drop in several there is a manifest rise of the water1 and you may drop in enough to make it overflow4 ,the same thing is true of the finest needle dropped in the water--it does not penetrate it1 but like the bullet displaces some of its particles and occupies their room2 and you can make the water overflow by dropping in many needles4 ,we can truly say1 then1 that water can not be penetrated even by a needle4 ,when any substance1 as sugar1 is dissolved in water1 its particles do not penetrate the water1 but go into the spaces between its particles4 ,so1 also1 when particles from odorous substances are diffused in the air1 they are not really in the air1 but are between its particles4 #dh4 ,inertia4--,matter has no power to put itself in motion4 ,when it is moved it is moved by some force which is either outside of the matter or is communicated to it in some way4 ,when your arm is moved it is not the matter in your arm that is the cause of its motion4 ,it is caused by a force in you which ,i will not dwell upon here1 because the subject belongs to ,physiology4 ,when air moves it is set in motion by some force acting upon it1 as when you blow it from your lungs or move it with a fan4 ,when the wind blows1 the air is set in motion by heat and the attraction of the earth1 as will be explained to you in another part of this book4 ,i might multiply examples to any extent1 showing that matter of itself can not move4 ,this property of matter is termed inertia4 #di4 ,inertia ,shown in the ,inability of ,matter to ,stop its ,motion4--,matter1 when once set in motion1 has no power to stop itself4 ,if it could stop itself it could not be said to be inert4 ,and as it is inert1 it would1 when once set in motion1 keep on moving forever unless stopped by some force4 ,when a stone falls to the ground1 it stops simply because the earth stops it4 ,if the earth were not in the way1 the stone would move straight on until it were stopped by something else4 ,so1 also1 a stone that is thrown up in the air would keep on1 and soon be out of sight1 and never return to the earth1 if it were not made to come down by forces acting upon it4 ,one of these forces is the resistance of the air1 which1 from the moment the stone starts1 is destroying its motion4 ,another force as constantly operating to retard the stone is the attraction1 or drawing force1 exerted by the earth upon it4 ,this powerful though unseen force will be treated of fully in the next chapter4 #ej4 ,matter ,equally ,inclined to ,rest and ,motion4--,it was formerly taught by philosophers that matter is more inclined to rest than to motion2 and this is the popular notion now4 ,this is because the chief causes that stop the motions that we see from day to day--viz41 the air and the attraction of the earth--are not visible4 ,for this reason1 until we investigate the subject1 it seems to us that motion has a natural tendency to stop1 or is spent1 as it is expressed4 ,when friction has an agency in arresting motion we see it plainly2 but the common idea is1 that in this case the motion is in part spent and in part destroyed4 ,but in no case is motion spent1 but it is always destroyed by obstacles4 ,the more thoroughly these are removed1 the longer will the motion continue2 and if they were wholly removed1 the motion would never cease4 ,perpetual motion1 then1 is not in itself impossible2 for matter has no more tendency to stop moving when once put in motion1 than it has to begin motion when it is at rest4 ,all motion would be perpetual if there were not forces opposing it4 ,if there were only one body in the universe1 and that were set in motion1 it would move forever through empty space in a straight line2 for there is no matter any where to resist its motion or to attract it away from its onward course4 #ea4 ,divisibility4--,though divisibility is a general quality of matter 7 #ah71 it is not an essential one4 ,for if it be true that every portion of matter is composed of atoms that remain whole 7 #ad71 this property can not be said to belong to these atoms4 ,it is only the bodies made up of these atoms that can be divided4 ,when we come to the atoms themselves the division must stop4 #eb4 ,weight4--,in speaking of the properties of matter ,i have said nothing about weight1 although in the popular mind this is thought of as being one of the most prominent of the properties of some kinds of matter4 ,this will be appropriately treated of when ,i come to speak of attraction1 for it is a mere result of attraction4 ,suffice it to state here1 that the weight of a body is the pressure occasioned by the attraction existing between it and another body4 ,if when a stone is raised from the ground the attraction between it and the earth could be destroyed1 the stone would remain there4 ,it would not press down1 and so would have no weight4 ,it is plain1 therefore1 that weight1 so far from being an essential property of matter1 is not really a property at all4 ,it is only an effect of a property--attraction4 ,if there were only one body in the universe1 it would have no weight1 for it would press in no direction because there is nothing to attract it4 ,but as it is1 all matter has weight1 for there is other matter to attract it4 ,,chapter ,,iv4 ,,attraction4 #ec4 ,nature of ,attraction4--,if you attempt to break a very tenacious solid substance1 why do you not succeed8 ,it is because the particles are so strongly fastened together4 ,but how8 ,by some kind of cement or glue1 or by some mechanical contrivances as nails or hooks8 ,no4 ,they are fastened together by some unseen force4 ,we know nothing of the nature of this force4 ,we know only that it exists1 and we call it attraction4 ,the name is a proper one1 for it simply expresses the fact that one particle attracts or draws another particle toward itself4 #ed4 ,newton's ,idea of ,attraction4--,it was stated in #bj that the particles of matter1 even in the densest substances1 are not in actual contact1 but have spaces around them4 ,now it was supposed by ,newton that there is some kind of ethereal substance pervading all these spaces1 which causes this attraction between the particles4 ,he supposed also that this ether was every where in space1 causing attraction between masses of matter4 ,but all this is mere supposition1 and we know not whether there is this sort of ethereal glue keeping the universe together1 or whether it is some property in the particles themselves that makes them thus attract each other4 ,but the fact of the attraction we know1 and we can observe the phenomena which it produces1 and discover the laws or rules by which this force is regulated in its action4 #ee4 ,attraction in ,solids4--,attraction is stronger in some solids than in others4 ,the mason with his trowel easily divides a brick2 but he can not do this with a piece of granite1 for its particles have a greater attraction for each other than those of the brick4 ,so a rap which would break a glass dish would not injure a copper one of the same thickness4 ,a weight that would hang securely from an iron wire would break a lead wire of the same size2 that is1 it would tear the particles apart1 because they are not strongly attracted to each other4 ,attraction has different modes of action in different solids4 ,it therefore fastens their particles together in different ways1 and thus produces all the various qualities1 already noticed1 which are so useful to us--tenacity1 hardness1 softness1 ductility1 flexibility1 etc4 #ef4 ,attraction in ,liquids4--,in a liquid the attraction between the particles is very feeble compared with that in solids4 ,the attraction of the particles of steel is in strength about three million times that of the particles of water4 ,we make the estimate in this way3 ,we find that a steel wire will sustain a weight equal to #ci1jjj feet of the wire4 ,but a drop of water hanging to the end of a stick can not be more than one-sixth of an inch in length2 that is1 water will hold together by the attraction of its particles only to this extent1 which is a little less than the three millionth part of the length of steel wire which could hang without breaking4 #eg4 ,freeness of ,movement of the ,particles of ,liquids4--,there is one prominent characteristic of liquids which is probably not entirely owing to the feeble attraction of their particles--,i mean the freeness with which these particles are moved among each other4 ,this is owing probably in part to some peculiar arrangement of the atoms in making the particles of a liquid4 ,i will illustrate this in a coarse way4 ,if the atoms of lead in shot were so arranged as to make irregular jagged forms1 they could not readily be moved among each other4 ,we suppose the ultimate atoms of a liquid to be so arranged in the formation of particles as to make them not only round but very smooth4 ,hence comes the great ease with which they circulate among each other4 ,illustration3 ,fig4 #i4 ,illustration3 ,fig4 #aj4 #eh4 ,globular ,shape of ,drops of ,liquids4--,as the particles of a liquid move thus freely among each other1 their attraction disposes them to assume a globular or round shape4 ,the reason of this can be made plain by ,figs4 #i and #aj4 ,the outside of a perfect sphere is all at the same distance from the centre4 ,so all the circumference of a circle is at the same distance from the centre1 as represented in ,fig4 #i4 ,but this is not true of all parts of the surface of a cube or of a square3 a1 for example1 is farther from the centre than b is4 ,now in a drop of liquid all the particles are attracted toward the centre1 for in that line from each particle lies the largest number of particles to attract it4 ,this can be made obvious by taking some point in the drop1 as represented in ,fig4 #aj1 and drawing lines from it through the centre and in other directions4 ,if a be the point in the drop1 it is plain that the line from it through the centre is longer than a b or a c4 ,therefore a particle1 a1 will be attracted toward the centre rather than in the direction a b or a c1 because there are more particles in the direction of the centre1 and the more particles there are the stronger is the attraction4 ,but this is not all4 ,the particles in the line a c1 tending to make a go toward c1 are balanced by the particles in the line a e1 tending to make it go toward e4 ,the two lines of particles therefore together tend to make it go in a middle line between them1 that is1 toward the centre1 just as two strings pulling equally1 the one to c and the other to e1 would make a body1 a1 move in a middle line between these two directions4 ,the same can be shown of the two lines of particles a b and a d1 and so of any other two alike in situation on each side of the line through the centre4 ,the tendency of every particle is1 then1 to go toward the centre1 and it would go there if there were not particles between to prevent it4 ,you see how this would operate in the case of the particles on the surface of the drop4 ,as these are all striving1 as we may say1 in obedience to attraction1 to get to the centre1 none of them will be raised up into an angle or a point1 as would be the case if the drop were in the shape of a cube4 ,if this should be done it would show that some of the particles were not as strongly attracted toward the centre as others are1 which is an impossibility4 #ei4 ,the ,globular ,form in ,different ,liquids4--,the disposition to form a sphere is seen more distinctly in mercury than in any other liquid4 ,if you drop a little of it upon a plate it separates into globules1 which roll about like shot4 ,why can not the same thing be done with water8 ,why do the drops of water hang upon the window-pane1 showing only in an imperfect way their disposition to the globular arrangement8 ,it is because the particles of water have a greater attraction for other substances1 and less attraction for each other1 than the particles of the quicksilver have4 ,water sometimes exhibits its disposition to the globular form in full on the leaves of some plants1 and rolls about in balls like mercury4 ,this is because there is something on the surface of the leaf which repels rather than attracts the water4 ,if you put your finger1 however1 on one of these drops1 it will spoil it1 and your finger will be moistened1 because there is an attraction between the particles of your skin and of the water4 ,take another illustration of this difference in attraction4 ,if you drop a little oil upon the surface of water it will float about in round drops4 ,this is because the water repels the oil1 as the surface of some kinds of leaves does water4 ,but when oil is spilled upon wood or cloth its particles have so strong an attraction for their particles that they unite with them1 instead of gathering up into little round companies as they do on the surface of water4 #fj4 ,manufacture of ,shot4--,we have a beautiful example of the tendency of fluids to the globular arrangement in the manufacture of shot4 ,the melted lead is poured into a large vessel in the top of the shot-tower4 ,this vessel has holes in its bottom1 from which the metal falls in drops4 ,each drop1 as it whirls round and round in its fall1 takes the globular form4 ,by the time that it reaches the end of its journey1 about two hundred feet1 it becomes so far cooled as to be solid1 and as it is received in a reservoir of water1 its globular form is retained4 ,bullets can not be made in this way1 because a quantity of melted lead sufficient to make a bullet will not hold together in a globular form4 #fa4 ,globular ,form of the ,earth and the ,heavenly ,bodies4--,it is supposed that the sun1 moon1 earth1 and all the heavenly bodies were once in a liquid state1 and that they owe their globular shape to this fact4 ,as they whirled on in this condition in their course1 the different solids were gradually formed1 and at length they acquired their present state4 ,how all the mighty changes could be effected in our earth1 converging it from a liquid into a body with a solid crust1 having such various substances in it1 and so variously arranged1 with its depressions containing water1 and the whole covered with its robe of air fifty miles in thickness1 we can not understand4 ,and yet there are some portions of the process which chemistry and geology have revealed to us1 giving us some glimpses of the wonders which1 during the lapse of ages1 ,god wrought in our earth in preparing it for the habitation of man4 ,illustration3 ,fig4 #aa4 ,fig4 #ab4 #fb4 ,crystallization4--,the arrangement of the particles of solid substances is different from that of liquids4 ,the tendency here is to straight lines and angles2 that is1 to crystalline forms4 ,alum or common salt1 when it becomes solid from a solution1 forms crystals4 ,so also does sugar4 ,the crystals of different substances are different4 ,in ,fig4 #aa you have the crystal of common salt1 and in ,fig4 #ab that of alum4 ,we see this crystalline tendency every where1 even in the rude rocks and common stones4 ,the rocks are disposed to exhibit regular layers1 or columns1 or battlements1 and always do so except when interfering circumstances prevent4 ,and when you examine their composition1 or that of the stone under your feet1 you see the same crystalline disposition in detail that you see in the mass4 #fc4 ,crystallization of ,water4--,water1 when it changes into a solid1 shows the same disposition1 of which the crystals of the snow and the frost-work on our windows are familiar examples4 ,when snow forms1 the water of the clouds is suddenly crystallized by the cold air1 the particles taking their regular places more readily and certainly than if they were guided by intelligence1 because in obedience to an unerring law established by the ,creator4 ,we sometimes have an example of this sudden crystallization of water under our eye4 ,the water in a pitcher may remain fluid1 although it is cooled down to the freezing point1 and even below it1 if it be kept perfectly still4 ,but on taking up the pitcher the water at once becomes filled with a net-work of ice-crystals4 ,the explanation is this3 ,the stillness of the water has prevented its particles from taking on the new arrangement needed for the formation of ice2 but the jostling of them in taking up the pitcher has served to make them do it thus suddenly4 ,illustration3 ,fig4 #ac4 #fd4 ,frost and ,snow4--,the frost-work on our windows is a wonderful exhibition of the variety of forms that crystallization can produce4 ,it sometimes presents figures like leaves and flowers1 such as we see chased on vessels of silver1 but much more delicate and beautiful4 ,so varied and fantastic are the forms in which these water-crystals are arranged1 that it is very natural to ascribe them1 as is done universally in the dialect of the nursery1 to the ingenuity of a strange and tricksy spirit4 ,every snow-flake is a bundle of little crystals as regular and beautiful as the crystals which you so much admire in a mineralogical cabinet4 ,and there is great variety in the grouping of these crystals4 ,you have some specimens of these groups in ,fig4 #ac as they appear on examining them with the microscope4 ,over six hundred different forms have been enumerated1 and a hundred have been delineated4 ,it is a very quick operation by which the particles of water in the clouds thus marshal themselves1 as if by magic1 in these regular forms4 ,but a quicker operation is that by which hail is formed--so quick that the particles have not time to set themselves in the crystalline arrangement1 but are huddled together without order4 ,the brilliant and glistening whiteness of the snow is owing to the reflection of light from its minute crystals4 ,in the arctic regions the beauty of the snow is often much greater than with us4 ,the snow crystals of last night1 says ,captain ,m',clintock in his ,discovery of the ,fate of ,sir ,john ,franklin1 were extremely beautiful4 ,the largest kind is an inch in length2 its form exactly resembles the end of a pointed feather4 ,stellar crystals two-tenths of an inch in diameter have also fallen2 these have six points1 and are the most exquisite things when seen under a microscope4 ,in the sun1 or even in moonlight1 all these crystals glisten most brilliantly2 and as our masts and rigging are abundantly covered with them1 the ,fox never was so gorgeously arrayed as she now appears4 #fe4 ,order in ,nature4--,we see in this general tendency to crystallization a striking illustration of the fact that ,god is a ,god of order4 ,disorderly arrangement is never seen except where there is an obvious necessity for it4 ,and even when there is apparent disorder1 a little examination generally shows that essentially there is order4 ,the rocks that give so much variety to scenery are not piled up in confusion1 and order has evidently reigned in their construction4 ,pick up a common stone1 and on breaking it you will see the crystalline arrangement in its interior4 ,nay1 more1 much of the very soil is made up of separated and broken crystals4 ,illustration3 ,fig4 #ad4 #ff4 ,particles ,must be very ,near to ,each ,other to ,adhere4--,why is it that when you have broken any thing made of glass1 however accurately you may bring the two parts together1 you can not make them unite in one again8 ,it is simply because the particles of substances will not attract each other enough to be united unless they are brought very near together4 ,now it is impossible to bring the particles on the two surfaces of a broken piece of glass as near together as they were before it was broken4 ,if you could do so1 no crack would be visible4 ,you can join them by some kind of cement4 ,this is because the particles of the cement1 while it is soft1 can be insinuated among the particles of the glass2 and thus1 when it dries1 it becomes a bond of union between the particles on each side of the breach4 ,for the same reason you can make the cut surfaces of some yielding substances adhere4 ,if you divide a piece of ,india-rubber with a clean cut1 you can make the two surfaces adhere by pressing them together firmly4 ,the particles in this case are not unyielding1 as those of the glass are1 and some of them are therefore brought into such near neighborhood as to attract each other sufficiently to unite together4 ,so1 too1 if you cut two bullets so as to have a very smooth flat surface on each1 you can make them adhere quite strongly by pressing them together1 especially if you give a little turning motion at the same time that you press1 for this will cause the particles on the two surfaces to be somewhat mingled together4 ,if you have quite large balls of lead with handles1 as represented in ,fig4 #ad1 it will require considerable force to separate them when they have been thus pressed together4 #fg4 ,other ,illustrations4--,silver and gold may be made to adhere to iron by a very great and sudden pressure4 ,the iron must be made very smooth1 and the silver or gold plate very thin4 ,a powerful blow brings the particles of the thin plate into such nearness to those of the iron that union is effected1 or1 in other words1 that they attract each other sufficiently to be united4 ,so1 also1 a sheet of tin and one of lead can be made to adhere so as to form one sheet by the pressure of the rollers of a flatting-mill4 ,two very smooth panes of glass laid one upon another may have so many particles brought into great nearness as to occasion some adhesion4 ,it will be slight1 however1 few comparatively of all the particles coming near enough to adhere1 for the smoothest glass is full of inequalities1 as may be seen by the microscope4 #fh4 ,strength of ,adhesion4--,in no case do particles come in actual contact 7 #bj71 and their adhesion depends on the nearness of their neighborhood to each other4 ,the strength of union1 then1 between two surfaces depends on the number of particles brought near enough to adhere together4 ,in the case of the two bullets or the lead balls1 if all the particles of the two surfaces were near enough to adhere1 the lead would be just as strong at the junction as any where else4 ,the reason that so strong adhesion takes place between portions of some substances when we soften them by heat is that the particles of the two softened ends are all brought near enough together for adhesion4 ,thus the two ends of a broken stick of sealing-wax may be firmly united by heating them and then pressing them together4 ,the same thing can be done with glass4 ,when iron is welded1 as it is termed1 some hammering is required to make the particles of the two softened ends of the iron unite4 ,illustration3 ,fig4 #ae4 #fi4 ,attraction ,between ,solids and ,liquids4--,the attraction which solids and liquids have for each other furnishes us with many interesting phenomena4 ,the adhesion of drops of water to glass and other solids is a familiar example of this attraction4 ,if you dip your hand into water1 it is wet on taking it out1 because your skin has sufficient attraction for the water to retain some of it4 ,a towel will retain more of it for two reasons--with the interstices between its fibres it presents much more surface to the water1 and it has none of the oily substance which on your skin1 though being in small quantity1 serves somewhat to repel the water4 ,the attraction of solids and fluids for each other is shown very prettily in the experiment represented in ,fig4 #ae 7p4 #dg74 ,a piece of wood is attached by the string1 a1 to one end of a balance1 and weights just sufficient to balance it are placed in the opposite scale4 ,if now the wood is brought in contact with the water in the vessel1 b1 it will require additional weight in the scale to separate the wood from the water4 ,illustration3 ,fig4 #af4 ,fig4 #ag4 ,illustration3 ,fig4 #ah4 ,illustration3 ,fig4 #ai4 #gj4 ,farther ,illustrations4--,when you see stems of plants rising above the surface of stagnant water you will observe that the water is considerably raised about them4 ,this is from the attraction between them and the water4 ,for the same reason water is not as high in the middle of a tumbler as it is at the sides4 ,if you immerse a piece of glass in water1 the water will rise at its sides as represented in ,fig4 #af4 ,if you immerse two together1 as in ,fig4 #ag1 the water will rise higher between than outside of them1 because the particles between are attracted by two surfaces1 while those outside are attracted only by one4 ,it is for the same reason that two men can raise a weight higher than one of them can alone4 ,and if the pieces of glass be brought quite near together1 as in ,fig4 #ah1 the water will be raised higher still1 because there is less to be raised by the two surfaces4 ,it is just as two men can raise a small weight higher than they can a large one4 ,the same thing may be beautifully illustrated in this way3 ,let two pieces of glass1 as represented in ,fig4 #ai1 be immersed in colored water1 with two of their edges joined together at ,a ,b1 the opposite edges at ,e ,d ,c being separated4 ,the height to which the fluid rises will make a curved line1 ,a ,f ,c1 it being lowest at the edges which are separated1 and highest at the edges which are joined together4 ,illustration3 ,fig4 #bj4 ,illustration3 ,fig4 #ba4 #ga4 ,rise of ,liquids in ,tubes4--,for the same reason that water rises higher between plates of glass than outside1 so it will rise higher in a tube than it will outside of it4 ,the diagram in ,fig4 #bj will make this clear4 ,i represent in this a transverse section of a tube1 enlarged so that the demonstration may be plain4 ,we will take a particle on the inside and the outside at equal distances from the glass4 ,it is clear that the particle a is not as near to as many particles of the glass as the particle b is4 ,the lines drawn show this4 ,the longest lines extending from the particles a and b to the glass are equal in length2 that is1 a e and a f are equal to b g and b h4 ,it is clear1 therefore1 that all the glass between the lines at c and d is as near to the particle b as the glass between the lines at e and f is to the particle a4 ,but this is not all4 ,the particle b is near enough to all the inside of the tube to be attracted by it1 while very little attraction is exerted upon a by any part of the glass beyond that which is included between e and f4 ,the same difference can be shown with regard to all the particles on the inside of the tube compared with those outside4 ,the former are nearer to more particles of the glass than the latter1 and therefore are more strongly attracted4 ,again1 as the nearer the plates of glass are 7 #gj7 the higher the water rises between them1 so the smaller the tube is the higher will the water rise in it4 ,you can try the experiment as represented in ,fig4 #ba4 ,it is obvious that the particle b 7,fig4 #bj7 would not be very strongly attracted by the part of the tube opposite if the tube were a large one2 but it would be if the tube were very small1 for then it would be quite near to that part4 #gb4 ,capillary ,attraction4--,the term capillary 7derived from the ,latin word capilla1 hair7 has been commonly applied to the attraction exhibited under the circumstances just noticed1 because it is most obvious and was first observed in tubes of very fine bore4 ,the same term is used when the attraction is seen in the rising or spreading of a liquid in interstices as well as in tubes4 ,thus capillary attraction causes the rising of oil or burning fluid in the wicks of lamps4 ,the liquid goes up in the interstices or spaces between the fibres1 as it does in the spaces of tubes4 ,i will give some other examples4 ,if you let one end of a towel be in a bowl of water1 the other end lying over upon the table1 the whole towel will become wet from the spreading of the water among the fibres in obedience to capillary attraction4 ,if you suspend a piece of sponge so that it merely touch the surface of some water1 or if you lay it in a plate with water in it1 the whole sponge will become wet4 ,so1 too1 if you dip the end of a lump of sugar in your tea1 and hold it there a little time1 the whole will be moistened4 ,in very damp weather the wood-work in our houses swells from the spreading of water in the pores of the wood in obedience to capillary attraction4 ,especially will this be so in basement rooms1 where the water can go up from the ground in the pores of the walls1 as well as from the damp air4 ,in watering plants in pots1 if the water be poured into the saucers1 it will pass up through the earth by capillary attraction4 ,for the same reason plants and trees near streams grow luxuriantly1 being abundantly supplied with water1 which rises to their roots through the pores of the soil4 ,the disposition of wood to imbibe moisture in its pores has sometimes been made use of very effectually in getting out millstones4 ,first a large block of stone is hewn into a cylindrical shape4 ,then grooves are cut into it all around where a separation is desired1 and wooden wedges are driven tightly into them4 ,these absorb moisture from the dews and rain1 and therefore swell so much as to split the stone in the direction of the grooves4 ,the blotter which you use furnishes an illustration of capillary attraction1 the ink being taken up among the fibres of the paper4 ,ordinary writing paper will not answer as a blotter1 because the sizing fills up the interstices between the fibres4 ,,chapter ,v4 ,,gravitation4 #gc4 ,attraction ,between ,masses4--,i have thus far treated of attraction as existing between the atoms or particles of matter when they are brought very near together1 which is called the attraction of cohesion4 ,but it exists also between any portions of matter that are separate from each other4 ,thus if two cork balls be placed on the surface of water near to each other1 their attraction will soon bring them together4 ,to have the experiment striking1 the balls must be varnished1 that they may glide easily over the water4 ,bubbles of glass will exhibit the same attraction4 ,so1 also1 floating pieces of wood are apt to be found together2 and when a ship is wrecked1 as soon as the sea becomes calm the parts of the wreck are in collections here and there4 ,now when a stone falls to the ground1 it does so for precisely the same reason that the two cork balls come together in the water4 ,the idea of all who have not been informed on such subjects is1 that the stone comes to the ground because the ground is down and the stone is up1 and there is nothing to support the stone in the air4 ,they have no idea that some power makes the stone come down4 ,there is such a power1 and it is the attraction which the earth and the stone have for each other4 ,if you hold the stone in your hand1 and thus prevent its falling1 you simply resist a power which is pulling it down4 ,if you could in any way suspend the attraction of the earth and the stone for each other1 you could let go of the stone1 and it would remain just there in the air1 and would not come down until the attraction is restored4 #gd4 ,attraction ,mutual4--,the cork balls move toward each other because their attraction is mutual4 ,so do the earth and stone really move toward each other for the same reason4 ,as the stone is drawn toward the earth1 so is the earth drawn toward the stone4 ,but the earth is so large a thing to be drawn that its motion is exceedingly small--so small that practically it may be considered as nothing4 #ge4 ,illustration4--,this may be clearly illustrated if we compare the force of attraction to the force of muscular action4 ,suppose a man in a boat pulls on a rope which is made fast to a ship lying loose at the wharf1 and in this way draws his boat toward it4 ,he does not dream that he moves the ship at all2 but he in reality does1 for if instead of one boat a hundred or more pull upon the ship1 they will move it so much as to make the motion apparent4 ,in the case of the single boat1 the ship as really moves as when a hundred boats are pulling on it1 but it is only the one hundredth part as much4 ,now let the ship represent the earth1 and the little boat some body1 as a stone1 attracted by it4 ,the earth and the stone move toward each other1 just as the ship and the boat do4 ,and if1 as we multiplied the number of boats1 we should multiply the bulk of the stone till it is of an immense size1 it would have by its attraction a perceptible influence upon the earth's motion4 ,observe in regard to the illustration1 that it makes no difference whether the man be in the boat or in the ship as he pulls4 ,in either case he exerts an equal force on the ship and the boat1 making them to approach each other4 ,so it is with the attraction between the earth and the stone4 ,it is a force exerted equally upon both4 ,its effect on the earth is not manifest1 because it is so much larger than the stone2 just as the effect of the man's pulling is not manifest upon the ship1 because it is so much larger than the boat4 #gf4 ,proportion of the ,mutual ,motions of ,attraction4--,let us pursue the illustration a little farther4 ,if a man stand in a boat1 and pull a rope made fast to another boat of the same size and weight1 both boats1 in coming together1 will move over the same space4 ,just so it is with the attraction between two bodies having the same quantities of matter or equal weights--they attract each other equally1 and therefore meet each other half way4 ,let now one boat be ten times as great and as heavy as the other4 ,the small boat would move ten times as much as the large one when the man brings them together by pulling the rope4 ,in like manner1 if a body one-tenth as large as the earth should approach it1 they would attract each other1 but in coming together the body would move ten times as far as the earth would4 ,in the case of falling bodies1 even though they may be of great size1 the earth moves so slightly to meet them that its motion is wholly imperceptible4 ,it has been calculated that if a ball of earth the tenth part of a mile in diameter were placed at the distance of a tenth part of a mile from the earth1 as the earth and this body would be moved by their attraction to meet each other1 the motion of the earth would be only the eighty thousandth of a millionth 7#a_/hj1jjj1jjj1jjj7 of an inch4 #gg4 ,attraction ,universal4--,the attraction of which ,i have been speaking exists between all bodies1 however distant they may be from each other4 ,sun1 earth1 moon1 and stars attract each other2 and in obedience to this attraction they have a tendency to come together in one great mass1 and would do so if another force acting in opposition to this did not prevent it4 ,this force will be treated of in another part of this book4 #gh4 ,the ,tides4--,one effect of the attraction between the earth and the moon is quite familiar4 ,i refer to the tides4 ,when the tide rises it is because the water of the ocean feels the attracting force of the moon4 ,the moon actually lifts the water toward itself4 ,the attraction of the sun sometimes increases and sometimes diminishes the tides1 according to its position in relation to the moon and the earth4 ,if the land were as movable as the water1 or1 in other words1 if its particles were held together by no stronger attraction than those of water1 there would be the same motion that there is in the ocean over the surface of the earth1 as in its revolution successive portions of it present themselves toward the moon4 #gi4 ,meaning of the ,word ,gravitation4--,the attraction thus existing between different bodies of matter separated from each other is called the attraction of gravitation or gravity1 in distinction from the attraction of cohesion treated of in the previous chapter4 ,this name was given to it because we have such common examples of its influence in the fall of bodies to the earth4 ,they are said to gravitate toward the earth4 ,and they are said to do so by the force or attraction of gravitation or gravity4 ,the term terrestrial gravitation is sometimes used in speaking of the earth's attraction1 in distinction from the same thing in operation in other planets4 ,illustration3 ,fig4 #bb4 ,illustration3 ,fig4 #bc4 #hj4 ,attraction ,toward the ,earth's ,centre4--,all bodies are attracted toward the centre of the earth4 ,this is because the earth is globular1 as may be made clear by ,fig4 #bb4 ,let the circle represent the earth1 and a a body attracted by it4 ,the lines drawn from the body to the earth represent the attractive force exerted by the earth upon the body4 ,it is obvious from these that there is as much attraction on the one side of the line drawn from the body to the earth's centre as there is on the other4 ,the attractive force1 then1 of the earth as a whole is exerted upon the body in the direction of this middle line4 ,it tends to draw it1 therefore1 toward the centre4 ,if1 therefore1 a weight be suspended by a string1 the line of the string continued would go to the centre of the earth4 ,this being so1 it is clear that two weights suspended by two strings do not hang perfectly parallel to each other4 ,the difference is so slight in an ordinary pair of scales that it can not in any way be perceived4 ,but if it were possible to suspend in the heavens a beam so long as to stretch over a large extent of the earth's circumference1 as represented in ,fig4 #bc1 the scales attached to it would be very far from hanging parallel to each other4 ,substances suspended in different parts of the globe are hanging in different directions1 and those which are hung up by our fellow-men on the opposite side of the earth1 are hanging directly toward us4 ,illustration3 ,fig4 #bd4 #ha4 ,up and ,down4--,all falling bodies fall toward the centre of the earth1 and the same remarks can be made on this point that ,i have made in relation to suspended weights4 ,up and down are merely relative terms--up being from the centre of the earth1 and down toward it4 ,as the earth moves round on its axis1 the same line of direction which we call upward at one time is downward at another4 ,this may be illustrated on ,fig4 #bd4 ,let the circle represent the circumference of the earth4 ,in the daily revolution we pass over this whole circle4 ,if we are at ,d at noon1 we are at ,e at six o'clock1 and at ,f at midnight4 ,if1 therefore1 the ball ,a be dropped from some height at noon1 the line in which it falls will be at right angles to a line in which it will fall if you drop it from the same height at six o'clock2 for this height will have moved in this time from ,a to ,b4 ,if it be dropped from the same height at midnight its line of direction will be directly opposite to what it was twelve hours before2 for the height will have moved in that time to ,c4 ,it is not always true that falling bodies tend exactly toward the centre of the earth4 ,it is nothing in the centre that attracts them1 but it is the substance of the whole earth2 and as this is irregular in its density and form1 the attraction will be irregular also4 ,thus it is found by accurate experimenting that a plumb line suspended in the neighborhood of a mountain is so attracted by it that it will not hang exactly parallel with another suspended at some distance from the mountain4 ,the difference is not1 however1 enough in any case to have any practical bearing4 #hb4 ,weight4--,i have said before 7 #eb7 that what we call weight is not a property of matter1 but merely the result of a property1 the attraction of gravity4 ,this ,i will now illustrate4 ,if two bodies are falling to the earth1 and one of them contains ten times as many particles of matter as the other1 ten times as much force of gravity is required1 and is actually exerted1 to bring it to the ground4 ,this will appear plain to you if you bear in mind that a body does not come to the ground because there is nothing to keep it there1 but because it is drawn down by the force of attraction1 and then compare this force to any other force1 as1 for example1 that of muscular action4 ,if you draw toward you two weights1 one of which is twenty times as heavy as the other1 or1 in other words1 has twenty times the quantity of matter that the other has1 you must exert twenty times as much strength on the former as you do on the latter4 ,just so it is with the force of attraction4 ,the earth attracts or draws toward itself a body having twenty times the quantity of matter that another has with twenty times the amount of force4 ,and the first body will have twenty times the weight of the other1 for it will make twenty times the pressure upon any thing that resists the force with which the earth draws it toward itself4 ,weight1 then1 is the amount of the pressure occasioned by the attraction existing between the earth and the body weighed4 ,if you place a substance in one side of a pair of scales1 it goes down because of the attraction between it and the earth4 ,by placing weights in the other side until the scales are balanced1 you find how much is needed to counteract the downward pressure caused by the attraction of the substance and the earth for each other2 or1 in other words1 you find out how much it weighs4 ,in doing this you use certain standard weights2 that is1 certain quantities of matter which have been agreed upon by mankind1 and are called by certain names1 as pounds1 ounces1 etc4 ,when a spring is used in weighing1 the spring has been tried by these standard weights1 and its scale has been marked accordingly4 #hc4 ,weight not ,fixed1 but ,variable4--,weight does not depend alone upon the density of the body weighed1 but also upon the density of the earth4 ,for the attraction causing the pressure which we call weight is a mutual attraction1 and is in proportion to the quantities of matter in both the body and the earth4 ,if1 therefore1 the density of the earth were increased twice1 three times1 or four times1 the weights of all bodies would be increased in the same proportion2 that is1 the force with which the earth would attract them would be twice1 three times1 or four times as great as now4 ,this would not be perceived by any effect on balances1 for the weights and the articles weighed would be alike increased in weight4 ,but it would be perceived in instruments that indicate the weights of bodies by their influence on a spring4 ,these would disagree with scales and steelyards just in proportion to the increase of the earth's density4 ,it would be perceived also in the application of muscular and other forces in raising and sustaining weights4 ,every stone would require twice1 three times1 or four times the muscular effort to raise it that it does now4 #hd4 ,weight ,varies with ,distance4--,the nearer two bodies are to each other the greater is their attraction4 ,the nearer a body is to the earth the greater is the attraction that presses it toward the earth2 in other words1 the greater is its weight4 ,the force of gravity1 or weight1 is greatest1 therefore1 just at the surface of the earth1 and it diminishes as we go up from the earth4 ,as we go from the earth1 the force of gravity lessens in such a proportion that it is always inversely as the square of the distance from the centre of the earth4 ,i will explain4 ,if the distance from the centre of the earth to its surface1 which is #djjj miles1 be called #a1 then #djjj miles from the earth would be called #b1 or twice as far from the centre1 and #hjjj miles from the earth would be #c1 and so on4 ,the squares of these numbers would be #a1 #d1 #i1 #af1 etc4 ,now as weight lessens so as to be inversely as the square of the distance1 a body weighing a pound on the surface of the earth would weigh but a quarter of a pound at the distance of #djjj miles1 and but the ninth part of a pound at #hjjj miles4 ,a body weighs less on the summit of a high mountain than it would in the valley below1 because it is farther away from the great bulk of the earth1 and therefore is not so strongly attracted4 ,the difference1 however1 is but small4 ,a man weighing two hundred and fifty pounds in the valley would weigh but half a pound less if on the summit of a mountain four miles high4 #he4 ,weight ,every ,where4--,i have spoken of weight only in relation to the earth4 ,but there is weight in bodies every where1 for there is attraction wherever there is matter4 ,the weight of substances on the surface of different heavenly bodies varies according to the quantities of matter in those bodies4 ,as the moon is much smaller than the earth1 what weighs a pound with us would weigh much less than a pound in the moon4 ,and as the sun is much larger than the earth1 what is a pound with us would be much more than a pound there4 ,if we knew the exact densities of the sun and the moon and the earth1 as well as their size1 we could estimate exactly the difference in the weights which any body would have in them2 for the attraction which causes the pressure that we call weight is as the quantity of matter1 and the quantity of matter depends on both density and size4 #hf4 ,cohesion1 ,capillary ,attraction1 and ,gravitation the ,same4--,the attraction of cohesion1 capillary attraction1 and gravitation are only different modes of action of the same power2 viz41 the attraction which matter every where has for matter4 ,at first thought it would appear that there is something peculiar in the attraction of particles when they are brought together so as to adhere4 ,for if we take any substance1 a piece of glass1 for example1 its particles seem to be held together by an attraction vastly stronger than that attraction which inclines different bodies to move toward each other4 ,if you break the glass1 however closely you may press the two pieces together1 they will not unite again4 ,it would seem1 at first view1 that there must be some peculiar arrangement of the particles which is destroyed by breaking the glass4 ,but we can readily account for the facts in another way4 ,the attraction between bodies of matter is the greater the nearer we bring them together4 ,the nearer1 for example1 is the moon to any portion of the earth1 the greater is the attraction which it exerts1 as seen in the tides2 and if it were much nearer to the earth than it is1 our tides would prove awfully destructive4 ,what is true of masses is also true of the particles of which they are composed4 ,though their attraction is comparatively feeble when at a distance from each other1 it increases1 not in the arithmetical but the geometrical ratio 7 #hd71 as they come nearer together2 so that when they are exceedingly near together the attraction is very powerful4 ,it must be remembered in regard to the pieces of broken glass that you can not bring the particles on their surfaces as near as they were before the glass was broken1 for the crack does not disappear4 ,and as the attraction is inversely as the square of the distance1 a little distance must make a great difference4 ,the particles of some substances you can bring so near together as to cause adhesion1 as you saw in the case of the two bullets 7 #ff74 ,that their adhering together depends merely upon their particles being brought near to each other appears from the fact1 that the smoother you make the surfaces the more strongly will they adhere4 ,and the reason that liquids and semi-liquids adhere so readily to solid substances is1 that their particles1 moving freely among each other1 have thus the power of arranging themselves very near to the particles of the solid4 ,thus1 when a drop of water hangs to glass1 all the particles of water in that part of the drop next to the glass touch1 or rather are exceedingly near to1 the particles of the glass4 #hg4 ,variety in the ,results of ,attraction4--,it is one and the same force1 then1 which binds the particles of a pebble together1 and makes it fall to the ground--which moulds the tear and bids it trickle from its source--which gives the earth and all the heavenly bodies their globular shape1 and1 in connection with another power hereafter to be noticed1 makes them revolve in their orbits4 ,how sublime the thought that this one simple principle that gives form to a drop extends its influence through the immensity of space1 and so marshals the host of heaven that1 without the least interruption or discord1 they all hold on their course from year to year and from age to age6 ,it is thus that ,omnipotence makes the simplest means to produce the grandest and most multiform results4 #hh4 ,opposition ,between the ,modes of ,attraction4--,although cohesion and gravitation are essentially the same thing1 we see them continually acting in opposition to each other4 ,abundant illustrations might be given1 but1 ,i will cite only a few4 ,illustration3 ,fig4 #be4 ,fig4 #bf4 #hi4 ,why ,pitchers have ,lips4--,if you pour water out of a tumbler there is a struggle between the attraction of cohesion and gravitation for the mastery--the attraction of cohesion tending to make the water adhere to the tumbler1 and run down its side1 as in ,fig4 #be1 and gravitation tending to make it fall straight down4 ,but when water is poured out of a pitcher1 as in ,fig4 #bf1 the lip of the pitcher acts in favor of the attraction of gravity2 for the water would have to turn a very sharp corner to run down the outside of the pitcher in obedience to the attraction of cohesion4 ,in pouring water from a tumbler1 we can often1 by a quick movement1 throw the water1 as we may say1 into the hands of gravity before the attraction of cohesion can get a chance to turn it down the tumbler's side4 ,if you can only make the water begin to run from the tumbler without going down its side there will be no difficulty2 for there is an attraction of cohesion between the particles of the water1 tending to make them keep together1 which in this case acts against the cohesion between the water and the glass1 and therefore acts in favor of gravitation4 ,it is cohesion that forms the drop on the lip of a vial as we drop medicine--cohesion between the particles of the liquid1 and cohesion between these particles and those of the glass4 ,it is gravitation1 on the other hand1 that makes the drop fall1 it becoming so large that the force of gravity overcomes the cohesion between the drop and the vial4 #ij4 ,size ,limited by ,gravity4--,were it not for the attraction of gravity there would be no limit to the size of drops of any liquid4 ,when the drop reaches a certain size1 it falls because it is so heavy2 or1 in other words1 because with its slight cohesion the attraction of the earth brings it down4 ,now if this attraction could be suspended1 and the attraction of cohesion left to act alone1 particles of water might be added to the drop to any extent1 and they would cling there4 ,you can see the struggling between cohesion and gravitation very prettily illustrated if you watch the drops of rain on a window-pane4 ,if two drops happen to be quite near together they unite by attraction1 and then1 being too large to allow of its being retained there by cohesion in opposition to gravitation1 the united drop runs down4 ,if it meet with no other drop it soon stops1 because by cohesion some portion of it clings to the glass all along its track1 and so at length lessens it sufficiently to allow it to remain suspended again4 ,it is from the influence of the attraction of gravitation that different kinds of liquids furnish drops of different sizes1 the heavier giving small1 and the lighter large ones4 ,thus you can drop from a vial a larger drop of alcohol than of water1 and a larger one of water than of nitric acid4 ,you have another illustration of a similar character in the adhesion of chalk to a black-board or any surface4 ,the chalk crayon itself can not adhere1 for the attraction of the earth does not permit it4 ,but small quantities of it can adhere for the same reason that water adheres to surfaces in small quantities4 ,so also dust clings to sides of furniture1 though a lump of dirt would not4 ,illustration3 ,fig4 #bg4 ,illustration3 ,fig4 #bh4 #ia4 ,illustrated in ,solid ,bodies4--,we can illustrate the limitation of size in solid masses by ,figs4 #bg and #bh4 ,suppose that a and b1 ,fig4 #bg1 are two projections of timber from a post1 b being twice as large as a4 ,it is evident that b can not support twice as much weight as a1 for gravitation is dragging it downward from its attachment to the upright post with twice the force that it does a4 ,the case is still stronger when1 as represented in ,fig4 #bh 7p4 #fd71 the larger timber is twice as long as the smaller4 ,here d has four times the bulk of c4 ,but it can not support four times as much weight at its end1 not only because its own weight presses it downward1 but because half of its weight is at a greater distance from the place of attachment than the smaller beam is4 ,gravitation here operates in opposition to cohesion in such a way that the projecting timber1 if carried to a certain size1 will fall by its own weight1 either breaking in two1 or tearing away from its attachment4 ,this tendency is very commonly resisted in buildings and other structures by braces1 as represented in ,fig4 #bi4 ,here the weight of the horizontal timber at some distance on each side of a1 is made to press upon the upright post instead of directly downward4 ,illustration3 ,fig4 #bi4 #ib4 ,farther ,illustrations4--,the size of bodies1 both animate and inanimate1 is limited by the ,creator in obedience to the principles above developed4 ,this is seen in the fact that there are no animals on the land to compare in size with the monsters of the deep4 ,a whale does very well in the water1 because he is buoyed up by that element2 but an animal as large as a whale could not well exist on land1 because gravitation would act so strongly in opposition to cohesion4 ,at least it would be necessary1 in order that he might walk about1 or even hold together1 that his great bulk should be made up of very firm and tenacious materials4 ,whenever any thing very large or tall is to be supported1 its support is always broad and composed of very cohesive materials4 ,we see this exemplified in the massive trunks of full-grown trees1 as compared with the slender trunks of trees of the same kinds in the nursery4 ,we see it exemplified in the fact that the highest mountains are built of the hardest rocks1 while the soft chalk formations are confined to those of small size4 ,there is a limit to the height even of the granite mountains in the influence of gravity4 ,if carried much higher than they are1 the attraction of the earth1 in its opposition to cohesion1 would tear them apart in their fissures1 or cause the immense weight to crush their foundations4 ,in the moon1 where gravitation is less than on the earth 7 #he71 mountains can be much higher without these results1 and accordingly the telescope shows them to be so4 ,in ,jupiter1 on the other hand1 which is much larger than the earth1 the mountains1 if there be any1 can not rise to any great height1 and if there be any living beings as large as we are in that planet they must be made of vastly firmer materials to prevent their being crushed by their own weight4 #ic4 ,the ,above ,principles ,transgressed by ,man4--,man often transgresses these principles in his structures4 ,for example1 a building settles because the foundation is not strong enough to bear the superincumbent weight2 in other words1 the force of gravitation is not sufficiently taken into the account4 ,when a very tall building is erected1 the lower portions ought to be made of very cohesive substances4 ,firm granite is therefore an appropriate material for the lower story of tall brick buildings4 ,at least should the walls of the lower stories of such buildings be made thicker than they ordinarily are1 to resist properly the force of gravitation in the weight above4 ,stores intended to bear much weight on their floors are often built without due regard to the cohesive force required to sustain the weight4 ,long timbers are sometimes supported only at the ends1 when their own weight1 to say nothing of what may be brought to press upon them1 requires that they should be supported at other points4 ,while in modern buildings the timbers are often too small1 in some old buildings the upper timbers are so heavy as to lessen rather than increase the strength of the structure4 ,especially is this true of the unsightly beams which in some ancient houses we see extending along the ceilings above4 ,many other examples could be given1 but these will suffice4 ,the practiced eye1 in looking at a building1 instinctively requires that every part should be seen to be suitably supported4 ,a gallery in a church1 therefore1 if without pillars or braces from the wall1 is displeasing to such an eye1 even though there may be really sufficient support provided in the mode of structure4 ,the same can be said of galleries supported by slender iron pillars1 especially if they be painted of some light color so as to look as if they were wood rather than iron4 ,for the same reason porticoes without pillars are unsightly4 ,so1 too1 the eye instinctively looks for a sufficient base to every pillar and pilaster4 ,the concealment of the base in any way1 or the substitution of any thing else for it1 is an unpleasant anomaly1 and yet such anomalies are sometimes seen even in expensive buildings4 #id4 ,attraction of ,natural ,philosophy and of ,chemistry4--,the attraction of which ,i have treated in this and the previous chapters is that which belongs to ,natural ,philosophy1 in distinction from that of ,chemistry4 ,its effects are only mechanical1 while the attraction of ,chemistry goes beyond this1 and affects the composition of substances4 ,for example1 the attraction between the two gases1 oxygen and hydrogen1 which makes them unite to form water1 belongs to ,chemistry2 while that which makes the particles of water cohere is in the province of ,natural ,philosophy4 ,,chapter ,,vi4 ,,centre ,,of ,,gravity4 ,illustration3 ,fig4 #cj4 ,illustration3 ,fig4 #ca4 ,fig4 #cb4 #ie4 ,centre of ,gravity ,illustrated4--,if you balance a ruler on your finger1 as in ,fig4 #cj1 it is balanced because there is just as much weight on one side as on the other4 ,now just over your finger1 in the middle of the ruler1 there is a point that we call the centre of gravity2 or1 in other words1 the centre of the weight of the ruler4 ,this point is indicated in the figure4 ,there is as much of the weight of the ruler on the one side of this point as on the other1 and also as much above it as below it4 ,if your finger should be a little to the one side or the other of this point1 the ruler would not be balanced1 and would fall4 ,when balanced it does not fall1 simply because this central point is supported by being directly over the end of the finger4 ,the weight of the ruler1 then1 may be considered practically as all being in that point1 for it is there that is exerted all the downward pressure of the ruler as it is balanced4 ,so1 also1 when the ruler is balanced on the finger1 as represented in ,fig4 #ca 7p4 #fh71 this same centre of gravity is directly over the point of the finger1 and is therefore supported4 ,if it be to the one side or the other1 as in ,fig4 #cb1 it is not supported1 and the ruler therefore falls4 ,you see1 then1 that if a body be balanced1 the centre of gravity is directly over the point of support4 ,if1 on the other hand1 a body is suspended1 the centre of gravity is directly under the point of support4 ,illustration3 ,fig4 #cc4 #if4 ,definition4--,if a plumb-line from the centre of gravity could be extended into the earth it would go directly to its centre4 ,the body may be considered as making all its pressure from its centre of gravity in that direction1 in obedience to the attraction of gravitation4 ,the best definition1 then1 that we can give of the centre of gravity is1 that it is that point in a body from which its pressure as a whole toward the centre of the earth proceeds4 ,it is that point1 therefore1 the support of which insures the support of the whole body4 ,and in speaking of the weight of a body1 or its downward pressure1 we may consider all the matter composing it as collected or concentrated in that point4 ,the body1 therefore1 can be balanced in any position in which this point is supported1 as shown in ,figs4 #cj and #ca4 ,and when a body is suspended1 it is at rest only when the centre of gravity is directly under the point of support4 ,thus1 if you have a circular plate suspended at ,e1 ,fig4 #cc1 it will not be at rest when it is moved to the one side or the other1 as represented by the dotted lines1 but only when the centre of gravity1 c1 is directly under the point ,e4 ,illustration3 ,fig4 #cd4 ,fig4 #ce4 #ig4 ,how to ,find the ,centre of ,gravity of a ,body4--,if you take a piece of board1 and suspend it at ,a1 ,fig4 #cd1 and suspend a plumb-line from the same point1 the centre must be somewhere in that line4 ,but exactly at what point it is you do not know4 ,how will you ascertain this8 ,mark the line ,a ,b on the board1 and suspend the board by another point1 as in ,fig4 #ce4 ,as the centre of gravity must be somewhere in the plumb-line as it now hangs1 of course it is at ,o1 where the two lines cross4 ,illustration3 ,fig4 #cf4 ,fig4 #cg4 #ih4 ,scales and ,steelyards4--,when two bodies are connected by a rod or bar1 the centre of gravity of the whole is somewhere in the connection4 ,if the two bodies be equal in weight1 as in ,fig4 #cf1 the centre of gravity is exactly in the middle of the rod1 as marked4 ,but if the bodies are unequal1 as in ,fig4 #cg1 the centre of gravity is nearer to the larger body than to the smaller4 ,in balancing a body in one scale with weights in another1 you have a case parallel to that of ,fig4 #cf4 ,the centre of gravity of the body weighed1 the weights1 and the scales1 as a whole1 is midway between the scales1 at the point of support4 ,in the steelyard you have the heavy body to be weighed nearer the centre of gravity than the small weight is on the long arm1 and so the case is parallel with that of ,fig4 #cg4 ,illustration3 ,fig4 #ch4 #ii4 ,the ,centre of ,gravity of a ,body not ,always in the ,body ,itself4--,the centre of gravity of a hollow ball of uniform thickness is not in the substance of the ball1 but it is in the centre of the space in the ball1 for the line of the ball's downward pressure would be from that point4 ,if the ball had a frame-work in it1 as represented in ,fig4 #ch1 the centre of gravity would obviously be at ,a1 the centre of this frame-work4 ,but if there were no frame-work1 and perpendicular lines were supposed to be drawn from different points of suspension1 ,c1 ,b1 ,d1 and ,e1 these would intersect at the point ,a1 showing that this is the centre of gravity1 according to the rule for finding it given in #ig4 ,so1 also1 the centre of gravity of an empty box1 or an empty ship1 would be an imaginary point in the space inside4 ,in a hoop it is the centre of the hoop's circle4 ,illustration3 ,fig4 #ci4 #ajj4 ,the ,centre of ,gravity ,seeks the ,lowest ,point4--,the centre of gravity always takes the lowest place which the support of the body will allow4 ,in a suspended body1 therefore1 it is always directly under the point of suspension4 ,to get to the one side or the other of this position it must rise4 ,this the attraction of gravity forbids1 and if by any force it is made to rise1 this attraction at once brings it back4 ,this is manifest in the case of a suspended ball1 ,fig4 #ci4 ,if the ball be moved to b1 it will1 on being let go1 return to its first position1 simply because its centre of gravity1 in obedience to the earth's attraction1 seeks the lowest place possible4 ,from inertia 7 #di7 it moves beyond this point1 and continues to vibrate back and forth for some time2 but when its motion is stopped1 it hangs perpendicularly2 that is1 in such a way that its centre of gravity shall have the lowest possible position4 ,i add a few other illustrations of the same point4 ,when a rocking-horse is at rest1 its centre of gravity is directly over the point at which it touches the floor1 for thus it has its lowest possible place4 ,if it be rocked1 the centre of gravity is moved to a higher point1 and for this reason it rocks back again4 ,the same is seen in the swing1 the cradle1 the rocking-chair1 etc4 ,most interesting illustrations of the same thing are found in the ,laggan1 or ,loggan ,stones as they are called1 several of which are seen on the rugged parts of the ,british coast4 ,an immense rock1 which has been loosened by some convulsion1 rests with a slightly-rounded base on another rock which is flat1 and it is so nicely balanced that one person alone can produce a perceptible rocking motion in it4 ,i saw1 many years ago1 a large rock near ,salem1 ,massachusetts1 thus situated4 ,there is one also in ,great ,barrington1 ,massachusetts4 ,illustration3 ,fig4 #dj4 ,fig4 #da4 ,illustration3 ,fig4 #db4 ,illustration3 ,fig4 #dc4 #aja4 ,farther ,illustrations4--,it is because the centre of gravity always seeks the lowest place that an egg lies upon its side4 ,when on its side1 the centre of gravity is at its lowest point1 as is manifest from comparing ,fig4 #dj with ,fig4 #da 7p4 #ga74 ,children often have a toy1 called a witch1 which illustrates the same thing in another way4 ,it is a piece of light substance1 as pith1 with a shot fastened in one end4 ,it always stands up on its loaded end1 and can not be made to lie down on its side1 because the centre of gravity would not then be at the lowest point4 ,there is an amusing ,chinese toy of the same kind4 ,it is a figure of a fat old woman1 ,fig4 #db1 loaded with lead at the bottom1 so that its centre of gravity is at a4 ,if the figure be thrust over to one side1 as shown by the dotted lines1 the centre of gravity is raised1 and the upright position is at once resumed4 ,if the toy were not loaded1 it would lie in the position represented in ,fig4 #dc1 just as the egg lies on its side4 ,illustration3 ,fig4 #dd4 ,illustration3 ,fig4 #de4 ,fig4 #df4 ,illustration3 ,fig4 #dg4 #ajb4 ,curious ,experiments4--,you can not hang a pail of water on a stick laid upon a table1 as represented in ,fig4 #dd1 for the centre of gravity is not supported4 ,but if you place another stick a as a brace1 in the manner represented in ,fig4 #de 7p4 #gc71 so as to push the pail under the table1 it will hang securely1 because the centre of gravity is now under the point of suspension4 ,the explanation of the following experiment is the same3 ,run a large needle through a cork2 fasten to the cork a fork1 and you can suspend the whole on the edge of a table1 as seen in ,fig4 #df4 ,here the centre of gravity is directly under the point of suspension1 which is at the point of the needle4 ,the same can be said of the very common toy represented in ,fig4 #dg4 ,the horse1 made of very light material1 stands securely1 because the centre of gravity of the whole is in the heavy ball1 which is under the point of suspension4 ,if the horse be made to rock back and forth1 the centre of gravity in the ball moves in a curved line1 as in the case of a ball suspended by a string 7,fig4 #ci74 ,it is at its lowest place only when the horse is at rest4 ,the hanging of a cane with a hook-shaped handle on the edge of a table is to be explained in the same way4 ,illustration3 ,fig4 #dh4 ,fig4 #di4 ,fig4 #ej4 ,illustration3 ,fig4 #ea4 ,illustration3 ,fig4 #eb4 #ajc4 ,stability of ,bodies4--,the firmness with which a body stands depends upon two circumstances--the height of its centre of gravity1 and the extent of its base4 ,the lower the centre of gravity1 and the broader the base1 the firmer does the body stand4 ,a cube1 represented in ,fig4 #dh1 is more stable1 that is1 less easily turned over1 than a body shaped as ,fig4 #di1 because it has a larger base4 ,the contrast is still greater between ,figs4 #dh and #ej4 ,the reason of the stability of a body with a broad base is found in the fact1 that in turning it over the centre of gravity must be raised more than in turning over one of a narrower base4 ,the curved lines indicate the paths of the centres of gravity as the bodies are turned over4 ,in the case of a perfectly round ball1 the base is a mere point1 and therefore the least touch turns it over4 ,its centre of gravity does not rise at all1 but moves in a horizontal line1 as shown in ,fig4 #ea4 ,the pyramid is the firmest structure in the world1 because it possesses in the highest degree the two elements--a broad base1 and a low position of the centre of gravity4 ,on both these accounts the centre of gravity must ascend considerably when the body is turned over1 as seen in ,fig4 #eb4 ,illustration3 ,fig4 #ec4 ,illustration3 ,fig4 #ed4 #ajd4 ,bodies not ,upright ,unstable4--,when a body does not stand upright1 its stability is diminished simply because only a portion of the base is concerned in its support4 ,in ,fig4 #ec the base is broad1 but the body is so far from being upright that the centre of gravity bears upon the very extremity of the base on one side1 as the perpendicular line from it indicates4 ,the least jostle will turn it over1 because the centre of gravity need not ascend the least when this is done4 ,you see1 then1 that the less upright a body is1 the less of the base is of service in its support1 because the farther is the line of direction of the downward pressure of the centre of gravity from the centre of the base4 ,the famous tower of ,pisa1 ,fig4 #ed1 one hundred and thirty feet high1 overhangs its base fifteen feet4 ,it was undoubtedly built intentionally in this way to excite wonder and surprise1 for what would otherwise have been a very unsafe structure is rendered stable and safe by the arrangement of its materials4 ,its lower portion is built of very dense rock1 the middle of brick1 and the upper of a very light porous stone4 ,in this way the centre of gravity of the whole structure is made to have a very low position4 ,illustration3 ,fig4 #ee4 #aje4 ,familiar ,illustrations4--,you see now the explanation of the fact which common experience teaches every one1 that the taller is a body1 and the narrower its base1 the more easily is it overturned4 ,this is exemplified in the two loads1 ,fig4 #ee4 ,the base is the space included by the wheels4 ,the centre of gravity is so high in the tall load that a perpendicular line drawn from it falls outside of the base if the cart come upon a considerable lateral inclination of the road4 ,but the smaller load1 under the same circumstances1 is perfectly secure from overturning4 ,a high carriage is more easily overturned than a low one1 for the same reason4 ,a stage1 if loaded on its top1 is very unsafe on a rough road4 ,stability is given to articles of furniture by making their bases broad and heavy1 as you see in tables supported by a central pillar1 candlesticks1 lamps1 etc4 ,the tall chairs in which children sit at table would be very insecure if their legs were not widely separated at the bottom1 thus widening the base of support4 ,in the ladder1 so commonly used now in picking fruit1 a broad base is furnished between the foot of the ladder and the two standards which are spread out to sustain its top4 ,illustration3 ,fig4 #ef4 ,fig4 #eg4 #ajf4 ,support of the ,centre of ,gravity in ,animals4--,the base of support which quadrupeds have1 viz41 the space included between their four feet1 is quite large2 and this is one reason that they walk so soon after birth4 ,a child does well that can walk at the end of ten or twelve months1 for the supporting base is quite small compared with that of a quadruped4 ,it consists of the feet and the space between them4 ,it requires skill1 therefore1 in the child to manage the centre of gravity in standing and walking1 and this is gradually acquired4 ,if one should grow up without ever standing on his feet1 he would find1 as the infant does1 that some training is necessary to enable him to do it4 ,it is on account of the smallness of the base furnished by the feet that the statue of a man is always made with a large base or pedestal4 ,although we exert considerable skill in walking1 by no means so much is requisite as the ,chinese ladies must put in exercise with their small feet4 ,still more skill is exercised by one who has two wooden legs1 or one who walks on stilts4 ,the base made by the feet can be varied much by their position4 ,if the toes be turned out and the heels brought near to each other1 the base will not be as large as when the feet are straight forward and far apart1 as is manifest in ,figs4 #ef and #eg4 ,it is for this reason that the child1 in his first essays at standing and walking1 instinctively manages his feet as in ,fig4 #ef4 ,illustration3 ,fig4 #eh4 ,illustration3 ,fig4 #ei4 ,illustration3 ,fig4 #fj4 ,fig4 #fa4 #ajg4 ,motions of the ,centre of ,gravity in ,walking4--,in walking1 the centre of gravity is alternately brought over one foot and the other1 and so moves in a waving line4 ,this is very manifest as you see people before you going down the aisle out of a church4 ,when two are walking together1 if they keep step the two waving lines of their centres of gravity run parallel1 as in ,fig4 #eh1 and they walk easily2 but if they do not keep step these lines run as in ,fig4 #ei1 and the movement is both awkward and embarrassing4 ,the line of movement of the centre of gravity is always slightly waving upward also1 as seen in ,fig4 #fj 7p4 #gh74 ,in the case of a man with wooden legs the line would not be gently waving1 but somewhat angular1 as represented in ,fig4 #fa4 ,illustration3 ,fig4 #fb4 ,fig4 #fc4 ,illustration3 ,fig4 #fd4 ,illustration3 ,fig4 #fe4 #ajh4 ,the ,centre of ,gravity and ,attitudes4--,the object of various attitudes assumed under different circumstances is to keep the centre of gravity over the base of support4 ,a man with a load on his back would not assume the position of ,fig4 #fc1 but that of ,fig4 #fb1 so that the centre of gravity of his load may be directly over his feet4 ,so a man carrying any thing in front leans backward1 as in ,fig4 #fd4 ,in ascending a hill a man appears to lean forward1 and in descending to lean backward2 but in fact he is in both cases upright in reference to the plain on which the hill stands1 as may be seen in ,fig4 #fe4 ,a perpendicular line drawn from his centre of gravity strikes the ground midway between the feet1 that is1 in the middle of the base1 and if prolonged would go straight to the centre of the earth4 ,when one rises from a chair he draws his feet backward1 and then bends his body forward to bring the centre of gravity over the feet4 ,unless this is done1 it is impossible to rise1 at least deliberately1 as you will find by trying the experiment4 ,a man standing with his heels close to a wall can not stoop forward and pick up any thing1 for the wall prevents him from moving any part of his body backward1 and therefore when he stoops forward1 the centre of gravity being brought far in advance of the base1 he loses his balance and falls4 ,a man who did not understand this undertook to stoop in this way to pick up a purse containing twenty guineas1 which he was to have if he succeeded1 the forfeiture in case of failure being ten guineas4 ,of course his centre of gravity made him lose his wager4 #aji4 ,rope-,dancers1 ,tops1 etc4--,great skill is exhibited by the rope-dancer in supporting the centre of gravity4 ,similar skill is seen in feats of balancing1 as1 for example1 in balancing a long stick upright on the finger4 ,in these cases the centre of gravity is very little of the time directly over the point of support4 ,it is kept in constant motion nearly but not quite over this point1 this unstable equilibrium1 as it is called1 being vastly less difficult to maintain than stable equilibrium2 that is1 keeping the balance in one unvarying position4 ,it is the motion of the top that makes it to stand upright upon its point--a very beautiful example of unstable equilibrium4 ,the centre of gravity revolves around a perpendicular line1 at exceedingly little distance from it at first1 but greater and greater as its motion becomes less rapid1 till at length the centre of gravity gets so far from this line that the top falls4 ,for a similar reason an intoxicated man may not be able to keep himself up if he undertake to stand still1 and yet may do so if he keep moving4 ,as in the case of the top1 his centre of gravity must be kept in motion1 or he must fall4 ,,chapter ,,vii4 ,,hydrostatics4 #aaj4 ,what ,hydrostatics ,teaches4--,hydrostatics is that branch of ,natural ,philosophy which treats of the pressure and equilibrium of liquids4 ,the phenomena which it brings to view all results from the influence of the attraction of the earth upon liquids4 ,it is for this reason that this subject calls naturally for our consideration after examining the general subject of attraction1 as we have done in the previous chapters4 ,in order to understand fully the phenomena of ,hydrostatics1 you must continually bear in mind the two grand characteristics of liquids4 ,one is1 that the particles move freely among each other 7 #i74 ,the other is1 that a liquid is almost entirely incompressible 7 #cf74 #aaa4 ,level ,surface of ,liquids4--,it is the influence of gravitation upon liquids that gives them a level surface whenever they are not agitated by any cause4 ,observe how this is4 ,a still body of water you may consider as being made up of layers of particles4 ,each layer will have all its particles equally attracted by the earth1 and must therefore be level4 ,if any of the particles were less attracted than their neighbors they would rise1 as is the case when heat is applied1 as you will see hereafter4 ,whenever the upper layers of the particles are disturbed by wind or any other cause1 as soon as the disturbance ceases the particles will again take their places in level layers under the influence of gravitation4 #aab4 ,a ,comparison4--,the particles of water may be compared to shot4 ,if you have shot in a vessel1 and they are heaped up in any portion of the surface1 on shaking the vessel those that are highest will roll down1 and the result will be a level surface4 ,they would do this without agitation if they were as smooth as the particles of water are4 ,if we could have a microscope strong enough to distinguish the shape of the particles of water1 the surface would probably appear like the level surface of shot in a vessel4 ,but the particles of water are so exceedingly minute that the surface of water1 when entirely free from agitation1 is so smooth as to constitute a perfect mirror1 often feasting our eyes with another world of beauty as we look down into its quiet depths4 ,water was man's first mirror1 and one of the most beautiful passages in the ,paradise ,lost is the description of ,eve's first waking after her creation at the side of a lake1 and seeing her form reflected in its smooth waters4 ,illustration3 ,fig4 #ff4 #aac4 ,surface of ,liquids not ,truly ,level4--,strictly speaking1 the surface of a liquid is not level1 but rounding4 ,but it is so little so that it can not be perceived unless we take into view a very large surface1 as the ocean4 ,here it is very manifest1 for whenever a ship comes into port the first thing seen from the shore is the topmost sail1 the rest of the ship being concealed by the water rounded up between it and the observer4 ,this is illustrated in ,fig4 #ff4 ,at a the ship is just in sight1 while at b it is so near shore that the eye sees the whole of it4 ,if the earth had no elevations of land1 or if there was water enough to cover them1 the water would make a perfectly globular covering for the earth1 being held to it by the force of attraction4 ,the reason for this is precisely the same as was given in #eh for the disposition of a drop of liquid to take the globular form4 ,as in that case1 so in this1 it can be demonstrated that each particle is attracted toward a common centre1 and that this will produce in the freely-moving particles a uniformly rounded surface4 ,what could thus be shown to be true if the earth were wholly covered with water1 is true of the portions of water which now fill up the depressions in the earth's crust2 and it can be perceived1 as shown in the first part of this paragraph1 in the case of any extended portion of it4 ,illustration3 ,fig #fg4 #aad4 ,spirit-,level4--,what we call a perfectly level surface is1 then1 one all parts of which are equally distant from the centre of the earth1 and is therefore really a spherical surface4 ,but the sphere is so large that any very small portion of it may be considered for all practical purposes a perfect plane4 ,a hoop surrounding the earth would bend eight inches in every mile4 ,in cutting a canal1 therefore1 there is a variation in this proportion from a straight level line4 ,as the variation is but an inch in an eighth of a mile1 it is of no account in taking the level for buildings4 ,levels are ascertained by what is called a spirit-level4 ,this consists of a closed glass tube1 ,fig #fg1 nearly filled with alcohol4 ,the space not occupied by alcohol is occupied by air4 ,the tube is placed in a wooden box for convenience and security1 there being an opening in the box at a4 ,now when the box with its glass tube is perfectly level1 the bubble of air will be seen in the middle at a2 but if one end be higher than the other1 the bubble will be at or toward that end4 #aae4 ,rivers4--,if a trough be exactly level1 the water will be of the same depth at one end as at the other1 for the surface of the water at both ends will be at the same distance from the centre of the earth4 ,but raise up one end1 and it is now deepest at the other end4 ,if it were not so1 the surface at the two ends would not be at the same distance from the centre of the earth4 ,now if1 with the trough thus placed1 water run in at the upper end and out at the lower1 you have exemplified what is taking place in all rivers--the water is in constant motion from the influence of gravitation1 causing it to seek to be on a level4 ,a very slight slope will give the running motion to water1 for the particles are so movable among each other that in obedience to gravity they flow down the inclined plane to seek a level4 ,three inches declivity in every mile in a smooth straight channel will make a river run at the rate of about three miles an hour4 ,the ,ganges1 which receives its waters from the ,himalaya ,mountains1 in running #ahjj miles falls #hjj feet4 ,the ,magdalena1 in ,south ,america1 falls only #ejj feet in running #ajjj miles between two ridges of the ,andes4 #aaf4 ,how some ,rivers have been ,made4--,changes are constantly produced in the earth by the disposition of water to seek a level4 ,in doing this the water carries solid substances of various kinds from elevated places into depressed ones1 tending to fill up the latter4 ,new channels are also sometimes made by the water4 ,the boy who makes a little pond with his mud-dam1 and lets the water overflow from it into another pond on a lower level1 as he sees a channel worked by the water between the two ponds becoming larger and larger1 witnesses a fair representation on a small scale of some extensive changes which have in ages past taken place in some parts of the earth4 ,it is supposed1 and with good reason1 that many rivers had their origin in the way above indicated4 ,for example1 where the ,danube runs its long course there was once a chain of lakes4 ,these becoming connected together by their overflow1 the channels cut between them by the water continually became larger1 until at length there was one long1 deep1 and broad channel1 the river1 while the lakes became dry1 and constituted the fertile valley through which that noble river runs to empty into the ,black ,sea4 ,it is said that a similar process is manifestly going on in the ,lake of ,geneva1 the outlet of it becoming continually broader1 while the washing from the neighboring hills and mountains is filling up the lake4 ,towns that a century ago lay directly upon the borders of the lake have gardens and fields now between them and the shore2 and ,dr4 ,arnot says1 ,if the town of ,geneva last long enough1 its inhabitants will have to speak of the river in the neighboring valley1 instead of the picturesque lake which now fills it4 ,illustration3 ,fig4 #fh4 ,illustration3 ,fig4 #fi4 #aag4 ,canals4--,the management of the locks of a canal is in conformity with the disposition of water to seek a level4 ,a ground view of a lock and a part of two adjacent locks is given in ,fig4 #fh4 ,the lock1 ,c1 has two pair of flood-gates1 ,d ,d and ,e ,e4 ,the water in ,a is higher than in ,c1 but the level is the same in ,c and ,b1 because the gates1 ,e ,e1 are open4 ,suppose now that there is a boat in the lock ,b that you wish to get into the lock ,a4 ,it must be floated into the lock ,c1 and the gates ,e ,e must be closed4 ,the water may now be made to flow from the higher level1 ,a1 into ,c1 till the level is the same in both ,a and ,c4 ,but this can not be done by opening the gates ,d ,d1 for the pressure of such a height of water in the lock ,a would make it difficult1 perhaps impossible1 to do this2 and besides1 if it could be done1 the rapid rush of water into ,c would flood the boat lying there4 ,the discharge is therefore effected by openings in the lower part of the gates ,d ,d4 ,these openings are covered by sliding shutters1 which are raised by racks and pinions1 as represented in ,fig4 #fi4 ,when the water has become of the same level in ,a and ,c1 the gates ,,dd can be easily opened1 and the boat may be floated from ,c into ,a4 ,if a boat is to pass downward in the locks1 the process described must be reversed4 ,canals are also extensively used for supplying water by side openings to turn water-wheels for the working of machinery4 ,the water turns the wheel by the force which gravitation gives it as it descends from the level of the canal to the level of the river4 ,illustration3 ,fig4 #gj4 ,illustration3 ,fig4 #ga4 #aah4 ,other ,illustrations4--,we see the tendency of fluids to be on the same level in other ways4 ,in a coffee-pot the liquid has the same level in the spout as in the vessel itself1 whatever may be its position1 as seen in ,fig4 #gj 7p4 #hf74 ,if it be turned up so far that the level of the fluid in the vessel is higher than the outlet of the spout1 the fluid runs out4 ,if two reservoirs of water be connected together the water will stand at the same height in both1 whatever the distance between them may be4 ,so1 also1 in the aqueduct pipes that extend from a reservoir1 the water will rise as high as the surface of the water in the reservoir itself4 ,if the outlets of the pipes be lower than this level the water will run from them1 as in the case of the coffee4 ,the cause of these and similar facts is the same as that of the level surface in vessels and reservoirs--the action of gravitation4 ,this may be made plain by ,fig4 #ga4 ,let the figure represent a vessel with divisions of different degrees of thickness1 these divisions1 however1 not extending to the bottom of the vessel4 ,water in this would stand at the same level in the different apartments1 just as it would if the vessel had no such divisions1 as represented4 ,this is simply because the attraction of the earth acts upon the water in the same way with the divisions as without them4 ,and you can see that it will make no difference whether these divisions be thick or thin1 or whether the apartments be near1 as you see here1 or far apart1 as they are when branch pipes extend from a reservoir4 ,a branch pipe may be considered as having the same relation to the reservoir1 as one of the narrow apartments in the figure has to the rest of the vessel4 ,the result is not at all affected by either the size or form of the tubes that may be connected with a common reservoir--a fluid will stand at the same height in all4 ,thus we have1 in ,fig4 #gb1 tubes of various size and shape1 a b c d e1 connected with a reservoir1 r1 and if water be poured into one of them it will rise to the same height in all1 just as in the different apartments of the vessel represented in ,fig4 #ga4 ,a man once thought that he had gained the great desideratum1 perpetual motion1 by a vessel constructed as in ,fig4 #gc4 ,he reasoned in this way3 ,if the vessel contain a pound of water1 and the tube only an ounce1 as an ounce can not balance a pound1 the water in the vessel must be constantly forcing that in the tube upward4 ,it therefore must constantly run out of the outlet of the tube1 and as it flows into the vessel the circulation must go on1 and the only hindrance to its being a perpetual circulation would be the evaporation of the water4 ,he was confounded when he found1 on pouring water into the vessel1 that it stood at precisely the same level in the vessel and the tube4 ,illustration3 ,fig4 #gb4 ,illustration3 ,fig4 #gc4 #aai4 ,aqueducts4--,the ancients built aqueducts of stone at immense expense1 in some cases spanning valleys at great heights1 to supply their cities with water4 ,at the present day the same object is effected at comparatively small expense with iron pipes laid under ground4 ,no matter how much lower than the reservoir a valley crossed by the pipes may be1 the water flowing through them will rise any where in their branches to the same height as it stands in the reservoir4 ,it is supposed by some that the ancients were not aware of this fact2 but by others that they were aware of it1 and built their immense aqueducts because they had no material for constructing large pipes4 ,illustration3 ,fig4 #gd4 #abj4 ,springs and ,artesian ,wells4--,the principles which ,i have developed in the previous paragraphs will explain the phenomena of springs1 common wells1 and ,artesian wells4 ,the crust of the earth is largely made up of layers of different materials1 as clay1 sand1 gravel1 chalk1 etc4 ,when these were formed they were undoubtedly horizontal1 but they have been thrown up by convulsions of nature in such a way that they present every variety of arrangement4 ,as some of these layers are much more pervious to water than others1 the rain which falls and sinks into the ground often makes its way through one layer lying between two others which are impervious to water1 and so may make its appearance at a great distance from the place of its entrance1 and at a very different height4 ,how this explains the phenomena of springs1 common wells1 and ,artesian wells may be made clear by ,fig4 #gd4 ,a ,a and ,b ,b ,b are designed to represent porous layers of earth lying between other layers which are impervious to water4 ,the water in ,a ,a will flow out at ,c1 making what is commonly called a spring4 ,if we dig a well at ,f1 going down to the porous layer1 ,b ,b ,b1 the water will rise to ,g1 because this is on a level with the surface of the ground1 ,h1 where the supply of water enters4 ,from this point it may be raised by a pump4 ,if the well be dug at ,d1 the water will rise not only to the surface but to ,e1 because this is on a level with ,h4 ,water is sometimes obtained under such circumstances from very great depths4 ,in this case the porous stratum containing the water is reached by boring1 and then we have what is termed an ,artesian well4 ,the name comes from ,artois1 in ,france1 where this operation was first executed4 ,there is a celebrated well of this sort in ,paris over #ahjj feet deep1 and the water rises #aab feet above the surface4 ,more than #fjj gallons are discharged every minute4 ,london1 says ,dr4 ,arnot1 stands in a hollow1 of which the first or innermost layer is a basin of clay1 placed over chalk1 and on boring through the clay 7sometimes of #cjj feet in thickness7 the water issues1 and in many places rises considerably above the surface of the ground1 showing that there is a higher source or level somewhere--probably among the ,surry hills or those north of ,london4 #aba4 ,pressure of ,liquids in ,proportion to ,depth4--,the pressure of a fluid is in exact proportion to its depth4 ,for1 as the particles are all under the influence of gravity1 the upper layer of them must be supported by the second1 and these two layers together by the third1 and every layer must bear the weight of all the layers above it4 ,the increase of pressure at great depths produces the most striking effects4 ,thus if an empty corked bottle be let down very deep at sea1 either the cork will be driven in or the bottle will be crushed in4 ,a gentleman tried the following experiment3 ,he made a pine-wood cork1 so shaped that it projected over the mouth all around4 ,he then covered this with pitch1 and fastened over the whole several pieces of tarpaulin4 ,the bottle1 thus prepared1 he let down to a great depth by attaching to it a weight4 ,on raising it up he found that it contained about half a pint of water strongly impregnated with pitch1 showing that the pressure of the water forced water through the several pieces of tarpaulin1 the pitch1 and the pores of the wooden cork4 ,when a ship founders near land1 the pieces of the wreck1 as it breaks up1 float to the shore2 but when the accident happens in deep water1 the great pressure forces water into the pores of the wood1 and thus makes it so heavy that no part of the vessel will ever rise again4 ,when a man dives very deep he suffers much from the pressure on his chest4 ,if we watch a bubble of air rising in water it is small at first1 but it grows larger as it approaches the surface1 because it sustains less pressure than when it was deep in the water4 ,the force with which a fluid is discharged from an opening in a vessel depends on the height of the fluid above the opening4 ,the difference in this respect between a full barrel and one nearly empty is very obvious4 ,most fishes1 probably1 can not bear the pressure of great depths1 and so are commonly found on the coast1 or on banks1 as they are called1 in the midst of the ocean4 ,illustration3 ,fig4 #ge4 #abb4 ,sluice-,gates1 ,dams1 etc4--,the application of the above principles in the construction of sluice-gates1 dams1 etc41 is a matter of great practical importance4 ,let us look at this4 ,as pressure in a fluid is always in proportion to the height of the fluid above the point of pressure1 the pressure upon any portion of the side of a vessel containing a fluid must be in proportion to its distance from the surface2 or1 in other words1 it is the weight of a column of water extending from this portion to the surface4 ,let ,a ,b ,c ,d 7,fig4 #ge7 represent a section of a cubical vessel1 that is1 one in which each side is of the same size with the bottom4 ,the pressure on the point a1 in the line ,a ,b1 is that of a column of particles1 ,a a4 ,but ,a a is equal to c b1 and c b is equal to b a4 ,therefore b a may represent the pressure on a4 ,in the same way it can be shown that e d represents the pressure on d1 n m the pressure on m1 ,c ,b that on ,b4 ,therefore the pressure on all the points in ,a ,b will be represented by lines filling up all the triangular space ,a ,b ,c1 and this is half of ,a ,b ,c ,d1 which represents the pressure on the line ,c ,b4 ,it is clear1 then1 that as the pressure on a vertical line in the side is half that on a line at right angles to it in the bottom1 the pressure on the whole side is half that on the whole bottom4 ,illustration3 ,fig4 #gf4 ,we see from the above demonstration why it is that a dam is built in the form represented in ,fig4 #gf4 ,we see1 also1 why in the monstrous vats in some of the ,english breweries 7some of them holding many thousand barrels7 the hoops and other securities at the lower part of them require to be made of very great strength4 ,it is manifest1 also1 that if a sluice-gate is to be kept shut by a single support1 this must be applied at one third of the distance from the bottom1 there being as much pressure1 as seen by ,fig4 #ge1 on the lower third as on the upper two thirds of the gate4 #abc4 ,lateral ,pressure in ,fluids4--,the pressure of a liquid on the side of a vessel1 of which ,i have spoken above1 is a lateral pressure1 and it is caused by the downward pressure of gravitation in the liquid4 ,but how8 ,the particles of a fluid are freely movable among each other1 and therefore are ready to escape from pressure in any direction4 ,the particles at a1 ,fig4 #ge1 pressed upon by the column of particles extending above them to the surface1 are ready to escape laterally1 and would do so if there were an opening made in the vessel at that point4 ,but if the vessel contained a block of ice1 fitting it as accurately as the body of water1 there would be no escape at the opening1 because the particles of the solid are so held together that the downward pressure of the earth's attraction occasions no lateral pressure4 ,illustration3 ,fig4 #gg4 ,illustration3 ,fig4 #gh4 ,the manner in which the downward pressure of the earth's attraction causes lateral pressure may be made clear by ,figs4 #gg and #gh4 ,we will suppose that the particles of solids and liquids are alike round1 and that a solid differs from a liquid only in having its particles firmly united by attraction4 ,let a1 b1 and c1 in ,fig4 #gg1 represent three particles of a solid4 ,as they are united firmly they will have a united pressure from the centre of gravity directly toward the centre of the earth1 as represented by the arrow4 ,let now d1 e1 and f1 ,fig4 #gh1 represent three particles of water4 ,these being but very slightly coherent1 will make each an independent pressure toward the earth's centre1 as indicated by the arrows4 ,it is plain that d tends to separate e and f1 and will do so if they are left free to move in a lateral direction4 ,for example1 if e be at the side of a vessel1 and an opening be made there1 the downward pressure of d will give e a lateral movement1 forcing it out of the opening4 #abd4 ,another ,view4--,to return to ,fig4 #ge1 observe that the lateral pressure at any point in the side of a vessel1 as a1 is occasioned wholly by the downward pressure of a vertical column of particles extending from that point to the surface4 ,the neighboring columns of particles have nothing to do with it4 ,the same thing is true in regard to any other point either in the line ,a ,b or another line drawn on the side of the vessel4 ,it is therefore true of the whole side1 that the pressure upon it is occasioned alone by the columns of particles that are in close proximity to the side1 and not at all by the other columns of particles in the vessel4 ,the number of these columns1 therefore1 in the vessel1 or1 in other words1 the breadth of the body of water in it1 makes no difference with the pressure on its side4 ,for this reason two flood-gates so little apart that a few hogsheads or even pails of water fill up the space between them1 are as much pressed upon as they would be if a lake or an ocean of water lay between them4 ,it has been objected to the project of digging a ship canal between the ,red ,sea and the ,mediterranean1 that as the water in the former is twenty feet higher than in the latter1 it would burst through the flood-gates with such force as to produce most disastrous results4 ,but according to the principle which ,i have illustrated1 there would be no more danger of this than there would be if two ponds were united by a canal1 in one of which the water is twenty feet higher than in the other4 ,illustration3 ,fig4 #gi4 #abe4 ,pressure in ,liquids ,equal in all ,directions4--,we are now prepared to go a step farther4 ,the pressure occasioned by gravitation in fluids operates equally in all directions when the fluid is at rest4 ,that is1 any particle of a liquid is pressed equally in all directions4 ,if it were not so it would not remain at rest1 but would be moved in the direction in which the superior pressure operates4 ,suppose that a1 ,fig4 #gi1 is a stratum of particles in a vessel containing water at rest4 ,the upward pressure on it being equal to the downward pressure1 the stratum neither rises nor falls4 ,if a body of liquid be disturbed by wind or any other cause1 those particles which are raised above the common level in waves are pressed downward more than upward or laterally in obedience to the action of gravitation4 ,they therefore move downward1 pushing laterally and upward the neighboring particles1 till the liquid regains its level surface and its state of rest4 ,so1 also1 if any particles become heated they are lighter than their neighboring particles1 and the latter being more strongly attracted than the former1 push them upward in order to take their places4 ,when all the liquid comes to have the same temperature it is at rest1 each particle having an equal pressure upon it in all directions4 ,illustration3 ,fig4 #hj4 #abf4 ,illustrations4--,if a bladder filled with water be compressed by the hand1 the water is pressed no more immediately under the hand than in any other part of the bladder1 and wherever an opening be made the water will rush out with equal readiness4 ,a hose-pipe as readily bursts upward as in any other direction4 ,a large cork1 if sunk in very deep water1 will be uniformly reduced in its dimensions1 showing that it has been pressed equally on all sides4 ,in the experiments with the closed bottles 7 #aba71 the result is the same if the bottle be so sunk as to have its mouth downward4 ,if two tubes1 shaped as in ,fig4 #hj1 be thrust down into water1 the water will rise with equal facility in both1 although in the straight one the pressure which carries up the water is wholly upward1 while in the bent one it is at the first downward4 ,illustration3 ,fig4 #ha4 #abg4 ,upward ,pressure as the ,depth4--,it has been shown that the downward and the lateral pressures are as the depth4 ,the same is true of the upward pressure1 for it is produced by the same cause--the attraction of the earth4 ,let us look at this4 ,why is any particle of a fluid pressed upward at all8 ,it is from the struggle on the part of the neighboring particles to get below it4 ,and why this struggle8 ,it is from the attraction of gravitation1 and so the greater this attraction the greater is the upward as well as the downward pressure4 ,the upward pressure therefore differs at different depths as the downward pressure does4 ,thus1 in ,fig4 #ha1 the upward pressure against the layer or stratum of particles1 b1 is greater than that against a1 for the same reason that the downward pressure on b is greater than that on a4 ,but the two pressures at b are equal1 and so are they at a1 and therefore each stratum remains at rest4 ,illustration3 ,fig4 #hb #abh4 ,experiments4--,some very neat experiments can be tried1 showing that the upward pressure varies with the depth4 ,take a large glass tube1 ,a ,b ,c ,d1 ,fig4 #hb1 and let there be fitted to one end a circular plate of brass1 which may be held there by a string1 ,f4 ,thus arranged1 plunge it quite deep into water1 and you will find that you will not need to hold on to the string1 for the brass disk will be held tight to the tube by the upward pressure of the water4 ,now draw up the tube slowly1 and at length the disk will fall from the end of the tube4 ,why8 ,because the end of the tube has come to a point where the upward pressure of the water is less than the downward pressure of the disk4 ,to have this experiment succeed1 the end of the tube where the disk is applied must be very even and smooth4 ,another experiment may be tried in this way4 ,tie to one end of a glass tube a piece of thin ,india-rubber or bladder1 and fill the tube partly with water4 ,the ,india-rubber will of course bulge out or be convex from the weight of the water4 ,press the closed end down a little way in a vessel of water1 so that the level in the tube shall be above the level in the vessel4 ,the ,india-rubber is still somewhat convex1 because1 as the upward pressure upon it is in proportion to its distance from the surface of the water outside of the tube1 it is not as great as the pressure downward of the higher water in the tube4 ,push the tube now so far down that the level in the tube is the same with that in the vessel4 ,the ,india-rubber is now flat1 because the downward and upward pressures upon it are equal1 just as would be the case with a stratum of water in place of it4 ,but press the tube lower down1 and the ,india-rubber bulges upward into the tube1 because the upward pressure is now greater than the downward4 #abi4 ,great ,effects from ,small ,quantities of a ,fluid4--,you are now prepared to understand the explanation of some very striking phenomena in the pressure of liquids4 ,if you take a perfectly tight cask1 and1 filling it with water1 screw into its top a long tube1 by pouring water into the tube you can burst the cask4 ,to understand this you must bear in mind two facts--that the fluid in the cask is not compressible1 and that its particles move freely among each other4 ,any pressure1 therefore1 exerted upon it is felt through the whole of it equally4 ,if the tube1 says ,dr4 ,arnot1 have an area of a fortieth of an inch1 and contain when filled half a pound of water1 this produces a pressure of half a pound upon every fortieth of an inch all over the interior of the cask2 which is more than a common cask can bear4 ,suppose a small reservoir of water exists in the side of a mountain wholly closed up1 and that water from a height above finds its way to it by a crevice1 it may by its pressure even burst open the side of the mountain4 ,and it matters not how large or small the crevice may be1 for pressure in a liquid is only as the height4 ,if the reservoir be ten yards square and an inch deep1 and the fissure leading to it be but an inch in diameter and two hundred feet in height1 it is calculated that the pressure of the water in the fissure would be equal in force to the weight of #ejjj tons4 ,illustration3 ,fig4 #hc4 #acj4 ,explanation4--,the manner in which these effects are produced may be made clear by ,fig4 #hc4 ,let ,a be a close vessel filled with water1 and let a tube1 b1 be made fast in it1 with a movable plug or piston at c4 ,if the surface of the water be pressed upon by this piston with the force of a pound1 as the water is incompressible and its particles are freely movable among each other1 the pressure will be extended equally through all the water1 and every portion of the vessel of equal extent with the tube's opening at c will be pressed upon with the force of a pound4 ,if another tube1 d1 of the same size were inserted with a piston1 i1 the force of a pound applied to the piston c would push upward the piston i with the same force4 ,and if there were several pistons of the same size1 by pushing upon one with the force of a pound they would all be pressed upward with exactly this force4 ,farther1 if e be a tube five times as large as b1 its piston1 n1 will be forced upward with a pressure of five pounds by the downward pressure of a pound upon c4 ,suppose now that a pound of water were substituted for the piston c1 the other pistons would be pressed upward as before4 ,and if all the pistons be removed1 the pound of water in b will press the water up the tube d with the force of a pound1 and up the tube e with the force of five pounds4 ,illustration3 ,fig4 #hd4 ,to make this still more clear ,i will present it in a little different form4 ,let ,b1 ,fig4 #hd1 be a close vessel with two tubes1 one of which is five times as large as the other4 ,if sufficient water be poured into the vessel to occupy a part of the tubes1 it will stand at the same height in both tubes1 as indicated4 ,if there be a pound of water1 then1 in the tube c1 there will be five pounds in a4 ,now if the five pounds of water in a made any more pressure on the whole body of water in ,b than the pound of water in c does1 it would press up the water in c to a greater height4 ,but this is impossible1 as has been shown in #aah4 ,observe that the five pounds of pressure in a is spread over five times the area or extent of surface that the pound's pressure in c is4 ,if the tube c have an area of an inch square1 the water in it will exert a pressure of a pound on every square inch in the vessel4 ,the water in a exerts a pressure of five pounds2 but it must be remembered that it does not press with this force on every square inch1 but on every space of five square inches1 and that therefore its pressure on every inch is the same as that in the tube c4 #aca4 ,hydrostatic ,paradox4--,you see in the phenomena and explanations given above that a small quantity of a fluid can1 under certain circumstances1 exert an enormous pressure4 ,this fact has been called the ,hydrostatic ,paradox4 ,it does seem1 at first view1 incredible or paradoxical1 when one asserts that a few ounces of water can be made to raise weights of hundreds or even thousands of pounds4 ,but the explanations which ,i have given show you that there is no unexplainable mystery in the fact4 ,the cause of it is the same as that which gives a level surface to liquids2 viz41 the force of gravitation acting upon a substance whose particles are freely movable among each other4 ,illustration3 ,fig4 #he4 #acb4 ,hydrostatic ,bellows4--,the instrument called the ,hydrostatic ,bellows is represented in ,fig4 #he4 ,it consists of two circular boards1 ,a and ,b1 united together by strong leather1 and having a tube1 ,c1 through which water can be poured into it4 ,the amount of weight which can be sustained on the bellows without forcing the water out of the tube depends on the size of the bellows4 ,if the area of the tube is only one thousandth of that of the top of the bellows1 a pound of water in the tube will balance a thousand pounds' weight on the bellows4 ,it is for the same reason that in ,fig4 #hd one pound of water in the tube c balances five pounds in a4 ,as the weight presses upon the top as a whole1 it is the same as if there was a vessel of the same size with the bellows resting upon it and containing a thousand pounds of water4 ,the water1 in that case1 would stand at the same height in the vessel and the tube4 ,this shows that the ,hydrostatic ,paradox is only one of the exemplifications of the great fact that a fluid1 from the influence of gravitation1 seeks to be on a level4 ,it is the water in the bellows seeking to be on a level with that in the tube that causes the upward pressure sustaining the weight4 ,when the weight on the bellows is less than is required to balance the water in the tube1 the weight can be raised continually by pouring water into the tube4 ,but observe that although the lifting force be so strong1 it is very slow in its operation4 ,if the comparative areas of the tube and the bellows be as above supposed1 the water must fall in the tube ten inches in raising the weight the one hundredth part of an inch4 ,illustration3 ,fig4 #hf4 #acc4 ,bramah's ,hydrostatic ,press4--,the principles which ,i have elucidated have been applied by ,mr4 ,bramah in his ,hydrostatic ,press4 ,this consists of a small metallic forcing-pump1 ,fig4 #hf1 in which the water1 a1 is pumped up by the piston1 s1 worked by the lever1 c b d1 and forced into a strong and large cylinder1 ,a4 ,in this cylinder is a stout piston1 ,s1 having a flat head1 ,p1 above4 ,between this plate and another1 ,r1 is placed the body1 ,w1 which is to be compressed4 ,it is obvious that the pressure exerted will be in proportion to the difference between the size of the pump1 a1 and the cylinder1 ,a1 just as in the case of the bellows1 it depended on the difference between the areas of the tube and of the top of the bellows4 ,in the press the force of a pump is substituted for the pressure of a very high column of water1 simply because it is more convenient4 ,this press is of great service in the mechanic arts4 ,it is used in pressing paper1 cloth1 hay1 cotton1 etc4 ,it has also been recently used in raising enormous weights4 ,the tubes of the celebrated bridge over the ,straits of ,menai were raised by a machine constructed on this principle4 ,,chapter ,,viii4 ,,specific ,,gravity4 #acd4 ,nature of the ,subject4--,we now come to a very interesting subject1 which is at least intimately connected with ,hydrostatics1 if it may not be considered a part of it4 ,the principles which have been developed in the chapter on ,hydrostatics in relation to liquids are to be applied here to various kinds of substances4 ,and as we proceed you will see that all the phenomena brought to view in this chapter are to be referred to the same cause with those of the previous chapter2 viz41 the attraction of gravitation4 #ace4 ,specific ,gravity ,defined4--,before proceeding with the investigation ,i will give you the definition of specific gravity4 ,the specific gravity of any substance is its weight as compared with the same bulk or volume of other substances4 ,water is taken as a standard1 and its specific gravity is for convenience called #a4 ,mercury1 then1 is said to have a specific gravity of #ac4e1 for it is thirteen and a half times as heavy as the same volume of water4 ,it is easy to see how the specific gravities of different fluids may be ascertained4 ,one mode1 and the most obvious one1 is to weigh in a vessel equal quantities of them4 ,in what way the specific gravities of solids are ascertained will be explained in another part of this chapter4 #acf4 ,action of ,gravity on ,solids in a ,liquid4--,the reason that a very heavy substance1 as a stone1 sinks in water is simply that the earth attracts it more strongly than it does the water1 and so drags the stone down through it4 ,if the stone lay upon a bladder filled with water1 it would press upon it with the force with which it is attracted by the earth4 ,but where water is not thus confined1 the stone thrusts its particles to the one side and the other till it gets to the bottom4 ,it is the attraction of gravity1 also1 that makes light substances1 as wood and cork1 rise in water4 ,in this case the water is attracted by the earth more strongly than the wood or cork1 and so gets below it1 and in so doing pushes the lighter substance up above itself4 ,illustration3 ,fig4 #hg4 ,but you will observe that the wood1 on rising in the water1 does not come completely out of it and lie upon the surface1 but a part of it remains immersed in the water4 ,the explanation of this will furnish you with the key to the understanding of many very interesting facts4 ,suppose that half of a block of wood1 ,a1 ,fig4 #hg1 weighing a pound1 is above the surface of water4 ,as it is attracted to the earth with the force of a pound1 it has pushed to the one side and the other just a pound of water1 and taken its place4 ,it is drawn down toward the earth with the same force with the pound of water on either side of it1 b or c4 ,if it were attracted any more than with the force of a pound1 that is1 if it weighed more than a pound1 it would displace more than a pound of water4 ,if it were of just the same weight with the same volume of water1 it would displace a volume of water equal to itself2 it would be wholly immersed1 and would stay any where in the water1 wherever you placed it1 because it is attracted by the earth with the same force that the same bulk of water is4 ,illustration3 ,fig4 #hh4 #acg4 ,farther ,explanation4--,suppose water in a vessel divided into equal portions of a pound each1 as represented in ,fig4 #hh4 ,now suppose that the portion a should at once change into solid ice without at all altering its bulk or weight4 ,it would not move from its position1 because it is attracted by the earth precisely as much as when it was water1 and as much as is each of the equal portions of water around it4 ,but as water on becoming ice does really increase in bulk1 and therefore become lighter1 this block of ice would rise so that a part of it would be above the surface4 ,illustration3 ,fig4 #hi4 ,the lighter a substance is that is immersed in water1 the more there will be of it above the surface4 ,take two blocks of wood of different weights though of the same size4 ,suppose the heaviest one1 ,a1 ,fig4 #hi1 is one third lighter than the same bulk of water4 ,one third of it will be above the surface4 ,if the other1 ,b1 is half the weight of water1 half of it will be above the surface4 ,we should say1 then1 that the specific gravity of the wood in the first block is two thirds of that of water1 and the specific gravity of the wood in the second is one half that of water4 #ach4 ,illustrations4--,there are many interesting facts that illustrate the principles which ,i have developed4 ,a stone is lifted much more easily in water than in air1 because of the support afforded by the upward pressure of the water4 ,a boy will often wonder why he can lift a very heavy stone to the surface1 but can get it no farther4 ,when a bucket of water is drawn up a well much less exertion is required to raise it through the water than through the air after it emerges from the water4 ,while it is in the water you raise only the bucket itself1 the water in it having no weight1 being sustained by the water around it4 ,but when it comes to the air you have the weight of the water added to that of the bucket4 ,when a person lies in a bath for some time1 on raising his arm from the water it seems to be very heavy4 ,the reason is1 that it has had for so long a time the support of the water that when it is lifted into the air the want of this support is sensibly felt1 just as we perceive the difference between raising a bucket of water through water and raising it through air4 ,it is said that ,archimedes took in the full idea of the principles of specific gravity as his limbs felt the liquid support of a bath1 and so overjoyed was he with the discovery1 that he ran home crying out all the way1 6 6--,i have found it6 ,i have found it6 ,it was a rational joy1 for he had found a principle of immense value to science and to the world4 #aci4 ,boats and ,life-,boats4--,a boat of iron will float with as much of it out of water as one of wood of the same size1 provided that the iron be made so thin that the boat is not heavier than the wooden one4 ,for what is it that floats8 ,not the iron or wood1 but a wooden or iron boat filled with air4 ,if it were filled with water instead of air it would sink1 the specific gravity of the materials of which it is built being on the whole of greater specific gravity than water4 ,life-boats have in their structure either a large quantity of cork or air-tight vessels of tin or copper1 and in this way they are made so light that they will float even when filled with water4 ,as the weight of a body can be estimated from the quantity of water which it displaces1 we can very readily estimate the weight of the load of a canal-boat1 as its form is so simple and regular4 ,in order to do this we must first know how far the boat sinks in the water when empty1 or1 in other words1 how much water it displaces4 #adj4 ,specific ,gravity of ,animals4--,birds have a much less specific gravity than animals that walk1 in order that they may mount up easily in the air4 ,their light feathers increase greatly their bulk1 as you may see whenever a bird is stripped of them4 ,besides this1 the bones are hollow and communicate with the lungs4 ,birds that swim1 as ducks1 swans1 etc41 have so small a specific gravity--that is1 are so large in proportion to their weight--that but a small part of the body is under water1 and the motion of their feet is not required at all to sustain them1 but only1 like the action of oars1 to propel them along4 ,insects are of small specific gravity1 those that fly the most swiftly being the lightest4 ,fishes are very nearly of the same specific gravity with water1 and hence require but little muscular effort to move about in their element4 ,they are assisted much in rising and falling by a contrivance by which they can instantaneously alter their specific gravity4 ,they have an air-bladder1 which they can dilate or contract at pleasure4 ,when dilated1 the bulk of the fish is increased and his specific gravity lessened1 and he rises easily and at once4 ,by compressing it he as readily sinks4 #ada4 ,specific ,gravity of the ,human ,body4--,the human body1 when the chest is filled with air1 is so much lighter than water that it will float with about half the head above the surface4 ,a knowledge of this fact1 with proper presence of mind1 might ordinarily save persons from drowning2 for if the body be put in the proper position1 the feet downward and the head thrown backward1 the nose and mouth will be out of the water4 ,so little is required in the way of support to keep the whole head out of water1 that persons who can not swim are often saved from drowning by catching hold of very small pieces of wood4 ,an oar would support half a dozen men1 if they would be satisfied with keeping only the head out of water2 but if each one struggle to get his whole body upon the oar1 they may all be lost4#a ,a life-preserver is a great aid in preservation from drowning1 for it diminishes the specific gravity of the body4 ,it is commonly an air-tight bag fastened round the upper part of the body1 which can be filled by blowing into it through a pipe which has a valve in it4 ,on the great rivers of ,china1 says ,dr4 ,arnot1 where thousands of people find it more convenient to live in covered boats upon the water than in houses on the shore1 the younger male children have a hollow ball of some light material attached constantly to their necks1 so that in their frequent falls overboard they are not in danger4 ,when a person is drowned the body sinks because in the struggle much of the air in the lungs is lost1 just as the fish sinks when his air-bladder is contracted4 ,it is1 however1 so little heavier than water after this is done1 that it very readily rises when any gas is produced in it by putrefaction4 ,it is a common popular notion that firing cannon over the water will raise the drowned4 ,but it can produce no effect1 unless perhaps the agitation caused by the concussion may hasten a very little the rising of a body which from commencing putrefaction is about to rise4 ,in wading a river the feet press upon the bottom with only a force equal to the weight of half the person's head1 this being the difference between the weight of the body and the weight of the same bulk of water4 ,now this pressure is not sufficient to give a sure footing against even a moderate current4 ,many persons have been drowned from ignorance of this fact4 ,a man carrying a load may often ford a river safely where without a load to press him down1 and thus give him a sure footing1 he would be carried down the stream4 ,so a man may walk in deep water upon broken glass with impunity4 ,illustration3 ,fig4 #ij4 #adb4 ,how to ,ascertain the ,specific ,gravity of ,solids4--,it results from the upward pressure of water that a body weighs less in water than in air4 ,take a piece of gold or any other substance1 a1 ,fig4 #ij 7p4 #ajg71 and weigh it suspended as you see from one of the scales4 ,introduce the gold now into a cup of water1 and you will find that a part of the weight must be taken from the opposite scale to preserve the balance4 ,the weight which you take from the scale will be the weight of a quantity of water equal in bulk to the piece of gold2 for the immersed body is supported with a force equal to the weight of the water it displaces 7 #acg74 ,by comparing1 therefore1 its weight in water with its weight in air we determine its specific gravity4 ,thus if a lump of gold weigh nineteen ounces1 and on being weighed in water weighs eighteen1 it will prove that gold is nineteen times as heavy as water4 ,and if a lump of copper weigh nine ounces in air and eight in water1 it is nine times as heavy as water4 ,calling water1 therefore1 #a1 the specific gravity of gold is #ai1 and of copper #i4 ,it is obvious that a body of the same specific gravity with water would weigh nothing when immersed in water1 for it would be supported with an upward pressure precisely equal to its own weight1 just as the same bulk of water is4 ,a pound of water1 therefore1 will weigh nothing in water4 ,the experiment can easily be tried4 ,weigh a glass bottle1 suspended on one arm of the scale-beam1 and then put a pound of water in it4 ,on immersing it in water it will be balanced if you take out the pound weight in the opposite scale4 #adc4 ,archimedes and the ,crown4--,hiero1 ,king of ,syracuse1 stipulated for a crown of pure gold4 ,but suspecting the maker of it of adulterating the gold1 he called upon ,archimedes to detect the imposture4 ,he did it in this way3 ,he procured two lumps of gold and silver of the same weight with the crown1 and observed the quantity of water which each displaced4 ,he then tried the crown1 and found that it displaced less than the silver and more than the gold1 and therefore concluded that it was an alloy of the two metals4 ,all this was suggested to him by his experience in the bath1 referred to in #ach4 ,illustration3 ,fig4 #ia #add4 ,how to ,ascertain the ,specific ,gravity of ,liquids4--,there are several modes of ascertaining the specific gravities of different liquids4 ,the instrument called a ,hydrometer furnishes the most common mode4 ,this is used chiefly in determining the quality of spirit4 ,the more alcohol and the less water spirit contains the less is its specific gravity4 ,the ,hydrometer consists of two bulbs of glass1 ,a ,b1 ,fig4 #ia1 with a slender stem1 ,c1 which is graduated4 ,in the lower bulb are a few shot or a little mercury1 to give the instrument its proper weight1 and to make its centre of gravity to be in the lower part4 ,the lighter the fluid to be tested is the lower will the instrument sink in it4 ,this is a very accurate instrument1 detecting the slightest adulteration in spirits4 ,dr4 ,arnot tells an amusing story of the detection of a ,chinese trader in liquors4 ,he had sold a quantity of liquor to the purser of a ship1 averring it to be of the same quality with a sample which he had given him4 ,the purser tried it with his ,hydrometer1 and found it to be of greater specific gravity than the sample4 ,the ,chinese promptly denied the fraud2 but on being told the exact quantity of water which he had added1 he was so much confounded that he immediately confessed his guilt1 and made ample amends4 ,when the ,hydrometer was shown to him he offered a large price for what appeared to him to be a magical instrument1 foreseeing that it would be of great advantage to him in his business4 ,in ,switzerland and in the north of ,italy1 where the peasants bring their milk to a common dairy1 and are allowed a quantity of cheese at the end of the season in proportion to the amount of milk which they have brought1 a ,hydrometer is used to test the quality of the milk4 ,there is a propriety in this1 not only as a safe-guard against adulteration1 but because there is a difference of quality in the milk of different cows1 some giving a much more watery milk than others4 #ade4 ,centre of ,gravity in ,floating ,bodies4--,the same principles which apply to the centre of gravity in bodies standing on a firm basis apply also to floating bodies4 ,that the centre of gravity may be low in a loaded vessel the heavy part of the cargo is put underneath1 and generally ballast of stone or iron is necessary for the same purpose4 ,in large flat-boats1 the base of support being extensive1 there is not the same need of taking care that the centre of gravity be low4 ,if a ship be laden in part with an article which will dissolve in water1 there is much danger1 if the ship should leak1 that this portion of the cargo shall be dissolved and be pumped out with the bilge-water1 thus altering the trim of the vessel1 or removing the centre of gravity from over the middle line1 and bringing it too far forward1 or carrying it too far back1 making the ship wholly unmanageable4 ,four large ,english ships1 in part loaded with saltpetre1 were supposed to be lost from this cause in #ahji off the ,isle of ,france4 ,the immense ice-islands1 or icebergs1 which float about in summer in the polar regions1 by melting irregularly often change the place of their centre of gravity1 and in turning over present one of the most sublime spectacles in nature4 ,a mountain of ice1 extending high in the air and deep in the sea1 suddenly turns over1 and produces a rolling of the ocean which is often felt at the distance of many leagues4 ,,chapter ,,ix4 ,,pneumatics4 #adf4 ,what ,pneumatics ,teaches4--,as ,hydrostatics treats of the pressure and equilibrium of liquids1 ,pneumatics treats of the same in air and the gases1 or aeriform substances4 ,the name comes from the ,greek word 1 meaning air1 breath1 spirit4 ,illustration3 ,fig4 #ib4 #adg4 ,air ,material and has ,weight4--,that air is a material substance has been already proved to you1 for it was shown in #df that it has impenetrability1 one of the essential properties of matter4 ,it has extension also1 for bodies of air can be obtained in various shapes confined in vessels1 so that we can speak of cubes and spheres of air2 and besides1 the ultimate atoms 7 #ae7 of air must have shape or extension4 ,that air has weight can be proved by weighing it as you would any other substance4 ,let a hollow globe1 ,a1 ,fig4 #ib1 having a neck with a stop-cock1 ,b1 be emptied of air and weighed4 ,if now you open the stop-cock1 and so let in the air1 the other beam of the scale will rise1 because the globe is heavier than it was before4 ,the additional weight required to make the scales balance will indicate the weight of the air which the globe contains4 ,it is one eight hundredth 7#a_/hjj7 of the weight of the same volume of water4 ,how the globe can be emptied of the air will be shown in another part of this chapter4 #adh4 ,air ,attracted by the ,earth4--,the weight of the air is simply the result of the attraction of the earth 7 #eb74 ,air is attracted by the earth just as water is2 and the water takes its place below air because it is attracted more strongly than the air4 ,it is from the attraction of the earth that air descends into any hollow spot in the earth when water is removed from it4 ,it takes the place of the removed water because from the influence of attraction it gets as near to the earth as possible4 ,if you put into a vial mercury1 water1 and oil1 the mercury will be at the bottom1 because it is more strongly attracted by the earth than the other fluids4 ,the water will be next1 then the oil1 and lastly1 over all1 there is air1 that being less attracted than any of the other substances4 ,it is this attraction of the air by the earth that gives us the chief phenomena of ,pneumatics4 #adi4 ,why ,some ,things ,fall and ,others ,rise in ,air4--,most substances fall in air for the same reason that very heavy substances sink in water4 ,they fall because the earth attracts them more strongly than it does the air4 ,the reason that some substances rise in air is precisely the same as that given in #acf for the rising of substances in water4 ,the air being attracted more strongly than they are pushes them up to get below them1 as cork or wood is pushed up by water4 ,thus a balloon filled with hydrogen gas rises in air for the same reason that a bladder filled with air rises in water4 ,so1 also1 smoke rises in air1 just as oil rises in water4 ,illustration3 ,fig4 #ic4 #aej4 ,thickness of the ,earth's ,air-,covering4--,the air makes a covering for the earth about fifty miles deep4 ,if the earth were represented by a globe a foot in diameter1 the air might be represented by a covering a tenth of an inch in thickness4 ,the line a1 ,fig4 #ic1 gives us the curve of the surface of such a globe1 and the space between a and b represents the comparative thickness of the covering of air4 ,this is ascertained by calculation from the pressure of the air upon the earth4 ,it is just as the depth of water may be calculated from the pressure which it makes4 ,we do not take this mode of ascertaining the depth of water1 because we can measure it from the surface by sounding4 ,but we should be obliged to adopt it if we lived at the bottom of water1 as we do at the bottom of the sea of air4 #aea4 ,how the ,air-,covering ,adheres to the ,earth4--,the earth flies on in its yearly journey around the sun at the rate of #aajj miles per minute1 and yet it holds on to this loose airy robe by its attractive force1 so that not an atom of it escapes into the surrounding ether4 ,of itself it is disposed to escape2 and it would do so1 and be diffused through space1 if the attraction of the earth for it were suspended4 ,for1 unlike liquids1 the air has no disposition to keep together2 that is1 there is no attraction between its particles4 ,on the other hand1 there is a repulsion1 so that they are disposed to keep far apart1 and are kept together only by pressure4 ,it is the pressure of the earth's attraction that keeps them together to the extent of fifty miles all around it4 #aeb4 ,compressibility of ,air4--,in looking at the influence of gravitation upon air1 it must be remembered that air is very compressible1 while water is very nearly incompressible4 ,while1 therefore1 in a body of water the particles are very little nearer together at the bottom than at the surface1 the particles of the air are much nearer together close to the earth than they are far away from it4 ,for as all the particles of the air are attracted or drawn toward the earth1 those below are pressed together by the weight of those above4 ,the air is therefore thinner as we go up from the surface of the earth1 and in the outer regions of the sea of air it is too thin to support life4 ,even at the tops of very high mountains1 or the heights sometimes reached by balloons1 disagreeable effects are often experienced from the thinness of the air4 ,the air has been compared1 in regard to its varying density at different heights1 to a heap of loose compressible substance2 as1 for example1 cotton-wool1 which is quite light at the top1 but is pressed more and more together as you go toward the bottom4 ,hydrogen gas has only one fifteenth the weight of air at the surface of the earth2 and therefore the hydrogen balloon rises till it reaches a height where the air is so thin that the balloon is of the same weight with an equal bulk of air1 and there it stops4 #aec4 ,in what ,aeriform ,substances and ,liquids are ,alike4--,you have seen in #cf and #ch how the air and gases differ from liquids4 ,but in one very important respect they are alike1 viz41 the movability of their particles4 ,hence pressure is in air1 as well as in water1 equal in all directions1 so that in the experiment with the bladder1 in #abf1 it makes no difference in the result whether there be water or air in it4 ,for the same reason pressure is as the depth in aeriform substances as in liquids1 and the laws of specific gravity apply to the one as well as to the other4 ,you are now prepared to understand the results of the action of gravitation upon air and the gases2 or1 in other words1 the principal phenomenon of ,pneumatics4 #aed4 ,pressure of the ,atmosphere4--,the amount of the pressure of the atmosphere is very readily estimated1 the mode of doing which ,i will speak of in another part of this chapter4 ,it presses with a weight of fifteen pounds on every square inch4 ,suppose that you extend your outspread hand horizontally in the air4 ,you feel no pressure upon it1 but there is a pressure of some two or three hundred pounds of air upon it4 ,if your hand be five inches long and three broad it presents a surface of fifteen square inches1 on every one of which the atmosphere is pressing with the weight of fifteen pounds4 ,that is1 there is a pressure on the upper surface of your hand of a column of air weighing #bbe pounds4 ,so1 also1 on the lid of a box only thirty inches square1 there is a pressure of #ac1ejj pounds4 ,the whole pressure on the body of a man of common size is about fifteen tons4 ,but why is it that the lid of the box is not broken in1 your hand not borne down1 and your body not crushed8 ,it is simply from the fact1 shown in the previous chapter in regard to liquids1 and in this in regard to aeriform substances1 that the pressure is equal in all directions4 ,the lid and the outspread hand are therefore balanced by an upward pressure equal to the downward1 and the body has the pressure on all sides the same4 ,if the air could be removed from within the box the lid would be crushed in2 if from under the hand1 that would be borne down2 and if from one side of the body1 the body would be forced violently in that direction till it met with an opposing pressure4 ,but besides this equal pressure of the air on all sides1 there is air within the pores and interstices of all bodies that are not very dense1 and its particles are subject to the same laws as are those on the outside4 ,all this can be made clear to you by the air-pump4 ,illustration3 ,fig4 #id4 #aee4 ,air-pump4--,in ,fig4 #id you have a representation of an air-pump as commonly arranged4 ,at a a are two pump-barrels1 the pistons in which are worked by means of the handle1 b4 ,those pumps are very nicely made1 and the frame-work1 d e d e1 to which they are attached1 is very strong and firm1 so that the pumps may work evenly4 ,there is a large1 smooth1 metallic plate1 f4 ,at c is a bell-shaped glass vessel1 close at the top1 but open at the bottom1 the edge of which is ground very true1 so that it may fit exactly on the metallic plate4 ,in the middle of the plate is an opening which leads to the pump-barrels1 and it is through this that the air is pumped out of the glass receiver1 c4 ,if we wish to let the air in after we have pumped it out we loosen the screw at g1 for from the opening here is a passage to the opening in the middle of the plate4 ,illustration3 ,fig4 #ie4 ,the operation of the air-pump can be made clear by the plan in ,fig4 #ie4 ,but one pump-barrel1 a1 is represented1 with a piston1 c1 working in it4 ,in the piston there is a valve1 i1 opening upward1 and also one at b1 in the beginning of the passage leading to the centre of the plate where is the receiver1 d4 ,the working of the instrument is thus3 ,if the piston be forced down1 the air under it1 being compressed1 will close the valve at b1 and will rush upward through the valve i in the piston4 ,let the piston now be raised2 the resistance of the air above it will close the valve i1 while the valve b will be opened by the air rushing from the receiver1 d1 through the passage1 e1 to fill the space between the piston and b4 ,you see1 then1 that every time that the piston is drawn up air passes out of the receiver through the valve b into the space between this valve and the piston4 ,none of this air which has passed out can go back again1 for the moment that you press upon it by forcing downward the piston the valve b is shut down1 and the air escapes from the pressure by passing out through the valve i4 ,each time1 therefore1 that you work the piston up and down you pump out some of the air from the receiver2 and if you pump for some time there will be exceedingly little air left in it1 and that will of course be diffused throughout the receiver4 ,it will be thin1 like that in the upper regions of the atmosphere4 ,illustration3 ,fig4 #if4 ,fig4 #ig4 ,illustration3 ,fig4 #ih4 #aef4 ,experiments4--,when the receiver is full of air it can be moved about on the plate easily1 and can be lifted from it4 ,but work the pumps a few strokes and you will find that the receiver is firmly fastened to the plate1 for the air within1 being made thin1 presses with little force compared with the air outside4 ,if the pumps be worked for some time no force could release the receiver from the pressure without breaking it4 ,but loosen the screw1 g1 and thus let the air in1 and the equality of the pressure on the outside and inside is at once restored4 ,take off now this large receiver1 and place a small glass jar1 open at both ends1 on the plate1 with the hand covering the upper opening1 as represented in ,fig4 #if4 ,on exhausting the air the hand is so firmly pressed into the glass that it requires considerable force to disengage it from the pressure4 ,if we tie a piece of bladder or ,india rubber over this jar1 as in ,fig4 #ig1 and then pump out the air1 the bladder at first is pressed in as represented1 and if we pump on it at length bursts with a loud report4 ,it would make no difference in the result of the experiment if the jar were shaped as in ,fig4 #ih1 for the pressure is the same in all directions4 ,the resemblance between air and liquids in this respect may be illustrated thus3 ,suppose that a flat fish covers with one of its sides the end of the tube of a pump4 ,he feels no uncomfortable pressure1 because the water in the pump and that below it press equally upon him4 ,if1 now1 the pressure of the water in the pump could be suddenly taken off by the piston1 the fish would be pressed upward into the tube1 as the bladder is pressed upward in ,fig4 #ih1 or downward in ,fig4 #ig1 or as the hand is pressed downward in ,fig4 #if4 ,the ,magdeburg ,hemispheres1 ,fig4 #ii1 illustrate very impressively the pressure of the atmosphere4 ,they consist of two hemispheres whose edges at ,a fit very accurately upon each other4 ,the air is exhausted through the stem where you see the stop-cock1 and then the handle ,b is screwed on4 ,the force required to pull these hemispheres apart depends upon the extent of their surface4 ,in the famous experiment at ,magdeburg1 in #afed1 by ,otto von ,guericke1 the inventor of the air-pump1 two strong hemispheres of brass of a foot in diameter were employed1 and it required the force of thirty horses to separate them4 ,in ,fig4 #ajj you see a receiver with an opening at the top4 ,cemented in this opening is a wooden cup1 a1 terminating in a cylindrical piece1 b4 ,if mercury be poured into the cup1 on exhausting the air from the receiver the mercury will be forced through the pores of the wood by the external air1 and will fall in a silver shower4 ,a tall jar1 c1 is placed there to receive it1 to prevent any of it from going down into the opening in the metallic plate4 ,illustration3 ,fig4 #ii4 ,illustration3 ,fig4 #ajj4 ,illustration3 ,fig4 #aja4 #aeg4 ,the ,sucker4--,the boy's sucker illustrates the pressure of the air4 ,it is simply a circular piece of leather with a string fastened to its centre1 as seen in ,fig4 #aja4 ,when the leather is moistened and pressed upon a smooth stone1 on pulling the string a vacuum is made between the middle of the leather and the stone1 and the leather adheres by its edges to the stone1 just as the receiver adheres to the plate of the air-pump when the air is pumped out4 ,there are many animals that have contrivances of a similar character4 ,the gecko and the cuttle-fish furnish interesting examples1 as noticed in my ,natural ,history1 pages #aih and #cbj4 ,snails1 limpets1 etc41 adhere to rocks by a like arrangement4 ,some fishes do the same4 ,there is one fish called the remora1 that attaches itself by suckers to the side of some large fish or a ship1 and thus enjoys a fine ride through the water1 without any exertion on his part4 ,in all such cases it is water instead of air that makes the pressure1 but the principle is the same4 ,flies and some other insects can walk up a smooth pane of glass1 or along the ceiling over-head1 because their feet have contrivances akin to the boy's sucker4 ,the hind-feet of the walrus are constructed somewhat like the feet of the fly1 enabling this huge animal to go up smooth walls of ice4 ,illustration3 ,fig4 #ajb ,illustration3 ,fig4 #ajc4 ,illustration3 ,fig4 #ajd4 #aeh4 ,density of the ,air ,dependent upon ,pressure4--,the fact that the degree of the density of the air is dependent on pressure has been already shown in #aeb4 ,the same thing can be shown in various ways with the air-pump4 ,if a small bladder1 partly filled with air1 ,fig4 #ajb1 and loaded with a weight so as to sink in water1 be placed in a jar of water1 and the whole be set under the receiver of the air-pump1 on exhausting the air the bladder will swell out with the expanded air in it1 and will rise as seen in the figure4 ,the reason is1 that the pressure being taken off the surface of the water1 the bladder bears only the pressure of the water1 and not that of the air with the water1 and so the air in it expands and becomes less dense4 ,if an ,india-rubber bag be partly filled with air1 ,fig4 #ajc 7p4 #aai71 and put under the receiver1 on exhausting the air1 the surrounding pressure being thus taken off from the bag1 the air in it becomes expanded1 that is1 rarefied4 ,for the same reason1 if a vessel with soap-bubbles in it be placed under the receiver1 on pumping out the air the bubbles will become much enlarged4 ,a very pretty experiment illustrating the same thing may be tried in this way4 ,let an egg with a hole made in its small end be suspended in a receiver1 as represented in ,fig4 #ajd1 a wine-glass being beneath it4 ,on exhausting the air the egg will all run out of the shell into the wine-glass1 and then1 on admitting the air1 it will run back again into the shell4 ,the explanation is this3 ,there is air in the large end of the egg4 ,as soon as the pressure of air is taken off from all about the egg the air in the egg expands1 forcing out the contents2 but when the air is admitted into the receiver the air in the egg is at once condensed into its former small bulk by the surrounding pressure4 ,illustration3 ,fig4 #aje4 ,illustration3 ,fig4 #ajf4 #aei4 ,hydrostatic ,balloon4--,the philosophical toy represented in ,fig4 #aje illustrates very beautifully the influence of pressure upon the density of the air4 ,the balloon in the jar of water is of glass1 with a small orifice at its lower part4 ,care must be taken in putting water in the balloon to have just enough to make it of a little less specific gravity than water4 ,in that case it will be at the top of the jar1 with a very little of its top above the surface of the water4 ,now tie a piece of ,india-rubber cloth over the top of the jar1 and the apparatus is complete4 ,on pressing upon the ,india rubber the balloon will go down in the jar1 and on taking off the pressure it will rise4 ,the explanation is this3 ,the pressure upon the ,india rubber is felt through the whole body of the water in the jar1 and forces a little more water into the orifice of the balloon1 condensing the air that is there4 ,the balloon consequently becomes heavier1 and has a greater specific gravity than water1 and sinks in it4 ,but when the pressure is taken off1 the condensed air in the balloon1 by its elasticity1 returns to its former bulk1 expelling the surplus water just introduced1 and the balloon1 becoming therefore as light as before1 rises4 ,grotesque figures of glass may be managed in the same way4 ,the ,cartesian image1 ,fig4 #ajf1 is an example4 ,this has air in its upper part1 a1 and water up to c d4 ,when pressure is made on the ,india rubber more water is forced into the image through the tail1 b1 and it goes down like the balloon1 to rise again when the pressure is taken off4 ,illustration3 ,fig4 #ajg4 ,illustration3 ,fig4 #ajh4 #afj4 ,air in ,substances4--,i have said that there is air in the pores and interstices of wood1 flesh1 and a great variety of substances4 ,in all these cases the presence of the air can be made manifest by taking off the pressure of the surrounding air1 and thus allowing the air in these substances to expand4 ,if an egg be placed in a jar of water1 ,fig4 #ajg1 under the receiver of an air-pump1 on exhaustion being made air-bubbles will constantly rise in the water from the egg4 ,so1 too1 a glass of porter1 ,fig4 #ajh1 will have its surface covered with foam1 the carbonic acid gas in it escaping freely when the pressure of the air upon it is taken off4 ,the same thing may be seen to some extent even in water1 for it always contains some air4 ,for the same reason a shriveled apple1 with the pressure of the air taken from it1 will become plump and fair1 but will shrink at once to its shriveled state when the air is admitted into the receiver4 #afa4 ,elasticity of the ,air4--,all the phenomena cited in #aeh1 #aei1 and #afj exhibit the elasticity of the air4 ,it is from this property that it is always disposed to expand4 ,it will do so whenever pressure is taken from it1 or when it can overcome pressure to which it is subjected4 ,this property is most strikingly exhibited when the air is much condensed by pressure4 ,and the greater the condensation the stronger is the expansive or elastic force4 ,illustration3 ,fig4 #aji4 #afb4 ,the ,condenser4--,in ,fig4 #aji you have the plan of an instrument called the ,condenser4 ,in ,a ,b1 a cylinder1 moves the piston1 ,p4 ,air is admitted to the cylinder at ,f1 and into the receiver1 ,v1 at ,g4 ,the valve at ,f prevents any air from escaping from the cylinder1 and the valve at ,g prevents it from escaping from the receiver4 ,the operation of the instrument is this3 ,if the piston be pressed downward1 the pressed air in the cylinder shuts the valve ,f and opens ,g1 and so enters the receiver ,v4 ,if now the piston be raised1 air rushes in at ,f to fill the space in the cylinder4 ,it can not come from ,v1 because the valve ,g is shut by its pressure4 ,by working the piston for some time you can get a body of air into ,v of very great density4 ,you see that this instrument is the very opposite of the air-pump4 ,in the receiver1 ,v1 you have condensed air1 while in the receiver of the air-pump you have rarefied air4 ,if you compare the two instruments you will see that the opposite results are owing to different arrangement of the valves4 ,illustration3 ,fig4 #aaj4 #afc4 ,the ,gasometer4--,gas is distributed in pipes from the gasometer at the gas factory by the agency of the elasticity occasioned by condensation under pressure4 ,the apparatus1 ,fig4 #aaj 7p4 #abb71 consists of a large round vessel1 ,g1 open below1 and sunk in a larger vessel of water1 w4 ,we will suppose the vessel1 ,g1 to be full of water4 ,gas is introduced into it through the pipe1 p r1 the gasometer rising as it fills with the gas4 ,p is a weight balancing the gasometer1 and so permitting it to rise as the gas enters4 ,the gasometer being filled1 the gas is to be distributed4 ,for this purpose weights are put upon the gasometer1 so that the gas may be compressed4 ,under this pressure it by its elasticity seeks for more room1 and obtains it by escaping through the pipe o b c4 ,as the pressure on the gas needs to be regulated1 there is sometimes a gauge1 h i1 attached1 which shows the amount of the pressure4 ,it is a bent tube with water in the bend4 ,you see at once that the greater the pressure upon the gas the higher will the water be in the branch1 h1 of the gauge4 ,illustration3 ,fig4 #aaa4 #afd4 ,air-,guns and ,pop-,guns4--,these illustrate the elasticity of condensed air4 ,the air-gun is constructed in this way3 ,a receiver1 like ,v1 ,fig4 #aji1 is made so that you can screw it on and off from the instrument4 ,after being charged with condensed air it is screwed upon the gun1 its stem communicating with the barrel4 ,in order to discharge the gun there is a contrivance connected with the trigger for raising the valve1 ,g1 so that some of the condensed air may enter the barrel4 ,on doing so1 it by its sudden expansion rapidly forces out the contents4 ,the principle on which the common pop-gun operates is the same4 ,there is air confined between the two corks1 a and b1 ,fig4 #aaa 7p4 #abc74 ,as the rod1 ,r1 is pushed quickly in1 the cork b is carried nearer to a1 so that the air between them is condensed4 ,with the condensation the expansive force is increased2 and when it becomes so great that the cork a can no longer resist it1 it throws the cork out1 and so quickly as to occasion the popping sound4 #afe4 ,powder and ,steam4--,the explosion of powder furnishes a good illustration of the expansive force of condensed air or gases4 ,these gases are produced so suddenly from the powder that at the instant they are in a very condensed state1 and therefore expand powerfully4 ,so1 also1 steam has power in proportion to its condensation4 ,when formed under the confinement of a boiler1 on being allowed to escape it expands with great force4 ,the application of the expansive power of steam will be treated of particularly in another part of this book4 ,illustration3 ,fig4 #aab4 #aff4 ,retardation by ,condensed ,air in ,gunnery4--,when a ball is fired it is constantly retarded in its flight by the resistance of the air1 for it has to push the air away on every side in order to make its way through it4 ,of course1 then1 the more condensed the air is the greater is the resistance4 ,now it is condensed air that the ball is obliged to remove2 for as it goes forward it1 by its rapid pressure1 condenses the air directly before it4 ,and the more rapid is its flight the greater is the condensation1 and therefore the greater the resistance4 ,besides1 the retarding effect is increased by the tendency to a vacuum behind the ball4 ,all this can be made clear by ,fig4 #aab4 ,let ,b be a ball going very rapidly in the direction indicated by the arrow1 the cloud representing the condensed air before it1 and the space included in the two lines the vacuum behind it4 ,it is obvious that the more rapidly the ball goes the less readily is the air pressed out of the way1 and therefore the more it is condensed in front of the ball4 ,at the same time the more rapid is the ball the less readily does the air close up behind it1 and therefore the greater is the tendency to a vacuum there4 ,for these reasons there is more retarding influence exerted by the air upon a ball in the first part of its course than in its latter part4 ,illustration3 ,fig4 #aac4 ,illustration3 ,fig4 #aad4 ,illustration3 ,fig4 #aae4 #afg4 ,pressure of the ,air on ,liquids4--,if you plunge a tumbler into a vessel of water1 and turning it over hold it so that its open part is just under the surface1 it will remain full4 ,the reason is that the weight of the air pressing upon the surface of the water in the vessel prevents the water in the tumbler from passing downward4 ,now if you introduce a bent tube under the tumbler1 as in ,fig4 #aac1 and blow through it1 the air that you force up into the tumbler presses the water down1 taking its place4 ,that is1 the pressure of the air acts in opposition to the pressure of the air outside upon the surface of the water in the vessel4 ,you take a jar1 a1 ,fig4 #aad1 and filling it with water1 turn it over with its open end downward1 the water will remain in the jar4 ,you have here a representation of the pneumatic trough used by the chemist in collecting gases4 ,to fill the jar a with gas he puts the mouth of the retort from which the gas issues under the jar a1 and the gas passing upward expels the water1 as the water is expelled by the breath from the tumbler in ,fig4 #aac4 ,in ,fig4 #aae 7p4 #abe7 is represented an experiment which shows not only that the pressure of the air sustains the column of water in the cases cited above1 but also that it makes no difference in what direction this pressure is exerted4 ,take a large tube1 a1 closed at one end and open at the other1 and fill it even full with water4 ,place1 now1 a piece of writing-paper over its mouth1 and carefully invert the tube1 as seen in the figure4 ,the paper will remain1 and the water will not run out4 ,it is the pressure of the air that sustains the water1 and the paper only serves to maintain the surface of the water unbroken4 ,if the paper were not there the particles of the air would insinuate themselves among those of the water1 and pass upward in the tube4 ,you can try this experiment with a wine-glass1 and may even succeed with a tumbler4 ,we see in these experiments the reason that a liquid will not run from a barrel when it is tapped1 if there be no vent-hole above1 unless there be so large an opening made as to let the air work its way in bubbles among portions of the liquid4 ,it is this entrance of the air that causes the gurgling sound in pouring a liquid from a bottle4 #afh4 ,amount of ,atmospheric ,pressure4--,if1 instead of the jar a1 in ,fig4 #aad1 you have a tube thirty-four feet high1 and closed at the top1 situated as the jar a is1 it will remain full of water4 ,if the tube be longer the water will stand only at thirty-four feet1 leaving a vacuum above it4 ,it makes no difference what the size of the tube is2 the result will be the same in all cases4#b ,that is1 a column of water thirty-four feet high can be sustained by the pressure of the atmosphere4 ,it is easy1 therefore1 to estimate the weight or pressure of the air4 ,the pressure of the column of water is found to be fifteen pounds to the square inch of its base1 and this1 of course1 is the amount of pressure or weight of the atmosphere which it balances4 ,mercury is thirteen and a half times as heavy as water1 and therefore the air will sustain a column of it only about thirty inches in height4 ,illustration3 ,fig #aaf4 #afi4 ,barometer4--,the weight of the atmosphere varies to some extent at different times1 and the barometer is an instrument for measuring these variations4 ,it is constructed on the principles developed in the previous paragraphs4 ,in ,fig4 #aaf is a representation of the instrument4 ,a ,b is a glass tube about #cd or #ce inches long1 closed at one end4 ,it has been filled with mercury1 and then inverted in a cup of the same liquid1 ,c4 ,the vacuum above the mercury is called the ,torricellian vacuum1 from ,torricelli1 an ,italian1 who first developed the principles of the instrument4 ,the mercury generally1 as stated in #afh1 stands at about the height of thirty inches4 ,but it varies from this with the weather4 ,when the weather is bright and clear the air is heavier than this1 and1 pressing upon the mercury in the vessel1 forces it up higher in the tube4 ,but when a storm is coming the air is apt to be lighter1 and therefore pressing less strongly on the mercury in the vessel1 the mercury in the tube falls4 ,the barometer is of great service1 especially at sea1 in affording the sailor warning of an approaching storm4 ,an incident is related by ,dr4 ,arnot which strikingly illustrates its value in this respect4 ,he was at sea in a ,southern latitude4 ,as the sun set after a beautiful afternoon the captain foresaw danger1 although the weather was perfectly calm1 for the mercury in the barometer had suddenly fallen to a remarkable degree4 ,he gave hurried orders to the wondering sailors to prepare the ship for a storm4 ,scarcely had the preparations been made when a tremendous hurricane burst upon the ship1 tearing the furled sails to tatters1 and disabling the masts and yards4 ,if the barometer had not been observed the ship would have been wholly unprepared1 and shipwreck1 with the loss of all on board1 would have been the result4 ,a water-barometer could be made1 but it would be an unwieldy thing1 for the tube must be over #cd feet long4 ,besides1 it would not answer in very cold weather1 as the water would freeze4 ,so short a column of the heavy fluid1 mercury1 balances the weight of the atmosphere that a barometer made with this is of very convenient size2 and then there is no danger of the mercury's freezing1 except in the extreme cold of the ,arctic regions4 #agj4 ,barometer a ,measurer of ,heights4--,the atmosphere1 as stated in #aeb1 diminishes regularly in density as we go upward4 ,the rate of this diminution has been accurately ascertained1 and therefore we can estimate heights by the amount of pressure on the mercury in the barometer4 ,at a height of #ejj feet the barometer will be half an inch lower than in the valley below4 ,at the summit of ,mont ,blanc it stands but half as high as at its foot1 indicating a height of #ae1jjj feet4 ,du ,luc1 in his famous balloon ascension from ,paris1 saw the barometer at one time standing at about twelve inches1 showing an elevation of #ba1jjj feet4 #aga4 ,relation of the ,air's ,pressure to the ,boiling ,point4--,water heated to #bab degrees of ,fahrenheit boils1 that is1 it becomes vapor4 ,now if water be heated on the summit of a high mountain it boils before it arrives at this degree of temperature4 ,on the top of ,mont ,blanc it boils at #ahj degrees1 that is1 #cb degrees below the boiling point of water at the foot of the mountain4 ,this is because the pressure of the air acts in opposition to the change of water in to vapor1 and the less the pressure is the less heat will be required to vaporize the water4 ,we may illustrate this influence of the pressure of air upon boiling by the following experiment4 ,let a cup of ether 7which boils at #ih degrees7 be placed under the receiver of an air-pump4 ,on rarefying the air by the pump the ether will boil4 ,the general effect of pressure upon boiling may be prettily illustrated by another experiment4 ,boil some water in a thin flask over a spirit-lamp4 ,blow out the lamp1 and1 corking the flask tightly1 let the boiling cease4 ,if1 now1 you pour some cold water over the flask the boiling will commence again with considerable force4 ,why8 ,because you condense the steam which is over the water by the application of cold1 and thus take off the pressure4 ,then1 again1 if1 while the water is boiling1 you pour hot water over the flask1 the boiling ceases1 because the heat favors the accumulation of steam1 and therefore renews the pressure on the surface of the water4 ,you can see from what has been stated that most liquids have the liquid form because of the pressure of the atmosphere upon them4 ,if there were no atmosphere1 ether1 alcohol1 the volatile oils1 and even water1 would fly off in vapor2 and the earth would be enveloped in a vaporous robe1 for the particles of the vapors would be held to the earth by attraction1 just as the particles of the air are now1 #aea4 ,illustration3 ,fig4 #aag4 #agb4 ,syphon4--,the pressure of air upon fluids is beautifully exemplified in the operation of the syphon4 ,this instrument is simply a bent tube having one branch longer than the other4 ,its operation is shown in ,fig4 #aag4 ,the tube having been first filled with the liquid1 has its shorter branch in the liquid of the vessel ,a1 which is to be emptied1 and the other in the vessel ,b1 which is to receive the liquid4 ,as you see it here1 the opening of the long branch is below the surface of the liquid in ,b4 ,it is manifest1 therefore1 that the air presses equally upon the surfaces in both vessels1 tending to support the fluid in the tube1 just as the fluid is supported in the jar in ,fig4 #aad4 ,but1 notwithstanding these equal pressures1 the liquid runs up the tube from ,a1 and down its longer branch into ,b4 ,why is this8 ,as the pressure of a column of fluid is as its height1 there is greater pressure or weight in the longer branch than in the other2 and it is this difference in weight that causes the flow from ,a into ,b through the syphon4 ,the difference in the columns in the two branches is not the difference in length of these branches1 but the distance between the levels of the fluid in ,a and ,b1 that is1 the distance from a to b4 ,the operation1 then1 of the instrument is this4 ,there is a constant tendency to a vacuum at ,c1 the bend of the tube1 from the influence of gravitation on the excess of fluid in the long branch over that in the short one4 ,this tendency is constantly counteracted by the rise of fluid in the short branch1 it being forced up by the pressure of the air upon the surface of the fluid in ,a4 ,illustration3 ,fig4 #aah4 ,if the syphon were so placed that the surface of the liquid in ,a is precisely on a level with that in ,b1 as represented in ,fig4 #aah1 the liquid would remain at rest1 for as pressure is as the height1 #aba1 and the pressures on the two surfaces are equal1 there would be an exact balance4 ,but let the surface in ,b be in the least lower than in ,a and the flow will begin4 ,and the greater the distance between the two levels the more rapid will be the flow1 for the greater will be the influence of gravitation in the long branch4 ,illustration3 ,fig4 #aai4 ,again1 if the end of the long branch of the syphon be free1 as in ,fig4 #aai 7p4 #acj71 the syphon will operate in the same way1 for the air1 pressing in all directions equally1 tends to support the column of fluid in the long branch by a direct upward pressure1 but is prevented from doing so by the excess of fluid in it above what is in the shorter one4 ,the operation of the syphon is commonly represented in this way2 but ,i have given first the arrangement in ,fig4 #aag1 in order that you might more clearly see the principle of the instrument4 ,illustration3 ,fig4 #abj4 #agc4 ,uses of the ,syphon4--,the syphon is used chiefly for discharging liquids from one barrel or vessel into another4 ,for convenience1 it is often constructed after the plan of ,fig4 #abj4 ,to the long branch1 ,b ,c1 is attached the tube ,,ed4 ,it is used in this way3 ,the end of the short branch1 ,a1 being introduced into the liquid to be drawn off1 you place your finger upon ,c1 and after filling the syphon by suction at ,e1 you remove the finger and let the liquid run4 ,the syphon has sometimes been used to drain pits and mines4 ,it of course can never be used where the elevation over which the tube is to bend is over #cd feet from the surface of the water to be discharged1 for then the air would not press the water up to the bend of the syphon4 ,illustration3 ,fig4 #aba4 #agd4 ,cup of ,tantalus4--,this cup1 ,fig4 #aba1 has a syphon in it1 the short branch1 b1 opening into the cup1 and the long branch1 d1 having its outlet in the bottom4 ,as you pour water into the cup it will remain there until you pour enough in to cover the bend of the syphon4 ,as soon as this is done1 the syphon being filled1 the water suddenly flows out from the outlet1 a1 of the long branch4 ,illustration3 ,fig4 #abb4 #age4 ,intermitting ,springs4--,the operation of an intermitting spring is essentially the same with that of the cup of ,tantalus4 ,you have a representation of such a spring in ,fig4 #abb4 ,there is a cavity in a hill1 supplied with water from a passage above4 ,there is also a passage from it which takes a bend upward like a syphon4 ,now when the water in the cavity is low it will not run out from the syphon-like channel2 but when the cavity becomes filled above the level of the bend the water will at once flow out1 just as it does from the cup of ,tantalus as soon as the bend of its syphon is covered4 ,illustration3 ,fig4 #abc4 #agf4 ,pumps4--,in ,fig4 #abc you have a plan of a common pump4 ,a tube1 ,c ,d1 extends down into the well1 ,w4 ,above this is the barrel of the pump1 ,a ,b1 in which the piston works up and down4 ,there is a valve1 ,f1 in the piston1 and another1 ,e1 at the bottom of the barrel4 ,both of them open upward4 ,we will suppose that the pump is entirely empty of water4 ,if1 now1 the piston descend1 the valve ,e shuts down1 and ,f opens1 letting the pressed air between the piston and ,e pass upward4 ,see what will happen when the piston rises4 ,the air above the piston can not get below1 for its pressure will shut the valve ,f4 ,but there will be a tendency to a vacuum below the piston as it rises1 and the air will go up through the valve ,e to fill up the space4 ,but why does the air rise8 ,because of the pressure of the air upon the surface of the water in the well4 ,this forces up in the pump the water and the air above it1 just in proportion as the downward pressure in the pump is lessened4 ,if the pumping be continued all the air will soon be expelled1 the water following it and flowing out at the opening1 ,g4 ,it is obvious that the pump will be useless if the valve ,e be over #cd feet above the surface of the water in the well1 as the pressure of the atmosphere will not sustain a column of water higher than this4 #agg4 ,suction4--,in common language1 the operation of the pump is attributed to what is called a principle of suction1 as if there was a drawing up of the water4 ,but the water1 you see1 is not drawn1 but forced1 up4 ,so it is with all operations of a similar character4 ,in sucking up a fluid through a tube the fluid is forced up1 because the pressure downward in the tube is removed4 ,but how is it removed8 ,it is done by a movement of the tongue downward from the roof of the mouth1 thus causing a tendency to a vacuum1 as the upward movement of the piston in the pump causes this tendency under it4 ,to fill the space made by the movement of the tongue the air is forced up the tube1 the liquid following2 and1 as in the case of the pump when the air is all expelled1 the liquid will begin to discharge into the mouth4 ,illustration3 ,fig4 #abd4 #agh4 ,forcing-,pump4--,the forcing-pump is constructed differently from the common pump4 ,its plan is given in ,fig4 #abd4 ,it has a pipe1 ,c ,d1 and a barrel1 ,a ,b1 like the common pump4 ,it has also the valve ,e at the bottom of the barrel4 ,but it has no valve in the piston4 ,connected with the barrel is another pipe1 ,f ,g1 from which the water issues4 ,this has a valve1 ,h1 opening upward4 ,the operation of the pump is obvious4 ,as the piston is drawn up ,e opens and ,h shuts1 and when it is forced down ,e shuts and ,h opens4 ,illustration3 ,fig4 #abe4 #agi4 ,fire-,engine4--,the fire-engine has commonly two forcing-pumps1 with a contrivance for making the water issue in a uniform stream4 ,this contrivance can be explained in ,fig4 #abe4 ,the discharging pipe1 ,l ,m1 extends down into a large vessel1 ,i ,k1 which is filled with air4 ,the uniformity of the stream depends upon the elastic force of compressed air1 as you will see if ,i explain the operation of the machine4 ,when the water is forced through the opening ,h1 it rises to the level ,n ,o1 compressing the air in ,i ,k1 for the tube ,l ,m is too small to allow all the water to escape that comes from the larger tube1 ,h ,e4 ,now the moment that the piston ceases to force the water through ,h1 the elastic force of the compressed air operates1 shutting down the valve ,h1 and forcing the water up ,l ,m4 ,the result1 you see1 is a continuous forcing up of the water through this tube1 and therefore a uniform stream4 ,,chapter ,x4 ,,motion4 #ahj4 ,universality of ,motion4--,the world is full of motion4 ,the rising and setting of the sun1 the changes of the seasons1 the falling of the rain1 the running of rivers into the ocean1 the ascent of water into the air by evaporation1 the wind moving in silence or rushing on in its might1 are familiar examples of motion constant and every where present4 ,but with all this motion1 sometimes in conflict and often variable1 order and regularity reign4 ,the causes of motion1 though various in their operation1 are kept by the ,creator from producing confusion and disorganization by a few simple laws1 which regulate the movements both of atoms and of worlds4 ,the principal of these causes ,i will now briefly notice4 #aha4 ,causes of ,motion4--,attraction is the most universal of the causes of motion in the universe4 ,while it binds atom to atom1 it also binds system to system throughout the immensity of space2 and while it makes the stone fall to the ground1 it moves the countless orbs forever onward in their courses4 ,it is this which causes the tides to flow and the rivers to run down their slopes to the ocean1 and thus by keeping up the never-ending motion of water all over the earth in seas1 lakes1 rivers1 and the millions of little streamlets1 diffuses life and beauty over the vegetable world1 and gives to man the vast resources which we see developed in the numberless applications of water-power and navigation4 ,heat pervades all matter1 and is every where uniting its influence with the other causes of motion4 ,it is heat that produces all the motions of the air1 termed winds4 ,it is heat that causes the rise of the water all over the earth in evaporation1 so that it may be collected in clouds1 again to descend to moisten the earth and keep the ever-flowing rivers full4 ,heat applied to water gives to man one of his best means of producing motion in machinery4 ,the agencies which ,chemistry reveals to us are ever at work causing motion among the particles of matter2 and though they generally work in silence1 they sometimes show themselves in tremendous explosions1 and in convulsions of nature4 ,busy life is every where producing motion1 more especially in the animal world4 ,it gives to the myriads of animals1 great and small1 that swarm the earth not only the power of moving themselves1 but also the power1 to some extent1 of moving the material world around them4 #ahb4 ,action and ,reaction ,equal4--,when any of the causes of motion act1 the action is met by an opposite and equal reaction4 ,if1 for example1 a blow be given1 an equal blow is received in return4 ,for this reason1 if one in running hits his head against the head of another both are equally hurt4 ,when a child knocks his head against a table1 there is sound philosophy in the common saying that he has given the table as good a blow as he has received1 though it may afford him no comfort4 ,many very interesting illustrations of this law of motion suggest themselves1 of which ,i will give a few4 ,illustration3 ,fig4 #abf4 ,a swimmer1 pressing the water downward and backward with his hands and feet1 is carried along forward and upward by the reaction of the water4 ,and in this case1 as in every other1 the greater the action the greater is the reaction2 in other words1 the more strongly he presses with his hands and feet1 the more rapidly is he borne along by the reaction of the water against the pressure4 ,a boat advances in proportion to the force with which the oars press against the water4 ,so the rapidity of a steamboat depends on the force with which the paddles drive the water astern4 ,birds rise in the air by the reaction of the air against their wings as they are pressed downward4 ,a sky-rocket pursues its rapid flight because a large quantity of gaseous matter issues from its lower end1 and1 being resisted by the reaction of the air1 by its pressure throws the rocket upward4 ,so if a ship fire guns from the stern its advance will be accelerated1 but if from the bow it will be retarded4 ,when a broadside is fired the ship inclines to the other side4 ,in ,fig4 #abf 7p4 #acf7 is represented the plan of ,barker's mill4 ,it consists of a cylinder1 c1 arranged in a frame in such a way that it can revolve on the point upon which it rests4 ,water runs into it by a tube1 p1 and escapes by the branches1 a and d4 ,these are so arranged that the reaction upon the issuing water makes the cylinder revolve rapidly1 causing the ends of the branches to whirl around as indicated by the dotted lines and arrows4 ,if a spring be compressed between two equal bodies1 it will throw them off with equal velocities4 ,if they are unequal1 the velocity of the smaller body will be greater than that of the larger1 and in proportion to its smallness4 ,for this reason1 when a ball issues from a cannon1 though the cannon and the ball are equally acted upon by the elastic or expansive force of the gases set free by lighting the powder1 the gun is moved but very little because the force is diffused through so large a mass1 while the ball being so much smaller moves with great velocity4 ,when a volcano throws stones from its crater the earth may be compared to the cannon1 the stones to the ball1 and the explosive materials throwing the stones to the exploding powder projecting the ball4 ,as the cannon is moved as much as the ball1 so is the earth moved as much as the stones1 the only reason that it does not move as far and as rapidly as the stones being that the force is diffused through so large a bulk4 ,these examples illustrate very well the relation of action and reaction1 for whenever there is an action of one body upon another it is as if a spring were between the two bodies1 acting equally upon both4 ,when a man jumps from the ground it is as if a spring were compressed between him and the earth1 and this expanding moves the earth exactly as much as it does the man4 ,he really kicks the earth away from him4 ,the motion of the earth is not obvious because it is diffused through so large a mass4 ,the case is parallel to that of the ball and cannon4 ,the same force is exerted upon the man and the earth1 but the man1 like the ball1 moves the most1 and in proportion to his comparative smallness4 ,so when a bird hops from the ground1 the earth moves as really as the bird4 ,if the bird hop from a twig1 you perceive that the twig is moved by the pressing down of the bird as it rises4 ,when it starts from the ground it exerts the same downward pressure1 and moves the earth as really as in the other case it did the twig4 #ahc4 ,inertia ,shown in the ,communication of ,motion4--,what is meant by the inertia of matter you have already learned in #dh4 ,this property is exemplified in the communication of motion to any body1 or1 in other words1 in setting it in motion4 ,of this ,i will give some illustrations4 ,when the sails of a vessel are first spread to the wind the vessel does not move swiftly at once1 for some time is required for the force applied to overcome the inertia of so large a mass1 and to put it in rapid motion4 ,horses make a greater effort to start a load than they do to keep it in motion after it is started4 ,if one be standing up in a carriage1 and the horses start off suddenly1 he falls backward1 because his body1 from its inertia1 does not readily and at once partake of the motion of the carriage4 ,if a person start forward quickly with a waiter filled with glasses in his hands1 the glasses will slide backward4 ,so if a person start quickly from his chair with a cup of tea in his hand1 the tea will be thrown backward upon him4 ,you see from the foregoing illustrations that it requires some time to communicate motion to any body4 ,i will give some illustrations of this fact of a more striking character4 ,if a ball be thrown against an open door it will move the whole door1 and perhaps shut it2 but the same ball if fired will pass through the door without moving it perceptibly4 ,in the latter case its velocity is so great that there is not time enough to communicate motion to the whole door1 and it moves only that part of it with which it comes in contact4 ,a bullet thrown with but little force against a window will crack a whole pane of glass2 but if shot from a pistol it merely makes a round hole4 ,so1 also1 a cannon-ball having a great velocity may pass through the side of a ship1 doing perhaps comparatively little damage1 while one moving with much less velocity may do vastly more damage by splintering the wood to a considerable extent4 ,for the same reason a rapid ball hitting a person may occasion less suffering and do less harm than a slow ball2 for a rapid ball kills merely the parts which it touches1 leaving the flesh around in a sound state1 while the slow ball bruises over a large space4 ,if a large pitcher filled with some heavy liquid be quickly taken up the handle will break1 leaving the pitcher behind4 ,large dishes are often broken in this way when heavily loaded4 #ahd4 ,inertia ,shown in the ,disposition of ,motion to ,continue4--,of this ,i will cite some illustrations4 ,as in the case of the ship1 in the first illustration in #ahc1 it takes time to communicate motion to the whole ship1 or1 in other words1 to overcome its inertia1 so when the ship is once in rapid motion it does not stop suddenly when the sails are taken down1 but its inertia tending to keep it moving is gradually overcome by the resistance of the water4 ,if one be standing up in a carriage in motion1 and the horses suddenly stop1 he will be thrown forward1 for his body has a motion in common with the carriage1 and from inertia is disposed to go on when the carriage stops4 ,when you strike your foot against any thing to get the snow off1 you give the foot and the snow a common motion together1 then arresting the motion of the foot1 the snow from inertia passes on4 ,the same thing is illustrated in striking a book against any thing to get the dust off4 ,if a ship strike upon a rock every thing on board which is loose is dashed forward4 ,the earth as it revolves on its axis has a velocity at the equator of about #ajjj miles an hour4 ,if this revolution should be suddenly arrested every thing loose on its surface1 having acquired the motion of the earth1 would be at once thrown eastward1 just as the furniture1 etc41 on board ship are dashed forward when it is stopped by running against a rock4 ,all the houses1 and monuments1 and structures of every kind would fall prostrate eastward4 ,all the cities on our ,atlantic coast would be plunged into the ocean2 and while the waters would leave the western shores of the ,atlantic1 they would overflow its eastern shores1 and deluge the continent of ,europe1 as water in a vessel on board a ship that had struck an obstacle would be thrown forward over its side4 ,illustration3 ,fig4 #abg4 #ahe4 ,an ,equestrian ,feat4--,in the feat represented in ,fig4 #abg the only exertion made by the rider is to raise himself sufficiently to pass over the cord1 and he comes down again upon the horse's back1 simply because of the motion which he has in common with the horse1 his feet going in the path represented by the dotted line4 ,if he should attempt to throw himself forward1 as in leaping from the ground1 he would go too far1 and perhaps strike upon the horse's neck instead of his back4 ,skill in jumping from a moving carriage consists in making proper allowance for the forward motion which is had in common with the carriage4 ,most persons are apt to overdo the matter1 and so come to the ground prostrate1 and with more violence than is necessary4 #ahf4 ,a ,case in ,court4--,a dashing young man driving a light phaeton ran against a heavy carriage4 ,his father was induced by his son's representations to prosecute the driver of the carriage for driving too fast4 ,a knowledge of motal inertia very readily decided the case4 ,the son and his servant both declared that the shock of the carriage was so great against the phaeton that they were thrown over the horses' heads4 ,they thus proved themselves guilty of the fast driving1 for it was their own rapid motion that threw them out when the phaeton was stopped by running against the carriage4 ,the following case is a parallel one4 ,if two boats1 the one of large size sailing slowly up stream1 the other a small one sailing rapidly down1 run against each other1 a man standing in the bow of the one going down will be thrown much farther forward than one standing in the bow of the other4 #ahg4 ,course of ,bodies ,thrown into the ,air4--,it results from the principle that ,i have illustrated that when any body1 as a stone1 is thrown1 as we say1 straight upward1 it does not1 in reality1 go up or come down perpendicularly4 ,if it did it would come down at a great distance from us4 ,suppose it takes two seconds for it to go up and to reach the ground4 ,if we are at the equator1 in that two seconds we move from the point where we threw up the stone nearly #cjjj feet eastward1 and therefore if the stone rose and fell perpendicularly it would fall #cjjj feet westward of us4 ,why1 instead of this1 does it fall at our feet8 ,because that when thrown into the air it has not only the upward motion given by the hand1 but also the forward motion of the earth4 ,it is a case similar to that of the rider in ,fig4 #abg1 the horse representing the surface of the earth and the rider the stone4 ,for the same reason a man on board of a steamboat1 though it move fifteen miles an hour1 tosses up his ball or orange and catches it as well as if he were on land4 ,this he could not do if both he and his orange did not have the same forward motion that the boat does4 ,so1 also1 if a man fall from a mast-head he reaches the deck at the foot of the mast when the vessel is sailing rapidly1 just as he would if it were lying still at the wharf4 ,if he did not by inertia retain the forward motion which he had in common with the vessel he would fall at some distance behind the mast4 #ahh4 ,the ,earth and the ,atmosphere4--,the air being held to the earth by attraction1 #aea1 it has a motion in common with the earth4 ,it revolves with the earth just as the tire of a wheel revolves with the wheel4 ,this being so1 our winds are nothing but slight variations of this constant rapid whirl of the aerial coating of the earth4 ,if the atmosphere were suddenly to stop whirling round with the earth we should move through it with a velocity of #aejj feet a second2 and the destructive effect upon us would be the same as it would were the earth standing still while the air moved over its surface with this fearful velocity4 ,a wise man1 not reflecting that the atmosphere moved with the earth1 proposed rising in a balloon1 and waiting till the country to which he wished to go should be passing under him4 #ahi4 ,motion and ,rest4--,though we use the term rest in opposition to motion1 it is obvious from some of the illustrations given that rest is merely a relative term1 for not a particle of matter in the universe is at rest4 ,though when we are sitting still we call ourselves at rest1 we are moving every hour1 by the revolution of the earth on its axis1 #ajjj miles eastward1 and #fh1jjj miles in our annual journey round the sun4 ,why1 then1 are we so insensible to these rapid motions8 ,it is partly because the motions are so uniform1 but chiefly because all things around us1 our houses1 trees1 and even the atmosphere1 are moving along with us4 ,if we were moving along alone1 even at a slow rate1 while all these objects were standing still1 we should be conscious of our motion1 as we are when1 as we ride along in a carriage1 we see the objects at the road-side not moving along with us4 #aij4 ,a ,comparison4--,the above can be made more clear and impressive by a familiar comparison4 ,a man on board of a steamboat1 by confining his attention to things within the boat1 may1 after a while1 be almost unconscious of the boat's moving1 if the water be smooth1 though the boat may be going at the rate of fifteen miles an hour4 ,if he be reading in the cabin he will think as little of his motion as he would were he reading in his parlor at home4 ,if he should be blindfolded1 and turned around a few times1 it would be impossible for him to tell the direction in which the boat is going4 ,now it is with a man on the earth as it is with the man in the boat4 ,he is unconscious of the motion of the earth for the same reason that the man in the boat is unconscious of the boat's motion4 ,all objects around him are moving along with him1 as the objects around the man in the cabin of the boat are moving along with him4 ,we can carry the parallel farther4 ,while the man sits in the cabin he knows not how fast the boat moves1 nor even whether it moves at all4 ,he must look out to decide this1 and even then he may not be able to tell whether the boat moves1 or whether he merely sees the water running by it4 ,we often are actually deceived in this respect4 ,a steamboat struggling against wind and wave may appear to those on board to be advancing when it is really stationary1 or even when it is losing ground4 ,so when we look at the sun we know not whether it is the sun or the earth that is moving4 ,mere vision1 without reasoning on the subject1 leads one to think that it is the sun that moves4 ,for the same reason1 if a child should be placed in a carriage for the first time without seeing the horses1 but with its eyes fixed on objects at the road-side1 he would probably think that all the fences and trees and rocks and houses are in motion4 #aia4 ,absolute and ,relative ,motion4--,the motion of a body is said to be absolute when it is considered without relation to the position of any other body4 ,its motion is said to be relative when it is moving with respect to some other body4 ,absolute rest is unknown1 for no body in the universe is known to be without motion4 ,but a body may be relatively at rest1 that is1 in a fixed relative position to other bodies4 ,every body is in a state of absolute motion1 and yet it may be in a state of relative rest4 ,all objects that appear to us to be at rest have a very rapid absolute motion4 ,they appear to be at rest merely because they have the same rapidity and direction of absolute motion that we have ourselves4 ,and all the motions which are apparent to the eye are only slight differences in the common absolute motions1 of which1 though they are so exceedingly rapid1 we are entirely unconscious4 ,thus1 if ,i stand still1 and another at my side walks at the rate of three miles an hour eastward1 we both of us have a common absolute motion of #ajjj miles in every hour1 and he merely adds three miles to his thousand--,i move #ajjj miles1 and he #ajjc4 ,so if ,i sit still in my parlor1 and my friend travels eastward at the rate of #bj miles an hour1 ,i move every hour #ajjj miles1 and he #ajbj4 ,and if he travel westward at this rate he really travels slower than ,i do--he has an absolute motion eastward of #ihj miles1 and ,i of #ajjj4 ,at the same time we are both whirling on in our annual journey around the sun at the rate of #fh1jjj miles an hour4 #aib4 ,obstacles to ,motion4--,as motion is naturally disposed to continue 7 #di and #ahd71 whenever it is stopped it does not spend itself1 but is stopped by obstacles4 ,the principal of these obstacles are3 gravitation2 the resistance of opposing substances--solids1 liquids1 and gases2 and friction4 ,when a stone is thrown into the air its upward motion is gradually destroyed by the attraction of the earth and the resistance of the air4 ,observe1 now1 why it descends4 ,it is from the action of one of the causes which arrested its upward flight--the attraction of the earth4 ,in its descent it is retarded by the resistance of the air1 as it was in its ascent4 ,this retardation is very obvious in the case of substances which present a large surface to the air1 as a feather4 ,a small piece of lead will outweigh many feathers1 and therefore1 as its quantity of matter is so much greater in proportion to its surface than that of a feather1 it will fall to the ground much more quickly4 ,that this is owing wholly to the resistance of the air can be proved with the air-pump4 ,illustration3 ,fig4 #abh4 ,suppose that you have a tall receiver1 ,fig4 #abh1 on the air-pump1 and a piece of lead and a feather are placed at its upper part in such a way that they can be made to fall at the same instant4 ,exhaust the air1 and then let them fall4 ,they will go down side by side1 as represented by the figure1 and reach the bottom of the receiver at the same time1 because there is no air there to resist the progress of the feather4 ,the toy called the water-hammer illustrates the same thing4 ,when water falls through the air the resistance of the air tends to separate its particles1 as we see in the falling of water thrown up by a fountain4 ,in the water-hammer1 which is a closed tube containing a little water and no air1 when the water is made to fall from one end to the other1 as there is no air to divide it1 it falls as one mass1 and gives a sharp sound like the blow of a hammer4 ,an instrument essentially like this can be made with a thin glass flask4 ,put a little water in it1 and1 after heating it to boiling over a spirit-lamp1 cork the flask tightly1 and then leave the water to cool4 ,as all the space above the water was filled with steam when the flask was corked1 it is a vacuum now that the steam is condensed4 ,illustration3 ,fig4 #abi4 #aic4 ,relation of ,bulk to the ,resistance of ,liquids and ,gases4--,you have already seen1 in #aib1 that the more surface a body has in proportion to its weight the greater is the resistance of the air to its motion4 ,this truth1 which applies to liquids as well as to airs or gaseous substances1 explains the fact that small bodies meet with proportionately more resistance than large ones4 ,the body ,b1 ,fig4 #abi1 you see is made up of eight cubes of the size of the cube a1 that is1 it has eight times the quantity of matter that a has4 ,now if ,b were moving through air or water1 any of its sides pushing the water before it would meet with only four times the resistance that a side of a would1 for its surface is only four times as large1 and yet the body is eight times as large as a4 ,and the greater the difference of size the greater is the difference of resistance4 ,if ,b were a cube twenty-seven times as large as a it would meet with only nine times as much resistance4 ,you see here the reason that shells and cannon-balls can be thrown much farther than bullets and small shot4 ,the sportsman does not throw away his shot by foolishly aiming at birds at great distances1 and yet shells and large cannon-balls can be thrown the distance of several miles4 ,the difference is not in the degree of velocity which the powder produces1 but in the resistance of the air4 ,it is for the same reason that rain falls with greater rapidity than drizzling mist4 ,as liquids and aeriform substances resist solids in motion in proportion to the amount of surface which the solids present to them1 so also when they strike against solids they cause motion in them in proportion to the amount of surface acted upon4 ,thus a violent wind could not move a lump of tin1 but could blow along a sheet of it1 or tear up a roofing of it if it got beneath4 ,so clouds of sand are raised into the air in the deserts of ,africa1 although the particles are of the same material as stones1 and therefore have the same specific gravity4 ,for the same reason dust1 feathers1 the down and pollen of flowers1 etc41 are blown about1 although they are heavier than the air4 ,a pebble is moved more easily by a current of water than a stone1 because it has a larger surface1 in proportion to its weight1 to be acted upon by the water4 ,for the same reason sand is moved more easily than pebbles1 and fine mud than sand1 though stones1 pebbles1 sand1 and mud may all be of the same material4 ,this explains why you will find mud where the current is slow1 sand where it is faster1 pebbles and stones where it is still faster1 and where the current is exceedingly rapid you find nothing but large rocks--sand1 pebbles1 and stones not being able to resist its force4 ,for the same reason1 in the process of winnowing1 the chaff is carried away by the wind2 while the grain1 presenting less surface in proportion to its weight to be acted upon by the air1 falls to the floor4 ,in all the above cases the moving water or air may be considered as acting in opposition to the attraction of the earth1 the latter pulling the substance down to the earth1 and the former pushing it away from the earth4 ,of course1 the more surface the water or air has to push upon the greater is the effect2 and it is to be remembered that the attraction of gravity is as the quantity of matter1 without any regard to amount of surface in the body attracted4 ,illustration3 ,a ,b ,c ,d ,fig4 #acj4 #aid4 ,relation of ,force to ,velocity4--,it would seem at first thought that the motion produced in any body must be in exact proportion to the force producing it2 that is1 that twice the force which produces a given velocity would double that velocity1 and three times would treble it1 etc4 ,this is true where there are no obstacles to motion1 as in the case of the heavenly bodies moving in their orbits4 ,but in all motions here upon the earth there are obstacles2 and as reaction is always equal to action1 the greater the velocity the greater is the reaction of the obstacle4 ,if1 therefore1 you increase the velocity of any body1 you not only have to communicate more motion to it1 but you must overcome also the increased reaction4 ,the rate of increase of force for increased velocities has been very accurately ascertained4 ,this ,i will explain4 ,a boat moving from ,b to ,a1 ,fig4 #acj1 we will suppose1 displaces a quantity of water represented by the space between the two lines extending from ,b to ,a4 ,now if it move from ,b to ,c1 it displaces twice the bulk of water ,b ,c2 and as it is displaced in the same time that ,b ,a was1 each particle is displaced with twice the velocity4 ,double the force is required to displace a double portion of water1 and to do this with double the velocity the force must be doubled again4 ,so if the boat is made to move three times as far in the same time1 that is from ,b to ,d1 three times the quantity of water is displaced1 and each of these three portions1 ,b ,a1 ,a ,c1 and ,c ,d1 is displaced with three times the velocity4 ,the force required1 then1 to do this is nine times that required to carry the boat from ,b to ,a in the same time4 ,it is plain1 therefore1 that with velocities represented by the numbers #a1 #b1 #c1 #d1 etc41 the forces requisite to produce these velocities must be as the squares of these numbers2 viz41 #a1 #d1 #i1 #af1 etc4 ,this law is a very important one in a practical point of view4 ,for example1 it shows us how much larger a quantity of coal is required to produce in steamboats a high velocity than a moderate one4 ,its application too to the science of gunnery is important4 #aie4 ,relation of ,shape to ,velocity4--,the resistance of air or water to a flat surface is greater than to a convex one1 because the latter readily turns the particles to the one side and the other4 ,so1 also1 a concave surface is resisted much more than a flat one1 because the particles of the air or water can not so easily escape sideways4 ,fishes are of a spindle-like and slender shape1 that they may have as little resistance as possible from the water4 ,it is for this reason that a fish has no neck1 for if it had one the upper portion of its body would1 from the resistance of the water striking against it1 prove a serious impediment to rapidity of motion4 ,mankind have in some measure imitated the shape of fishes in their boats and ships4 ,boats which are intended to bear light burdens and go swiftly are made very long and narrow4 ,the webbed feet of water-fowls1 when they are moved forward1 are folded up so as to meet with as little resistance as possible2 but when they are moved backward they are spread out so as to press against the water a broad concave surface4 ,for the same reason the wings of a bird are made convex upward and concave downward2 and when it moves its wing upward it makes it cut the air somewhat edgewise1 but in moving it downward it presses directly with the whole concave surface4 #aif4 ,friction4--,friction is generally an obstacle to motion4 ,when we roll a ball1 the more rough is the surface on which we roll it the greater is the friction and the sooner is the ball stopped4 ,friction lessens the rapidity of motion in machinery1 and to prevent this as far as possible oiling and other expedients are employed4 ,but sometimes friction is a cause of motion1 as1 for example1 the friction of the driving-wheels of a locomotive upon the rails4 ,in this case the wheel pushes backward on the rail at each successive point of contact4 ,to make this clear1 suppose a common wheel is deprived of its rim and is made to revolve on the ends of its spokes4 ,the end of each spoke gives a backward push as it strikes the ground4 ,now the rim of a wheel makes the same pushes1 but they are more numerous--they are continuous1 being made by all the successive points in the rim4 ,sometimes the rails of a railroad are too smooth from frost or some other cause1 and then sand is thrown upon them to give the locomotive a start4 ,the sand serves to prevent the wheels from sliding by enabling them to get some hold upon the rails in their backward pushes4 #aig4 ,friction of ,liquids in ,tubes4--,so easily does water flow along that we should not at first view suppose that it would be delayed much from friction as it passes through pipes or along channels4 ,but the retarding influence is considerable4 ,an inch tube #bjj feet long1 lying horizontally connected with a reservoir1 will discharge water not one quarter as fast as an inch orifice in the side of the reservoir4 ,sudden turns in a pipe should be avoided1 because they occasion so much friction against the sides of the pipe and among the particles of water by disturbing the regularity of the current4 ,in the entrance of the arteries into the brain1 in order to prevent the blood from flowing too rapidly into this organ1 there are sudden turns in the arteries to retard the blood2 and in grazing animals1 as there is special danger that the blood will flow too freely to the brain as the head is held down in eating1 there is a special provision to prevent this in a net-work of arteries4 ,if the arteries of the brain in such animals were straight tubes they would continually be dying of congestion of the brain or of apoplexy4 ,illustration3 ,fig4 #aca4 ,friction in a small pipe is greater in proportion to its size than in a large pipe4 ,in a pipe an inch in diameter water will not move more than one-fifth as fast as in a tube two inches in diameter4 ,this may be made clear by ,fig4 #aca1 in which is represented the area of a small tube inside of the area of a tube of twice its diameter4 ,suppose the effect of the friction in the large tube to extend in to a4 ,in the small one it will extend in as far1 that is1 to b4 ,but e a is about five times as long as e b1 so that there is full five times as much water clear of friction in the large tube as there is in the smaller one4 ,illustration3 ,fig4 #acb4 #aih4 ,friction in ,streams4--,the retarding effect of friction is very obvious in brooks and rivers4 ,the water in the middle of a stream runs much more rapidly than it does near its banks4 ,when a river is very shallow at its sides the water there scarcely moves1 though in the middle the water may be running at a rapid rate4 ,a tide1 therefore1 flowing up a river1 moves more freely near its banks than it does in the middle of the stream1 because it meets with less resistance there from the downward current4 ,water moves less rapidly at the bottom of a river than it does at the surface4 ,for this reason1 if a stick be so loaded at one end as to stand upright in water1 in the current of a river its upper end will be carried along faster than its lower end1 and therefore it will incline forward1 as in ,fig4 #acb4 ,as the sea rolls in over a beach1 each wave at length pours over its crest and breaks1 because the lower part of the wave is retarded by friction on the beach4 ,were it not for the constant retardation of friction at the sides and bottom of rivers1 and at their bends1 those rivers which have their rise at a considerable height above the level of the sea would acquire an immense velocity4 ,thus the ,rhone1 drawing its waters from #ajjj feet above the level of the ocean1 would pour them forth with the velocity of water which had fallen perpendicularly the same height1 that is1 at the rate of #agj miles an hour1 did not friction continually diminish the velocity4 #aii4 ,waves4--,waves are generally formed by the friction of air upon water4 ,observe how they are formed4 ,as soon as any portion of water is raised above the general surface it tends by gravity to fall to a level with the water around it1 and in doing so the portion next to it is forced upward1 forming another wave2 and so one wave produces another1 each one being smaller than the preceding1 till at length the motion is wholly lost4 ,this is always the process when the cause of the motion is a single impulse1 as when a stone is dropped into the water4 ,but when the waves are produced by a succession of impulses1 as when wind makes them1 they are mostly of the same size4 ,it is quite a common notion that the water moves as rapidly as the waves appear to do2 but the water really remains nearly stationary1 rising and falling1 while merely the form of the wave advances4 ,the same wave is made up continually of a succession of different portions of water1 or rather it is a succession of different waves4 ,this is very well illustrated by the waving of a rope or carpet4 ,in an open sea a wave slopes regularly on either side2 but when it comes near the shore1 for the reason given in #aih1 it grows more and more nearly perpendicular on the side toward the shore1 till at length it falls over1 and if it be very large the roar thus caused by its breaking is heard to a great distance4 #bjj4 ,height of ,waves4--,so awful1 says ,dr4 ,arnot1 is the spectacle of a storm at sea that it is generally viewed through a medium which biases the judgment2 and lofty as waves really are1 imagination pictures them loftier still4 ,now no wave rises more than ten feet above the ordinary sea-level1 which1 with the ten feet that its surface afterward descends below this1 gives twenty feet for the whole height from the bottom of any water-valley to an adjoining summit4 ,this proposition is easily verified by a person who tries at what height on a ship's mast the horizon remains always in sight over the top of the waves--allowance being made for accidental inclinations of the vessel1 and for her sinking in the water to much below her water-line1 at the time when she reaches the bottom of the hollow between two waves4 ,the spray of the sea1 driven along by the violence of the wind1 is of course much higher than the summit of the liquid wave2 and a wave1 coming against an obstacle1 may dash to a great elevation above it4 ,at the ,eddystone ,light-house1 when a surge breaks which has been growing under a storm all the way across the ,atlantic1 it dashes even over the lantern at the summit4 #bja4 ,momentum4--,the momentum of a body is its force when in motion4 ,in estimating the momentum of any body two things must be considered--its velocity1 and its quantity of matter or weight4 ,a bullet fired from a gun has a vastly greater force1 or power of overcoming obstacles1 than one thrown by the hand1 from its greater velocity4 ,now suppose the weight or quantity of matter to be increased ten times1 and that it moves with the same velocity as before1 it will have ten times as much force as before1 and will overcome ten times as great an obstacle4 ,for this reason a small stone dropping upon a man's head may do but little harm1 while one ten times as large1 falling from the same height1 may stun and perhaps kill him4 ,but if the large stone could fall with only one-tenth of the velocity of the small one1 the effect of both would be the same4 ,let this example illustrate the rule for calculating the momentum of moving bodies1 viz41 multiply the quantity of matter into the velocity3 ,let the weight of the small stone be #a ounce1 and that of the large one #aj ounces4 ,if they fall from a height of #af feet the force with which the large one will strike will be expressed by #afj 7#afaj71 that of the small one by #af 7#aaf74 ,suppose1 now1 that by some force in addition to gravity the small one could be made to move ten times as fast as the large one1 the force with which it would strike would be equal to that of the large one1 and would be expressed by the number #afj4 ,illustration3 ,fig4 #acc4 ,i will illustrate this in another way4 ,let a and b1 ,fig4 #acc1 be two balls of clay of equal size hanging over a graduated arc4 ,now if b be let fall from the top of the arc1 #f1 on striking against a it gives half of its motion to a1 and they both move on together4 ,but how far will they go8 ,to #c1 on the other side of the arc4 ,why8 ,let the quantity of matter in each ball be called #a1 and the motion of b #f4 ,the momentum will therefore be #f4 ,now the momentum of the two together will be the same after the blow as that of b was before it4 ,but the quantity of matter is twice as great1 and must be called #b4 ,therefore the motion must be represented as #c1 to make the momentum #f 7#bc74 ,but suppose that b is twice as large as a4 ,falling from #f1 its momentum would be represented by #ab 7#bf74 ,after it has struck a1 the momentum of the two together would be the same as that of b before the stroke2 but the quantity being #c1 the motion would be represented by #d4 ,they would therefore move to #d on the arc4 #bjb4 ,examples4--,a few examples illustrating momentum as being compounded of quantity of matter and velocity will suffice4 ,if a musket-ball of an ounce weight were so much spent as to move with only a velocity of a foot in a second1 its force would be so small that if it hit any one it would do no harm4 ,but a cannon-ball weighing a thousand ounces moving at this slow rate would have a very great force--equal1 in fact1 to the momentum of an ounce ball moving #ajjj feet in a second4--,if a plank push a man's foot against a wharf he will scarcely feel it2 but if the plank1 instead of being alone1 is one of a thousand planks fastened together in a raft1 and the whole move with the same velocity1 the force will be increased a thousand-fold1 and the plank will crush the foot4 ,so1 also1 if the one plank when alone should move a thousand times as fast as the whole raft1 the same result would follow4--,so soft a substance as a candle can be fired through a board from the momentum given to it by an immense velocity4--,perhaps there is no better example of the great force given to a substance by an enormous velocity than we have in the wind4 ,so light a thing is air that people think of it as almost nothing4 ,but let it be set in rapid motion1 and the velocity gives to it a force1 a momentum1 which will drive ships upon the shore1 throw over buildings1 and tear up trees by the roots4 ,in this last example we see beautifully illustrated the meaning of the expression quantity of motion4 ,in the moving air each particle does its share of the work in the destructive effects mentioned4 ,each particle1 therefore1 may be considered as a reservoir of motion1 and the quantity of motion in any case depends upon the quantity which each particle has and the number of the particles4 #bjc4 ,production of ,great ,velocities4--,when there are no obstacles to motion great velocities may be produced by a single impulse4 ,thus at the beginning the ,creator gave a single impulse to each of the heavenly bodies1 producing enormous velocities1 which continue unaltered year after year and age after age1 because these bodies fly in their orbits through space where there is no resistance of any thing like air to retard the motion4 ,but in all the motions that we see around us there are obstacles continually retarding them2 and therefore no very rapid motion is produced by any single impulse1 but a succession of impulses is required to accumulate sufficient momentum so to overcome the obstacles as to secure a great velocity4 ,i will give a few examples in illustration4 ,one of the best examples we have in the fall of bodies to the earth4 ,you know that the greater the elevation from which a body falls the greater is its velocity1 and therefore the greater the force with which it strikes4 ,why is this8 ,if it fell because of a single impulse making it go toward the earth1 this would not be the case1 and if there were no air in the way the velocity would be uniform2 but the resistance of the air would retard the velocity1 so that if a number of bodies should receive the same impulse at different elevations1 the one the farthest off would be the most retarded1 and therefore come down slower than all the rest4 ,in this case1 the higher the elevation from which a man should fall the less would be the injury4 ,but a body does not come to the ground by a single impulse1 but by a succession of impulses1 or rather a continued impulse4 ,every moment that the body is coming down it is drawn upon by the attraction of the earth1 and this continued action of the cause of the motion makes it continually increase in rapidity4 ,it is on the same principle of continued action that a man lifts his hammer high when he wishes to inflict a heavy blow4 ,in this case both gravitation and the muscular power of the arm exert their force on the hammer through the whole space4 ,a horse in kicking does the same thing1 and by the great length of the leg the velocity given to the foot by this continued action of the muscles is very great4 ,an arrow is not shot by a single momentary impulse of the bow-string1 but the string1 by following it through a considerable space1 gives it a continued impulse4 ,the action of gunpowder upon a bullet issuing from a gun is apparently an instantaneous and single impulse1 but it is not really so4 ,the great velocity given to the bullet is given to it by the continued impulse of the expansive force of gases produced from the powder1 and it therefore depends much on the length of the barrel4 ,if this be short1 the force of the powder is not confined long enough to the bullet to give it a great velocity4 #bjd4 ,arrest of ,great ,velocities4--,as a continued force is required to produce great velocities1 so a continued resistance is necessary to arrest them4 ,it is by the gradual or continued resistance of the air that the motion of a cannon-ball is destroyed4 ,now if instead of this gradual resistance any hard substance1 as a block of granite1 were opposed to the progress of the ball1 it would be at once broken asunder4 ,we see then the reason that a hard substance of moderate thickness does not offer so effectual a resistance to a body moving very rapidly as some substance of a more yielding kind and of greater bulk4 ,for example1 a bale of cotton will arrest a ball which would pass through a plank1 for the cotton yielding easily permits the force of the ball to be felt and resisted by a larger bulk1 while the wood1 not yielding1 opposes but a small portion of its whole bulk to the force of the ball1 and therefore does not arrest it2 in other words1 the momentum of the ball is communicated to a much larger quantity of matter in the cotton than in the wood4 ,these principles afford a ready explanation of a feat which is sometimes performed4 ,a man lies upon his back1 and1 having an anvil carefully placed upon his chest1 allows some one to strike a heavy blow with a hammer upon the anvil1 and no injury is received4 ,why8 ,because the momentum or force of the hammer is diffused throughout the bulk of the anvil1 and then again through the bulk of the yielding chest4 ,the man takes good care to have his lungs well filled with air at the moment of the blow1 for this increases the bulk and elasticity of the chest1 and thus promotes the diffusion of the momentum4 ,if the blow of the hammer were received directly upon the chest great injury would be done1 for the force would now be spent upon one small spot alone4--,the principles above elucidated are applied by men instinctively in their common labors and efforts4 ,you see a man catching bricks that are tossed to him4 ,as he receives the bricks into his hands he lets his hands and the bricks move together a little way1 so that he may gradually arrest the motion of the bricks4 ,to do it suddenly would give him a painful lesson on momentum4 ,so when a man jumps from a height he does not come to the ground in a straight position4 ,this would cause a sudden and therefore a painful arrest of the motion of the whole body4 ,to avoid this he comes to his feet with all the great joints of his body bent1 so that the different portions approach the ground successively1 his head having its motion arrested last4 ,illustration3 ,fig4 #acd4 #bje4 ,communication of ,motion in ,elastic ,bodies4--,momentum is transferred from one body to another very differently in elastic from what it is in non-elastic bodies4 ,as you saw in #bja1 when one non-elastic body strikes upon another the momentum is divided between them1 and both move on together4 ,now if a and b1 ,fig4 #acc1 were elastic bodies1 as ivory balls1 and b should be let fall against a1 it would give all its momentum to a4 ,therefore b would stop1 and a would move on to the same height from which b came4 ,the reason is1 that the velocity lost by b and received by a is just double what it would be if the balls were non-elastic4 ,for the same reason1 if a and b1 being elastic1 meet each other from equal heights on the arc1 they will both rebound1 and return to the same heights from which they came4 ,but if non-elastic they simply destroy each other's momentum and stop4 ,the effect produced in the former case is just twice as great as in the latter1 as you may see by reckoning on the arc4 ,for the same reason1 too1 if you have a row of elastic balls1 as in ,fig4 #acd1 and let a fall from the point i upon b1 it will stop there2 and communicating all its momentum to b1 this momentum will pass from b to c1 and so on through all the row of balls to e1 the last one1 which will fly off to the point h1 at the same height with i1 the point from which a fell4 ,if b be held still1 and a be let fall upon it1 a will rebound to the height from which it fell1 for then the compressed elastic spring 7 #ci7 of each ball1 as b is immovable1 communicates all the motion to a4 ,it is for this reason that an elastic ball1 on being thrown against any thing fixed1 rebounds4 ,if what it is thrown against be perfectly elastic it rebounds with a force equal to that with which it is thrown4 ,illustration3 ,fig4 #ace4 #bjf4 ,reflection of ,motion4--,if an elastic body be thrown perpendicularly upon a surface it rebounds in the same path in which it is thrown4 ,but if it hit the surface obliquely it is thrown off or reflected in a different direction4 ,thus a ball thrown from b upon c1 ,fig4 #ace 7p4 #aeh71 will return in the line drawn to b4 ,but if it be thrown from d it will be reflected in the line c a4 ,now the angle d c b1 called the angle of incidence1 is always equal to b c a1 the angle of reflection4 ,the same1 you will find in other parts of this book1 is true of sound and light and heat4 #bjg4 ,uniformity of ,motion4--,since motion1 when once begun1 is disposed to continue unless arrested by obstacles1 it is naturally uniform both in its velocity and its direction4 ,i will speak now only of velocity4 ,suppose a body to be set in motion1 and to meet with no opposition from friction1 or the resistance of air1 or attraction1 it would move on forever1 and with the same velocity with which it began4 ,now precisely these circumstances we have in the motion of the heavenly bodies in their orbits4 ,they are1 it is true1 under the influence of attraction1 but in such a way1 as you will soon see1 as not to interfere with the uniformity of their motion4 ,were it not for this uniformity we should have no regularity of times and seasons4 ,it is only by the uniform motion of the earth round the sun1 and round its own axis1 that we can calculate for to-morrow1 or next week1 or next year4 ,if these motions were irregular it would throw confusion into all our calculations for the future and all our recollections of the past4 ,we can measure time by nothing else but regular motion1 and were there no regular motion we should have merely the very inaccurate measure furnished by our sensations4 ,to measure time with accuracy we take some great and extensive uniform motion as our standard4 ,thus1 the revolution of the earth around the sun we take as one division of time1 and call it a year4 ,we observe that during this time it whirls around on its own axis #cfe times1 and the time occupied by each of these revolutions we call a day4 #bjh4 ,the ,pendulum4--,various modes of measuring time have been adopted by mankind4 ,at first time was inaccurately divided by merely observing the sun4 ,but after a while man resorted to various contrivances to measure short periods of time with accuracy4 ,all of these depend upon the uniformity of motion alone4 ,the sun-dial measures time by the uniform movement of the shadow on its face1 caused by the uniform movement of the earth in relation to the sun4 ,the hour-glass measures time by the uniform fall of sand produced by the attraction of gravity4 ,the best measurement of time is by the comparatively modern invention of clocks and watches1 in which time is divided into very minute periods by the uniform motion of the pendulum or the balance-wheel4 ,the pendulum furnishes an interesting example of motion kept up by the influence of gravity4 ,it was not till the time of ,galileo1 less than three centuries ago1 that its operation was understood and appropriated to the measurement of time4 ,he observed that chandeliers hanging from lofty ceilings vibrated very long and uniformly after they were accidentally agitated1 and the thought of the philosopher evolved from this phenomenon the most important results4 ,though it had been before men's eyes in some shape or other since the creation1 it was reserved for ,galileo to observe its significance1 and the result is that the pendulum has become man's time-keeper over the whole earth4 ,illustration3 ,fig4 #acf4 #bji4 ,explanation of its ,operation4--,a pendulum consists commonly of a ball or weight at the end of a rod suspended so as to vibrate with little friction at the point of the suspension4 ,let a b1 ,fig4 #acf1 represent such a pendulum4 ,when it is at rest it makes a plumb-line hanging toward the centre of the earth4 ,if it be raised to c and be left to fall1 the force of gravity will not only carry it to b1 but1 by the accelerated velocity or accumulated momentum which it gives it in its descent1 it will carry it to d4 ,the same would be true of its return from d4 ,and it would vibrate forever in this way if it could be entirely freed from the resistance of the air and friction4 ,but1 as it is1 the pendulum left to itself gradually loses its motion from these obstacles4 ,in the common clock the office of the weight is to counteract the influence of these obstacles1 and keep the pendulum vibrating4 ,in the watch the mainspring performs the same office to the balance-wheel4 ,illustration3 ,fig4 #acg4 ,the times of the vibrations of a pendulum are nearly equal whether the arc it describes be great or small4 ,for when the vibration is a large one the velocity which the pendulum acquires in falling is greater than when the vibration is of small extent4 ,the reason is that the higher it rises the more steep is the beginning of its descent4 ,thus a c1 ,fig4 #acg1 is steeper than c b4 ,illustration3 ,fig4 #ach4 ,fig4 #aci4 #baj4 ,gridiron ,pendulum4--,the longer a pendulum is the longer time does its vibration occupy4 ,it requires a pendulum of the length of a little over thirty-nine inches to vibrate seconds4 ,cold weather1 by contracting the pendulum1 makes it vibrate quicker than in summer1 and so makes the clock go faster4 ,various contrivances have been resorted to in order to counteract the variation of length in pendulums by heat and cold1 but what is called the gridiron pendulum is the best4 ,in this pendulum an ingenious use is made of the fact that heat expands brass nearly twice as much as it does steel4 ,a simple form of this pendulum is given in ,fig4 #ach4 ,the middle rod is made of brass1 and the side rods1 b and c1 of steel4 ,suppose that the brass rod expands or increases in length half an inch4 ,the rod c would be drawn upward by it1 and the rod b downward1 each one quarter of an inch2 but this effect is counteracted by the expansion of each steel rod1 which is half that of the brass1 that is1 one quarter of an inch4 ,the ball d1 therefore1 always retains the same distance from the point of suspension1 e4 ,in ,fig4 #aci you have a gridiron pendulum of a more compound character1 a part of the bars being steel1 and a part brass4 #baa4 ,motion ,disposed to be ,straight4--,when a body is set in motion1 if it be left to itself--that is1 if nothing interfere with its motion--it will move in a perfectly straight line4 ,it requires some interference from some force to bend the motion4 ,you will readily see from the views which ,i have given you that there never is any motion that is1 strictly speaking1 straight1 because every motion is in some measure compound2 that is1 each cause of motion is modified in its action by other causes of motion4 ,but we can approximate very nearly to straight motion by making one cause preponderate very much over other causes4 ,this ,i will illustrate4 ,if we fire a bullet horizontally from a gun it is acted upon by three forces3 the propulsive force of the powder1 the resistance of the air1 and the attraction of the earth4 ,the action of the second of these is in direct opposition to the first1 and therefore only retards the motion1 and does not tend at all to turn it from its straight course4 ,this is seen in the fact that the ball is turned neither to the right hand nor to the left4 ,but the third force tends to make the ball bend its course toward the ground4 ,it does this from the instant that the ball leaves the gun throughout its flight1 but so slightly that practically we can consider the ball as going straight for short distances4 ,when we take a long range we must make allowance for this bending down of the motion4 ,accordingly1 for the sake of precision1 a double sight is provided in modern guns1 as seen at ,a and ,b1 ,fig4 #adj4 ,this you see secures the pointing of the gun a little above the level of the object aimed at1 that level being indicated by the dotted line4 ,illustration3 ,fig4 #adj4 ,illustration3 ,fig4 #ada4 ,the greater is the propulsive force the more nearly to a straight line is the path of the propelled body4 ,this may be seen very clearly in ,fig4 #ada1 representing the issuing of water at different points from a vessel4 ,as pressure in a liquid is as depth1 #aba1 the force with which the water is thrust out is greater at ,c than at ,b1 and at ,d than at ,c4 ,the issuing stream1 therefore1 is most nearly straight at the lowest point1 ,d4 ,the motion of projectiles1 thus alluded to1 will be more particularly noticed farther on4 ,illustration3 ,fig4 #adb4 ,illustration3 ,fig4 #adc4 #bab4 ,compound ,straight ,motion4--,we call that motion compound which is produced by two or more forces acting upon the body4 ,this may be straight or curved4 ,i will first speak of the straight4 ,if a man attempt to row a boat straight across a river1 the point which he will reach will not be directly opposite to that from which he started1 but below4 ,two forces act upon the boat3 the current tending to carry it straight down the stream1 and his rowing tending to carry it straight across4 ,the boat will go in neither of these directions1 but in a line between them4 ,let ,a ,b1 ,fig4 #adb1 represent the bank of the river1 from which he starts at ,a1 with the bow of the boat pointing to ,c1 on the opposite bank4 ,suppose now that in the time that it takes him to row across the current would carry him down to ,b if he did not row at all4 ,he will in this time1 by the two forces together1 reach the point ,d1 opposite to ,b1 his course being the line ,a ,d4 ,so if the wind blow upon a vessel in such a way as to carry it eastward1 and a current is pushing it southward1 the vessel will run in a middle line1 viz41 southeast4 ,for the same reason if a boy kick a foot-ball already in motion1 it will not be carried in the direction in which he kicks it1 but in a line between that direction and the direction in which its former motion was carrying it4 ,in swimming1 flying1 rowing1 etc41 we have examples of compound motion1 the middle line between the directions of the forces always being taken by the body moved4 ,illustration3 ,fig4 #add4 ,illustration3 ,fig4 #ade4 ,if we take ,fig4 #adb1 illustrating the movement of the boat1 and draw two lines1 one from ,a to ,c and the other from ,b to ,d1 we shall have the parallelogram ,a ,c ,d ,b1 ,fig4 #adc1 in which the line ,a ,c represents the force of the rowing1 ,a ,b the force of the current1 and ,a ,d the path of the boat4 ,you see1 then1 that if we wish to find in what direction and how far in a given time a body acted upon by two forces will move1 we are to draw two lines in the direction of these forces1 and of a length in proportion to the distances to which they would move it in that time2 then by drawing two lines parallel to these we shall have a parallelogram1 and the diagonal of this will represent the distance and the course of the moving body4 ,if a body be acted upon by two equal forces and at right angles to each other1 the figure described will be a square1 as you see in ,fig4 #add4 ,if they vary from being at right angles to each other the figure will vary in the same proportion from the square figure1 as seen in ,figs4 #ade and #adf4 ,in the three figures ,a ,b and ,a ,d represent the two forces1 and ,a ,c the resulting motion4 ,you observe by these diagrams that the nearer the two forces come to being in the same direction the farther will they move the body4 ,illustration3 ,fig4 #adf4 ,illustration3 ,fig4 #adg4 ,you see this in the different lengths of the diagonals in ,fig4 #add and ,fig4 #adf4 ,the more nearly1 therefore1 the wind coincides with the current the more rapidly will a vessel be carried along before the wind4 ,when1 on the other hand1 the angle at which two forces act upon a body is much greater than a right angle1 they will propel it but a small distance4 ,thus if two forces act on a body in the directions ,d ,a and ,d ,c1 ,fig4 #adg1 they will move it only the distance represented by the diagonal ,d ,b4 ,this diagram represents the motion of a vessel sailing almost directly against a current by a wind the force of which is equal to that of the current1 while ,fig4 #adf represents the motion of a vessel where wind and current being of equal force very nearly coincide4 ,in the above diagrams ,i have supposed the forces to be equal2 but the same truth can be shown in regard to unequal forces as seen in ,fig4 #adc4 #bac4 ,curved ,motion4--,no single impulse can produce a curved motion4 ,neither can two or more impulses communicated at one time4 ,in both of these cases the motion would be in a straight line4 ,curved motion may be produced by two forces1 one of which gives it a single impulse1 and the other acts upon it continuously4 ,a familiar example you have in a ball whirled around at the end of a string4 ,you can give it an impulse1 and then1 holding it in your hand1 let it whirl4 ,here the impulse you give the ball is one force1 and the tension of the string is the other1 the latter acting continuously4 ,your hand holding the end of the string is the centre about which the motion revolves2 the impulse which you have given the ball tends to make it fly away from the centre in a straight line1 and hence is called the centrifugal force2 the tension of the string keeps it from thus flying off1 and so is called the centripetal force4 ,when the earth1 at the creation1 was put in motion it would have moved in a perfectly straight line1 were it not constantly drawn toward the sun by attraction1 the continuous action of this latter force being the same as the tension of the string in the case of the whirling ball4 ,the force of attraction1 then1 is the centripetal force of the earth1 and the impulse which was given to it by the ,creator in the beginning is its centrifugal force2 and1 balanced between these two forces1 the earth and all the heavenly bodies move uniformly onward in their orbits4 ,the centrifugal force you see in these illustrations is simply the tendency of motion to a straight line from the inertia of matter2 and this is constantly counteracted by the centripetal force4 ,illustration3 ,fig4 #adh4 #bad4 ,illustrations of ,centrifugal ,force4--,when a wet mop is whirled the water flies off in every direction by its centrifugal force4 ,on the same principle a dog1 coming out of the water1 shakes off the water by a semi-rotary motion4--,when a suspended bucket of water is turned swiftly around the water rises high on its sides1 and leaves a hollow in the middle4 ,it is the tendency to fly away from the centre of motion that causes this4--,large wheels1 revolving with great velocity1 have been broken by the centrifugal force of its particles1 and hence the necessity of having such wheels made very strong4 ,the immense grindstones used in gun-factories have sometimes been broken through in the middle1 or have flown into pieces from the same cause4--,a man riding horseback on turning a sharp corner inclines his body toward the corner1 to avoid being thrown off by the centrifugal force4 ,so1 in the feats of the circus1 a man standing on a horse running at full speed around the ring inclines his body strongly inward1 as you see in ,fig4 #adh 7p4 #afg74 ,the horse also instinctively inclines in the same direction for the same reason4 ,if the rider finds himself in danger of falling1 by making the horse go a little faster1 thus adding to the centrifugal force1 the difficulty is relieved4--,the centrifugal force is made use of in milling4 ,the grain is admitted between two circular stones by a hole in the centre of the upper one1 and as the stone revolves it constantly moves toward the circumference1 and there escapes as flour4 ,illustration3 ,fig4 #adi4 #bae4 ,bends in ,rivers4--,we see the operation of the centrifugal force in the bends of rivers4 ,when a bend has once commenced in a river it is apt to increase1 for as the water sweeps along the outer bank of the bend it presses strongly against it1 just as the water in the whirled bucket1 #bad1 presses against its sides1 by its centrifugal tendency1 or1 in other words1 its tendency to assume a straight motion4 ,of course the result is a wearing away of this outer bank1 and in proportion to the looseness of the material of which it is composed and the velocity of the river's current4 ,and when one bend is formed another is apt to form below1 but in an opposite direction4 ,the water1 by sweeping along the bend a1 ,fig4 #adi1 is directed by it toward the opposite bank at b1 and makes a bend there also4 ,illustration3 ,fig1 #aej4 ,it is in this way that a river1 running through a loose soil1 the ,mississippi1 for example1 acquires a very serpentine course4 ,as the water in the whirled bucket rises around the sides1 so in the river the water will be higher against the bank a than on the opposite side4 ,eddies and whirlpools are produced on the same principles1 when water is obliged to turn quickly around some projecting point4 ,if a current were moving swiftly along the shore a toward the point b1 ,fig4 #aej1 it would be directed outward by the resistance of this projection1 and so a depression would be left at c1 just behind it1 and this depression would be surrounded by a revolving edge of water4 ,illustration3 ,fig4 #aea4 #baf4 ,application of the ,centrifugal ,force in the ,arts4--,much use is made of the centrifugal force in the arts1 but ,i will give but two examples4 ,in the art of pottery the clay is made to revolve on a whirling table1 the workman at the same time giving the clay such shape as he chooses with his hands and various instruments4 ,in doing this he constantly has reference to the centrifugal force1 giving the table a velocity proportioned to the amount of this force which is needed in each stage of the operation4 ,the most beautiful application of this force that ,i have ever witnessed is in the manufacture of common window-glass4 ,the glass-blower gathers up on the end of his iron tube a quantity of the melted glass1 and blows it out into a large globe4 ,when it is of sufficient size and thinness he places it on a rest1 as you see in ,fig4 #aea 7p4 #afi74 ,a second man now comes with a rod having some melted glass on the end1 and attaches this to the globe at a point opposite to that where the tube of the first man is joined to it4 ,there now comes a boy1 and1 giving this tube a quick blow1 severs its connection with the globe1 leaving a hole in the globe where the glass breaks out4 ,the second man1 having the globe attached to his rod1 carries it to a blazing furnace1 and resting the rod on a bar at its mouth1 puts the globe directly into the flame4 ,the glass is soon softened1 and he whirls the globe continually around4 ,the hole in the globe enlarges by the centrifugal force1 and at length by this force the globe is changed into a flat1 circular disk4 ,panes of glass which are called bull's-eyes are cut from the centres of these disks4 ,illustration3 ,fig4 #aeb4 #bag4 ,steam-,governor4--,the operation of the centrifugal force is beautifully exemplified in this regulator of the steam-engine4 ,it consists of two heavy balls1 ,fig4 #aeb1 suspended by bars from a vertical axis1 the bars being connected to the axis by hinges4 ,the bars have also a hinged connection at their lower ends with two smaller bars1 and these latter have a similar connection with a collar that slides up and down on the axis4 ,now the faster the axis turns the farther the balls fly out from it1 from the centrifugal force1 and the higher the collar slides up on the axis4 ,from the collar extends1 as you see1 a lever4 ,this is connected with a valve in the steam-pipe1 and so regulates the amount of steam that enters the working part of the engine4 ,the object of this ingenious contrivance is to make the engine regulate its own velocity4 ,when it is not working too fast the valve in the steam-pipe is wide open4 ,but the moment that it works too rapidly the balls extend out far from the axis1 so that the collar rises1 and by the lever partly closes the valve4 ,less steam1 therefore1 can come to the engine1 and the engine working in consequence less rapidly1 the balls fall again1 opening the valve4 ,you see1 then1 that the regulation of this valve by the governor effectually prevents the action of the engine from becoming too rapid4 ,illustration3 ,fig4 #aec4 ,illustration3 ,fig4 #aed4 #bah4 ,shape of the ,earth ,influenced by the ,centrifugal ,force4--,if the potter should make a ball of soft clay revolve rapidly around on a stick run through it1 the ball would bulge out at the middle1 where the centrifugal force is greatest1 and would be flattened at the ends where the stick runs through it4 ,this is precisely what has happened to the earth4 ,at the equator1 where the centrifugal force is greatest1 it has bulged out about thirteen miles1 while it is flattened at the poles4 ,this shape was of course assumed before the earth became solid4 ,in ,fig4 #aec we have the shape of the earth represented1 ,n ,s being the polar diameter1 and ,e ,e' the equatorial diameter4 ,the tendency to take this shape from the centrifugal force may be illustrated by the instrument represented in ,fig4 #aed4 ,it consists of a set of circular hoops of brass1 with an axis1 b a4 ,the hoops are fastened to the axis at a1 but are left free at b4 ,by a little machinery at the top they can be made to revolve rapidly1 and bulging out at the sides by the centrifugal force1 they slide down on the axis at b4 ,illustration3 ,fig4 #aee4 #bai4 ,projectiles4--,i have already spoken of projectiles in #baa4 ,you saw there that any body1 as a cannon-ball1 which is projected horizontally1 falls to the earth in a curved line4 ,two forces act on the ball2 viz41 the projectile force given by the powder and the force of gravitation4 ,the force of gravity being always the same1 the shape of the curve which the projected body describes must depend on the force with which it is projected4 ,this is very strikingly exemplified in the curves described by the different streams of water in ,fig4 #ada4 ,but whether the projectile force be great or small1 the moving body thrown horizontally will in every case reach the ground in the same time4 ,thus if two cannons stand side by side on a height1 one of which will send a ball a mile and the other half a mile1 the two balls1 if fired together1 will reach the ground at the same instant1 though at first thought it would seem that the ball which travels twice as far as the other would take a longer time to do it in4 ,this is because the horizontal force of the ball does not oppose in the least the downward force of gravity4 ,if it were thrown upward instead of horizontally1 the projectile force would be opposed to gravity1 and in proportion as the direction came near to being vertical4 ,as horizontal force does not interfere with the action of the force of gravity1 it follows that a ball dropped at the instant at which another is fired will reach the ground at the same instant that the fired ball does4 ,this can be made clear by ,fig4 #aee4 ,suppose it takes three seconds for a ball to fall from the top of a tower to its foot4 ,in the first second it falls to a4 ,the ball projected horizontally from the cannon1 being operated upon by the same force of gravity1 will fall just as far1 and will be on a level with it at b4 ,both balls fall farther and farther each second1 both being accelerated in the same degree because it is done by the same force4 ,the projected ball will reach d when the falling ball is at c1 and the plain at f when the falling ball is at e1 the foot of the tower4 ,the same holds true in all cases4 ,a bullet dropped from a level with the barrel of a gun1 paradoxical as it may seem1 will fall to the ground no sooner than one which is shot from the gun4 ,illustration3 ,fig4 #aef4 #bbj4 ,all ,falling ,bodies really ,projected4--,when a body falls from any height1 it does not1 as you have already seen in #ahg1 fall in a straight line1 as it appears to do4 ,it falls in a curved line1 for1 like all projectiles1 it is acted upon by a horizontal force as well as the force of gravity4 ,but what is this horizontal force8 ,it is the motion which the body has in common with the earth in its rotation on its axis4 ,in this rotation the height from which the body falls goes to the eastward #aejj feet in a second4 ,if1 therefore1 the body did not partake of the motion of the earth1 and went to the ground in a straight line in a second1 it would be when it reached the ground #aejj feet westward from the foot of the height from which it fell4 ,but it does partake of the earth's motion1 and goes eastward as fast as the height does1 and so describes the curved line of a projectile4 ,suppose a ball falls from a height ,a1 ,fig4 #aef1 and in a second of time that height passes to ,c4 ,the forward or projectile force would tend to carry the ball to ,c1 and the force of gravity would tend to carry it to ,b4 ,but both forces acting together1 it pursues a middle path1 and this path is a curved line1 because one of the forces is a continued force1 #bac4 ,for the same reason1 if a ball be dropped from a railway car in motion1 and it takes a second for it to fall1 it will be at the end of that second just under that part of the car from which it fell4 ,although the car may have moved a considerable distance1 the dropped ball1 partaking of its motion1 goes along with it in its fall4 ,so a ball dropped from a mast-head when a ship is in motion goes along with the ship in its fall4 ,the ball in each of these cases describes in its fall a curved line4 #bba4 ,motion in ,orbits4--,why is it1 let us ask1 that a cannon-ball shot horizontally from some great height will not revolve around the earth like the moon4 ,it has the same two forces acting upon it as the moon has--viz41 a projectile force1 and the attraction of the earth--and both ball and moon describe a curve in their motion4 ,but the curve of the ball bends to the earth1 while that of the moon ever sweeps around the earth4 ,why is this8 ,first1 there is the resistance of the air continually retarding the velocity of the ball4 ,but1 secondly1 even if the ball could be projected from an elevation sufficiently high to be outside of the atmosphere1 the force of the projection would not be great enough4 ,we know1 from the rate of progress of the heavenly bodies in their orbits1 that it would require an immense velocity to keep the ball from being brought to the earth by its attraction4 ,the ,creator of these worlds1 when he launched them into their orbits1 gave them precisely that impulse which is needed to balance the centripetal force of attraction1 and so they pursue a middle course between the two directions in which these two forces tend to carry them4 ,and as their velocities have never been retarded by the resistance of air or any other substance1 they have been ever the same from the beginning4 ,,chapter ,,xi4 ,,the ,,mechanical ,,powers4 #bbb4 ,machines not ,sources of ,power4--,the ,mechanical ,powers1 as they are termed1 are six in number--viz41 the ,lever1 the ,wheel and ,axle1 the ,inclined ,plane1 the ,screw1 and the ,wedge4 ,they are not1 strictly speaking1 powers2 for1 as you will see in the course of our investigation1 they are merely means of applying power to advantage1 and are not in reality sources of power4 ,the true sources of power are the causes of motion treated of in #aha4 ,the instrument or machine can not create power1 and the only use of all the variety of tools and machinery is to enable us to apply power in such a manner1 with such a velocity1 and in such a direction1 as to effect the objects which we have in view4 ,the term ,mechanical ,power1 then1 is not strictly proper as applied to those contrivances which commonly have this name2 but the term is in so general use that it would not be well to alter it4 ,every instrument1 however simple and insignificant1 and every machine1 however large or complicated1 is an example of some one of the six ,mechanical ,powers1 or of a combination of them4 ,i will proceed to consider each of these separately4 ,in doing this certain terms will be used which ,i will first explain4 ,power is the force by which a machine or instrument is moved4 ,weight is the resistance to be overcome4 ,if the resistance be in some other form than that of weight it is called technically by this name4 ,so what is called ,power may be in the form of weight4 ,the fulcrum is the point on which the instrument or machine is supported while it is in motion4 #bbc4 ,the ,lever4--,the ,lever is the most simple of all the ,mechanical ,powers1 and is therefore in universal use4 ,though the savage makes use of but few tools in comparison with the civilized man1 he uses the lever almost constantly in some form or other4 ,the wedge is the only one of the other ,mechanical ,powers that he uses to any great extent4 ,levers are of three kinds1 which ,i will notice separately4 ,illustration3 ,fig4 #aeg4 ,illustration3 ,fig4 #aeh4 #bbd4 ,lever of the ,first ,kind4--,in the lever of the first kind the fulcrum or prop is between the weight and the power4 ,the common crow-bar or hand-spike is a familiar example1 as seen in ,fig4 #aeg--the stone1 ,s1 or other heavy body to be moved being the weight1 the stone or block of wood1 ,f1 on which the bar rests being the fulcrum1 and the pressure of the hand1 ,h1 the power4 ,the nearer the fulcrum is to the weight1 or the farther is the power from the fulcrum1 the greater is the force of the lever4 ,this may be illustrated on ,fig4 #aeh4 ,here the short arm of the lever1 as it is called1 ,c ,w1 is one eighth of the length of the long arm1 ,a ,c4 ,if the weight hanging at the end of the short arm be #gb pounds1 a weight of #i pounds1 or the force of a hand amounting to this1 will balance it at the end of the long arm4 ,but if the power should be applied at only four times the distance from the fulcrum at which the weight is1 then it would require a force of #ah pounds to balance the #gb pounds on the short arm4 ,similar variations can be made by altering the length of the short arm4 ,the power and the weight will balance each other if the weight multiplied by the length of the short arm1 and the power multiplied by the length of the long arm1 give equal products4 #bbe4 ,scales and ,steelyards4--,in the common scale-beam we have a lever1 the two arms of which are equal1 and therefore equal weights suspended at the ends balance4 ,if they be not exactly equal1 a heavier weight will be necessary on the shorter arm than on the longer4 ,the inequality will injure the buyer if the prop be too near the scale in which the weights are placed1 and the seller if it be too near that which holds the article to be sold4 ,any difference can be easily detected by changing the places of the article and the weights4 ,whenever cheating is practiced by the false balance1 it is of course done in a small way1 to avoid any observation by the eye of the inequality of the two arms of the scale-beam1 and the weight of the scale hanging from the shorter arm is made a little greater than that of the other1 so that they may balance4 ,scales may be rendered very accurate by making the fulcrum or pivot of hardened steel1 and of a wedge shape1 with a sharp edge1 in order to avoid friction as much as possible4 ,the steelyard differs from the scale-beam in having the arms of different lengths4 ,the principles on which this instrument is constructed were developed in what ,i said of ,fig4 #aeh4 ,when either with the balance or the steelyard two weights balance each other the centre of the weights and the apparatus taken together is just over the fulcrum1 #aie4 ,we see in this the reason that it is necessary to have the prop near the large weight when we wish to balance it by a small one4 #bbf4 ,other ,examples4--,scissors are double levers of the first kind4 ,the fulcrum is the rivet1 the weight or the resistance to be overcome is the article to be cut1 and the power is applied to the long arms of the levers by the fingers4 ,with large shears hard substances can be cut4 ,even plates of iron are cut like paper by shears which are worked by a steam-engine4--,pincers are double levers4 ,the hinge1 or rivet1 is the fulcrum4--,the common hammer1 as used in drawing nails1 is a good example of the power of this kind of lever4 ,though crooked1 it acts in the same way with a straight lever4 ,the fulcrum is the point on the board where the hammer rests1 and this is between the resistance to be moved1 the nail1 and the power1 that is1 the hand which grasps the handle4 ,illustration3 ,fig4 #aei4 #bbg4 ,no ,gain of ,power in this ,lever4--,i will now illustrate the truth that there is no gain or saving of power in this lever1 though at first thought it would seem that there is4 ,let a b1 ,fig4 #aei1 represent a lever1 and e its fulcrum4 ,let the arm a e be twice as long as e b4 ,a pound1 therefore1 suspended from a will balance two pounds at b4 ,if1 now1 when the weights are suspended1 the long arm be raised so that the lever shall be in the position represented by the line c d1 and then let go1 the one pound at c1 balancing the two pounds at d1 will bring the lever again to the position a b4 ,it will be perceived that the end of the long arm of the lever moves through the space a c1 which is larger than b d1 through which the end of the short arm moves1 in the same time4 ,the one-pound weight1 in fact1 falls two feet in raising the two-pound weight one foot1 and it moves twice as far as a one-pound weight suspended at i would4 ,if a one-pound weight could raise a two-pound weight without thus moving through twice as much space we might then say that there is an actual gain of power in the lever4 ,but it evidently makes no difference whether one pound moves through two feet or two pounds through one foot2 the force is the same in both cases4 ,for the momentum or force of a moving body is in proportion to its weight and velocity1 #bja2 and therefore the pound weight1 moving through two feet1 has as much momentum as the two-pound weight moving through one foot in the same time4 ,the small weight does the same amount of work that the larger one would by moving twice as far in the same time as the larger1 just as a boy1 who carries a load half as large as a man1 will do as much work as the man if he carry it twice as fast4 ,illustration3 ,fig4 #afj4 #bbh4 ,the ,see-,saw4--,we see the same thing illustrated in the see-saw1 ,fig4 #afj4 ,the man1 being much heavier than the boy1 is nearer the prop1 and as they move up and down the boy passes over a much larger space than the man1 describing an arc in a much larger circle4 #bbi4 ,archimedes's ,lever4--,archimedes said that if he could have a lever long enough and a prop strong enough he could move the world by his own weight4 ,but he did not think how far he would have to move to do this1 from the vast difference between his weight and the weight of the earth4 ,he would have required1 says ,dr4 ,arnot1 to move with the velocity of a cannon-ball for millions of years to alter the position of the earth by a small part of an inch4 #bcj4 ,an ,analogy4--,you will remember that in the case of the ,hydrostatic ,paradox1 the ,hydrostatic ,bellows1 and ,bramah's ,press 7 #aca1 #acb1 and #acc71 great effects are produced by a small power4 ,but this small power has to execute an extensive motion in order to produce these effects4 ,thus1 as stated in #acb1 if the area of the top of the ,hydrostatic ,bellows be one thousand times the area of the tube1 though the water poured into the tube will raise a very great weight on the bellows1 the water in the tube must fall ten inches in raising the weight the hundredth part of an inch4 ,so when the pressure of the hand on the long arm of a lever moves a great weight1 as a heavy stone1 the weight is moved but a little1 while the extent of the hand's motion is comparatively very great4 ,illustration3 ,fig4 #afa4 ,illustration3 ,fig4 #afb4 #bca4 ,lever of the ,second ,kind4--,in the second kind of lever the weight is between the fulcrum and the power1 as you see in ,fig4 #afa4 ,the same rule of equilibrium applies here as in the case of the lever of the first kind4 ,the #gb pounds of weight can be sustained by #h pounds of power1 because the power acts on the lever at #i times the distance from the fulcrum that the weight does1 for #agb #ih4 ,the common wheel-barrow1 ,fig4 #afb 7p4 #ahj71 is an example of this kind of lever4 ,the point at which the wheel presses on the ground is the fulcrum1 and the weight is the load1 its downward pressure from its centre of gravity being indicated at ,m4 ,of course the nearer the load is to the fulcrum the easier it is1 on starting1 to raise the handles4 ,the crow-bar can be used as a lever of this kind when its point is placed beyond the weight to be raised4 ,the chipping-knife1 ,fig4 #afc1 is another example4 ,the end1 ,f1 attached to the board1 is the fulcrum1 the hand pressing at ,p the power1 and the resistance of the substance ,r1 which is to be cut1 is the weight4 ,nut-crackers have a similar arrangement4 ,in shutting a door by pushing it near its edge we move a lever of this kind4 ,the hinge is the fulcrum1 and the weight is between this and the hand4 ,illustration3 ,fig4 #afc4 ,we see1 then1 the reason that the slight push of a hand shutting the door may even crush a finger when caught in it at the side where the hinges are4 ,the finger is a resistance so near the fulcrum that the power moving through a great space acts upon it with immense force4 ,the same explanation applies to the severe bite of the finger when it is caught in the hinge of a pair of tongs4 ,the oar of a boat is a lever of this kind1 the weight to be moved being the boat1 which is between the power1 the hand of the rower1 and the fulcrum1 the resisting water4 ,illustration3 ,fig4 #afd4 #bcb4 ,lever of the ,third ,kind4--,in the third kind of lever the power is between the fulcrum and the weight1 as seen in ,fig4 #afd4 ,in the first two kinds of lever the power may be less than the weight1 but in this the power must always be greater than the weight4 ,this lever has1 then1 no mechanical advantage1 as that expression is commonly used4 ,applying the same rule here as in the other levers1 see what is the result4 ,if the weight1 as in ,fig4 #afd1 be #i times as far from the fulcrum as the power is1 it will require a power equal to a weight of #fdh pounds to sustain a weight of #gb pounds1 for #igbafdh4 ,illustration3 ,fig4 #afe4 #bcc4 ,examples4--,when a man puts his foot against the end of a ladder1 and raises it by taking hold of one of the rounds1 the ladder is a lever of this kind4 ,it is evident that he spends his force upon it at a great mechanical disadvantage1 for the power is applied much nearer to the fulcrum than the weight of the ladder1 taken as a whole1 is4 ,if you push a door to by placing your hand very near the hinges1 you do not shut it as easily as when you take hold of it at its edge4 ,in the first case it is a lever of the third kind1 and the hand moves through a small space1 and therefore must exert a considerable force2 while in the latter case the door is a lever of the second kind1 and the hand1 moving through a greater space1 puts forth less force4 ,when we use a pair of tongs we use a pair of levers of the third kind4 ,they are an instrument in which convenience rather than power is needed4 ,we can not grasp any thing very firmly with them because the power is so much nearer to the fulcrum than the weight to be lifted4 ,for this reason a pinch with the ends of the tongs is nothing compared with one in the hinge4 ,the most beautiful example of this lever we have in the moving apparatus of animals4 ,take1 for example1 the principal muscle which bends the elbow1 as represented in ,fig4 #afe 7p4 #ahb74 ,this comes down from the shoulder in front of the bone of the arm1 and is inserted just below the elbow-joint into one of the bones of the forearm4 ,it pulls upon the forearm very near the fulcrum1 which is the elbow-joint1 and so acts at a great mechanical disadvantage4 ,the object of this arrangement is to secure quickness of movement1 which is here1 as in almost all muscular motions1 of more importance than great strength4 ,when great weights are lifted the fact that the muscles act at such mechanical disadvantage makes the exhibition of power wonderful4 ,illustration3 ,fig4 #aff4 ,illustration3 ,fig4 #afg4 #bcd4 ,compound ,levers4--,when several levers are connected together we call the whole apparatus a compound lever4 ,let each of the levers in ,fig4 #aff be #c inches long1 the long arms being #b inches1 and the short ones #a inch4 ,one pound at ,a will1 according to the rule1 balance #b at ,b1 and #b at ,b will balance #d at ,c1 and #d at ,c will balance #h at ,d4 ,therefore #a pound at ,a will balance #h pounds at ,d4 ,and you see that an equilibrium is effected when the power is to the weight as the product of all the short arms is to the product of all the long arms4 ,the compound lever is used in weighing heavy loads--as hay1 coal1 etc4 ,you have a representation of the arrangement in ,fig4 #afg4 ,the load1 ,w1 stands on a platform1 ,a ,b1 which rests upon two levers1 ,e ,d and ,e ,c4 ,the long arms of these levers are ,e ,g and ,e ,f1 and the short arms are ,g ,d and ,f ,c4 ,the ends of the long arms press upon the fulcrum of the lever1 ,h ,i4 ,the pressure is now transmitted from the end of the long arm by the rod1 ,i ,k1 to a small lever1 ,k ,l1 where a small weight or power1 ,p1 balances the weight of the heavy load1 ,w4 ,the two objects secured by this arrangement are accuracy and the occupation of a small space4 ,illustration3 ,fig4 #afh #bce4 ,wheel and ,axle4--,the mechanical power next in simplicity to the lever is the ,wheel and ,axle4 ,the most familiar applications of this power we see in drawing water and in raising heavy articles in stores4 ,the principle of this power is the same as that of the lever1 as may be shown in ,fig4 #afh1 which represents a section of the wheel and axle4 ,the power1 ,p1 hangs by a cord which goes round the wheel1 and the weight1 ,w1 by a cord around the axle4 ,we may consider the power as pulling on a lever represented by ,a ,b1 the long arm of which is ,a ,c1 and the short arm ,b ,c4 ,you see that the wheel and axle1 then1 may be viewed as a constant succession of levers1 and it is therefore sometimes called the perpetual lever4 ,and the same rule of equilibrium applies here as in the simple lever4 ,illustration3 ,fig4 #afi4 #bcf4 ,windlass4--,in the common windlass the power is applied to a winch or crank1 ,d ,c ,b1 ,fig4 #afi1 instead of a wheel4 ,in estimating the power of this arrangement ,b ,c must be considered the long arm of the lever1 and half of the diameter of the axle1 ,b ,a1 as its short arm4 ,illustration3 ,fig4 #agj4 ,illustration3 ,fig4 #aga4 #bcg4 ,capstan4--,in the capstan1 represented in ,figs4 #agj and #aga1 the axle is in a vertical position4 ,the top of it is pierced with holes1 into which levers are introduced4 ,in ,fig4 #agj you see the instrument as it is commonly used in moving buildings4 ,sometimes horse-power is applied at the ends of the levers4 ,great power is exerted by this instrument2 but we have the same fact here as in all cases where a small force produces a great effect--the effect is slow1 and the force passes over a great space in producing it4 ,the moving of a building a foot requires many circuits of the horse around the axle4 ,fig4 #aga gives us the capstan as it is commonly on board ship4 ,the head of it is circular1 with many holes for levers1 so that many men can work together in raising a heavy anchor4 ,illustration3 ,fig4 #agb4 #bch4 ,fusee of a ,watch4--,in the fusee of a watch we have a wheel and axle of a peculiar construction4 ,when we wind up a watch the chain is wound around the spiral path-way on the fusee1 ,b1 ,fig4 #agb1 and at the same time the spring is coiled up tightly in the round box1 ,a4 ,the spring1 in gradually uncoiling itself1 turns this round box around1 and thus pulls upon the chain1 c1 making the fusee to revolve1 and so give motion to other parts of the machinery4 ,now the spring1 in its effort to uncoil1 acts strongest at first2 and therefore if the fusee were of uniform size the watch would go fastest when first wound up1 and go gradually slower as it run down4 ,this difficulty is obviated by giving the power a small wheel to pull on at first1 and gradually enlarging the wheel as the spring uncoils4 ,this is because1 in order to produce a certain effect on a given weight by a power1 the less the power is the longer must be the arm of the lever on which the power acts4 ,illustration3 ,fig4 #agc4 ,illustration3 ,fig4 #agd4 ,fig4 #age4 #bci4 ,the ,pulley4--,the third mechanical power is the ,pulley4 ,pulleys are fixed or movable4 ,in ,fig4 #agc you have a fixed pulley4 ,there is no mechanical advantage in this pulley1 for its action may be conceived of as the action of successive levers of equal arms1 ,b ,f and ,a ,f1 and therefore equilibrium requires an equality of the power and weight4 ,but this pulley is often a great convenience4 ,for example1 a man can raise himself or some weight to any desired elevation1 as seen in ,fig4 #agd4 ,it is used also in effecting descents4 ,with two fixed pulleys a horizontal force may be used in raising a weight vertically1 as seen in ,fig4 #age4 ,in using a fixed pulley either one or the other of two objects is attained--applying force where we could not otherwise apply it1 and changing the direction of its application4 ,illustration3 ,fig4 #agf4 #bdj4 ,movable ,pulley4--,you have a representation of a movable pulley in ,fig4 #agf 7p4 #ahf74 ,it is evident here that the force of the weight is equally divided between the cords1 ,a ,b1 so that the cord ,b1 extending over the fixed pulley1 needs to have a weight1 ,p1 but half of the weight ,w to balance it4 ,a movable pulley is sometimes called a runner1 and a fixed pulley is commonly connected with it1 in order to give the desired direction to the force4 ,many pulleys are often connected together in various ways1 as seen in ,fig4 #agg4 ,it is easy to estimate in such cases the relation of the power to the weight on the principles developed in relation to the lever4 ,if1 for example1 in the system of pulleys on the left1 the weight be #cf pounds1 the two cords of the first pulley will each sustain a weight of #ah pounds1 those of the next pulley each #i pounds1 and those of the next each #d pounds4 ,the weight ,w then will be balanced by the weight ,p if it weigh #d pounds4 ,illustration3 ,fig4 #agg4 ,illustration3 ,fig4 #agh4 #bda4 ,inclined ,plane4--,the fourth mechanical power is the ,inclined ,plane4 ,this being a very simple contrivance is much used1 especially when heavy bodies are to be raised only a small height1 as in getting large boxes and hogsheads into stores4 ,the mechanical advantage of the inclined plane may be illustrated on ,fig4 #agh4 ,the line ,a c represents an inclined plane4 ,if a weight be drawn up this plane it is raised only the height ,b c4 ,a smaller power is requisite to draw the weight up the plane than to raise it perpendicularly2 and the power necessary will be the less the longer the plane4 ,a power which would balance a weight on an inclined plane would be to the weight as the height of the plane to its length4 ,thus if ,a c be twice as long as ,b c1 a weight of four pounds on the plane may be balanced by a two-pound weight suspended by a cord passing from the weight over the summit of the plane4 ,a flight of stairs is an inclined plane in regard to the principle on which the ascent is effected1 the projections in it being for the purpose of affording a sure footing in making the ascent or the descent4 ,so likewise hogsheads are let down the steps of a cellar-way by ropes1 and it makes no difference in the principle of the operation whether the steps have or have not planks laid along them4 ,it is supposed that the immense stones in the pyramids and other massive ,egyptian structures were put into their position by means of the inclined plane4 ,roads1 when they are not level1 are inclined planes1 and the steeper the inclination the more power is required to draw a load up the road4 ,great mistakes were formerly made in carrying roads too frequently over high hills4 ,besides failing to take advantage of the principles of the inclined plane1 in many cases the horse in going over a hill passes over quite as much space as he would if the road were made to go round the base of the hill1 and sometimes even more4 ,if the hill were a perfect hemisphere1 a road over it would be just equal in length to a road around its base to the opposite point4 ,illustration3 ,fig4 #agi4 #bdb4 ,the ,wedge4--,this is the fifth of the mechanical powers4 ,it may be considered as two inclined planes placed with their bases together1 as seen in ,fig4 #agi4 ,indeed1 sometimes the wedge has one side only inclined1 it being only half of the ordinary wedge4 ,the difference between the inclined plane and the wedge in operation is1 that in the first the inclined plane is fixed1 and the weight is made to move up along its surface1 while in the latter the weight1 that is1 the resistance1 is stationary1 and the surface of the plane is made to move along upon it4 ,the power of the wedge is estimated just as the power of the inclined plane is1 that is1 by comparing the thickness of the wedge with the length of its side4 ,the less the thickness of the wedge compared with its length1 obviously the more powerful is the wedge as a penetrating instrument4 ,the wedge is used for splitting blocks of wood and stone1 for producing great pressures1 for raising heavy bodies1 etc4 ,all cutting and piercing instruments1 knives1 razors1 axes1 needles1 pins1 nails1 etc41 act on the principle of the wedge4 #bdc4 ,the ,screw4--,this is the sixth mechanical power4 ,the principle of it is essentially that of the inclined plane4 ,the thread running around the screw is an inclined plane which is spiral instead of straight1 and so is also the corresponding part in the nut an inclined plane running in the opposite direction4 ,in the common screw the nut is fixed1 and the screw is made to play up and down in it2 but sometimes the screw is fixed1 and the nut is made to play around it4 ,the screw acts like a wedge1 and has the same relation to a straight wedge that a road winding up a hill has to a straight road of the same length and rise4 ,especially does the comparison hold when the screw is forced into wood2 the wedge goes straight into the wood1 but the edge of the screw's thread enters the wood spirally4 ,illustration3 ,fig4 #ahj4 ,to estimate the force of the screw we compare the length of one turn of the thread around it with the height to which the thread rises in going round4 ,let a b1 ,fig4 #ahj1 represent one turn of the thread1 and b c the height to which it goes4 ,it is clear from the figure that the principle which applies to the inclined plane and to the wedge applies here also4 ,as the less is the height of the plane the easier it is for a weight to be drawn up it2 and as the less is the depth of the wedge the less is it resisted2 so1 also1 the less the height of the turn of the screw's thread the easier is it to move the screw1 and the greater is the force which it exerts4 ,hence the prodigious power of a screw with a thread which rises very slowly in its spiral turns4 ,screws are much used when great pressure is required1 as in pressing oils and juices from vegetable substances1 in compressing cotton into bales1 in bringing together with firm grasp the jaws of the vice1 etc4 ,in turning the screw a bar is used1 so that we have in this instrument the combined advantages of the screw and the lever4 ,that you may have some idea of the power of these two instruments acting together ,i will suppose a case4 ,let the weight to be raised by a screw be #aj1jjj pounds4 ,let a turn of the screw be #aj inches long1 and the rise be but one inch4 ,then1 so far as the screw is concerned1 the power requisite to raise the #aj1jjj pounds will be #ajjj--the ratio of the height of the thread's turn to its length4 ,but the power of the lever is yet to be estimated4 ,let the length of the lever1 passed through the head of the screw so that it is equal on each side1 be #cj inches4 ,the diameter of the screw is about three inches1 or one-tenth of the diameter of the circle described by the end of the lever4 ,it will now take but a power of #ajj pounds to raise the weight1 the ratio of the radius of the screw to half the length of the lever4 #bdd4 ,truly but ,three ,mechanical ,powers4--,the ,wheel and ,axle1 you have seen1 is merely a modification of the ,lever1 and the ,wedge and the ,screw are modifications of the ,inclined ,plane4 ,the ,mechanical ,powers are1 then1 in reality but three--the ,lever1 ,pulley1 and ,inclined ,plane4 ,and these are the elements of all machinery1 from the simplest tool that is used for the most common purposes to the most complicated and powerful engine which the ingenuity of man ever designed4 ,the principle upon which a pin is shaped is identical with that of the wedge1 by which large masses are cleft in two2 and the instrument by which the finest textures are cut by delicate fingers is arranged on the same principle with those varied contrivances by which immense weights are raised by a comparatively small power1 viz41 the principle of the lever4 #bde4 ,friction in ,machinery4--,you have seen1 as we have proceeded1 that the ,mechanical ,powers1 though thus named1 do not generate power4 ,so far from this1 there is really a loss of power in their use1 chiefly from friction4 ,in raising a weight1 for example1 directly by the hand1 there is no loss from this cause2 but if you use a pulley you have the friction of the cord upon it1 and a loss of power in proportion to the amount of friction4 ,in some cases the loss of power from this cause is so great as to call for a considerable variation from such calculations as we have made in this chapter in regard to the relations of power and weight in machinery4 ,in the operations of the screw friction has a great influence in diminishing the power of the instrument4 #bdf4 ,the ,real ,advantages of the ,mechanical ,powers4--,if there is then no saving1 but a loss of power in tools and machinery1 what1 let us inquire1 are their advantages8 ,if one man can do alone by the aid of some instrument what would otherwise require the exertion of many men1 though he be slow in doing it1 yet it is a great advantage4 ,thus one man can with a lever move a stone which perhaps it would require thirty men to move without it1 and though it take him thirty times as long1 it saves him the trouble of getting a company of men to help him4 ,so if a man can raise his goods by a wheel and axle to the upper loft of his store1 though he raise them slower than several men would lift them directly by ropes1 it is an advantage to him1 as it saves the hiring of a company of laborers4 ,a few men by a capstan can raise an anchor which could be raised without it only by a large company of men4 ,another advantage often is that there may be intervals of rest in applying the force without any loss4 ,this is obvious in the case of the pulley1 but still more so in the case of the screw4 ,it is friction in both these cases which enables the workman to rest4 ,it saves to him all that he has gained by opposing any tendency to slip back4 ,we see the same thing in the wedge4 ,when this is driven into wood1 it remains because it is prevented from returning by the friction of the wood against its sides4 ,it is the same cause which holds a nail in its place1 and opposes any effort to draw it out4 ,in driving the wedge the workman can have as long intervals as he pleases between his blows1 because friction saves all that is gained4 ,this effect is very well exemplified in the capstan1 ,fig4 #agj4 ,it requires but little exertion of the man who sits there to hold the rope1 because the few turns of it around the axle prevent its slipping easily4 ,a third advantage which often attends the use of tools and machines is that force may be made to produce motion at various distances1 in various directions1 and in various degrees of velocity4 ,thus as to distance1 a man standing on the ground can raise a weight to the top of a house by a pulley4 ,so1 also1 a water-wheel may by the connections of machinery produce motion at considerable distances from it4 ,then as to direction1 horizontal motion may be converted into vertical1 rotary into straight1 etc4 ,the velocity of motion is generally varied by cog-wheels4 ,thus a wheel of #fj cogs revolving once in a minute1 playing on a wheel of #aj cogs1 will make it revolve once in #f seconds4 ,illustration3 ,fig4 #aha4 ,another advantage of tools and machines is that they secure a better mode of applying power than we otherwise could have4 ,thus when several men are pulling on a rope much power is lost by their pulling irregularly1 a difficulty which is removed by the pulley4 ,the same can be said of applying pressure by the screw4 ,one man presses more steadily1 and therefore more effectually1 than fifty men would without the screw4 ,the arrangements of tools and machines are so made as to provide convenient ways of applying our strength4 ,an instrument1 for example1 for moving a weight by hand is so shaped as to hold the weight well1 and also to afford a good handle for the hand to grasp4 ,the common claw hammer is a very good illustration4 ,we grasp the nail by an iron claw1 with the handle we can apply not merely the force of the hand1 but that of the whole arm1 and then we have the immense lever power of the instrument4 ,we have a good illustration of convenience in an instrument1 in what is called a ,lewis1 represented in ,fig4 #aha4 ,it is used for raising blocks of stone in building4 ,it has three parts1 ,a ,b ,c4 ,it is used in this way3 ,a hole is made in the upper part of the block of stone to be raised in shape like the instrument2 then ,a and ,c are inserted1 and ,b is pushed in between them4 ,with the ring1 ,d1 bolted through the instrument the stone is raised to its place by the ordinary machinery4 ,the principle of the instrument1 you see1 is that of the wedge4 #bdg4 ,man a ,tool-,making ,animal4--,though there is no actual saving of power in the tools and machines which man uses1 yet so great are the advantages which he reaps from them1 that more than two thousand years ago a philosopher thought that man could not be better distinguished from brutes than by calling him a tool-making animal4 ,if the distinction was so striking in the time of ,aristotle1 when tools and machines were so few in number and so rudely contrived1 and so few of the sources of power were appropriated by man to his use1 how much more striking is it now1 with all the variety and perfection of instruments and machinery1 and with the ever-extending appropriation of the sources of power furnished by the elements4 ,the power which air and water and gravitation give is applied constantly with more and more variety and effect2 and the appropriation of that mighty source of power1 steam1 is wholly a modern invention4 ,,chapter ,,xii4 ,,sound4 #bdh4 ,what ,sound is4--,sound is such a vibration of substances as can1 on being transmitted to the ear1 act upon the sense of hearing4 ,i say such a vibration1 because there may be vibrations which will not produce the sensation of sound1 ,vibrations which are either very slow or very quick will not do it4 ,thus if a plate of metal or a string make less than #ae or more than #dh1jjj vibrations in a second1 no effect is produced upon the ear4 ,the capacity of hearing differs1 however1 in different persons1 so that although few can hear vibrations which are beyond the range which ,i have mentioned1 there are many whose capacity falls much within it either at one end or both ends of the scale4 ,the range for animals is not the same as that for man4 ,thus the lion and the elephant can hear a sound when the vibrations are too infrequent to make any impression upon our ears2 while small animals have a susceptibility in the organ of hearing for vibrations so quick that we can not hear them1 and at the same time are not susceptible to the slower vibrations4 ,how far the range varies in different animals has not been ascertained to any extent4 ,illustration3 ,fig4 #ahb4 #bdi4 ,the ,vibration of ,sounding ,bodies ,manifest to the ,senses4--,if we place the hand upon a large bell that has been struck we can feel the vibration4 ,if we strike one of the ends of a tuning-fork upon some hard body we can see the vibration1 as represented in ,fig4 #ahb by the dotted lines4 ,if we look in upon the strings of a piano as it is played1 the vibration of the larger strings is very observable to the eye4 ,if we rub the edge of a drinking-glass so as to produce a musical sound1 the water which is in it will be thrown into waves from the vibration of the glass4 ,illustration3 ,fig4 #ahc4 #bej4 ,wind ,instruments4--,in wind instruments1 as the flute1 horn1 etc41 it is the vibration of the body of air in the instrument which causes the sound4 ,in the common tin whistle or bird-call1 ,fig4 #ahc1 the sound is produced by the vibration imparted to the contained air by the impulse of the breath through the orifice1 ,b4 ,illustration3 ,fig4 #ahd4 #bea4 ,an ,analogy4--,the vibration of a sounding body is much like that of a pendulum4 ,the end of the tuning-fork1 ,fig4 #ahb1 on being struck passes to b1 and in returning passes by the point of rest1 ,a1 as the pendulum does1 and reaches a4 ,so1 also1 if a string1 ,a ,b1 ,fig4 #ahd1 be drawn aside to ,d1 as it flies back to ,c it will by its inertia pass on to ,e1 and so will continue to vibrate back and forth for some time4 ,the same rule also applies to the extent of the vibrations here as in the case of the pendulum1 #bji4 ,the quickness of the vibration is not at all affected by its width4 ,the farther the string1 ,a ,b1 is drawn to one side the greater will be the force with which it will return1 and hence it will arrive at its position on the other side of the middle line as soon when drawn far away from this line as it would if drawn but little away4 ,the same thing is true of the vibrations or waves of air1 though it can not so easily be made plain to you4 #beb4 ,how the ,sensation of ,sound is ,produced4--,the vibration of a sounding body is transmitted to the ear ordinarily through the air1 and there strikes upon a little drum1 a membrane at the bottom of the external cavity of the ear just like a common drum-head4 ,here the vibration of the air is communicated to this drum1 and from this to a chain of very small bones4 ,from the last of these bones it is transmitted to another very small drum1 and from this to a fluid in some very complicated passages in the most solid bone in the body4 ,these may be called the halls of audience4 ,in the fluid contained in them are spread out the branches of the nerve of hearing1 which receive the impression of the vibration1 and transmit it to the brain1 where the mind takes knowledge of it4 ,observe that the vibration1 transmitted first through the air1 then through the drum1 then the chain of bones1 then another drum to a fluid1 stops at the fluid4 ,what is transmitted from this to the brain by the nerve we know not1 and so we call it an impression4 #bec4 ,sound ,transmitted through ,various ,substances4--,in ordinary hearing sound1 as you have seen1 is transmitted through various substances before the vibration arrives at the liquid in the halls of audience4 ,but sound need not take this course in all cases to arrive at the nerve of hearing4 ,if1 for example1 you place a watch between your teeth1 the sound will go through the solid teeth and the bones of the jaw directly to the halls of audience by a short cut1 instead of going round through the outer ear-passage to the drum1 and so through the chain of bones4 ,fishes in hearing receive the vibration through water4 ,if you place your ear at the end of a timber1 while some one scratches with a pin at the other end1 you hear the sound distinctly1 for the vibration is transmitted through the timber2 as in the case of the watch between the teeth1 it goes through the solid bone4 ,illustration3 ,fig4 #ahe4 #bed4 ,sound not ,transmitted through a ,vacuum4--,as sound is a vibration of some substance it can not be transmitted through absolute space4 ,this can be proved by an experiment with the air-pump1 as represented in ,fig4 #ahe4 ,the apparatus in the receiver is so arranged that the bell1 a1 can be struck by pressing down a sliding rod1 h4 ,if it be struck before the air is exhausted the sound is heard through the glass4 ,but the more you exhaust the air the fainter will be the sound2 and at length1 if you keep on pumping1 it can not be heard at all4 ,the same experiment can be tried with a music-box4 ,it is from the thinness of the air on high mountains1 and at the great heights reached by balloons1 that all sounds are so faint4 ,the report of a pistol fired off on top of ,mont ,blanc is a mere crack compared with its report when fired in the valley below4 #bee4 ,motion of the ,heavenly ,bodies without ,noise4--,sound is often heard at a very great distance on the earth4 ,the sound of an eruption of a volcano has been heard in one case at the distance of #igj miles4 ,but suppose that the same sound should occur at the same distance from the earth1 that is1 over #ijj miles beyond the atmosphere that enrobes the earth1 no inhabitant of our world could hear it1 for the same reason that you do not hear the bell ringing in an exhausted receiver4 ,if1 therefore1 any sound1 however loud1 should be given forth by any of the heavenly bodies we could not hear it4 ,the course of these bodies in their orbits is noiseless1 because they meet with no resistance from any substance4 ,bodies passing rapidly through our atmosphere cause sound1 from the resistance which the air gives to their passage4 ,the whizzing of a ball is an example of this4 ,it is the passage of the electric fluid through the air which produces the thunder4 ,but the heavenly bodies1 having no such resistance1 make no sound in their course1 though their velocity be so immense4 ,in the expressive language of the ,bible1 their voice is not heard4 #bef4 ,velocity of ,sound4--,the velocity of sound varies in different media4 ,thus it passes through water four times as rapidly as it does through air4 ,dr4 ,franklin1 with his head under water1 heard distinctly the sound of two stones struck together in the water at the distance of more than half a mile4 ,sound passes through solids much more easily1 and therefore more rapidly1 than through liquids4 ,thus its velocity through copper is twelve times and through glass seventeen times greater than through air4 ,if you place your ear against a long brick wall at one end1 and let some one strike upon the other end1 you will hear two reports1 the first through the wall and the second through the air4 ,indians are in the habit of ascertaining the approach of their enemies by putting the ear to the ground4 ,when the eruption of a volcano is heard at a great distance the sound comes through the solid earth rather than through the air4 ,the ready transmission of sound through solids furnishes us with a very valuable means of examining diseases of the lungs and heart4 ,the sounds occasioned by the movement of the air in the lungs and by the action of the heart are very distinctly heard through the solid walls of the chest4 #beg4 ,measurement of ,distances by ,sound4--,it makes no difference with the velocity of sound whether it be loud or not4 ,thus the sounds of a band of music at a distance all reach your ear at the same time1 the sounds of the instruments that can scarcely be heard keeping exact pace in the air with the sounds of the loudest4 ,so1 also1 the velocity of sound is uniform throughout its whole course1 being just as rapid when it is about to die away as it was when it began4 ,it is from this uniformity in the velocity of sound that we can estimate the distance of the object by which any sound is made4 ,we do it by a comparison between light and sound4 ,sound moves at the rate of #aabj feet in a second4 ,now light moves #aib1jjj miles a second1 and therefore1 for all ordinary distances on the earth1 we need make no allowance of time for light in comparison with sound4 ,if we see1 then1 the operation by which a sound is produced we can estimate its distance from us by the length of time which elapses between what we see and what we hear4 ,in this way we can estimate very accurately the distance of a cannon that we see fired1 or the distance of a flash of lightning4 #beh4 ,loudness of ,sound4--,the loudness of sound depends upon the width of the vibrations producing it4 ,the harder you strike the end of the tuning-fork1 ,fig4 #ahb1 the farther will it vibrate the one way and the other1 and the louder will be the sound4 ,the same thing is true of the strings of a piano4 ,a round bell1 when it is struck1 tends in its vibration to take an oval form1 and the extent of its vibration back and forth as it does this governs the loudness of the sound4 ,as sound passes from the sounding body the vibration gradually lessens1 and at length dies away4 ,it is like the successive vibrations or waves of water produced by dropping a stone in it4 ,the louder the sound is the larger are the first vibrations1 and the farther will the vibrations extend1 as in water a large stone dropped into it will produce larger waves than a small one1 and the waves will extend over a greater space4 #bei4 ,diffusion of ,sound4--,when there is no hindrance sound spreads equally in all directions4 ,it is in this respect with the vibrations or waves of air as it is with the waves of water when a stone is dropped into it4 ,light is also diffused in the same manner1 as you will see in another chapter4 ,illustration3 ,fig4 #ahf4 #bfj4 ,reflection of ,sound4--,as waves of water striking against any object bound off1 so it is with the vibrations or waves of sound4 ,and the same is true of this as of all motion1 as stated in #bjf1 that the angle of incidence is equal to the angle of reflection4 ,the reflection of sound is the cause of echoes4 ,in order that an echo be perfect the sound must be reflected back to the ear from some plane surface of some size4 ,sometimes when there are successive plane surfaces of rocks along a river there are successive echoes4 ,thus in ,fig4 #ahf 7p4 #bjj7 is represented a locality on the ,rhine where a sound is reflected at successive places1 #a1 #b1 #c1 #d4 ,the rolling of thunder1 though sometimes caused by the different distances of parts of the same flash of lightning1 is commonly owing to reflections of the sound among the clouds4 ,from this cause the report of a cannon is more apt to be a rolling sound when there are clouds above than when the sky is clear4 ,sound is continually reflected in every variety of direction from obstacles with which it meets4 ,thus in a room it is reflected from the walls and from all the objects in the room2 and the more varied are the surfaces the more varied and confused are the reflections4 ,you know that a voice has a very different sound in a room when it is empty from what it has when the room is filled with an audience4 ,indeed1 a blind speaker can estimate very nearly the size of his audience by the sound of his own voice4 ,the explanation is1 that with a full audience the surfaces for reflection are vastly multiplied1 and so deprive the sound of the sharp and ringing character which is given to it by reflection from comparatively few surfaces which are plane and firm4 ,the effect produced by an audience upon the voice of the speaker is quite analogous to that of muffling upon the sound of a drum4 ,illustration3 ,fig4 #ahg4 #bfa4 ,whispering ,galleries4--,the reflection of sound from curved surfaces gives us some interesting phenomena4 ,the waves of sound in being reflected from a concave surface are gathered together to some point4 ,if the surface be a perfectly spherical one1 and the sound issue from the centre1 the reflection will be from all points to the centre4 ,but suppose the concave surface have the curve of an ellipse1 as represented in ,fig4 #ahg4 ,this1 instead of having a centre1 has two foci1 c and g4 ,now if a sound proceed from one focus1 c1 the waves of sound1 as represented by the lines c d1 c e1 c f1 c h1 will all be reflected to the other focus1 g2 so that if a person speak in a very low tone or even whisper at c1 he may be heard distinctly by another at g1 though persons at other points may hear nothing4 ,we may have this result with a curved wall extending even several hundred feet2 and such structures are called whispering galleries4 ,if in one of these galleries a person standing in one focus speak loudly he will be heard by others at any point by the direct waves of sound2 but the reflected sound will be added to the direct in the case of one standing at the other focus4 ,illustration3 ,fig4 #ahh4 ,illustration3 ,fig4 #ahi4 #bfb4 ,concentration of ,sound4--,it is by the reflection of sound that it can be concentrated in various ways4 ,thus in using a speaking-trumpet the waves of sound1 instead of moving in all directions as soon as they escape from the mouth1 are reflected by the sides of the instrument toward a central line as represented in ,fig4 #ahh4 ,the waves or vibrations1 being thus concentrated have more intensity and are thrown to a greater distance than if they issued directly from the mouth4 ,so a speaking-tube1 confining the vibrations1 carries the voice to distant parts of a building4 ,for the same reason the voice can be heard much farther through a narrow street than in an open space4 ,so1 also1 a speaker can be heard more distinctly in a hall than when addressing an audience of the same size in the open air4 ,the sounding-board1 once so fashionable in churches1 was really of considerable service in preventing the escape of the vibrations of the voice of the preacher upward1 and directing them downward upon the audience4 ,in the hearing-trumpet1 ,fig4 #ahi1 the vibrations are collected in the broad open end of the instrument1 and by reflection are thrown together into a narrow compass before they enter the ear to strike upon the drum4 ,we often instinctively make the palm of the hand act as an ear-trumpet when we do not hear distinctly4 ,many animals have the external ears movable1 so that they can direct their concave surface toward the point from which they wish to hear4 ,such ears are movable ear-trumpets4 #bfc4 ,difference ,between a ,musical ,sound and a ,noise4--,the difference between a musical sound and a noise is very analogous to the difference between a crystal and the same substance destitute of the crystalline arrangement4 ,in both there are vibrations1 but in the musical sound they have perfect regularity1 while in a noise the vibrations are irregular1 and there is confusion4 ,indeed so regular are the vibrations of musical sounds that the rules and principles of music have all the rigid exactness of mathematics4 #bfd4 ,how ,different ,notes are ,produced4--,the quicker is the vibration the higher is the note4 ,thus a short and small string on a violin or in a piano gives a higher note than a long and large string1 because its vibrations are quicker4 ,the tension of the string also has an influence1 the note being raised by increasing the tension4 ,in tuning a violin the right pitch is given to each string by lessening or increasing the tension by means of the screws to which the strings are attached4 ,in playing upon it various notes are made upon each string by shortening the vibrating portion more or less by pressure of the finger4 ,illustration3 ,fig4 #aij4 ,in wind instruments the note depends on the length and size of the column of air contained in them4 ,this may be illustrated by an organ-pipe1 ,fig4 #aij 7p4 #bjc74 ,it is one of the pipes of what is called the flute-stop4 ,it is constructed very much like a boy's willow whistle4 ,the air from the bellows of the organ enters at ,p1 and causes a vibration of the whole column of air in the pipe1 the sound issuing at t4 ,in the upper end is a movable plug1 s1 by which1 in tuning1 the note of the pipe is regulated4 ,if the note be too grave this plug is pushed downward1 so as to shorten the column of air4 ,it is from difference in rapidity of vibration that a large bell gives a graver note than a small one4 ,so1 too1 when musical sounds are produced by passing the moistened fingers over the edges of glass vessels1 the larger the vessel the graver is its note4 ,a tumbler will give a graver note than a wine-glass4 #bfe4 ,human ,voice4--,the principles which ,i have developed in relation to musical instruments apply to the voice4 ,the musical instrument of man1 by which the voice is produced1 is contained in a very small compass4 ,it is that box at the top of the throat commonly called ,adam's apple4 ,across this1 from front to rear1 stretch two sheets of membrane1 leaving a space between their edges4 ,in our ordinary breathing these membranes are relaxed1 and the space between their edges is considerable1 to allow the air to pass in and out freely4 ,but when we speak or sing these membranes1 or vocal chords1 as they are termed1 are put into a tense state by muscles pulling upon them1 and the opening between them is lessened4 ,the voice is produced by the air that is forced out from the lungs1 which1 striking on the chords1 causes them to vibrate4 ,the nearer their edges are together1 and the more tense they are1 the higher is the note4 ,the sounds are produced precisely as those of the olian harp are1 the air causing in the one case a vibration of strings1 and in the other of edges of membranes4 #bff4 ,harmony4--,when notes1 on being sounded at the same time1 are agreeable to the ear1 they are said to harmonize4 ,now this harmony depends on a certain relation between the vibrations4 ,the more simple is the relation the greater is the harmony4 ,for example1 if we take the first note1 termed the fundamental note1 of what is called the scale in music1 it harmonizes better with the octave than with any other of the eight notes1 because for every vibration in it there are just two in the octave4 ,take in contrast with the octave the second note4 ,here to every eight vibrations of the first note we have nine of the second1 and the consequence is a discord when they are sounded together4 ,the difference between the two cases is this3 ,in the first case the commencement of every vibration in the fundamental note coincides with the commencement of every second vibration in the octave4 ,but in the other case there is a coincidence at only every eighth vibration of the first note with every ninth of the second4 ,next to the octave1 the most agreeable harmony with the fundamental note is that of the fifth note of the scale4 ,here we have three vibrations to every two of the first note1 and so every second vibration in the first note coincides with every third vibration of the fifth4 ,next comes the harmony of the fourth1 there being here a coincidence at every third vibration of the fundamental note4 ,the more frequent1 you see1 are the coincidences between the vibrations the greater is the harmony4 ,in the three cases just stated the coincidence is in the first at the commencement of every vibration of the fundamental note1 in the second case at the commencement of every second vibration1 and in the third at the commencement of every third vibration4 #bfg4 ,the ,diatonic ,scale4--,in order that you may see the relative numbers of the vibrations for each of the notes ,i will give them for the whole scale4 ,they are as follows3 #a #i_/h #e_/d #d_/c #c_/b #e_/c #ae_/h #b ,c ,d ,e ,f ,g ,a ,b ,c4 ,according to this the note ,d has nine vibrations to every eight vibrations of ,c1 ,e has five to every four of ,c1 etc41 the octave ,c having just twice the number of vibrations that the fundamental note ,c has4 ,you have here expressed the proportion between the numbers of vibrations in the different notes4 ,suppose1 then1 that you know the number of vibrations in a second that ,c1 the fundamental note1 has1 you can readily calculate the number of vibrations of each of the other notes4 ,it is done by multiplying the number which ,c has by the fractions over the other notes4 ,thus if the number of vibrations in a second in the fundamental note be #abh1 by this process we make the vibrations of all the notes to be thus3 ,c ,d ,e ,f ,g ,a ,b ,c #abh #add #afj #agj #aib #bac #bdj #bef4 ,there are really but seven notes in what is called the diatonic scale1 the eighth note1 ,c1 being truly the first of seven other notes above1 having relations to each other similar to those of the notes below1 and constituting another octave4 ,so we may have several octaves1 one above another4 ,it is interesting to observe that the proportionate lengths of strings required to produce the eight notes of the scale have an exact numerical relation1 but the reverse of that of the numbers of the vibrations4 ,thus if you have eight strings of the same size1 their vibrating lengths required for the notes are as follows3 ,c ,d ,e ,f ,g ,a ,b ,c #a #h_/i #d_/e #c_/d #b_/c #c_/e #h_/ae #a_/b4 ,for the notes of the octave above the lengths are thus3 ,c ,d ,e ,f ,g ,a ,b ,c #a_/b #d_/i #b_/e #c_/h #a_/c #c_/aj #d_/ae #a_/d4 #bfh4 ,unison4--,in tuning instruments so as to make them harmonize the result is obtained when the corresponding parts of the instruments have the same number of vibrations4 ,thus the string in one violin that gives any particular note must vibrate just the same number of times in a second that the strings giving the same note in other violins do1 or it will not be in perfect unison with them4 ,the same is true of other strings for other notes1 and also of the corresponding parts of all kinds of instruments which are to be played together4 ,when1 in tuning instruments together1 it is said that a string of a violin1 for example1 is too flat1 the difficulty is that it does not vibrate with sufficient rapidity1 and it is therefore screwed up to make its note sharp enough1 as it is expressed1 to be in unison with the note of the corresponding strings or parts of other instruments4 #bfi4 ,mysteries of ,sound and ,hearing4--,there are many things of a mysterious character in relation both to sound and the manner in which it causes the sensation of hearing4 ,i will barely notice but two of these4 ,the effect1 or rather the chain of effects1 resulting in hearing is wholly mechanical1 until we come to the nerve of hearing1 which branches out with minute fibrils in the halls of audience of the internal ear4 ,it is merely a series of vibrations4 ,now how it is that the mere agitation of a fluid inclosed in hard bone can communicate through fine white fibres to the brain1 and through that to the mind1 the idea that we have of all the various sounds that are produced1 is a great mystery4 ,all that we know is that the nerve is the medium of the communication1 but of the manner in which it performs its office we know absolutely nothing4 ,again1 while it is sufficiently mysterious that this information can thus be given to the mind when one sound after another communicates its vibration to the liquid in the ear1 the mystery is greatly enhanced when various sounds come to the ear at one and the same time4 ,to get a distinct idea of the very compound and wonderful character of the process of hearing in such a case we will suppose that a full band of music is playing1 and at the same time mingled with its sounds there are various other sounds heard1 some of them perhaps discordant4 ,what a diversity of vibrations we have here6 ,we have the slow vibrations produced in the grave notes1 and the quick vibrations of the higher ones1 all traveling together through the air to the ear1 and each preserving its distinctive character4 ,and more than this1 after they arrive at the ear they are communicated unaltered through the drum1 the chain of bones1 the second drum1 and the liquid where the nerve is1 so that a correct report of each of all the notes is given through the nerve to the mind4 ,then1 too1 if there be any discord its vibration travels along with the rest1 and so do the vibrations of other sounds1 as the roaring of the wind1 the report of cannon1 and the noise of the people4 ,and besides all this1 in the multiplicity of the vibrations thus transmitted through so many different substances the mind gets a true report of the comparative loudness of the sounds1 and even of their character1 so that the sounds of drum1 fife1 trumpet1 etc41 are all accurately distinguished4 ,in view of such wonders how significant is the question1 ,he that planted the ear1 shall he not hear8 ,,chapter ,,xiii4 ,,heat4 #bgj4 ,heat and ,cold4--,in common language we speak of heat and cold as two distinct and opposite things4 ,that this is not strictly correct may be shown by the following experiment3 ,take three vessels1 and fill the first with ice-cold water1 the second with hot water1 and the third with tepid water4 ,if you place your right hand in the first and the left in the second1 and let them remain a little time1 on taking them out and plunging them together into the third vessel1 the water in it will feel warm to the right hand and cold to the left4 ,so the air of a cellar seems warm to you in winter and cold in summer in contrast with the air outside4 ,for the same reason water of a temperature that would ordinarily be refreshingly cool to us seems warm when drank after eating ice-cream4 ,it is manifest1 then1 that there is no fixed dividing-line between heat and cold4 ,there is1 in fact1 no such thing as cold4 ,substances are cold from being deprived of heat2 and no substance ever has all its heat taken from it4 ,sir ,humphrey ,davy proved that there is heat in ice by rubbing two pieces together in a very cold room4 ,they were gradually melted4 ,now this was not done by the air1 for that was at a temperature below the freezing point4 ,the heat which melted the ice came from the ice itself by means of the rubbing4 #bga4 ,nature of ,heat4--,there are two theories in regard to the nature of heat4 ,one is that heat is an imponderable 7 #af71 and of course a very subtile substance1 which pervades all matter4 ,its particles are supposed to repel each other strongly1 and hence they have a tendency to diffuse themselves1 and to separate the particles of matter from each other4 ,it is in this way that they are supposed to occasion the expansion of substances4 ,the other supposition1 which is most commonly received1 is that heat is a vibration of the particles of bodies1 and that it passes from these to bodies less warm through a subtile fluid called ether1 supposed to fill all space4 ,you see that if this be the true theory1 there is some analogy between heat and sound4 ,illustration3 ,fig4 #aia4 #bgb4 ,sources of ,heat4--,the principal of the sources of heat on our earth is the sun1 though that body is ninety-five millions of miles distant from us4 ,as the heat1 in traveling all this long journey1 is becoming more and more diffused2 or1 in other words1 as its rays are all the way separating from each other more and more1 we can have no conception of the concentrated heat that exists in the sun itself4 ,we can1 however1 approximate to the idea by observing the effects of heat when some of its separated rays are gathered to a point by a powerful lens1 as represented in ,fig4 #aia4 ,a lens which concentrated the heat ten thousand times melted platinum1 gold1 quartz1 etc41 in a few seconds4 ,and as the heat at the sun is supposed to be thirty times more concentrated than this1 none of the most solid substances of our earth would remain solid if they were there1 but would be some of them liquid1 and others even in a state of vapor4 ,the heat which the sun constantly radiates to the earth pervades all substances1 producing motion1 and awakening life every where1 so that1 in the expressive language of the ,bible1 ,there is nothing hid from the heat thereof4 ,another source of heat is within the earth itself4 ,it has been found as we go down into the earth there is a constant increase of temperature the farther we go4 ,this internal heat is attributed in part to subterranean fires and various chemical actions4 ,we see here and there external evidences of the operation of these causes in the eruptions of volcanoes1 the boiling springs1 the jets of steam and sulphureous vapors1 etc4 ,but that the heat in our earth which comes from these subterranean sources is small compared with that which comes from the sun1 is seen in the fact that the rate of increase of heat at great depths is much less than it is nearer the surface4 ,this would seem to show that although fires within the earth may have considerable influence in heating its crust1 on which we live1 it derives the most of its heat from the sun1 at least to a very great depth4 ,how great a source of heat electricity is we know not1 but that considerable heat comes from this source is evident from the melting and burning effect which we often see resulting from the passage of the electric fluid4 ,another very common source of heat is chemical action4 ,we see it continually produced in chemical experiments4 ,combustion1 which1 as will be shown to you in the ,second ,part of this ,series1 is nothing but an example of chemical action1 is the most common of all the chemical sources of heat4 ,animal heat is also1 for the most part1 a result of chemical action4 ,mechanical action is a common source of heat4 ,the rubbing of a match producing heat enough to occasion flame is a familiar example4 ,the spark produced in what is called striking fire is the burning of a particle of steel set on fire by the blow4 ,the ,indian was accustomed to light his fire by the rubbing together of two dry sticks till he learned an easier way from civilized neighbors2 and the blacksmith1 previous to the invention of phosphorus matches1 often lighted his fire by touching a sulphur match to a nail made red-hot by rapid and continued hammering4 ,machinery has sometimes been set on fire by friction1 and the water around a mass of metal has been so heated by boring as even to boil4 ,if you stretch a piece of ,india rubber several times in quick succession1 and then apply it to your lips1 you will perceive that the motion has warmed it4 #bgc4 ,relations of ,heat and ,light4--,heat is sometimes alone1 and is sometimes in intimate union with light4 ,all substances have some amount of heat1 and it passes from them to other bodies in their neighborhood that happen to have less heat in them4 ,in doing this it may or may not have the company of light4 ,in the radiation of heat from a stove1 unless it be heated to redness1 there is no light with the heat2 but from an open1 burning fire the light and heat come together4 ,but the rays of the sun give us the best example of the union of light and heat4 ,traveling together at an equal pace they are most curiously mingled1 as you will see when ,i come to speak particularly of light4 ,i will now proceed to notice the principal effects of heat2 viz41 expansion1 liquefaction1 and vaporization4 ,illustration3 ,fig4 #aib4 #bgd4 ,expansion in ,solids4--,heat1 you have seen in #bc1 acts in opposition to the attraction of cohesion1 tending to separate the particles1 and so produces an expansion of any substance4 ,this may be exemplified in the experiment represented in ,fig4 #aib1 in which ,a ,b is an iron rod1 which is of such a size that at the ordinary temperature it will fit into the space1 ,c ,d1 in a bar of iron1 and easily pass through the hole1 ,e4 ,if the rod be heated it will be enlarged or expanded in all directions1 so that it will neither fit into ,c ,d nor pass into the hole1 ,e4 ,when the wheel-wright puts a tire upon a wheel he uses the expansion of heat to make it fit tightly and firmly4 ,the tire is made a little too small to have it fit upon the wheel as it is4 ,but by being heated it is so expanded that it will readily go on to the wheel1 and then in contracting as it cools it so compresses the fellies as to hold on very tightly4 ,water is poured on to cool the iron quickly1 and thus prevent it from burning the wood4 ,iron hoops are put on barrels in a similar manner1 the compression caused by their contraction binding the staves together very strongly4 ,so in fastening the plates of boilers together1 the rivets are put in red-hot1 so that in their contraction they may press the plates closely together4 ,if an iron gate just shuts into its place in cold weather1 its expansion will prevent its shutting when warm weather comes4 ,in order to avoid this difficulty1 calculation must be made in fitting it for its place for the expansion to which it will be subjected by heat4 ,so in laying the rails of a railroad in cold weather care must be taken not to put the ends too near together4 ,in constructing iron bridges the expansion by heat must be calculated for in the arrangement4 ,nails often become loose after the lapse of years from the wear of the wood around them1 occasioned by their alternate expansion and contraction4 ,the leaking of gas-pipes in the earth is often undoubtedly caused by the loosening of the joints from contraction and expansion of the pipes by varying temperatures of the soil1 especially where they are not laid very deep4 ,if a stopper stick fast in a bottle it can be loosened by the application around the neck of a cloth dipped in hot water1 because the neck becomes expanded at once by the heat4 ,a similar expedient was once very ingeniously made use of in repairing the machinery of the steamer ,persia at sea1 and was perhaps the means of saving the vessel and the lives of all on board4 ,the accident which occurred was the breaking of the port crank-pin of the engine4 ,the problem to be solved was the removal of this pin1 which weighed nearly a ton1 and the substitution of a sound one which they had on hand in its place4 ,but it was found impossible to start the broken pin from its socket with all the force which could be brought to bear upon it by a sort of battering-ram constructed extemporaneously for the purpose4 ,it was determined now to try the expansive force of heat4 ,an iron platform was built under the socket1 and a brisk fire made upon it4 ,the socket soon expanded1 and the pin was now readily knocked out by the battering-ram1 just as the stopper of the bottle is easily removed when the neck is heated4 ,the walls of a very large building in ,paris1 which had bulged out and were in danger of falling1 were restored to their upright position by the expansion of heat4 ,illustration3 ,fig4 #aic4 ,it was done in this way3 ,long rods of iron were run through the walls after the plan represented in ,fig4 #aic 7p4 #bac71 their ends being made with a screw-thread1 with nuts fitted to them4 ,the rods marked a were first heated1 and as they lengthened the nuts were screwed up tight to the walls4 ,on cooling1 their contraction would of course draw the walls together4 ,the other bars1 b1 were now heated and managed in the same way4 ,the one set1 you see1 were made to hold on by their nuts to what had already been gained1 while the other were expanding4 ,by many repetitions of this process the walls were righted and the building saved4 ,the same mode has been adopted successfully in other cases of a similar character4 #bge4 ,expansion in ,liquids4--,liquids are expanded by heat more than solids are4 ,but they are very unequally expanded by it4 ,thus water is expanded more than twice as much as mercury1 and alcohol six times as much4 ,we have a frequent example of the expansion of water by heat in our kitchens4 ,if the tea-kettle be put over the fire filled to the brim1 it will run over long before the water begins to boil4 ,all liquids occupy more space in summer than in winter1 and in the former case weigh less--that is1 have less of real substance in them than in the latter4 ,if1 therefore1 alcohol1 or oil1 or molasses be bought by the gallon in winter and sold in summer1 there will be a profit afforded by the expansion4 ,twenty gallons of alcohol in winter becomes twenty-one in mid-summer4 ,illustration3 ,fig4 #aid4 ,the influence of the expansion of heat upon the specific gravity of liquids may be very prettily shown by the following experiment3 ,let some little bits of amber--a substance which is nearly of the same specific gravity with water--be thrown into water in a glass vessel1 and let the water be heated1 as represented in ,fig4 #aid1 by a spirit-lamp4 ,that portion of the water which is heated passes upward because it is made specifically lighter1 and colder water continually comes down to take its place4 ,the upward and downward currents are as indicated by the arrows1 the upward passing up in the middle1 the downward coming down at the sides4 ,this will be made manifest by the little bits of amber4 #bgf4 ,thermometers4--,it is the expansion of liquids by heat that in the thermometer gives us the measure of temperature4 ,the liquid metal mercury is commonly used for this purpose1 and answers well except in the extreme cold of the arctic regions4 ,there1 as mercury becomes solid at #ci degrees below zero1 it is necessary to use a thermometer with alcohol in it1 as this fluid can not be frozen by any degree of cold4 ,the operation of the thermometer is simply this3 ,heat expands the fluid in the bulb1 and the only way in which it can occupy more space as it expands is by rising in the tube4 ,the abstraction of heat1 on the other hand1 causes contraction1 and of course a proportionate falling of the fluid4 #bgg4 ,fahrenheit's ,thermometer4--,the thermometer was invented in the beginning of the seventeenth century1 but it is not decided who was the inventor4 ,there may have been in this case1 as in others1 more inventors than one1 the same ideas having1 perhaps1 entered several inquiring minds at the same time4 ,various fluids were used by different persons4 ,sir ,isaac ,newton used linseed oil4 ,fahrenheit1 a native of ,hamburg1 who flourished in the first part of the last century1 was the first to use mercury4 ,though various propositions were made by ,newton and others in regard to the measurement of heat by thermometers1 no thermometric scale seems to have met with general reception till that of ,fahrenheit's1 which was put forth about #agbj4 ,the plan of it is this3 ,his zero is the point at which the mercury stood in the coldest freezing mixture that he could make2 and he supposed that this was the greatest possible degree of cold1 as it was the greatest that he knew4 ,he next found the point at which the mercury stood in melting ice4 ,this he called the freezing point1 because the temperature is the same in water passing into the solid from the fluid state as in water passing into the fluid state from the solid4 ,in other words1 this point in the scale marks the transition line between the two states4 ,from this point ,fahrenheit marked off #cb equal spaces or degrees down to zero4 ,he now found the point at which the mercury stands in boiling water1 and called this the boiling point4 ,marking off the space on the scale between this1 and the freezing point in the same manner1 there are #ahj degrees--that is1 the boiling point is #bab degrees above zero4 ,the degrees above zero are commonly designated by the mark 1 plus2 and those below by the mark -1 minus4 ,thus1 #cb signifies #cb degrees above zero1 and -#cb signifies #cb degrees below4 ,illustration3 ,fig4 #aie4 #bgh4 ,other ,thermometers4--,fahrenheit's thermometer is the one commonly used in this country4 ,but there are several other thermometers on different scales1 as the ,centigrade1 ,reaumur's1 and ,de ,lisle's4 ,in ,fig4 #aie you see the plans of the scales of these thermometers placed side by side4 ,in the ,centigrade thermometer1 which is in use in ,france1 and indeed in a large part of ,europe1 the zero1 you see1 is placed at the freezing point2 and the space between this and the boiling point is divided into #ajj degrees1 which gives it the name ,centigrade4 ,reaumur's1 which is in use in ,russia1 has the same zero1 but he has only #hj degrees from this to the boiling point4 ,de ,lisle's1 which has gone entirely out of use1 has its zero at the boiling point4 ,the arrangement of ,fahrenheit1 although its zero is a mere arbitrary point1 is1 on the whole1 the best1 because its degrees are of such a size that they mark differences of temperature with sufficient minuteness for all practical purposes of an ordinary character without resorting to fractional parts4 ,illustration3 ,fig4 #aif4 #bgi4 ,expansion in ,aeriform ,substances4--,heat produces a vastly greater expansive effect in air1 the gases1 and vapors1 than it does in liquids4 ,the expansion of air by heat may be shown very prettily in this way3 ,take a glass tube that has a bulb on one end1 and1 placing the other open end in water 7as represented in ,fig4 #aif71 apply the palm of your hand to the bulb4 ,the heat of the hand being communicated to the bulb will expand the air1 and so1 as you see1 bubbles of air will escape through the water4 ,on removing the hand1 and allowing the bulb to cool1 the air in it will be condensed1 and water will pass up in the tube in proportion to the amount of air which has escaped4 ,a bladder partly filled with air will be made to swell out to plumpness if it be heated sufficiently1 and a full one may be so heated as to burst from the expansion of the air4 ,porous wood1 as chestnut1 snaps very much when burned1 because the heat expands the air contained in the pores4 #bhj4 ,balloons4--,the first balloons that were used were filled with heated air4 ,you have already seen1 in #adi1 why it is that balloons rise4 ,now in the hot-air balloon it is the expansion of the air by heat that makes it lighter than the surrounding air4 ,of course such a balloon is not as effective as the gas balloon1 for the air within it loses its comparative lightness as it becomes cooled2 while the gas which is used1 being very much lighter than air at the same temperature1 does not lose its lightness as the balloon goes up4 ,you learned in #aeb that the atmosphere becomes thinner as we go upward4 ,the gas balloon1 therefore1 rises until it arrives at that point where the air is of about the same specific gravity with the gas1 and there it stops4 ,it is made to descend by letting out some of the gas from a valve4 ,gas was not used for balloons till #aghb4 ,hydrogen gas was employed at first1 being over fourteen times lighter than air4 ,of late the common burning gas1 carbureted hydrogen1 has been generally used1 because it can be so readily obtained where there are gas-works4 ,illustration3 ,fig4 #aig4 #bha4 ,currents in the ,air from ,heat4--,heat is the grand mover of the atmosphere4 ,any portion of it that becomes warmer than surrounding portions rises1 or rather is pushed up1 for the same reason that a hot-air balloon rises1 the only difference between the two cases being that in the one the air is confined1 and in the other is left free1 and so becomes diffused4 ,and it is this rising of the air from expansion that causes nearly all the movements that we witness in the air4 ,we see this exemplified in various ways wherever there is a fire4 ,the air that is heated by the fire is forced upward by the colder air1 which1 on the principles of specific gravity1 seeks to get below the warmer and lighter air4 ,the hot air that comes through the registers of a furnace is pushed up by colder air below4 ,for the same reason the heated air around a stove-pipe is constantly going upward4 ,this is very prettily shown by the toy represented in ,fig4 #aig 7p4 #bah71 which is a paper cut spirally1 and suspended1 as you see1 upon the point of a wire4 ,the upward current makes the paper revolve rapidly around the wire4 ,it is from the rising of warm air that the galleries of a church are warmer than the space below4 ,in a common room the disposition of the air is continually to have its warmest portions above and the colder below4 ,it is for this reason that we have our arrangements for producing or introducing heat at as low a point as possible4 #bhb4 ,chimneys4--,we speak of the draught of a chimney1 and we say of one that does not smoke that it draws well1 as if the smoke were in some way actually drawn up4 ,but the same principles apply here as are developed in #bha4 ,the smoke1 which is a combination of heated air and gases with some solid matters in a fine state1 is forced up the chimney4 ,when a chimney does not draw well we open a door or a window for a little while until the fire gets thoroughly agoing4 ,why is this8 ,it is that we may have denser air than there is in the room1 so that the smoke may be pushed up more forcibly4 ,when the chimney becomes well heated there is ordinarily no difficulty1 because then the smoke in it is not obliged to part with much of its heat to the walls of the chimney1 and therefore is so much lighter than the air in the room that it is very easily forced upward4 ,the principal reason that a stove-pipe generally draws better than a chimney is that there is much less heat expended in establishing and maintaining the upward current4 ,especially is this true if the chimney be a large one4 ,in such a case there are both a great extent of brick and a large body of air to be heated to establish the upward current1 and these must be kept warm in order to maintain it4#c ,illustration3 ,fig4 #aih4 ,illustration3 ,fig4 #aii4 #bhc4 ,winds4--,if you open a door of a heated room a candle held near the floor will have its flame blown inward1 while one held near the top of the door will have its flame blown toward the cold entry4 ,here you have a good illustration of the manner in which winds are produced4 ,wherever the wind blows it is air pushing out of the way other air that is warmer1 in order that it may1 in obedience to gravitation1 get as near the earth as possible4 ,take1 for example1 the land and sea breezes1 as they are called4 ,during a hot summer's day the sun heats the earth powerfully1 while the ocean receives but little of its heat4 ,the heated land heats the air above it2 and as the air over the ocean is cooler1 and therefore heavier1 it pushes upward the air of the land1 for the same reason that water pushes up oil2 and as this goes on continuously a regular current is established4 ,the wind blows in upon the land1 as represented in ,fig4 #aih1 while the warmer air passes upward into the higher regions of the atmosphere1 and turns toward the sea4 ,the arrows show the course of the currents4 ,the resemblance of all this to the effect upon the candle held near the open door is very obvious1 the cold air from the entry blowing in below representing the breeze from the ocean1 and the warm air of the room blowing out above representing the passage of the warm air of the land out toward the ocean4 ,at night this is apt to be reversed4 ,the earth becomes cooled1 and with it the air that is over it4 ,the result is that the cooled air of the land now pushes upward the warmer air of the sea1 as seen in ,fig4 #aii4 ,illustration3 ,fig4 #bjj4 ,illustration3 ,fig4 #bja4 #bhd4 ,winds as ,affected by the ,rotation of the ,earth4--,the heat of the vertical sun upon the tropics causes a rise of heated air into the upper regions1 while there is a rush of colder air toward the equator from both north and south4 ,this effect is represented in ,fig4 #bjj 7p4 #bba71 ,e being the sun1 ,n the north pole1 and ,s the south pole4 ,an effect similar to that represented in ,figs4 #aih and #aii is produced here1 but it is on a much larger scale4 ,but the diagram does not present the matter in its true light in all respects4 ,the prevailing winds in the equatorial regions are not north and south winds1 as would appear from this diagram2 but they are from the northeast and southeast4 ,i will explain this by ,fig4 #bja4 ,as the earth turns on its axis it is plain that there is no part of the surface of the earth that moves so rapidly as the equator1 ,e ,w1 for that moves in a larger circle than any other part4 ,and the nearer you go to either pole1 ,n or ,s1 the less is the rapidity of the revolution4 ,now the atmosphere1 as stated in #ahh1 partakes of the motion of the earth4 ,the air1 therefore1 at the equator is moving from west to east with the rotation of the earth faster than it is any where else1 and the nearer you go to either pole the slower is its motion4 ,it follows from this that any portion of air blowing from the north or the south toward the equator1 as it comes from where it was moving east slower than air at the equator is1 would from its lesser momentum lag behind the air of the equator1 the wind would be curved toward the westward1 as indicated by the arrows4 ,the result would be that the northern wind would be converted into a northeaster1 and the southern into a southeaster4 ,all this can be made more clear with a globe1 or1 indeed1 with any round object4 #bhe4 ,liquefaction4--,the change of solids into liquids is one of the most observable effects of heat4 ,this change requires different degrees of heat in different substances4 ,thus while iron melts at the high heat of #bghf1 lead melts at #fcc1 sulphur at #bci1 ice at #cb1 and mercury at #ci below zero4 ,mercury is never found in a solid state1 but it sometimes becomes solid in the arctic regions when carried there and exposed in the open air4 ,we are apt to think of water as being in a more natural state when liquid than when it is solid1 just as we think of iron as being naturally solid and mercury as naturally liquid4 ,but in all these cases the state of the substance depends on its temperature1 and this is varied by circumstances4 ,water at the equator is always liquid1 and the idea of ice there is exceedingly unnatural2 while near the poles it is the reverse1 ice and snow reigning every where throughout the whole year4 #bhf4 ,evaporation4--,there are two ways in which the change of a liquid into a vapor occurs4 ,one is a rapid change when heat is so applied as to raise the liquid to its boiling point4 ,this is commonly termed vaporization4 ,the other mode is the ordinary gradual evaporation which goes on from the surface of the liquid4 ,this process is going on continuously1 not requiring any particular degree of heat1 but occurring under all degrees of the temperature of a liquid4 ,its rapidity1 however1 is in proportion to the degree of heat1 as may be seen by the rise of vapor from water that is being heated1 long before it begins to boil4 ,the same thing can also be seen in a bright summer's morning1 when the heat of the sun causes the moisture gathered from rain or dew to rise so abundantly from fences1 and boards1 and roofs as to be visible like smoke4 #bhg4 ,solution of ,water in ,air4--,evaporation is constantly going on from every wet surface1 except when the air is so loaded with moisture that it can take up no more4 ,the vapor is not ordinarily visible1 the particles of water passing quietly upward among those of the air1 being dissolved in the air just as some solids are dissolved in water4 ,it becomes visible only when so much of it rises that the solution of the water in the air is not readily effected4 ,the readiness with which the solution takes place depends much upon the temperature of the atmosphere4 ,some very common phenomena illustrate this4 ,in a very cold day the breath of animals1 as it comes out of the mouth1 seems to be loaded with moisture4 ,why8 ,it is not because there is more moisture in it than in warm weather1 but because cold air can not hold in solution so much water as warm air can4 ,the same explanation applies to the smoking of wet fences and roofs in the sun of a summer's morning4 ,the moisture is heated by the sun1 but the air1 not having become very warm as yet1 can not readily dissolve all the moisture that rises4 ,the phenomenon is not apt to occur when the hot sun shines after a shower at mid-day or in the afternoon1 because then the air is warm enough to take up all the moisture that is sent up into it4 ,how water1 being heavier than air1 rises in the atmosphere is a mystery4 ,it has been supposed by some that it was owing to a kind of affinity existing between water and air4 ,but in opposition to this is the fact that evaporation takes place more rapidly under the exhausted receiver of an air-pump1 where there is almost no air1 than it does where it is freely exposed to the atmosphere4 #bhh4 ,clouds4--,the water which goes up in the air in evaporation is variously disposed of4 ,some of it is deposited as dew or frost4 ,some of it forms fog4 ,some of it also mounts far upward and forms the clouds1 which are really collections of fog made high up in the air4 ,in fog and in clouds the water which in its evaporation is invisible becomes visible4 ,let us see how this is4 ,there is always more or less of water in clear air1 but the particles are so minutely divided and so thoroughly mingled with the particles of the air that they can not be seen4 ,but in a fog or cloud the particles of water are gathered together in little companies1 as we may express it4 ,and it is supposed1 some think ascertained1 that each of these companies of particles is globular and hollow4 ,if so1 then we may regard every cloud as a vast collection of minute bubbles or balloons careering through the air4 ,illustration3 ,fig4 #bjb4 ,illustration3 ,fig4 #bjc4 #bhi4 ,shapes of ,clouds4--,clouds have a very great variety of shape1 the causes of which are for the most part not understood4 ,they are generally divided into four classes3 ,cirrus1 ,cumulus1 ,stratus1 and ,nimbus4 ,the ,cirrus is represented in ,fig4 #bjb 7p4 #bbe74 ,it is a light1 fleecy cloud1 having graceful turns like curls1 and hence its name1 which is the ,latin word for curl4 ,such clouds are commonly very high up in the air4 ,the ,cumulus 7,latin for heap7 you see in ,fig4 #bjc 7p4 #bbe74 ,clouds taking this form appear as heaps rounded upward1 and often appear like mountains of snow when they are illuminated by the sun4 ,we see such clouds mostly in summer4 ,the ,stratus 7,latin for covering7 is seen in the same figure under the ,cumulus4 ,clouds of this form lie low in the horizon1 stretched along like a sheet4 ,they often form in the latter part of the day1 and increase in the night1 but the rising sun dissipates them4 ,the ,nimbus1 or rain-cloud1 is represented in ,fig4 #bjd 7p4 #bbf74 ,it has a uniform gray or dark color4 ,we often have two forms of cloud mingled together4 ,thus in ,fig4 #bje 7p4 #bbf7 we have a mixture of the ,stratus and the ,cirrus1 termed ,cirro-,stratus4 ,this is commonly called the mackerel-sky1 and is quite a sure prognostic of rain4 ,then we have the ,cirro-,cumulus1 ,fig4 #bjf 7p4 #bbg71 and the ,cumulo-,stratus1 ,fig4 #bjg 7p4 #bbg74 ,illustration3 ,fig4 #bjd4 ,illustration3 ,fig4 #bje4 ,water is gathered into clouds undoubtedly1 in part at least1 from the influence of attraction4 ,but what the circumstances are that give them all these various shapes we know not4 ,whatever they are1 they sometimes operate very extensively1 giving a similar shape to all the clouds that cover the whole arch of the heavens2 and at other times they operate variously in different localities1 producing different shapes1 sometimes even in near neighborhood to each other4 ,sometimes the edge of a cloud is irregular1 or curved1 or feathery2 and at others it is a well-defined line1 stretching along over a large portion of the horizon4 ,in all these cases we have only divers arrangements of the same thing--a collection of vesicles of water containing air1 which is made lighter than the air outside of the cloud by means which ,i shall speak of in another part of this chapter4 ,illustration3 ,fig4 #bjf4 ,illustration3 ,fig4 #bjg4 #bij4 ,rain1 ,snow1 and ,hail4--,when it rains the vesicles or minute bubbles of which the clouds are composed are broken up1 and each drop of rain contains the water which came from a multitude of these vesicles4 ,but let us see exactly how this result is produced4 ,rain comes from the contraction of the clouds by cold4 ,a cold current of air coming in contact with a cloud will condense its bubbles into drops1 and these of course will fall4 ,the same result occurs if a cloud passes into a cold stratum of air4 ,but let us look at the process more minutely4 ,let us see what the effect of cold is upon the bubbles4 ,illustration3 ,fig4 #bjh4 ,the first effect may be made clear by ,fig4 #bjh4 ,if a bubble be contracted by the influence of cold1 the water of its wall being made thicker1 there will be a gathering of it from gravitation at the lower part1 as represented by the dotted line4 ,you often see a similar effect in the soap-bubble4 ,it rises filled with the warm air from your lungs1 and as it goes up it is contracted by the colder air which is around it4 ,this contraction makes the water hang downward from the bottom of it4 ,and as the soap-bubble at length perhaps bursts in the air from the weight of this water1 so it is with the vesicles in the cloud4 ,and many of these1 united together by attraction1 form a drop4 ,when the cold is sufficiently severe it makes the water of the ruptured vesicles of the cloud arrange itself in snow-crystals instead of drops4 ,and when the cold acts with great rapidity upon a cloud it presses the particles of the water together so suddenly that there is not time for the crystalline arrangement1 and hail is formed4 #bia4 ,vaporization4--,the production of vapor by boiling differs in some respects from quiet evaporation4 ,here the liquid is raised in temperature to its boiling point1 and the formation of vapor is not confined to the surface4 ,in water the boiling point is #bab1 but it varies more or less from this in other liquids4 ,thus the boiling point of alcohol is #agc1 of ether #ie1 oil of turpentine #efh1 and mercury #feb4 ,illustration3 ,fig4 #bji4 #bib4 ,influence of ,pressure upon the ,formation of ,vapor4--,pressure restrains the production of vapor1 whether it be formed by evaporation or vaporization4 ,we know by experiments with the air-pump that the less pressure of air there is upon the surface of a liquid the more rapidly will evaporation from it go on4 ,i have already spoken of the influence of pressure upon the boiling of liquids in #aga4 ,i will give here a few additional illustrations4 ,ether boils when it is heated to #ie1 three degrees below the heat of the blood in our bodies4 ,if we place some of it in a vessel under the receiver of an air-pump1 by exhausting the air we can so take off the pressure that the ether will boil at the ordinary temperature of the air in a room4 ,the restraint of pressure upon boiling is very strikingly shown in the digester1 ,fig4 #bji4 ,this is a strong boiler1 a1 partly filled with water4 ,a thermometer1 d1 is fastened into it so as to indicate the heat of the water4 ,there is also a tube1 c1 extending to near the bottom of the boiler into a small quantity of mercury which is there4 ,let1 now1 the boiler be heated till the water boils1 the air being left to escape by the stop-cock1 b4 ,if the stop-cock be shut1 and we continue to apply the heat1 we can raise the water to a very high temperature without having it boil at all1 because of the pressure of the condensed steam upon its surface4 ,an apparatus somewhat after this plan1 called ,papin's digester1 has been used sometimes in cooking4 ,the great heat to which water can thus be raised causes it to extract the nutritious matter from bones and cartilages1 affording material for soup from what is commonly thrown away4 ,to guard against the danger of explosion a safety-valve is provided1 having a weight upon it which will keep it shut until a certain amount of pressure accumulates1 and then it is forced open1 letting out some of the steam4 #bic4 ,steam4--,the cloud of steam1 so called1 which you so often see escaping from a locomotive is not really steam4 ,steam is transparent and invisible4 ,you can see that it is so if you observe it issuing from the spout of a tea-kettle4 ,it is only after it gets an inch or more from the spout that it becomes visible1 and then it is really changed from steam into water by the condensing influence of the cold air4 ,and the water in the cloud thus formed is probably in the same condition with the water in the clouds above1 as described in #bhh4 ,illustration3 ,fig4 #baj4 #bid4 ,the ,steam-,engine4--,as compressed or condensed air has great power by its elasticity1 as seen in the air-gun1 #afd1 so also has condensed steam4 ,it is steam condensed1 and endeavoring1 therefore1 in proportion to its condensation1 to expand itself1 which constitutes the moving force of the steam-engine4 ,the steam is generated in a boiler1 having1 like the boiler of ,papin's digester1 a valve with a weight attached to it4 ,this valve is called a safety-valve1 because when the steam has reached a certain degree of condensation it lifts the valve1 and1 as some of the steam escapes1 such an increase of pressure as would occasion an explosion is prevented4 ,the expansive force of steam in a boiler is estimated in pounds by the weight on the valve1 and hence the common expression that there are so many pounds of steam on4 ,but the boiler is only the generator of steam1 and it remains to show how the steam is used in moving machinery4 ,this is done by allowing the steam to pass from the boiler into a cylinder1 and then move a piston back and forth by its expansive force4 ,the manner in which it does this may be made clear by the diagram1 ,fig4 #baj 7p4 #bca74 ,let e be a piston in a cylinder1 f1 which has four openings1 a1 b1 c1 and d4 ,these all have valves4 ,the steam is supplied from the boiler to the cylinder through a and c1 and makes its escape from b and d4 ,suppose1 now1 the piston is near the bottom of the cylinder1 as represented4 ,the valve at a is now opened that steam may enter to push up the piston1 and the valve at b shuts that the steam may not escape4 ,at the same time1 that pressure may be taken off from the upper surface of the piston1 d opens that the steam may escape1 and c shuts that none may enter4 ,when the piston is to be forced downward all this is reversed--c opens to admit the steam1 d shuts to prevent its escaping2 and below1 b is opened to let the steam escape1 and a is shut to prevent any from entering4 ,this is the plan of what is called the high-pressure engine4 ,the low-pressure engine differs from it in having the steam1 as it escapes from the cylinder1 pass into water to be condensed4 ,the latter requires less pressure of steam to work it1 and therefore is the safest4 ,the manner in which the motion of the piston is made to work various kinds of machinery ,i need not stop to explain1 especially as exemplifications of it may be seen in every quarter4 #bie4 ,communication of ,heat4--,heat has a constant tendency to an equilibrium4 ,if therefore any warm substance be in the neighborhood of one which has less heat1 a flow of heat from the former toward the latter takes place4 ,now this communication of heat occurs in three different ways1 called ,convection1 ,conduction1 and ,radiation4 ,i will speak of each of these separately4 #bif4 ,convection4--,this mode of diffusion of heat is in operation in those substances whose particles are movable among each other--viz41 liquids and aeriform substances4 ,i have already alluded to examples of this mode in speaking of the movements which heat causes in these substances4 ,the heat goes along with the particles which are moved1 or is conveyed along with them1 and hence the term convection4 ,in this movement the heated particles always ascend1 for the reason given in #bge4 ,of the multitude of examples of convection ,i will present but a few4 ,in the upward current about a stove-pipe you have an example of convection1 the heat generated being carried upward by the particles of this current4 ,this being so1 the heat of a stove has no effect upon the air below it by convection1 though it does have by radiation1 as you will soon see4 ,any hot fluid becomes cool chiefly by convection4 ,the air coming in contact with it taking some of its heat rises1 and other air comes in its turn to be also heated1 and so on till the fluid becomes of the same temperature with the air1 and then the currents of air cease4 ,the liquid cools more rapidly by stirring it1 because the air is brought into contact with a greater extent of surface1 and so the heat is conveyed away more rapidly4 ,the result is the same whether we disturb the surface by stirring it or by blowing upon it4 ,in the latter case1 however1 the effect is increased by making the air to come more rapidly upon the disturbed surface4 ,so in fanning1 it is the bringing of the air faster upon the surface of the body that causes the more rapid1 convection of heat from it4 ,every one must have observed the fact that a buckwheat cake cools much more quickly than a flour or rice cake4 ,it is because it has so many pores and little projections1 and so presents a much larger amount of surface to the heat-conveying air than the smoother and more solid cakes4 ,viscid fluids1 as molasses1 oil1 etc41 when heated do not cool as readily as water1 because their particles are not as movable1 and therefore heat is not conveyed as rapidly upward to be given off to the air4 ,illustration3 ,fig4 #baa4 #big4 ,conduction4--,in this mode of diffusion the heat goes through or among the particles of substances4 ,for example1 if one end of a bar of iron be held in the fire1 it travels through or among the particles to the other end4 ,the gradual progress of the heat may be seen by the following simple experiment3 ,take a rod of iron and attach to it1 as seen in ,fig4 #baa1 some little balls of wood by means of wax4 ,by heating one end with a lamp the balls will drop one after another as the heat passing along melts the wax which holds them4 ,illustration3 ,fig4 #bab4 #bih4 ,conductors and ,non-,conductors4--,heat is conducted more rapidly through some substances than through others4 ,there is great variety in this respect4 ,there is considerable among those which are reckoned as good conductors1 as is shown by the experiment represented in ,fig4 #bab4 ,here are cones of the same size of seven different substances--copper1 iron1 zinc1 tin1 lead1 marble1 and brick--all tipped with a little wax1 and placed on a stove4 ,the wax will melt on the copper cone first1 showing that this is the best conductor of them all2 and on the brick one last1 showing that this is the poorest conductor4 ,the conducting powers of the rest are according to the order in which ,i have mentioned them4 ,those substances which allow heat to pass through them very slowly are called non-conductors4 ,the term1 though convenient1 is not a strictly correct one1 for there are no substances which do not conduct heat in some degree4 ,wood is one of these poor conductors1 and hence wooden handles are put upon various instruments and vessels that are used about fires1 as the soldering irons of the tinman1 the metallic tea-pot1 etc4 ,as cloth is a non-conductor1 the holder is used in taking off the tea-kettle and in using the flat-iron4 ,glass is so poor a conductor that if you hold a rod or tube of it across the flame of a spirit lamp or gas burner1 and heat it even to redness1 you can place your fingers very near to the heated portion with impunity4 ,i had occasion to-day to bend a small glass tube in this way1 and ,i observed some water in it quite near to the heated part which remained undisturbed through the process4 ,it is the non-conducting quality of glass that makes it so liable to break1 when it is thick1 if it be exposed to any sudden change of temperature4 ,for example1 if hot water be poured into a thick glass vessel1 the inner surface is quickly expanded2 but the outer surface not expanding with it1 because the heat is not readily conducted through1 this irregularity in expansion causes a fracture4 ,it is for this reason that the flasks1 retorts1 etc41 used by the chemist are made very thin1 especially where the heat is to be applied4 ,illustration3 ,fig4 #bac4 #bii4 ,davy's ,safety-,lamp4--,one of the most beautiful applications of the conduction of heat we have in the ,safety-,lamp of ,sir ,humphrey ,davy1 an invention which has been the means of saving the lives of multitudes of miners4 ,it is represented in ,fig4 #bac4 ,with this lamp one can go into the midst of the most explosive gases with impunity4 ,now all that prevents the flame within from setting on fire the gases without is a covering of wire-gauze4 ,this1 being a good conductor1 conducts off the heat of the flame within so rapidly that it can not go through the openings as flame1 and so does not set fire to the gas without4 ,the fact upon which the construction of this lamp was based was discovered by trying many experiments4 ,among them were the following3 ,a piece of wire-gauze was held over a candle so that its flame struck against it4 ,the smoke issued above1 but no flame4 ,then a stream of gas was allowed to pass through the gauze1 as seen in ,fig4 #bad1 and was set on fire above4 ,it burned without inflaming the gas below4#d ,illustration3 ,fig4 #bad4 #cjj4 ,relation of ,density to ,conduction4--,generally the more dense a substance is the better is its conduction of heat4 ,thus the metals are better conductors than wood1 marble than brick1 the solids than liquids1 and liquids than aeriform substances4 ,we have frequently a good illustration of the difference between stone and brick as conductors in the melting of snow on sidewalks4 ,if a light snow fall in the spring1 after the earth has become somewhat warm1 you will see it melted from the stone walks much before it is from the brick ones4 ,this will be especially the case if the snow be melted mostly by the warmth of the earth without the agency of the sun4 ,the explanation is obvious4 ,the stone is a better conductor than the brick1 and therefore the heat of the earth comes up through the former more rapidly than through the latter4 ,illustration3 ,fig4 #bae4 ,illustration3 ,fig4 #baf4 #cja4 ,conduction in ,liquids4--,that liquids are poor conductors of heat may be shown by an experiment or two4 ,if a thin glass tube closed at one end be filled with water1 and the heat of a spirit lamp be applied to its upper portion1 as seen in ,fig4 #bae1 though the water at this portion may be made to boil1 there will not be the least movement in the lower part4 ,this will be very obvious if you have some amber-dust in the water4 ,again1 let a little water be frozen in the lower part of the tube by placing it in a freezing mixture1 and introduce a little oil1 and then over that some alcohol4 ,hold now the tube over the chimney of a lamp1 as represented in ,fig4 #baf1 until the alcohol boils4 ,the ice in the bottom of the tube will not be in the least affected1 and the oil will be but slightly heated4 ,if the heat were to be applied in either of the above cases at the lower part of the tube the result would be different1 because convection would then operate in the diffusion of the heat4 #cjb4 ,air as a ,non-,conductor4--,heat is rapidly diffused in air by convection2 but it is only when the air is free that this can be done4 ,when the air is confined in spaces or pores1 or among fibres1 heat makes its way through it very slowly1 for it can be diffused through it then only by conduction4 ,the variety of ways in which air is of service to us as a non-conductor is almost endless4 ,i will notice some of them4 #cjc4 ,double ,windows4--,the efficacy of double windows depends upon the confined air between them4 ,in the case of the single window a great deal of the heat inside is lost in this way3 ,the warm air of the room which comes in contact with the window imparts to it some of its heat1 and1 being thus cooled and therefore condensed1 passes downward4 ,as this process goes on continually this downward current by the window is constant4 ,the current outside is in the opposite direction4 ,the heat imparted to the window is taken up by the cold air1 and as it thus becomes warmer it passes upward4 ,and this upward current outside is as constant as the downward current inside4 ,now nearly all this is prevented by the non-conducting quality of confined air in the case of double windows4 ,if a pane were taken out from the upper part of the inner window1 and another from its lower part1 the inner window would be of little use1 for then the heat of the air in the room would be continually diminished by convection1 as when the window is single4 ,the warm air would pass in at the upper opening1 and1 being cooled1 pass down through the lower one4#e #cjd4 ,air as a ,non-,conductor in the ,walls of ,buildings4--,the spaces included between the outer wall of a building and the plastering inside being filled with confined air1 prevent the heat of the air in the apartments from passing off readily through the wall4 ,a house built of brick or stone1 with the plastering placed directly upon the inside of the wall1 would be kept warm with difficulty in winter1 because the solid wall would so readily conduct off the heat to the external air4 ,so1 also1 such a house would be very warm in summer1 because the heat of the sun and of the external air would be so rapidly communicated to the air of the house4 ,in this connection ,i will mention a contrivance to prevent the spreading of fires in blocks of buildings1 which1 though very effectual1 is seldom made use of1 partly because it occasions some trouble and expense1 and partly because it takes up a little room4 ,it is this3 ,a small space is left in the division wall between each two houses from top to bottom1 containing1 of course1 a body of confined air1 that is1 if the space be entirely shut in1 which is as essential here as in the case of the double windows4 ,with such an arrangement the interior of one house may be entirely consumed without communicating sufficient heat through the confined air to set on fire the other4 #cje4 ,fur1 ,hair1 and ,feathers4--,animals that live in cold climates are provided with suitable coverings for their protection4 ,quadrupeds1 for example1 are covered with fur1 and birds have an abundance of downy feathers4 ,these coverings have no warmth in themselves1 though in common language we speak of them as being warm4 ,they are simply non-conductors1 and so prevent the heat which is made in the body of the animal from escaping as fast as it otherwise would4 ,but why are they non-conductors8 ,it is not because the substance of which they are made is a non-conductor1 but because among their numberless fibres is partially confined that great non-conductor1 air4 ,let the fur or down be condensed into a thin hard plate upon the animal1 and it would prove of little service as a protection against cold4 ,down is much more abundant on the birds of cold climates than on those in warmer regions1 because more air can be confined among the fibres of down than among those of common feathers4 ,quadrupeds that are natives of warm climates generally have hair instead of fur4 ,when therefore the horse is taken to a cold climate he requires in winter the defense of a blanket2 and the ox needs under the same circumstances to be better housed than he ordinarily is4 ,as the elephant is a native of a climate positively hot1 his hairs are scanty and coarse4 ,formerly there were elephants in the cold regions of ,siberia1 as has been ascertained by remains found there4 ,but the elephant of ,siberia had under its hair1 close to the skin1 a fine wool to protect it against the cold4 ,animals that live in cold climates have their coverings become finer in fibre in the cold season of the year1 to give them the additional protection which they then need4 ,and animals with a furry covering1 if they are carried into a warm climate1 have their fur become coarse1 and approximate the condition of hair4 #cjf4 ,clothing4--,man has no covering to guard him against cold1 because he is capable of contriving clothing suitable to the various degrees of temperature to which he may be exposed4 ,the object of clothing is not to make the body warm1 but to keep it so4 ,the heat of the body is generated continually within itself1 and under all circumstances this heat is maintained quite uniformly at #ih4 ,this1 you see1 is a much higher degree than the atmosphere ordinarily has4 ,we are all the time1 then1 giving off heat to the air around us1 except when the air gets up to #ih4 ,we are comfortable only when we are giving off heat to a considerable amount1 for the point of temperature which is most agreeable to us when we are at rest is #gj or a little less1 that is nearly thirty degrees below the temperature of our bodies4 ,when the temperature is below this we need extra clothing4 ,in making choice of clothing for various degrees of temperature we practically apply the principles which ,i have developed4 ,those articles of clothing which can confine or entangle1 as we may say1 the largest quantity of air among their fibres are the best non-conductors1 or1 in common language1 are the warmest4 ,so1 too1 loose clothing is warmer than tight1 on account of the amount of air between the clothing and the body4 ,thus a loose glove is much warmer than a tight one4 ,the same general fact is exemplified in the coatings of straw which we put upon tender trees and shrubs in winter4 ,it is the air that is confined in the tubes of the straw which makes these coverings so effective a defense4 ,it is probably the air in the pores of the brick which makes it a poorer conductor than stone1 as illustrated by the fact stated in #cjj4 #cjg4 ,cocoons4--,many insects pass through their pupa or transition state in cocoons4 ,when this is done during warm weather1 as in the case of the silk-worm1 the cocoon is simple4 ,but when the pupa state lasts through the winter special provisions are made in the arrangements of the cocoon to guard the insect against the cold4 ,i will cite as an example the cocoon of one of our largest moths1 the ,cecropia4 ,this cocoon1 fastened to some shrub1 keeps its inmate secure from the rigors of the winter by a very beautiful arrangement4 ,the real cocoon is similar to that of the silk-worm2 but it has a very dense air-tight outer covering1 and the space between these two coverings of the pupa is filled with a loose substance1 which has air1 of course1 mingled with its fibres1 and acts1 therefore1 the part of a blanket for the insect4 #cjh4 ,buds of ,plants in ,winter4--,in the latter part of summer buds are formed on trees and shrubs1 and these contain the germs of the branches1 leaves1 and flowers which are to come out the next year4 ,these of course must be guarded against the cold of winter1 and it is done very much as the pupa is guarded in the cocoon4 ,each bud you can see has a covering of scales which is air-tight1 and inside of this there is a soft downy substance1 the blanketing of the bud4 ,in these coverings1 which have been called by some one the winter-cradles of the buds1 the infant vegetation of another year rocks back and forth in the wintry winds secure from the cold1 till the warm sun of spring wakes its hidden life into activity4 #cji4 ,snow a ,protection to ,plants4--,snow is a good blanket to the earth1 keeping its warmth from escaping into the cold air4 ,this is because it contains mingled with its feathery crystals such a quantity of air4 ,if snow come early1 before the ground and the plants in it have become frozen1 it will keep them from freezing through the winter1 if it remain during all that time4 ,it is curious to observe the peculiar arrangement of the snow in the arctic regions for the preservation of vegetation4 ,first in the autumn come soft light snows covering up the grasses1 and heaths1 and willows4 ,then as winter advances there are laid on top of these the denser snows1 making a compact1 stout roof over the lighter snows in which the scanty but precious vegetation of those regions is imbedded4 ,on top of this roof are deposited the snows of spring4 ,as these melt the water runs off from the icy roof down the slopes1 leaving untouched the plants underneath1 which lie there alike secure from the rush of waters and from the nightly frosts until the season is sufficiently advanced to bring them out with safety from their concealment4 ,then the icy roof melts1 and with it the light snows that have so long encircled the plants1 and the sun wakes them from their long sleep to a new life4 #caj4 ,influence of the ,conduction of ,heat on ,sensation4--,if you place your hand upon fur hanging at the door of a fur-store it does not feel as cold as the wood from which it hangs1 and the wood does not feel so cold as the iron bar of the shutter close by4 ,why is this1 when these substances are exposed to the same atmosphere1 and really have the same temperature8 ,it is because the iron conducts the heat from your hand more readily than the wood1 and the wood more readily than the fur4 ,so the iron handle of a wooden pump feels colder than the pump1 and the pump colder than the snow around it4 ,for the same reason1 in a cold room the rug or the carpet will not feel as cold as the poker and the hearth4 ,if water has stood long enough in a room to be of the same temperature with the air of the room your hand will feel colder in the water than in the air1 because the water is the better conductor4 ,so much for the sensation of cold4 ,on the other hand1 when substances are so heated as to give us the sensation of heat1 the conductors do this more than the non-conductors4 ,as they receive heat readily they also readily impart it4 ,for this reason1 with a brisk fire the hearth-stone feels very hot1 while the rug before the fire does not4 #caa4 ,radiation of ,heat4--,every substance sends heat into space constantly in straight lines in every direction4 ,these lines are radii1 and hence the term radiation is applied to heat diffused in this way4 ,it is very obvious in regard to the sun that it radiates heat in all directions4 ,the same can be seen in the case of a heated iron ball4 ,in whatever direction you hold your hand1 above1 below1 or laterally1 you feel the heat4 ,and it makes no difference whether the ball be red-hot or not4 ,that is1 heat is radiated either with or without light4 ,when a room is warmed by a furnace it is warmed altogether by convection2 but when it is warmed by a fire1 either in a fire-place or a stove1 we have both convection and radiation4 ,the heat which we receive from the sun comes altogether by radiation4 #cab4 ,connection between ,heat and ,light4--,the heat and light of the sun pass together through transparent substances1 as air1 glass1 water1 etc41 without heating them to any extent4 ,thus1 when the heat is transmitted through a lens1 #bgb1 the lens is little heated1 that is1 it lets almost all the heat pass through it4 ,the air is heated by the sun1 but not directly to any amount4 ,it is heated indirectly in this way3 the rays of the sun passing through the air heat the earth1 and then the air receives a part of this heat from the earth1 which is diffused through it by convection4 ,it is otherwise with heat that comes from a common fire4 ,it does not seem to be so thoroughly united with the light1 and therefore readily parts company with it1 as we may say4 ,while the heat and light of the sun go together through all transparent bodies the heat of a fire will not go with its light through all of them4 ,so while the heat of the sun does not warm the glass through which it passes the heat of a fire will warm it1 and therefore glass is an effectual screen against it4 ,in some operations in the arts a mask of glass is sometimes worn to ward off the heat4 ,the connection of light and heat will be farther noticed when ,i come to treat of light4 #cac4 ,relation between ,radiation and ,absorption4--,all surfaces that radiate will absorb also equally well the heat that is radiated upon them4 ,all rough and dark surfaces both absorb and radiate freely2 but all light-colored and polished surfaces do both slowly4 ,for this reason the black1 rough tea-kettle is well fitted to heat water in2 but it is not fitted to retain the heat in the water4 ,on the other hand1 the bright1 polished tea-pot absorbs heat poorly1 but retains it well4 ,illustration3 ,fig4 #bag4 #cad4 ,reflection of ,heat4--,radiated heat is reflected2 and here1 as in the case of motion1 #bjf1 and of sound1 #bfj1 the angles of incidence and reflection are equal4 ,some interesting experiments in relation to the reflection of heat can be tried with concave metallic mirrors4 ,thus1 if we take two such mirrors1 as represented in ,fig4 #bag1 and place in the focus of one a thermometer1 and in the focus of the other a small flask of hot water1 or a heated iron ball1 the mercury in the thermometer will rise1 although the mirrors may be many feet apart4 ,observe how the effect is produced4 ,rays of heat go from the flask directly toward the thermometer1 as represented by the lines in the figure2 but that the effect does not come from these can be proved by removing the mirrors1 leaving the flask and thermometer just as they are4 ,when the experiment is tried in this way no effect is produced on the thermometer1 because it is too far from the source of heat1 the flask1 to receive any perceptible influence in this way4 ,the effect comes from the rays of heat which go to the mirror near the flask1 and are reflected to the other mirror1 and then are reflected upon the thermometer1 all of which is represented by the dotted lines4 ,there is another way1 besides that already mentioned1 of showing that it is not the direct rays that produce the effect4 ,after arranging the apparatus put a screen between the thermometer and the mirror near it1 and the effect will be prevented because the reflection is cut off4 ,if a piece of ice be substituted for the flask of hot water the thermometer will fall--an effect opposite to that produced in the previous experiment4 ,this would seem to show that cold is radiated1 but as there is no such thing really as cold1 #bgj1 the effect must be attributed to the radiation of heat from the thermometer to the ice4 ,if a hot ball be placed in the focus of one mirror and a piece of phosphorus in that of the other1 the phosphorus will be set on fire1 though the mirrors may be twenty or more feet apart4 ,illustration3 ,fig4 #bah4 ,the reflection of heat may be exhibited very prettily with the experiment represented in ,fig4 #bah4 ,a sheet of bright gilt paper is rolled up in the shape of a funnel1 with the metallic side inward4 ,holding the larger end toward a fire1 the rays of heat coming from the fire into the funnel are reflected toward a central line1 and so pass out of the smaller end of the funnel concentrated4 ,if1 now1 a bit of phosphorus or a lucifer match be held a little distance from this end of the funnel it will be set on fire4 #cae4 ,formation of ,dew4--,it is by the radiation of heat that dew is formed4 ,the earth is constantly radiating heat into space as well as the sun4 ,in the daytime it receives a great deal more than it radiates4 ,but at night this is reversed1 and the earth is cooled4 ,the cooled earth condenses the moisture in the air which is in contact with it1 and so the moisture is deposited4 ,if the weather be very cold this is frozen1 and then we have frost instead of dew4 ,you observe that the dew does not fall1 though this is the ordinary expression4 ,its formation is analogous to the deposit of moisture which we so often witness in a hot day in summer on the outside of a tumbler containing cold water4 ,as the cold tumbler condenses the moisture in the air1 so does the earth at night1 it being cooled by radiation1 condense the moisture which has accumulated in the air by evaporation during the heat of the day4 ,there are some circumstances which have an influence upon the deposition of dew and frost4 ,less is deposited under a tree than outside of it1 because all the heat which radiates vertically upward from under the tree is radiated back again by the tree4 ,hence the efficacy of a covering over plants as a defense against frost4 ,clouds operate in the same way1 and for this reason no dew or frost is deposited in a cloudy night4 ,neither is any deposited in a very windy night1 because the moving air promotes evaporation1 and thus prevents the accumulation of moisture4 ,dew is deposited in different amounts on different substances4 ,this is owing to a difference in radiation4 ,grass and leaves radiate heat better than earth1 and earth better than stone2 and therefore while stones and gravel-walks may be dry or nearly so1 the loose earth may be moist and the grass and leaves thoroughly wet4 ,so you see that not even the dew1 plentiful as it is1 is wasted by the ,creator1 but is deposited just where it is wanted to refresh the parched earth and its vegetation4 #caf4 ,gideon's ,fleece4--,if you lay a fleece of wool upon the ground1 it is so poor a radiator of heat that no dew will be deposited upon it1 although there may be an abundance of it on the grass and leaves in its neighborhood4 ,but this was reversed in the case of ,gideon's fleece4 ,the laws of nature were set aside1 and the fleece was wet with dew while all around was dry4 #cag4 ,dew-,point4--,what is called the dew-point of the air is that degree of temperature to which any substance must be brought down in order to have dew deposited upon it4 ,this depends upon the amount of water there is in the atmosphere4 ,the more there is the higher is the dew-point4 ,when water condenses on a cold tumbler in a hot day there is much more water in the air1 and the dew-point is higher1 than when no moisture is condensed upon the tumbler4 ,so after a very hot clear day the earth needs not to be much cooled to produce a deposit of dew1 because the air has become so highly charged with moisture through the evaporation of the earth under the hot sun4 ,we can very readily at any time ascertain the dew-point4 ,take a glass of water1 and1 having a thermometer in it1 drop into it some pieces of ice1 and watch the outside of the glass4 ,as soon as it begins to be dimmed with moisture look at the thermometer1 and you have the dew-point4 #cah4 ,freezing ,mercury4--,mercury can be frozen by radiation when the cold is excessively severe1 although the thermometer may indicate a temperature considerably above -#ci1 the degree at which mercury freezes4 ,suppose that in a clear1 still night the temperature of the air is at -#bj4 ,in order to freeze the mercury it must be cooled down #ai degrees below this4 ,now this can be done by surrounding the mercury with some good non-conductor1 as charcoal4 ,this cuts off the supply of heat to the mercury1 while it is all the while giving off heat into space by radiation4 ,in like manner can ice be formed in an atmosphere that is above the freezing point1 and this is often done in warm climates4 #cai4 ,latent ,heat4--,you have seen1 #bgj1 that our sensations do not inform us accurately of the amount of heat in any substance4 ,the same is also true of the thermometer4 ,this only indicates the sensible or free heat4 ,there may be a great deal of heat locked up1 as we may say1 in the substance1 which can be brought out or made free by some change in the substance4 ,this heat thus locked up is called latent heat4 ,illustration3 ,fig4 #bai4 #cbj4 ,capacity for ,heat4--,the more heat a substance can take in and render latent the greater is its capacity for heat1 as it is expressed4 ,thus water has a much greater capacity for heat than mercury4 ,this can be proved by various experiments4 ,thus1 if we take two vessels just alike1 and having1 the one a certain quantity of water in it1 and the other the same quantity of mercury1 and expose them to the same degree of heat1 it will take much longer to raise the water to any specified temperature than the mercury4 ,why is this1 when they are both receiving the same amount of heat8 ,it is because the water renders a much larger portion of the heat latent than the mercury does4 ,we can reverse this experiment4 ,take these same vessels with their contents raised to the same temperature1 as indicated by the thermometer1 and allow them to cool in the air side by side4 ,the mercury will cool much faster than the water1 because it has much less of latent heat to part with4 ,the difference in capacity for heat between water1 oil1 and mercury may be shown by the experiment represented in ,fig4 #bai4 ,a pound of water is put into one ,florence flask1 a pound of oil into another1 and a pound of mercury into a third4 ,they are all heated to #bab1 and are then placed in funnels filled with pounded ice1 the funnels resting in glass jars of the same size4 ,now in cooling these fluids down to a certain point1 say #cb1 different amounts of the ice will be melted1 in the proportions of #ajj and #ej and #c4 ,this shows the proportions of latent heat in them which become sensible or free as their temperatures are lowered4 ,illustration3 ,fig4 #bbj4 #cba4 ,relation of ,latent ,heat to ,density4--,the more dense a substance becomes the less is its capacity for heat4 ,the heat produced by hammering iron is the latent heat rendered free by condensation1 this lessening the capacity of the iron for heat4 ,the same thing can be better illustrated in the condensation of a very compressible substance1 as air4 ,in ,fig4 #bbj you have represented a glass syringe with a closed end4 ,if there be placed in this end a little bit of cotton wool moistened with ether1 and the piston be forced downward very quickly1 the ether will be set on fire4 ,this is because the compression of the air lessens so much its capacity for heat that a great deal of its latent heat is made sensible or free4 ,the heat which is concealed in it in its ordinary state is1 as we may say1 fairly squeezed out1 as you would squeeze out the water that is concealed in the interstices of a sponge4 #cbb4 ,coldness of ,air at ,great ,heights4--,you learned in #aeb that the atmosphere is thinner the farther you go from the earth4 ,it is very thin1 therefore1 on the summits of high mountains4 ,this is the chief reason why it is so cold there1 for the rarer the air is the greater is its capacity for heat1 and the more of sensible or free heat therefore can it render latent4 #cbc4 ,relation of ,latent ,heat to the ,forms of ,substances4--,whether a substance shall be in the form of a solid1 liquid1 or gas depends upon the amount of heat which is latent in it4 ,if you take a piece of ice and melt it in a vessel1 the ice and the water in the vessel that comes from the melting of the ice are both at #cb until the ice is all melted4 ,but all this time heat is being communicated to the ice and water4 ,what becomes of it8 ,it is all taken up by the ice as it changes from its solid to its fluid state1 and becomes latent in it4 ,in fact every particle of ice must have just so much of latent heat in order to become fluid4 ,so1 also1 if water be heated to the boiling point1 #bab1 and be kept boiling1 the water will remain at that point till it is all vaporized4 ,all this time the water is receiving heat1 which1 instead of raising its temperature1 is becoming latent in the particles as they change from their liquid to their vaporous state4 ,as ,i said of the change from the solid to the liquid state1 so here1 every particle of the liquid must have just so much of latent heat in order to become aeriform4 ,whenever therefore any solid substance becomes liquid1 or liquid becomes aeriform1 heat is absorbed and becomes latent4 ,so1 on the other hand1 whenever any aeriform substance becomes liquid1 or liquid becomes solid1 latent heat is given out1 and becomes free and sensible4 ,the freezing of water1 then1 is a source of warmth to the air in its neighborhood--a fact which is practically made use of when tubs or pails of water are placed in conservatories to keep plants from freezing2 and the thawing of snow and ice is a source of cold1 as is exemplified by the chilliness of the air occasioned by this process4 #cbd4 ,clouds and ,latent ,heat4--,the water of which clouds are composed is heavier than air4 ,why1 then1 does it remain suspended8 ,why is it necessary that it should be collected into drops in order to have it fall8 ,this question can be answered by looking at the manner in which clouds are formed4 ,a cloud1 ,i have stated in #bhh1 is made up of minute vesicles or bubbles containing air4 ,now the air in these bubbles is lighter than the air that is around the cloud1 because it is warmer4 ,but how does it get its heat8 ,in order to understand this observe what the bubble is made from4 ,it is made from the water which was in the air in a state of vapor1 or in its aeriform state1 for this is the state of water that is evaporated and dissolved in the atmosphere4 ,but when it forms the vesicle it goes out of this state and becomes a liquid1 for the wall of the vesicle is a liquid wall1 just as the wall of a soap-bubble is4 ,now in passing out of the aeriform into the liquid state some latent heat must be made sensible4 ,what becomes of this sensible heat8 ,it just heats the air in the vesicle1 and so makes it like a heated air-balloon4 ,so all clouds are collections of innumerable heated air-balloons1 and the reason that some clouds are higher up than others perhaps is that their balloons have warmer and therefore lighter air in them4 #cbe4 ,freezing ,mixtures4--,the intense cold produced by these mixtures is the result of the change of free or sensible heat into latent4 ,for example1 when salt and snow are mingled together a melting of the two is quickly produced4 ,in this sudden change of a solid into a fluid a great quantity of heat must be rendered latent1 and therefore there will be a great loss of sensible heat by whatever the freezing mixture comes in contact with4 ,the process here1 you see1 is the opposite of solidification in relation to latent heat4 ,a portion of the snow1 after melting with the salt1 becomes solid ice4 ,why is this8 ,it is because it gives up its sensible or free heat to portions of the snow that are in the process of melting and are therefore making heat latent4 #cbf4 ,cold from ,evaporation4--,if you pour a little ether into the palm of your hand it will rapidly disappear in vapor1 producing a very cold sensation4 ,this sensation occurs because1 in the change of the liquid into the vaporous or aeriform state1 some of the sensible heat of your hand is abstracted to become latent in the vapor4 ,the evaporation of water also produces cold1 though not as decidedly as ether1 because its change into vapor is not so rapid at ordinary temperatures4 ,we make a practical use of the evaporation of water in many different ways4 ,thus we sprinkle water in a hot day upon the floors of piazzas1 steps1 etc41 that the evaporation may make much of the sensible heat about our houses latent4 ,for the same purpose1 in hot climates1 apartments are often separated from each other by mere curtains1 which are occasionally sprinkled with water4 ,so the inhabitants of such climates often cool their beverages by keeping a wet cloth for some time wrapped around the vessels that contain them4 ,evaporation is an important remedy for many cases of disease4 ,for example1 if the head be hot1 a steady application of a wet cloth to the forehead1 though a simple remedy1 is generally effectual1 and sometimes is very important4 ,most people make the application in a wrong manner4 ,they put on several thicknesses of cloth1 when a single thickness is the best1 because it will best secure the evaporation1 which is the cause of the relief afforded4 ,illustration3 ,fig4 #bba4 #cbg4 ,freezing in the ,midst of ,boiling4--,it is from the quantity of heat rendered latent by evaporation that water can be frozen in the midst of boiling ether2 and1 paradoxical as it may seem1 the boiling of the ether is the cause of the freezing4 ,the experiment is performed in this way3 ,place a test-tube or a little thin vial with water in it in the midst of some ether in a shallow vessel under the receiver of an air-pump4 ,on exhausting the air the ether will boil1 evaporation taking place rapidly because the pressure of the air is taken off from the ether4 ,now as the ether passes into vapor it extracts so much free heat from the vial of water that the water is cooled down to the freezing point1 and so becomes solid4 ,water can be frozen even by its own evaporation4 ,it is done in this way3 ,let a shallow vessel1 b1 ,fig4 #bba1 contain a little water1 and the vessel c oil of vitriol or sulphuric acid4 ,when the air is exhausted1 the pressure of air being taken off from the water1 vapor rises from it freely4 ,as the sulphuric acid has a great attraction for water it absorbs this vapor1 and so vapor continually rises from the water the more rapidly because what is formed is absorbed1 instead of remaining to make pressure on the water4 ,the result is that this rapid formation of vapor1 requiring that a great quantity of heat should be made latent1 at length abstracts so much heat from the water that remains that this becomes solid4 #cbh4 ,degree of ,heat ,endurable by ,man4--,it was formerly believed that the human body could not endure with impunity1 even for a short time1 a much higher degree of temperature than that which is met with in hot climates4 ,but in the year #agfj it was accidentally discovered that a much higher temperature than this could be endured4 ,an insect was destroying at that time the grain gathered in some parts of ,france1 and it was found that if the grain was subjected to a certain high degree of temperature the insect was killed1 and yet the grain was not injured4 ,in trying some experiments in regard to this matter the experimenters wished to know the point at which the thermometer stood in a large oven4 ,a girl attending on the oven offered to go in and mark the thermometer4 ,she did so1 remaining two or three minutes1 and the thermometer was at #bfj1 that is1 #dh above the boiling point of water4 ,as she experienced no great inconvenience from the heat she remained ten minutes longer1 when the thermometer rose to #gf above that point4 ,these facts were published1 and prompted scientific men to try other experiments4 ,in ,england1 ,dr4 ,fordyce1 ,sir ,charles ,blagden1 and others1 went into rooms heated even to #bdj and #bfj1 and remained long enough to cook eggs and steaks1 and yet themselves suffered little inconvenience4 ,the pulse was quickened1 the perspiration was very profuse1 but the heat of the body1 as ascertained by putting the thermometer under the tongue the moment they came out1 was scarcely raised at all4 ,the air in which they were roasted eggs quite hard in twenty minutes1 and when it was applied by a pair of bellows to a steak it cooked it in thirteen minutes4 ,the question arises1 how is it that this high degree of heat did not produce more effect upon the body8 ,one reason is that the heat of the air in the immediate neighborhood of the body was continually reduced by the evaporation of the free perspiration1 sensible heat being thus converted into latent4 ,another reason is that the air is not a good conductor1 and therefore did not communicate its heat readily to the body4 ,dr4 ,fordyce and his friends found that they could not touch with impunity any good conductor1 as the metals1 and they were obliged to wear upon their feet some non-conducting substance4 #cbi4 ,formation of ,ice4--,before dismissing the subject of heat ,i must notice the grand exception which we have to some of the operations of heat in the formation of ice4 ,heat generally produces expansion4 ,but in the case of water this law of expansion is set aside1 and the reverse is established4 ,this is done1 however1 only within a small range of temperature1 viz41 from the freezing point up the scale about seven degrees4 ,in all degrees above that the usual expansion by heat takes place4 ,the exception occurs at this part of the scale for a special purpose1 viz41 that water1 in distinction from other substances1 shall become more bulky1 and therefore lighter1 as it takes the solid form4 ,illustration3 ,fig4 #bbb4 #ccj4 ,description of the ,process of ,freezing4--,in order to make the process of freezing clear to you ,i will describe it as it ordinarily occurs1 that is1 from the action of cold air upon the surface of water4 ,the uppermost layer of the water imparts some of its heat to the air in contact with it4 ,this air rises and colder air takes its place1 which being warmed in its turn rises to make way for more of the cold air4 ,you have therefore a constant current of warmed air upward from the water4 ,in the mean time there is a current of a different character in the water--a downward one4 ,as fast as the water at the surface parts with heat to the air it falls1 other warm water taking its place1 to cool in its turn and go down4 ,this falling of the cooled water goes on regularly until a portion of water becomes cooled down to #ci1 that is1 #g above the freezing point4 ,this layer does not sink1 but remains at the surface1 for it is lighter than the warmer water below4 ,this is because the law that heat expands matter is now reversed4 ,beyond this point of the thermometer the colder the water is the lighter it is4 ,as the cooling now goes on from the air coming1 as before1 in successive layers to the water1 the cooled water at the surface continually increases4 ,at first it is a mere single layer of particles1 but after a while it is quite a body of cold water lying on the warmer water below4 ,at length some of it is cooled down to #cb1 the freezing point1 and a thin film of ice now forms4 ,the state of things just at this stage of the process may be represented by a simple diagram1 ,fig4 #bbb4 ,let the line a represent the film of ice4 ,the space between a and b is the portion of water cooled down below #ci4 ,the space below b is occupied by the water which is above this temperature4 ,in the space between a and b the cooler the water is the nearer it is to the surface4 ,that is1 from the line b1 where the water is exactly at #ci1 as you go upward1 the water lessens in temperature1 it being successively #ch1 #cg1 #cf1 etc41 till1 just in contact with the film of ice1 a1 it is at #cb4 ,the ice goes on to thicken gradually by additions below4 ,but it is to be remembered that ice is a good non-conductor1 so that the very first layer of ice makes the cooling of the water proceed more slowly than before4 ,and the thicker the ice becomes the slower is the cooling4 ,this secures against too great a formation of ice4 #cca4 ,why the ,above ,exception to ,expansion by ,heat ,exists4--,that we may see the reasons in part for the grand exception to the general law of expansion by heat which ,i have illustrated1 let us see what would be some of the results if the exception did not exist4 ,in that case the process of freezing would be as follows3 ,the water would communicate its heat from the surface to the air1 as before described1 and there would be a constant downward current of the cooled water4 ,when any portion of the water became cooled by the air down to #cb1 it would become ice1 and would sink to the bottom4 ,and after the process of freezing had once begun1 there would be a continual accumulation of ice at the bottom so long as the air remained cold enough to cool the water with which it comes in contact down to #cb4 ,the result may be stated in the general thus3 ,freezing would not begin so quickly as it now does2 but when once begun it would prove very destructive4 ,it would not begin as soon1 because the whole of any body of water must be cooled down to just this side of #cb before it could begin4 ,this would not take long where the water is shallow1 but it would where it is deep4 ,all shallow bodies of water1 then1 would be frozen up quite early in the winter2 and as water is a poor conductor1 and thawing must go from above downward1 some of them would not be thawed out again fully till quite into the next summer1 if even then4 ,and where the water is quite deep ice would at length begin to form1 and when formed it would be exceedingly slow in thawing4 ,in some cases it would never be thawed with such a body of non-conducting water to guard it against the warmth above4 ,it is easy to see that the heat of spring and summer would not thaw out any thing like the quantity of ice that it now does4 ,the reign of ice and snow on our earth would therefore be vastly more extensive than now1 and what is worse1 it would be extended more and more every year4 ,under such circumstances there would be great destruction of both animal and vegetable life4 ,i will mention1 however1 but a single item1 as it would occupy too much space to go into this subject with any fullness4 ,in the water under the ice1 which is always above #ci1 except that which is close to the ice when freezing is going on1 there is a vast amount of busy life which would be destroyed if ice were formed at the bottom1 chilling all the water above4 #ccb4 ,why the ,freezing ,point is at #cb4--;,if the freezing point of water were higher than #cb1 freezing would occur so early in the autumn1 and the ice and snow would last so late in the spring1 that the season would be too short to raise our supplies of fruits and grains4 ,if1 on the other hand1 it were at a lower point1 the earth would not have the protection of its light coat of snow1 but instead would be chilled by rains so cold that barrenness would be the result4 ,the multitudes of animals1 too1 that now live so securely in the water1 some of them even with the ice above them1 would all perish with the cold4 #ccc4 ,force of ,expansion in ,ice4--,as ice occupies one seventh more room than the water from which it is formed1 it exerts in its formation an expansive force which under various circumstances produces varied and often remarkable results4 ,of the many experiments which have been tried to show the force of this expansion ,i will mention but one4 ,a bomb-shell was filled with water1 at ,montreal1 and closed with an iron plug which was driven in with great force4 ,the plug was thrown a distance of #djj feet by the expansion when the water froze4 ,this expansion is sometimes an inconvenience to us1 as in bursting water-pipes2 but besides the great service which it does in the earth1 already noticed1 it is of service also in loosening the soil1 and in supplying it with requisite ingredients from the rocks by breaking them up and pulverizing them in small quantities from year to year4 ,,chapter ,,xiv4 ,,light4 #ccd4 ,nature of ,light4--,we do not know what light is4 ,there are two suppositions in regard to it4 ,one is that of ,sir ,isaac ,newton1 called the theory of emission4 ,according to this light is a substance1 but so ethereal that it has no weight1 and is capable of passing through various substances of even great density4 ,the other supposition is what is called the undulatory theory4 ,the advocates of this1 which is now quite generally received1 believe light to consist of undulations1 waves1 or vibrations in an ether which is supposed to exist every where1 pervading all space and every substance4 ,you perceive an analogy here to sound1 the vibrating medium in the case of sound1 however1 being always some palpable substance--solid1 fluid1 or aeriform4 ,heat is supposed1 as stated in #bga1 to be a vibration of the ethereal substance1 as light is1 though the two vibrations must of course be somewhat different in character4 ,any body that is capable of communicating the light-vibration to this ether is said to be luminous4 #cce4 ,sources of ,light4--,the chief source of light to our earth is the sun1 which is a permanently luminous body4 ,then we have the light of combustion in its various forms4 ,electricity is another source of light4 ,light is sometimes emitted during decay or putrefaction of some substances4 ,some animals--as fire-flies1 glow-worms1 and phosphorescent animals in the sea--have the power of emitting light4 #ccf4 ,light ,moves in ,straight ,lines4--,light1 like heat and sound1 radiates in straight lines in all directions from its source4 ,we can see this to be true by admitting rays of light into a darkened room through small openings in the shutters1 the rays making straight lines across the darkness1 as may be seen by the motes which are flying in the air4 ,the fact is recognized by the marksman in taking aim1 and by the engineer in making his levels4 ,the carpenter acts upon it when he tests the smoothness of any surface by letting the light pass along over it to his eye4 ,illustration3 ,fig4 #bbc4 ,illustration3 ,fig4 #bbd4 #ccg4 ,diffusion of ,light4--,as light passes in all directions from any body or point1 the farther we go from its source the less will the light be4 ,if we take any two rays of light1 the farther we trace them from their source the farther are they separated from each other1 and what is true of any two rays is true of all the rays4 ,it follows that the farther removed any surface is from a source of light the less light will there be upon it4 ,this decrease of light in proportion to distance is a perfectly regular decrease1 and it is as the square of the distance2 or1 in other words1 the intensity of light is inversely as the square of the distance4 ,take a screen1 ,fig4 #bbc1 and a candle1 placing a square piece of pasteboard between them at one foot from each4 ,the shadow on the screen1 you see1 covers a space four times as large as the pasteboard4 ,that is1 the light that shines on the pasteboard1 if allowed to pass on to the screen1 would be diffused over four times the space1 and therefore would have only one-quarter of the intensity4 ,so if as shown in ,fig4 #bbd1 the screen be placed at twice the distance from the pasteboard that the light is1 the shadow will cover a space nine times as large as the pasteboard1 and therefore the light there would have one-ninth of the intensity which it has where the pasteboard is4 ,again1 it is seen by ,fig4 #bbe that if the screen be placed at the distance of three feet the intensity of the light is one-sixteenth of that which it is at the pasteboard4 ,while the distances1 therefore1 are as #a1 #b1 #c1 #d1 etc41 the intensity of the light is inversely as the numbers #a1 #d1 #i1 #af1 etc41 that is inversely as the squares of the distance4 ,illustration3 ,fig4 #bbe4 #cch4 ,velocity of ,light4--,the velocity of light is so great that within any ordinary distances it may be considered as instantaneous4 ,thus when we measure the distance of a cannon by the difference between the time of its flash and the report1 we do not reckon the light to consume any time in its passage to the eye4 ,but when we come to look at objects as distant as the sun and other heavenly bodies1 we reckon in our calculations the time of the passage of light4 ,it takes light eight minutes to travel from the sun to us1 a distance of ninety-five millions of miles4 ,with the telescope stars have been seen which have been ascertained to be at such a distance that it requires over ten years for their light to come to the earth4 ,others have been seen which are much farther off1 but their distances have not been absolutely ascertained4 ,some have been seen supposed to be at such a distance that the light coming from them to the eye of the astronomer was a hundred thousand years in its passage4 ,illustration3 ,fig4 #bbf4 #cci4 ,roemer's ,observations4--,the velocity of light was first determined by ,roemer1 a ,danish astronomer1 in #afgf4 ,it was done in his calculations and observations of the eclipse of one of ,jupiter's moons4 ,after making the calculation of the time it would take for the satellite to pass through the shadow of the planet1 he observed its passage1 and found that it did not come out from the shadow as soon as his calculation required by fifteen seconds4 ,what was the difficulty8 ,if the earth had remained in one spot from the beginning to the end of the passage of the satellite1 the observation would have come out exactly according to the calculation4 ,but the earth had moved during this time 7about forty-two hours and a half7 the immense distance of #b1hhj1jjj miles4 ,the light of the emerging satellite therefore had to travel over this additional distance to overtake the earth1 and it took fifteen seconds to do it4 ,if we divide1 then1 this distance by #ae we get the distance which light travels in a second1 which is #aib1jjj miles4 ,all this can be made clear by the diagram1 ,fig4 #bbf4 ,let ,s be the sun1 ,j ,jupiter1 and ,c one of its moons emerging from its shadow4 ,let ,a be the earth as it is when the eclipse of ,jupiter's moon begins4 ,when it emerges the earth has passed to ,b1 and the light from the satellite has to travel as much farther to reach it now as ,b ,c is longer than ,a ,c4 ,roemer made other observations with the earth at some other parts of her orbit with the same result4 ,illustration3 ,fig4 #bbg4 #cdj4 ,reflection of ,light4--,light1 like sound and heat1 is reflected in straight lines when it strikes upon any resisting substance4 ,we can see this to be the case when it strikes upon any smooth and plane surface4 ,and it is true of light1 as it is of heat1 that the angles of incidence and reflection are equal4 ,thus if c1 ,fig4 #bbg1 be a reflecting surface1 and b c a line perpendicular to it1 then a ray of light1 d c1 will be reflected in the line c a1 and the angle of incidence1 d c b1 will be equal to the angle of reflection1 b c a4 #cda4 ,how we ,see4--,we see the various objects around us by the light which is reflected from them4 ,every point of every surface that we see reflects rays or vibrations of light to our eyes4 ,thus if we see a person there are rays of light reflected into our eyes from every part of him4 ,these rays form an image of him in the back part of each eye1 and it is by this image that we see him1 as will be explained in full in another part of this chapter4 ,reflected light is painting the images of objects in the eye every moment in great abundance and variety4 ,if a speaker have an audience of a thousand persons all looking at him1 his image is at the same time in two thousand eyes1 and in each of these two thousand images every motion and every changing expression are faithfully depicted4 ,illustration3 ,fig4 #bbh4 #cdb4 ,mirrors4--,that reflected light does thus form images of objects you see in the common mirror4 ,the image formed in it of any object comes from the light reflected from that object into the glass4 ,then in seeing the image light is reflected from it into the eye1 there to form a similar image1 though of much less size4 ,by using two or more mirrors the reflections of the image can be multiplied1 and by some arrangements of them to a very great extent4 ,that the image appears to be at the same distance beyond the surface that the object is before it1 is owing to the fact that the reflected rays come from the glass at the same angle that the incident rays strike upon it4 ,this may be shown from ,fig4 #bbh 7p4 #bfc74 ,suppose m m' is a looking-glass1 and an arrow1 ,a ,b1 is before it4 ,rays of light come from it at all points to the glass4 ,we will take only two of these rays at each end of the arrow4 ,the ray ,a g will be reflected to the eye at the same angle in the ray g o1 and the ray ,a f will be reflected in the ray f ,e4 ,and the reflected rays will have the same rate of divergence as the incident rays4 ,the same can be shown in regard to rays from ,b or any other point on the arrow4 ,now if the lines o g and ,e f be extended1 they will meet at the point a1 which is at the same distance behind the mirror as ,a is before it4 ,the same thing can be shown of the rays from ,b or any other point4 ,therefore the image of the arrow will appear to the eye to have the same relative position behind the glass that the arrow itself has before it4 ,illustration3 ,fig4 #bbi4 #cdc4 ,the ,kaleidoscope4--,i have already noticed the multiplication of the images of objects by using two or more mirrors4 ,in the kaleidoscope1 by a particular arrangement of mirrors1 the images are multiplied1 and by changes in the position of the objects the relative positions of the images are infinitely varied4 ,fig4 #bbi will serve to explain the operation of the instrument4 ,let ,a ,b and ,b ,c be two plane mirrors placed at right angles to each other1 and a an object before them4 ,let ,i be the position of the eye looking at the mirrors4 ,the rays a f and a g will be reflected to ,i as represented1 and the eye will see two images1 which appear to be at b and ,e4 ,but the ray a ,k will be reflected to c1 and then to ,i1 so that a third image will be seen at d4 ,here is but a single second reflection1 or reflection of an image2 but by placing the mirrors at an angle of #fj1 #de1 and #cj the images may be increased to six1 eight1 and ten1 having a circular arrangement4 ,in the kaleidoscope two mirrors are placed in a tube at an angle of #cj1 and variously-colored pieces of glass in the farther end of the instrument1 changing their relative position with every movement of it1 give an endless variety of images symmetrically arranged4 ,illustration3 ,fig4 #bcj4 ,illustration3 ,fig4 #bca4 #cdd4 ,curved ,mirrors4--,these may be concave or convex4 ,the action of a concave mirror upon light may be illustrated by ,fig4 #bcj4 ,if parallel rays1 as represented1 strike upon the mirror they will1 in their reflection1 be made to converge1 or come together1 at the focus1 a4 ,but suppose the light comes from this focus1 the rays of course diverging1 or going away from each other2 then the rays1 as reflected1 will be parallel4 ,if the light or object be nearer to the mirror than the focus1 and the rays of course be more diverging1 then the effect of the mirror will be to lessen the divergence when the rays are reflected4 ,you see that the tendency is to make the rays converge4 ,and hence concave reflectors are much used when it is desired to throw a great amount of light in one direction4 ,the effect of the concave mirror upon the apparent size and position of objects placed before it varies with the relation of their position to the focus4 ,the action of a convex mirror upon light is the opposite of that of the concave4 ,its tendency is to make the rays diverge4 ,thus 7,fig1 #bca71 if parallel rays strike upon a convex mirror they diverge1 as if they came from a focus behind the mirror1 as b1 as indicated by the dotted lines4 ,illustration3 ,fig4 #bcb4 ,illustration3 ,fig4 #bcc4 #cde4 ,refraction of ,light4--,when light passes from one medium into another it is bent from its course4 ,this may be illustrated by ,fig4 #bcb1 in which ,a ,b ,c ,d is a box1 into which a candle1 ,e1 is shining4 ,the candle is so placed that the shadow of the side ,a ,c falls at ,d4 ,but let the box be filled with water1 and now the shadow is removed to d1 as if the candle were at e4 ,this is because the rays of light from the candle1 in passing from the air into the water1 are bent or refracted so as to take a different direction4 ,here we have light passing from a rarer into a denser medium4 ,let us see now how it is when light passes from a denser medium into a rarer4 ,this can be illustrated on ,fig4 #bcc4 ,let the vessel1 ,a ,b ,c ,d1 be empty1 and let a coin be placed at ,o4 ,let the eye1 ,e1 be in such a position that a straight line1 ,o ,g ,e1 from the coin to the eye would strike the side of the vessel a little below the edge1 or1 in other words1 that the edge of the vessel would prevent the eye from seeing it4 ,if now1 keeping the eye in this position1 water be poured in up to a certain level1 say ,f ,g1 the coin comes into view4 ,this is because light coming from the coin to ,l is bent into another direction1 ,l ,e1 and the coin therefore appears to the eye to be at ,k4 ,in this case the refraction is from the perpendicular1 ,p ,q1 let down through the point ,l1 where the light emerges from the denser into the rarer medium4 ,but ,when light passes from a rarer into a denser medium the refraction is reversed--it is toward the perpendicular4 ,it is from this refraction of light that a stick partly immersed in water appears to the eye to be broken just at the surface of the water4 ,illustration3 ,fig4 #bcd4 #cdf4 ,dawn and ,twilight4--,the light of the sun1 in passing from space into our atmosphere1 is refracted4 ,if it were not we should have no daylight preceding the rise of the sun1 or twilight after its setting2 but light would burst upon the darkness of night at once when the sun appeared above the horizon1 and darkness would suddenly succeed to the light of day at sunset4 ,as it is1 in the morning the light bends toward us as it strikes across the atmosphere long before we see the sun1 and after the sun has disappeared from view at evening its light bends toward us in the same manner4 ,and farther1 we really see the sun in the morning before it gets above the horizon1 and in the evening after it has gone below it4 ,this may be made clear by ,fig4 #bcd4 ,let the central ball represent the earth4 ,now as the atmosphere is most dense near the earth1 and is rarer as you go outward from the earth1 it is represented in the figure as having different layers in order that the operation of the refraction may be more clear to you4 ,the outermost layer is exceedingly rare1 and each layer is more dense than the previous one as you go in toward the earth4 ,the light coming from the sun1 ,s1 below the horizon into the first layer of air1 instead of passing on straight to a1 as indicated by the dotted line1 bends toward the earth4 ,then in entering the second layer1 instead of passing on to b1 it will be bent or refracted still more1 as this layer is denser2 and so on through all the layers1 being refracted in each more than in the previous one4 ,the result is1 that as every object is seen in the direction in which the rays from it at length reach the eye1 the sun1 though really below the horizon1 appears to be above it1 as represented4 ,the path of light from the sun1 as it passes through the air1 is a curved line4 ,this is because the air1 instead of being of uniform density1 lessens in density as we go from the earth4 ,if it were of uniform density the light would be refracted in straight lines1 as in the experiments in #cde4 #cdg4 ,mirages4--,sometimes inequalities occur in the density of the lower portions of the atmosphere1 causing1 of course1 unequal refraction1 and producing some strange appearances1 termed mirages4 ,for example1 at ,ramsgate1 on the coast of ,england1 there was seen1 at one time1 as represented in ,fig4 #bce 7p4 #bfh71 a ship at such a distance that only her topsails were visible2 and above in the air there were two complete images of the ship1 the uppermost being erect and the under one inverted4 ,captain ,scoresby1 in a voyage to ,greenland1 saw an inverted image of a ship so well defined that he decided that it was the image of his father's ship1 the ,fame1 which was afterward verified4 ,the ship itself was at that time at a distance of #cj miles4 ,an incident in the early history of the author's place of residence may be cited as an example of mirage4 ,a ship left for ,england freighted with a valuable cargo1 and having on board a large number of the best citizens of the colony4 ,some time after there was immense excitement in ,new ,haven1 because the inhabitants saw1 with great distinctness1 what they supposed to be this vessel1 at only a little distance1 apparently sailing against the wind4 ,but it soon disappeared from view1 part after part1 until the whole was gone4 ,the ship itself was never heard from1 and it was supposed at the time that this appearance was a manifestation of ,providence for the purpose of informing the colonists what had become of their friends4 ,but what was seen was undoubtedly the reflected image of this or some other ship4 ,it is such appearances as these that have given rise to the stories which have been sometimes told of phantom ships4 ,mirages are very common in the extensive deserts in hot climates1 exhibiting to the eye of the traveler various deceptive appearances1 as islands1 lakes1 etc4 ,in ,bonaparte's campaign in ,egypt such an appearance caused whole battalions of thirsty soldiers to rush forward1 supposing at the moment that a plentiful supply of water was at hand4 ,illustration3 ,fig4 #bce4 ,the most astonishing instance of mirage of which ,i have ever heard is thus narrated3 ,the cliffs on the ,french coast are #ej miles distant from ,hastings1 on the coast of ,sussex1 and they are actually hidden from the eye by the convexity of the earth2 that is to say1 a straight line drawn from ,hastings to ,calais or ,boulogne would pass through the sea4 ,a year or two ago1 however1 a ,fellow of the ,royal ,society1 who was residing at ,hastings1 was surprised to see a crowd of people running to the sea-side4 ,upon inquiry as to the cause of this he was informed that the coast of ,france could be seen by the naked eye4 ,he immediately went down to the shore to witness so singular a sight1 and there discovered distinctly the ,french cliffs extending for some leagues along the horizon1 and so vividly that they appeared to be only a few miles off4 ,the sailors and fishermen1 with whom ,mr4 ,latham walked along the water's edge1 could hardly at first be persuaded of the reality of the appearance2 but as the cliffs gradually became more elevated they were so convinced that they pointed out to ,mr4 ,latham the different places they were accustomed to visit--such as the bay and the wind-mill at ,boulogne1 ,st4 ,vallery1 and other places on the coast of ,picardy1 even as far as ,dieppe1 all the ,french shores appearing to the ,english sailors as if they were sailing at a short distance from them toward the harbors4 ,with the aid of a telescope the ,french fishing-boats were plainly seen at anchor2 and the different colors of the land upon the heights1 together with the buildings1 were perfectly discernible4 ,the day when this occurred is said to have been extremely hot1 without a breath of wind stirring1 and the phenomenon continued visible in the highest splendor until past eight o'clock in the evening1 having been seen for three hours continuously4 ,illustration3 ,fig4 #bcf4 ,illustration3 ,fig4 #bcg4 #cdh4 ,visual ,angle4--,in order that you may understand the operation of lenses in relation to vision ,i must first explain to you what is meant by the visual angle4 ,in ,fig4 #bcf 7p4 #bgj7 are represented arrows of the same size at different distances from the eye4 ,from the ends of each of the arrows are drawn lines to the eye4 ,the angle which these lines make in each case as they meet at the eye is termed the visual angle4 ,now the apparent size of an object depends upon the size of this angle4 ,the degrees of the angles are marked upon the figure4 ,thus the visual angle of the nearest arrow is #abj degrees1 and that of the second is #fj1 only half as large4 ,the first arrow therefore appears twice as large as the second4 ,for the same reason it appears four times as large as the third1 eight times as large as the fourth1 and twelve times as large as the fifth4 ,the same thing is illustrated in another way in ,fig4 #bcg4 ,here the arrows e f1 g h1 and i k appear to the eye as large as ,a ,b1 because they have the same visual angle1 and for this reason make an image of the same size in the eye1 as you see is indicated in the figure4 ,it is hardly necessary to say that what is true of objects as a whole is true also of any part of them4 ,each part1 however small1 has its visual angle1 and this governs its apparent size4 ,illustration3 ,fig4 #bch4 ,illustration3 ,fig4 #bci4 #cdi4 ,lenses4--,transparent bodies having curved surfaces are called lenses4 ,there are six kinds1 represented in ,fig4 #bch4 ,the lenses in most common use are the double convex and double concave4 ,the explanation of the mode in which these act upon light will sufficiently illustrate the operation of the others4 ,they act by refraction1 the convex collecting the rays1 or bringing them nearer together1 and the concave putting them farther apart4 ,you can at once see1 then1 that a convex lens by causing the rays coming from an object to converge more1 increases the visual angle1 and therefore makes the object to appear larger than it otherwise would4 ,this effect is illustrated by ,fig4 #bci4 ,the rays of light coming from the arrow are made by the lens so to converge as to meet at a1 instead of b1 where they would meet if they did not pass through the lens4 ,that is1 by passing through the lens they have a larger visual angle1 and therefore the object is magnified4 ,the distance between1 c and d shows the size which the arrow would appear to have to the eye placed at a4 ,illustration3 ,fig4 #bdj4 #cej4 ,microscopes and ,telescopes4--,what has been said of the action of the convex lens upon the visual angle will serve to explain the operation of the microscope4 ,this instrument may be single or compound4 ,the compound microscope has more than one lens1 and is used to magnify very minute objects4 ,its operation may be seen by the diagram1 ,fig4 #bdj4 ,rays from the object1 ,e ,f1 passing through the first lens1 or object-glass1 as it is called1 form a magnified inverted image1 ,g ,h1 which is still more magnified by the eye-glass1 ,c ,d4 ,in the telescope we have also convex lenses1 but they are arranged differently from those of the microscope1 as the objects to be magnified are distant4 ,illustration3 ,fig4 #bda4 #cea4 ,magic ,lantern4--,this is an instrument by which pictures made upon slips of glass with coloring substances which allow the light to pass readily are thrown upon a screen magnified4 ,it is a metallic lantern1 ,a ,a1 ,fig4 #bda1 with a concave reflector1 p q1 and two convex lenses1 m and n4 ,at c d is a space between the lenses into which the pictures are introduced4 ,l is a strong light1 which is in the focus both of the mirror and the lens m4 ,the picture is therefore illuminated strongly by the rays reflected from the mirror and passed through the lens4 ,the lens n which is movable1 is so adjusted as to throw a highly magnified image of the picture upon the screen4 ,as the image is an inverted one the pictures must be inserted upside down1 that the images on the screen may be upright4 ,the solar microscope is1 in its essential parts1 like the magic lantern1 the sun being used as the illuminator4 ,illustration3 ,fig4 #bdb4 ,illustration3 ,fig4 #bdc4 #ceb4 ,camera ,obscura4--,this instrument differs from the magic lantern in giving us diminished images of objects4 ,an instrument of this kind can be arranged extemporaneously any where4 ,thus1 if into a darkened chamber light be admitted through a small opening1 inverted images of any objects in front of the opening will be formed upon a white screen in the opposite part of the chamber4 ,such an arrangement is represented in ,fig4 #bdb 7p4 #bgc71 ,c ,d being the chamber1 ,l the opening1 and a b the image of the object ,a ,b4 ,the images in such a case1 however1 are faint1 because the opening must necessarily be small1 and therefore but few rays1 comparatively1 come from the objects4 ,by making the opening larger1 and gathering the rays that enter it with a double convex lens1 we can have well-defined and bright images of objects4 ,though the camera obscura may have various forms1 ,i have described what is essentially the arrangement of the instrument4 ,one form of it1 for sketching either single objects or groups of them in landscapes1 is represented in ,fig4 #bdc4 ,here the rays of light coming from objects strike upon a mirror1 ,a ,b1 and are reflected through a convex lens1 ,c ,d1 upon white paper on the bottom1 ,e ,f1 of the box1 where the outlines of the images are traced by the sketcher4 ,the light can enter only at the opening above1 for on the side of the box which is open there hangs down a curtain on the back of the artist as he sketches4 ,illustration3 ,fig4 #bdd4 #cec4 ,the ,eye4--,the eye is essentially a camera obscura4 ,it is a dark chamber in which images are formed upon a screen in its back part1 and the light which comes from objects is admitted through an opening in front1 where there is a double convex lens4 ,that you may understand the manner in which the images are formed1 ,i give you1 in ,fig4 #bdd1 a map of the eye4 ,at a is the thick1 strong white coat called the sclerotic coat1 from a ,greek word meaning hard4 ,this1 which is commonly the white of the eye1 gives to the eyeball its firmness4 ,into this is fastened in front1 like a crystal in a watch-case1 e1 the cornea4 ,the sclerotic and cornea1 you see then1 make together one coat of the eye1 the outer one4 ,the cornea is the clear1 transparent window of the eye through which the light enters4 ,next to the sclerotic coat comes the choroid coat1 which is dark1 to prevent too much reflection back and forth in the eye4 ,then you have a very thin membrane1 c1 the retina1 the screen on which the images are formed4 ,this is composed chiefly of the fine fibres of the nerve of sight1 d4 ,to return to the front of the eye where the light enters--behind the cornea is the iris1 g g1 which is immersed in a watery fluid1 f1 called the aqueous humor4 ,the light passing through the cornea and the aqueous humor comes to the crystalline lens1 h1 which1 you see1 is a double convex lens4 ,passing through this and through a jelly-like substance1 called the vitreous humor1 which fills all that large space i1 it strikes upon the retina1 c1 where it forms the images of the objects from which it came4 ,you see now how the eye is like a camera obscura4 ,you have in it the dark chamber with its screen1 the opening through the iris1 the pupil1 for the admission of the light1 and just behind this opening the lens for gathering or concentrating the light before it falls upon the retina4 ,the refraction of the light is not1 however1 done wholly by this lens4 ,the projecting cornea1 with its contained aqueous humor1 refracts it considerably1 for it forms a convex lens4 ,illustration3 ,fig4 #bde4 ,illustration3 ,fig4 #bdf4 #ced4 ,distinct ,vision4--,in order that vision may be perfectly distinct1 it is necessary that the rays coming from each point of the object which is seen should1 on converging1 meet together1 or be brought to a focus on the screen of the eye1 the retina4 ,thus1 in ,fig4 #bde1 the rays which come from a1 the end of the arrow1 meet on the retina at b1 and those from c1 the other end1 are brought to a focus at d4 ,now the muscles of the eye have considerable power in adjusting the eye to objects at different distances1 so as to bring the rays in most cases together exactly at the retina4 ,they fail to do it with objects that are very near4 ,you can see that this is so if you bring any object1 as your finger1 nearer and nearer to the eye4 ,you will at length find that you can not see it distinctly4 ,the reason is1 that the rays from it diverge so much that the cornea and lens can not make them converge enough to meet at the retina4 ,this divergence of rays at different distances is illustrated in ,fig4 #bdf4 ,suppose that you are looking at some very minute object4 ,the nearer you bring it to the eye the better you can see it1 till you come to a certain point4 ,there the rays are so divergent1 as you can readily see by the figure1 that the lenses of the eye can not make them converge sufficiently for distinct vision4 ,now just here the microscope comes in to help the eye by causing these divergent rays to come nearer together before they enter the window of the eye1 the cornea4 ,illustration3 ,fig4 #bdg4 ,illustration3 ,fig4 #bdh4 #cee4 ,near-,sighted and ,far-,sighted4--,some persons have their eyes so shaped that they can not fully adjust them to objects at different distances4 ,thus the near-sighted can see with distinctness only objects that are near4 ,the reason is that the rays converge too much1 and are brought to a focus before they arrive at the retina1 as represented in ,fig4 #bdg4 ,the images therefore of distant objects are indistinct4 ,if the retina could in any way be brought forward a little the difficulty would be obviated4 ,but as this can not be done1 concave glasses are resorted to1 which counteract the effect of the too highly refractive power of the eye4 ,in the far-sighted the difficulty is of an opposite character4 ,the refractive power is so feeble that when near objects are viewed the rays are not brought to a focus soon enough1 as seen in ,fig4 #bdh4 ,convex glasses are used in this case1 making the divergent rays of near objects less divergent before they enter the cornea4 #cef4 ,images in the ,eye ,inverted4--,the images formed on the retina are inverted4 ,this can be proved by taking the eye of an ox and carefully paring off the back of it1 leaving little else than the retina itself4 ,holding now a candle before the eye1 its image may be seen inverted upon its rear part4 ,the question arises why it is that we see objects erect when their images on the retina are inverted4 ,on this point ,i will quote from my ,human ,physiology3 ,it has been supposed by some that we really see every thing reversed1 and that our experience with the sense of touch1 in connection with that of vision1 sets us right in this particular4 ,and this it is supposed is the more readily done from the fact that our own limbs and bodies are reversed as pictured on the retina1 as well as objects that are around us1 so that every thing is relatively right in position4 ,but if this be the true explanation1 those who have their sight restored after having been blind from birth should at first see every thing wrong side up1 and should be conscious of rectifying the error by looking at their own limbs and bodies4 ,but this is not so4 ,the above explanation of erect vision1 and other explanations of a similar character1 are based upon a wrong idea of the office which the nerve performs in the process of vision4 ,it is not the image formed upon the retina which is transmitted to the brain1 but an impression produced by that image4 ,the mind does not look in upon the eye and see the image1 but it receives an impression from it through the nerve2 and this impression is so managed that the mind gets the right idea of the relative position of objects4 ,of the way in which this is done we know as little as we know of the nature of the impression itself4 #ceg4 ,single ,vision4--,whenever we see any object with both eyes there is an image formed in each eye1 and impressions go from both eyes by the optic nerves to the brain4 ,and yet with these two impressions there is no double vision so long as the two eyes correspond with each other in situation4 ,this is because the image in one eye occupies the same place on the retina that the image in the other eye does4 ,the correspondence is ordinarily perfect1 the two eyes turning always together in the same way1 upward1 downward1 or laterally1 without the least variation4 ,you can observe the effect of a want of this correspondence by pressing one of the eyes in some direction with the finger while the other is left free to move in obedience to the muscles4 ,when this is done every object appears double1 because its image occupies in one eye a different part of the retina from what it does in the other1 and so two different impressions are carried to the brain4 ,the same thing occurs in squinting1 in which the action of the muscles of the two eyes does not agree4 ,ordinarily in squinting there is not double vision1 because the mind has the habit of disregarding the impressions that come from the defective eye4 ,but when squinting occurs suddenly from disease there is double vision1 for it takes a little time to form the habit referred to4 #ceh4 ,stereoscope4--,the images of objects in the two eyes1 though always similar1 are not generally perfectly alike4 ,they are so only when the object presents a simple surface1 as in the case of pictures4 ,when the object presents two or more surfaces to the sight the images are more or less unlike4 ,this can be illustrated in a very simple way4 ,hold a book up straight before your eyes with its back toward you4 ,you see the back and both sides4 ,now if you shut your right eye you will see with the left the back of the book and the left side4 ,that is1 these two parts of the book are imaged on the retina of the left eye4 ,by shutting the left eye it will appear that the image in the right is different1 for you see now with the back the right side of the book4 ,here you have the explanation of the stereoscope4 ,in the right side of this instrument you have the picture of the object as the object itself would appear to the right eye1 and in the left side you have the picture of it as it would appear to the left eye4 ,thus1 if a book in the position alluded to above were the object1 in the right picture there should be represented the back together with the right side of the cover1 and in the left the back with the left side of the cover4 ,the two impressions1 carried to the brain by the optic nerves1 give together the impression of a solid book4 ,the same principles apply to the representation of all solids in the stereoscope4 ,illustration3 ,fig4 #bdi4 #cei4 ,thaumatrope4--,each impression made upon the optic nerve by light lasts about the eighth part of a second4 ,no distinct impressions can be made1 therefore1 upon the retina unless they succeed each other with less rapidity than this4 ,if1 for example1 in the revolution of a wheel1 eight or more spokes pass by one point in a second1 they can not be seen as distinct spokes1 but will be mingled together1 producing one continuous impression4 ,so1 too1 if a light revolve so as to describe a circle in an eighth part of a second it will appear to the eye as one unbroken circle of light4 ,it is this continuous impression on the retina that makes small objects1 as the cars pass swiftly along1 appear to run in long lines along with us4 ,the fact thus developed is made use of in the contrivance of a toy called the thaumatrope4 ,a picture is made on each side of a circular card1 and whirling the card around very rapidly by means of two strings fastened to it1 the two pictures are made to mingle together as one4 ,thus in ,fig4 #bdi are represented the two sides of such a card1 on the one side there being the picture of a dog1 and on the other that of a monkey4 ,when made to revolve rapidly the monkey will be seen sitting on the back of the dog4 ,illustration3 ,fig4 #bej4 #cfj4 ,light ,compound4--,i have thus far spoken of light as if it were a simple thing4 ,but it is compound4 ,every ray of white light has in it seven different colors4 ,that this is so we can prove by taking a beam of light by itself and dissecting it1 as we may say1 or separating it into its seven parts4 ,i will show you how this can be done4 ,let ,d ,e1 ,fig4 #bej1 a beam of the sun's light1 pass through a small opening in a shutter into a dark room4 ,the rays will pursue a straight course1 and if a screen be placed at ,f they will make a spot of white light4 ,but if a glass prism1 ,a ,b ,c1 be held in the position represented the rays will be refracted1 and when received upon the screen ,m ,n the light will be separated into seven colors in the order which is given4 ,the figure thus produced is called the solar spectrum4 ,observe why it is that the colors are separated4 ,it is because they are refracted unequally4 ,if they were equally refracted the light upon the screen would be white1 as before it was refracted4 ,the violet rays are most refracted1 the indigo next1 the blue next1 etc41 and the red are the least refracted of all4 #cfa4 ,proportion of the ,colors in ,light4--,the colors in light are not equal in amount4 ,if we divide the spectrum into #cfj equal parts the proportion in the colors will be as follows3 red1 #de2 orange1 #bg2 yellow1 #dj2 green1 #fj2 blue1 #fj2 indigo1 #dh2 violet1 #hj4 ,some suppose that there are really but three simple colors1 red1 yellow1 and blue1 the other colors being produced by a combination of these4 ,thus red and yellow will together form orange1 and yellow and blue will form green4 ,illustration3 ,fig4 #bea4 #cfb4 ,recomposition of ,light4--,after decomposing light by passing it through a prism we can bring the separated colors together again and form from them white light4 ,the manner in which this is done is represented in ,fig4 #bea4 ,the beam of light1 after passing through the prism ,s ,a ,a'1 instead of proceeding in the direction indicated by the dotted lines to form the spectrum1 is made to pass through the prism ,s' ,b ,b'1 placed in a reversed position1 and its rays are refracted so as to assume their original relation1 making a white beam1 ,m4 ,here the second prism counteracts the effect of the first1 because its position is exactly the reverse4 ,illustration3 ,fig4 #beb4 ,newton very justly considered the decomposition and the recomposition of light as affording the most positive proof that white light contains all the seven colors4 ,he tried various experiments to prove the same thing4 ,thus he mingled together intimately seven powders having the seven prismatic colors1 and found that the mixture had a grayish-white aspect4 ,he also painted a circular board with these colors1 and found that on whirling it so rapidly that the colors could not be distinguished the whole board appeared to be white4 ,in order to have this succeed perfectly the proportion between the colors must be observed1 as in ,fig4 #beb4 ,a very pretty way of illustrating the composition of light is to have a top painted in this way4 ,when the top is whirling rapidly it is white1 but as it slackens its motion the seven colors appear4 #cfc4 ,colors of ,objects4--,the color of any object depends upon the manner in which it reflects light4 ,thus1 if it be red1 it reflects the red rays of the spectrum1 absorbing the other rays2 and if it be green1 it reflects the green rays1 etc4 ,if it reflect all the colors together1 it is white2 and if it reflect none1 or almost none1 of the light1 it is black4 ,you can readily see why the color of an object varies with the kind of light that shines upon it4 ,if an object which is red in sunlight be exposed to a yellow light1 as a yellow flame1 or sunlight that has passed through a yellow-colored glass or curtain1 it loses its red color1 for there are no red rays in the light to be reflected by it into our eyes4 ,a person exposed to such a light has a deathlike paleness1 the lips and skin losing entirely their red color4 ,this effect can be witnessed at any time by mixing alcohol with a little salt on a plate and setting fire to it4 ,you see in what has been said the reason that1 in examining goods in the evening1 especially by candle-light1 we find the colors often differ somewhat from those which they have in the day4 ,in some substances the colors are changeable with varying positions1 though the light be the same4 ,we see this often in shells and minerals4 ,we see it also in some fabrics1 as changeable silk4 ,this is owing to the arrangement of the particles1 it being such as to occasion variety in reflection with changes of position4 #cfd4 ,colors of the ,clouds4--,there is no more gorgeous display of colors than we sometimes see in the clouds at morning or evening1 especially the latter4 ,these colors are occasioned simply by refractions and reflections in the minute vesicles 7 #bhh7 of which the clouds are composed4 ,how simple are the materials1 light1 water1 and air1 and yet how grand and diversified are the results6 ,illustration3 ,fig4 #bec4 ,illustration3 ,fig4 #bed4 #cfe4 ,the ,rainbow4--,in producing the colors of the rainbow the materials are less even than in producing those of the clouds4 ,they are only light and water4 ,the colors come from the reflection and refraction of light in the drops of the falling rain4 ,i will illustrate the manner in which these reflections and refractions take place4 ,take a single drop1 represented in ,fig4 #bec4 ,let ,s be a beam from the sun4 ,this entering the drop at ,a1 is refracted1 and passes to ,b1 at the farther side of the drop4 ,here a portion of it is lost by its proceeding on in the line ,b ,c4 ,the remainder is reflected to ,d1 and passes to ,e1 being refracted as it thus passes out into a rarer medium1 the air4 ,here you have a single reflection and two refractions4 ,but in the second bow1 which is sometimes formed1 there are two reflections as well as two refractions1 as represented in ,fig4 #bed4 ,the beam of light1 ,s1 from the sun enters the drop at ,a1 is refracted1 and passes to ,b4 ,here a portion proceeds on in the direction ,b ,c4 ,the other portion is reflected to ,d4 ,then this is lessened by a part of it proceeding on in the line ,d ,e4 ,what remains is reflected to ,e4 ,you see here the reason that the second bow is not so bright as the primary one4 ,in the latter there is but one reflection in each drop1 and therefore there is but one point where there is loss of light by its passing on out of the drop2 while in the former there are two reflections1 and therefore loss at two points4 ,illustration3 ,fig4 #bee4 #cff4 ,circumstances under which ,rainbows are ,seen4--,a rainbow is seen when the spectator stands between the sun and falling rain4 ,this commonly can not be the case1 except in the latter part of the day4 ,it sometimes1 though very rarely1 happens that a shower passes from the east to the west in the morning1 and then a rainbow can be seen in the west4 ,fig4 #bee is intended to show under what circumstances a rainbow is seen4 ,let a horizontal line be drawn from ,o1 the observer1 to ,p1 a point directly under the middle point of the arch4 ,if this line were extended backward from the observer it would be precisely in the direction of the sun from him4 ,that is1 the sun is directly opposite the middle of the bow4 ,now if the drop at ,a reflect a red ray to the eye of the spectator all other drops similarly situated in the arch will reflect red rays4 ,so if ,b reflect a green ray all other drops similarly situated will do the same4 ,and so of ,c1 reflecting the violet ray4 ,for the sake of clearness there are only three reflections represented1 but the same is true of all the seven colors4 ,in the secondary bow the arrangement of the colors is reversed1 the red being at the inner part of the bow and the violet at the outer part4 ,the double reflections are manifest in the drops ,d1 ,e1 and ,f4 ,what ,i have described as taking place in a few drops takes place in countless multitudes of them in forming the bow4 ,as the exact place of the rainbow depends not only upon the direction of the rays of the sun but also the position of the spectator1 it is clear that no two spectators see precisely the same bow1 for the drops that form it for the one are not the same drops that form it for the other4 ,this is very obvious if the two be quite distant from each other2 but it is equally true if they are very near together1 although in this case the bow for the one would be very nearly coincident with the bow for the other4 ,it is also true that the rainbow of one moment is not the rainbow of the next1 for as the drops that reflect it are falling drops there must be a constant succession of them in any part of the bow4 #cfg4 ,colors in ,dew-,drops and ,ice-,crystals4--,we often see something very analogous to the rainbow in the dew4 ,as the sun rises1 if1 with our backs to it1 we look at the dew-drops1 we see all the colors of the rainbow glistening every where before us1 as if the grass were filled with gems of every hue4 ,here we have the same refraction and reflection in drops of water1 and the resemblance fails only in the regularity of arrangement which the rainbow presents4 ,we see the same thing also if the ground is strewed with bits of ice which have fallen from the branches of the trees1 and the sun shines aslant upon them4 #cfh4 ,heat and ,light4--,we have not yet finished our dissection of the beam of light1 begun in #cfj4 ,in the beam of light which is separated into the seven colors there is heat also2 and in the separation it is found1 as represented in ,fig4 #bef1 that the rays of heat are most abundant just beyond the red rays1 while they are very sparing indeed at the other end of the spectrum4 ,the greatest degree of light is at the boundary between the orange and the yellow rays4 ,illustration3 ,fig4 #bef4 #cfi4 ,chemistry of ,light and the ,daguerreotype4--,there is a chemical power also in light1 producing every where1 quietly but thoroughly1 important effects4 ,the chemical rays are most abundant at the end of the spectrum opposite to that where the heat-rays abound4 ,it is these which do the work in ,daguerreotyping4 ,in this art light has been said to be the painter2 but this is not strictly true4 ,light makes the image of the object1 just as it does in the camera obscura and in the eye1 but it has no power to fasten that image upon the metallic plate4 ,this is done by the chemical rays1 which1 like the rays of heat1 go along with the light4 ,without going into particulars1 which will be given in ,part ,second1 the process of ,daguerreotyping is simply this3 ,a metallic plate is so prepared that the chemical rays of light shall act upon it sensibly4 ,then the object to be taken--a person or any thing else--being before the instrument1 a slip of ground glass is inserted1 and when the operator gets the lens so adjusted that a good image of the object is seen on the glass he takes this out and puts in its place the metallic plate4 ,rays of light coming from the object make the image1 and the chemical rays bound up with the light act upon the plate so as to fix the image there4 ,,chapter ,,xv4 ,,electricity4 #cgj4 ,origin of the ,term4--,the ancients observed that when certain substances were rubbed together singular phenomena were produced4 ,one of these substances was amber1 and as the ,greek name for this is 1 the power which is thus excited into action has been called electricity4 ,illustration3 ,fig4 #beg4 #cga4 ,attraction and ,repulsion in ,electricity4--,one of the most common effects of electricity is attraction4 ,if we rub a tube or rod of glass with woolen or silk it will attract light articles1 such as cotton1 feathers1 lint1 etc41 so that they will adhere to it4 ,but repulsion is also an effect of electricity under certain circumstances4 ,in order that the explanation of these two opposite effects may be clear to you1 ,i will detail some of the experiments which exhibit both4 ,suppose that we have a pith ball1 ,a1 ,fig4 #beg1 suspended by a silk thread1 ,b1 from a stand1 ,c4 ,i must premise that silk does not readily let electricity pass over it1 or is a non-conductor1 and therefore any electricity communicated to the pith ball will remain there unless something be brought in contact with it or very near it4 ,if now you rub a glass tube1 thus exciting electricity upon it1 and then bring it near the ball1 it will attract the ball to it1 and then in a moment repel it1 so that it will stand off from the tube and retreat from it if you follow the ball with the tube4 ,why is this8 ,it is supposed that there is a subtile fluid on the electrified glass1 some of which passes to the ball as it touches the glass1 so that the ball and the glass are in a similar condition4 ,but the particles of the fluid repel each other2 and this is the reason that the ball is repelled from the glass as soon as it becomes charged with a part of the electricity of the glass4 ,for the same reason if two pith balls hanging from a standard become electrified from a glass tube or rod they will repel each other1 for they are in the same electrical condition4 #cgb4 ,vitreous and ,resinous ,electricity4--,suppose now that you rub a rod of sealing-wax with woolen or silk and hold it near a pith ball which has been electrified from glass4 ,it will attract the ball4 ,the reason is that an electricity is excited on the sealing-wax of a different kind from that which is excited on the glass4 ,the former is called resinous1 and the latter vitreous electricity4 ,they are supposed to be two fluids1 which have a strong attraction for each other1 while1 on the other hand1 the particles of either fluid are repellent to each other4 ,it is this attraction between the two fluids which causes in the case just stated the sealing-wax to attract the ball to itself4 ,we can illustrate this attraction in another way4 ,take two pith balls and electrify them1 the one from glass and the other from sealing-wax4 ,brought near together they will attract each other1 because they have two unlike electricities4 ,this1 you see1 is just the reverse of the effect produced in an experiment cited at the conclusion of #cga1 in which the electricities were alike in the two pith balls4 ,again1 if you bring the rubbed sealing-wax near to the ball electrified from glass1 the ball will be attracted1 and the same effect will follow if you bring the rubbed glass near to the ball electrified from sealing-wax4 #cgc4 ,franklin's ,theory4--,in #cgb is developed the theory now commonly received in regard to electricity4 ,the theory of ,franklin was different4 ,he supposed that there is but one electric fluid1 and that all bodies are in their usual state charged with a certain portion of it1 some having more than others1 according to their capacity for electricity4 ,while a body is in its usual state there is no manifestation of electricity4 ,the fluid is in a quiescent condition1 because its particles are prevented from repelling each other by the attraction which exists between them and the particles of the substance4 ,but this quiescence can be disturbed by friction and other causes4 ,thus if a glass rod be rubbed with a piece of silk1 the natural equilibrium is disturbed1 the glass having an excess and the cloth a deficiency of electricity4 ,the glass is therefore said to be positively and the cloth negatively electrified4 ,the equilibrium can be restored in the case of a positively electrified body by having its excess drawn off1 and in the case of a negatively electrified body by having its deficiency made up by receiving electricity from other bodies4 ,though this theory is discarded1 the terms positive and negative derived from it are retained1 being applied to the two fluids#f or electricities1 and they are often designated by the two signs and -4 #cgd4 ,upon ,what the ,kind of ,electricity ,excited ,depends4--,it depends on what a substance is rubbed with whether vitreous or resinous electricity is excited in it4 ,thus smooth glass rubbed with woolen cloth or silk will be positively electrified2 while if it be rubbed upon the back of a cat it will exhibit negative or resinous electricity4 ,so1 also1 if a resin1 as gumlac or sealing-wax1 be rubbed with silk or woolen cloth1 it will be charged with resinous electricity1 but it will be charged with vitreous or positive if it be rubbed with sulphur4 ,the terms vitreous and resinous are therefore incorrect1 for they are based upon the idea that one kind of electricity is always excited on glass1 whatever the friction may be made with1 and that the other kind is always excited on resins4 ,the most decided illustration of the incorrectness of these terms we have in the fact1 that while smooth glass rubbed with silk or woolen cloth becomes charged with positive 7vitreous7 electricity1 roughened glass rubbed with the same gives us negative 7resinous7 electricity4 ,below ,i give a table of substances1 any one of which has positive electricity developed on it when it is rubbed with any substance below it on the list1 and negative when rubbed with any substance above it3 #a4 ,cat-skin4 #b4 ,polished glass4 #c4 ,woolen cloth4 #d4 ,feathers4 #e4 ,wood4 #f4 ,paper4 #g4 ,silk4 #h4 ,sealing-wax4 #i4 ,amber4 #aj4 ,roughened glass4 #aa4 ,sulphur4 #cge4 ,conductors and ,non-,conductors4--,electricity passes over the surface of some substances very readily2 while over others it moves with very great difficulty1 and therefore very slowly and sparingly4 ,the former are termed conductors1 and the latter non-conductors4 ,as in the case of heat1 so with electricity there are no substances which are wholly non-conducting4 ,the best of all the conductors are the metals1 those least liable to oxydation being the most perfect4 ,next come charcoal1 water1 living substances1 flame1 smoke1 steam4 ,the best non-conductors are gumlac and gutta-percha4 ,then come amber1 resins1 sulphur1 glass1 silk1 wool1 hair1 feathers1 cotton1 paper4 ,non-conductors are sometimes called insulators1 from the ,latin word insula1 as they serve to confine electricity within certain bounds1 and prevent its escaping4 ,thus in the experiments with pith balls1 already cited1 the silk threads by which they are suspended prevent the electricity from escaping from them4 ,so the glass knobs on which the wires of the telegraph rest are insulators1 preventing the electric fluid from escaping down the poles into the ground4 ,illustration3 ,fig4 #beh4 #cgf4 ,electricity ,always on the ,surface4--,there is a marked difference between heat and electricity in the manner in which they are disposed of4 ,heat pervades all the particles of substances1 and in its conduction spreads through them1 while electricity in its ordinary movements operates altogether on the surface4 ,a hollow ball1 therefore1 can contain as much electricity as a solid1 and a hollow conductor of electricity is just as effectual as a solid one4 ,the following experiment exhibits in a very striking manner this disposition of electricity to occupy the surface alone3 ,let a1 ,fig4 #beh1 be a metallic ball supported by a glass stand1 b2 and let c c be metallic caps which will just cover the ball1 having non-conducting handles1 either glass or gumlac4 ,now1 after having charged the ball with electricity1 let the caps held by the insulating handles be carefully placed over the ball4 ,on withdrawing them it will be found that the electricity of the ball has all passed to the outer surface of these caps4 #cgg4 ,electrics and ,non-,electrics4--,it will be observed1 on looking over the list of conductors and non-conductors1 that among the non-conductors are those substances in which electricity is easily excited by friction1 such as glass1 amber1 silk1 etc4 ,these were therefore called electrics4 ,the conductors1 on the other hand1 were called non-electrics1 it being supposed that electricity could not be excited with them4 ,but this has been found not to be true4 ,for example1 if a metal be insulated by being placed on a pillar of glass or of gumlac1 so that the electricity1 when excited1 can not pass off readily1 its generation can be made manifest4 ,it is probably true that every substance is more or less an electric1 it being difficult to make this manifest in the case of conductors1 because the electricity passes off as fast as it is generated4 #cgh4 ,electricity ,every ,where ,active4--,i have said that there is electricity in all substances1 each having its own capacity for it1 but that in the usual condition of substances the electricity is in a state of equilibrium1 and therefore of quiet4 ,we see this quiet disturbed whenever there is a thunder-storm1 when we rub glass or silk1 or a cat's back1 or when we work an electrical machine4 ,but the active state of electricity is not limited to such palpable demonstrations as these4 ,electricity is undoubtedly in action every where and always1 although we can seldom appreciate and measure its action4 ,wherever there is motion there is a disturbance of the equilibrium of electricity1 and a consequent return to this equilibrium4 ,and this change from the one state to the other must be the constant cause of important changes and operations in the world around us1 and in our own bodies4 ,let us look at some of the indications of this universality electrical action4 ,the friction of any electric upon another awakens it4 ,the friction of the belts upon the drums in cotton factories does it quite freely4 ,every stroke of ,india rubber upon paper as you erase a pencil mark excites electricity4 ,the blowing of air upon glass does the same4 ,so1 also1 does the blowing off of steam from an engine4 ,electricity has been excited even upon ice by rubbing it when cooled down to #ac below zero4 ,experiments upon the air have shown that there is usually some free electricity in it1 the atmosphere being generally in a positive state1 especially when the air is dry and clear4 ,it is constantly generated from one source and another4 ,it is generated every where by evaporation4 ,every gust of wind1 causing friction of the particles of the air upon various substances1 generates it4 ,motion of every kind probably generates it4 ,chemical action1 as you will see in another part of this chapter1 generates it every where4 ,it is generated also in the operations of life1 and in some animals there are special organs--electrical batteries--for the generation of this agent4 ,illustration3 ,fig4 #bei4 #cgi4 ,induction4--,a remarkable influence is exerted by an electrified body upon another body in its usual state when brought near it1 and this influence is called induction4 ,i will illustrate this by ,fig4 #bei4 ,let ,a be a metallic ball standing on a glass pillar1 and charged with positive electricity4 ,let ,b be a metallic cylinder supported upon two glass pillars4 ,now if ,a be placed near ,b1 but not near enough for the electric spark to pass from it to ,b1 it will destroy the equilibrium of the two electricties in ,b1 the negative electricity being accumulated at the end near ,a1 and the positive at the remote end4 ,this is because the positive electricity in ,a repels its like in ,b and attracts the unlike fluid4 ,you observe that there is a pair of pith balls suspended at each end of ,b1 and also at the middle4 ,the two balls at the positive end repel each other because they are charged with the same electricity1 and so with the balls at the negative end4 ,but the balls hanging from the middle are not affected1 because they are on middle ground between the two electricities4 ,here is no communication of electricity from ,a to ,b1 but only an influence upon the quiescent balanced electricities of ,b4 ,accordingly1 if the surplus electricity of ,a be discharged by putting the hand or any good conductor upon it the influence will cease1 the equilibrium in ,b will be restored1 and the pith balls will all hang straight down4 ,the same effect will be produced if ,a be withdrawn to a distance from ,b1 and the influence will be renewed if ,a be brought near again4 ,illustration3 ,fig4 #bfj4 ,if instead of one conductor we use two1 ,b and ,c1 ,fig4 #bfj1 and have them in contact1 we shall have the negative electricity on ,b and the positive on ,c4 ,now if we withdraw ,c from ,b we may have the two electricities separate1 ,b being charged with the negative and ,c with the positive4 ,illustration3 ,fig4 #bfa4 #chj4 ,electrical ,machine4--,you are now prepared to see how the common electrical machine operates4 ,there are two kinds--the plate and the cylindrical4 ,the plate machine1 ,fig4 #bfa1 has at p a large plate of glass1 and at r a rubber which consists of two brass plates lined with leather which is stuffed1 the pressure of which upon the glass is regulated by a screw4 ,above this rubber is a brass ball1 d1 and a brass chain connects the rubber and the ball with the floor1 or1 in other words1 with the earth4 ,at c is what is called the prime conductor--a hollow brass cylinder with rounded ends1 having attached to it a rod with points1 as seen at a4 ,there is a similar rod attached to it on the other side of the glass plate4 ,the different parts of the instrument are supported on glass pillars1 g g g1 standing on a wooden platform4 ,the lower part of the plate is covered with a case of silk1 which1 being a non-conductor1 prevents the electricity on the glass from being lost in the air1 and also serves to keep the plate free from dust4 ,the rubber is covered with an amalgam of tin1 zinc1 and mercury1 this being found very effectual in exciting electricity4 ,the operation of the machine is this3 ,as the plate revolves positive electricity is collected upon the glass1 and negative electricity upon the rubber4 ,the former1 as it comes to the points at a1 goes to them and passes on by the rods to the prime conductor1 while the latter passes from the rubber by the chain to the earth4 ,the points at a are of great service in collecting the electricity1 because the fluid is always much more ready to go to points than to conductors of a blunt shape4 ,illustration3 ,fig4 #bfb4 ,the cylinder machine is represented in ,fig4 #bfb1 a a being a glass cylinder1 which can be turned rapidly by the multiplying wheel1 b b4 ,at c is a piece of silk1 and on the rear part of the cylinder is the rubber4 ,at d is the prime conductor4 #cha4 ,experiments4--,many experiments may be tried with the electrical machine4 ,i will cite a few of them3 ,if pith balls be attached to the prime conductor1 as seen in ,fig4 #bfa1 they will stand out from each other as soon as the machine is worked1 because they are both charged with the same kind of electric fluid4 ,let a small figure with its head covered with hair be placed upon the prime conductor4 ,as soon as the conductor becomes charged with electricity the hair stands out1 as represented in ,fig4 #bfc1 for the same reason that the pith balls diverged in the previous experiment4 ,illustration3 ,fig4 #bfc4 ,fig4 #bfd4 ,so1 also1 if you place on the conductor a figure having attached to it strips of tissue-paper1 they will diverge in the manner shown in ,fig1 #bfd4 ,illustration3 ,fig4 #bfe4 ,let a metallic plate1 a1 ,fig4 #bfe1 be suspended by a chain to the prime conductor1 and another plate1 b1 be supported upon a conducting stand4 ,if figures of paper or pith be placed between these plates as the machine is worked they will move about briskly between the plates1 being alternately attracted and repelled by the communication of the electricity4 ,illustration3 ,fig4 #bff4 ,the experiment represented in ,fig4 #bff is a very beautiful one4 ,let a b be a brass rod with an arch1 g1 by which it can be suspended from the end of the prime conductor4 ,to this rod are suspended three bells1 the two outer ones by chains1 and the middle one by a silk thread2 also two clappers1 d and e1 by silk threads4 ,the middle bell has a chain1 f1 connecting it with the table--that is1 with the earth4 ,the operation of the apparatus is this3 ,as soon as the outer bells become electrified they attract the clappers4 ,these1 on touching the bells1 receive a portion of their electricity1 and are repelled4 ,they therefore strike against the middle bell1 to which they impart the electricity which they received from the outer bells4 ,they swing back again then in the same state that they were in at first1 and now are attracted again by the outer bells4 ,this goes on so long as the electricity is communicated4 ,illustration3 ,fig4 #bfg4 ,let there be pasted upon a slip of glass a continuous line of tin-foil1 going back and forth1 as represented in ,fig4 #bfg1 and let there be a ball1 ,g1 connected with one end of1 the foil4 ,the word light is made upon it by cutting out with a sharp knife little portions of the foil4 ,if now with your finger on one end of the line of foil at a1 you present the ball ,g to the prime conductor1 the electric fluid will run along the whole length of the line from ,g to a4 ,in doing this the letters are beautifully illuminated1 a spark being produced at each interruption of the line4 ,so rapid is the passage of the electricity that the whole appears to the eye simultaneously illuminated4 ,illustration3 ,fig4 #bfh4 #chb4 ,the ,insulating ,stool4--,this consists of a wooden top1 a1 ,fig4 #bfh1 supported by glass legs1 c c4 ,it can be made simply by boring holes in the four corners of a piece of board sufficiently large to admit the necks of bottles4 ,many amusing experiments can be tried with this4 ,a person standing upon it can be highly charged with electricity by holding a chain connected with the prime conductor4 ,the hair will rise up as represented in ,fig4 #bfc1 and he can give electric shocks to other persons from any part of his body4 ,illustration3 ,fig4 #bfi4 #chc4 ,electricity ,discharged from ,points4--,i have already1 in giving an account of the electrical machine1 spoken of the readiness with which electricity is received by points4 ,it is discharged from them with equal readiness2 so that1 if a metallic point be attached to the prime conductor1 the electricity will be carried off into the air nearly as fast as it is received upon the conductor4 ,and as it passes off it creates a current in the air as it strikes upon it4 ,the reaction of the air upon the electrical currents can be very prettily exhibited with the apparatus represented in ,fig4 #bfi1 which consists of a cap1 ,a1 resting upon the point of a rod1 and having pointed wires branching out from it in a wheel-like arrangement4 ,you observe that the points are all bent one way4 ,if this apparatus be set upright upon the prime conductor1 the wheel can be made to revolve rapidly by working the machine4 ,as the reaction of the air against the gases issuing from the rocket makes it rise1 so the same reaction against the electricity issuing from these points causes the circular motion4 ,if electricity be discharged from a point in a darkened room it appears like a brush of light1 as represented in ,fig4 #bgj4 ,illustration3 ,fig4 #bgj4 ,illustration3 ,fig4 #bga4 #chd4 ,leyden ,jar4--,the ,leyden jar1 ,fig4 #bga1 is so called because it was contrived at ,leyden4 ,it was suggested by an accidental result of an experiment tried there with the electrical machine4 ,it consists of a glass jar coated upon the inside and the outside to near the top with tin-foil1 and having a metallic rod passing through the cork1 with one end touching the inner coating1 and the other surmounted by a brass ball or knob4 ,the jar is charged by holding the knob near to the prime conductor while the machine is worked4 ,the electricity passes by the metallic rod to the inside coating of the jar1 and accumulates there4 ,this is positive electricity4 ,in the mean time there is an accumulation of negative electricity on the outside coating4 ,but how is this8 ,it is by the repulsion of positive electricity for itself1 and its attraction for its opposite1 negative electricity4 ,as you hold the jar in your hand positive electricity is repulsed from its outside through your arm earthward1 while negative electricity is attracted to it by the positive which is within4 ,the two fluids get as near to each other as possible4 ,they are prevented from coming actually together by the non-conducting quality of the glass4 ,if a slip of tin-foil were made to connect the inside foil with the outer1 there would be no accumulation of electricity on the inside1 for as fast as it passed from the prime conductor to the inside it would pass out over the bridge of foil to the outside1 and down your arm and body to the earth4 ,illustration3 ,fig4 #bgb4 ,if there were no communication of the outside with the earth the jar would not be charged4 ,no electricity would pass to it1 because the positive electricity which is on the outside can not be driven off1 and no negative electricity can be received4 ,to make this plain1 suppose that the jar1 a1 ,fig4 #bgb1 having a bent rod1 is suspended to the prime conductor1 b4 ,here you have the inside tin-foil connected with the source of positive electricity4 ,but the outside is insulated4 ,no electricity can pass from it or to it4 ,it has both positive and negative electricity1 but they are in equilibrium4 ,if there were a preponderance of negative electricity there1 it would attract positive electricity to it as near as possible1 and so the latter would enter the jar from the conductor4 ,but there is no such preponderance1 and so1 though a little may enter--a spark or two--there will not be enough to charge the jar sensibly1 because there is no attraction in that direction4 ,but bring now another jar1 c1 near to the outside coating of a1 and there is a movement at once in the electricities4 ,the positive electricity has a chance now to pass off from the outside of a to the inside of c1 leaving therefore a preponderance of negative electricity on the outside of a1 which exerts an attractive influence on the positive electricity of the conductor drawing it to the inside of the jar4 ,illustration3 ,fig4 #bgc4 #che4 ,discharge of the ,leyden ,jar4--,the jar may be discharged by making a communication between the inside and outside by means of any conductor4 ,it may be done with the discharging-rod 7,fig4 #bgc74 ,this has two slender metallic rods1 with brass knobs at their ends1 and jointed at a1 so that the knobs can be separated to different distances4 ,the handle is glass1 so that as the electricity passes through the rods none of it may be communicated to the hand4 ,in discharging the jar one knob is placed upon the outside foil1 and the other is brought near to the knob of the jar4 ,the two fluids now rush together from their attraction1 and in doing so a bright flash is produced1 going from the knob of the jar to that of the discharging-rod1 and with this a report4 ,you can yourself be the conductor to discharge the jar4 ,if1 having one hand upon the outside of the jar1 you bring the other near its knob1 the fluids meet in you as they do in the discharging-rod1 and a shock will be experienced in proportion to the amount of charge in the jar4 ,any number of persons can together receive the same shock4 ,to do this they must join hands1 and the person at one end of the row must touch the knob of the jar while the person at the other end has his hand upon the outside4 ,you may touch either the knob of the jar or the outside coating separately1 and the power that is in it remains quiet2 but the moment that you touch both it bursts forth1 because a bridge is made upon which the two fluids can meet4 ,in a dry air the charge in the jar can be retained for some time1 the communication between the two electric fluids being very slow through the medium of air4 ,it is otherwise when there is much moisture in the air1 for water is a good conductor4 ,for this reason1 if you let the moisture from your breath come upon the jar between the outside coating and the rod1 the jar will be discharged soon1 though imperceptibly1 the moisture making a medium of communication between the inner and outer electricities4 ,illustration3 ,fig4 #bgd4 #chf4 ,the ,electrical ,sportsman4--,in this contrivance1 ,fig4 #bgd1 the discharge of the ,leyden jar is very prettily exhibited4 ,the jar1 c1 has a rod with two branches4 ,on the end of one of these1 ,b1 are suspended pith balls cut in the shape of birds4 ,on the other is a knob by which the jar can receive its charge from the prime conductor4 ,after it is charged it is placed on the stand with its knob1 b1 near the gun1 a1 of a metallic figure4 ,the suspended birds1 you observe1 stand out from each other1 because they are charged with the same fluid1 positive electricity1 and therefore are repellent4 ,now when the chain1 e1 which is connected with the outside of the jar1 is made to touch the foot of the metallic image1 the connection between the inside and outside of the jar is established4 ,of course there is an instantaneous flash between a and b1 and the birds1 losing their electricity1 fall1 and hang as they did before the jar was charged4 ,illustration3 ,fig4 #bge4 #chg4 ,electrical ,battery4--,by combining together a number of jars1 having the insides all connected together1 as seen in ,fig4 #bge1 with metallic rods1 and the outsides connected together in a similar manner1 we have what is termed an electrical battery4 ,by such an arrangement we can accumulate a large amount of electricity1 which can be discharged in the same way essentially as in the case of the single jar4 #chh4 ,light of ,electricity4--,the light produced by electricity is not occasioned by any thing like combustion4 ,it depends obviously upon the resistance which is offered to its passage4 ,thus when the electric fluid passes through air from the prime conductor to the knob of the ,leyden jar it causes a flash of light1 but when it arrives at the knob the flash ceases4 ,what is the reason of the difference8 ,in both cases it has the resistance of the air1 for when it comes to the knob it passes over the surface of the knob and rod2 but in the latter case it is so diffused in its conduction over the metallic surface that it meets with much less resistance from the air4 ,by experiments with the air-pump it is found that the denser the air is the more vivid is the spark2 and if electricity be passed through a glass vessel from which the air has been mostly exhausted we have the streams of light seen in the aurora borealis1 which are so strikingly in contrast with the vivid flashes of the lightning4 ,in the experiment1 #cha1 in which the word light is made by the passing electricity1 we have a striking illustration of the production of the spark by the resistance of the air4 ,if the foil were one continuous surface the electricity would be diffused over it without giving any light4 ,it is only where the electric fluid has to leap through the air from one portion of foil to another that the light is seen4 #chi4 ,sound of ,electricity4--,the report of electricity is a sort of crack or snap from the sudden condensation of the air by the rapid passage of the fluid4 ,the rolling of thunder is occasioned by the reverberation of the first sound among the clouds4 ,the nearer the flash is to us the more like a crack is its first sound as it comes to our ears4 #cij4 ,mechanical ,injuries from ,electricity4--,when any great amount of electricity meets in its passage with any imperfect conductor it does much violence to it4 ,thus it rends wood1 scatters water1 breaks glass1 etc4 ,various experiments have been tried illustrating the manner in which mechanical injuries result from electricity4 ,thus if it be made to pass through a card or several leaves closely pressed together1 there is a burr on each side of such a character as to show that two forces moving in opposite directions have made their passage4 #cia4 ,heat ,produced by ,electricity4--,electricity always produces in its passage some amount of heat1 probably by its mechanical effect4 ,when it is diffused over a large conducting surface the heat is not sufficient to be observable2 but if it be confined to the surface of a small wire the heat may be sufficient to melt or even burn it4 ,various effects can be produced by the heat thus caused by the passage of electricity4 ,gunpowder may be exploded by it4 ,alcohol and ether may be readily ignited by it1 especially the latter4 ,gas can sometimes be lighted by pointing the finger to an opened burner after walking across the room two or three times briskly1 rubbing the feet upon a thick carpet4 ,illustration3 ,fig4 #bgf4 #cib4 ,franklin's ,discovery4--,it had very early been conjectured that the electricity produced by the electrical machine is identical with lightning2 but it was reserved for our countryman ,franklin to prove the fact4 ,a tall spire which was being erected in ,philadelphia in #ageb he conceived might be used in his investigations1 but before it was completed the sight of a boy's kite in the air suggested to him another plan4 ,he made a kite by stretching a silk handkerchief over a frame1 and sent it up as he saw a thunder-shower rising1 his only companion being his son4 ,having raised the kite1 he attached to the end of the hempen string a key1 and also a silk ribbon1 by which he insulated his apparatus1 as seen in ,fig4 #bgf4 ,he now watched with much anxiety the result4 ,a cloud arose1 which he supposed1 from its appearance1 was well charged with electricity1 and yet no effect was seen4 ,franklin began to despair2 but he soon saw some loose fibres of the hempen string bristling up1 and1 applying his knuckle to the key1 received just such a spark as he had often received from the conductor of an electrical machine4 ,the discovery was made1 and ,franklin was at once overcome with emotion at the thought of the immortality which it would give his name4 ,he felt very much as ,archimedes did when1 after making one of his grand discoveries as he lay in a bath1 he went home saying all the way1 6 6 ,the fame of the discovery1 made in a manner so simple and yet so original1 spread every where1 and prompted to many experiments by other philosophers4 ,one1 ,professor ,richman of ,st4 ,petersburg1 fell a victim to his investigations4 ,while he was attending a meeting of the ,academy of ,sciences he heard the sound of distant thunder1 and hastened home to make some observations with an apparatus which he had erected4 ,while doing this a charge of electricity flashed from the conducting rod1 and piercing his head killed him instantly4 ,his assistant1 who stood near1 was struck down1 and remained senseless for some time1 and the door of the room was torn from its hinges4 #cic4 ,lightning-,rods4--,it was the discovery of ,franklin which led to the custom of attaching lightning-rods to buildings4 ,the object of a lightning-rod is to conduct any electricity in a cloud that may come over the building down into the ground4 ,for this purpose the rod should terminate in the air in points1 as these1 as you saw in #chj1 so readily receive the electric fluid4 ,the rod should be separated from the house by wooden supports1 and it should pass so far into the ground as to have its end in the midst of continual moisture4 ,the points should be gilt1 in order to preserve from corrosion1 or they may be made of silver or platina4 ,lightning is very apt to go down in chimneys1 as smoke is a very good conductor2 and therefore it is well to have the rods go up by chimneys1 especially if they are to have fire in them during the summer4 ,lightning-rods often undoubtedly are of service when there is no obvious passage of the lightning down them1 by quietly and continuously receiving electricity upon their points1 and passing it down into the earth4 ,illustration3 ,fig4 #bgg4 #cid4 ,galvanic or ,voltaic ,electricity4--,this form or mode of electricity ,i will barely notice here1 reserving its full consideration for ,part ,second1 where it appropriately belongs4 ,the history of its discovery is interesting4 ,the first dawning of ,galvanism is to be found in an experiment noticed by ,sulzer1 a citizen of ,berlin1 in #agfg4 ,he states that if a piece of zinc be placed under the tongue1 and a piece of silver upon it1 on being brought in contact a metallic taste is perceived1 and a shock is felt by the tongue4 ,sulzer attributed the effect to some vibratory motion occasioned by the contact of the metals1 and1 satisfied with this fanciful explanation1 pursued the inquiry no farther4 ,the statement excited but little notice until other facts of a similar character were brought out in #agij by ,galvani1 professor of ,anatomy at ,bologna4 ,he observed that the legs of some frogs1 which had been obtained for his invalid wife1 were convulsed1 when near an excited electrical machine1 on touching the nerves with a knife4 ,in contrast with the example of ,sulzer1 he was led to examine the matter further4 ,he found that the effect was produced when no electricity was communicated from the machine1 by establishing a connection between the nerves and the muscles by some conductors4 ,for example1 when a strip of zinc was placed in contact with the nerve which goes to the lower extremities1 and a strip of copper in contact with the legs1 on bringing the two together at the other end the legs would be convulsed1 being drawn up1 as represented in ,fig4 #bgg 7p4 #cjf74 ,but ,galvani did not get at the true explanation4 ,he supposed this to be an exhibition of animal electricity1 regarding the muscles as being a sort of ,leyden jar1 the nerve being the medium of communication with the inside4 ,illustration3 ,fig4 #bgh4 #cie4 ,volta's ,pile4--,the observations of ,galvani awakened much interest in all scientific minds1 and of course there was much of inquiry1 observation1 and experiment4 ,professor ,volta1 of ,pavia1 went a step farther than ,galvani toward the true explanation1 in referring the effects to the contact of dissimilar metals1 and he was led by this view of the subject to construct his pile or battery--called after him the voltaic pile--the object of which was to produce a much greater amount of electricity than could be obtained by the contact of only two pieces of metal4 ,the pile is made of circular pieces of copper1 zinc1 and cloth1 the cloth being moistened with salt-water4 ,they are arranged as represented in ,fig4 #bgh4 ,first a disk of copper is laid down1 then upon this one of zinc1 then one of cloth1 and so on in the same order1 the top of the pile ending in a plate of zinc4 ,if you touch one end of the pile with a moistened finger and the other end with a finger of the other hand1 you will feel a shock like that from a ,leyden jar4 ,the communication between the two ends of the pile may be made by wires1 as seen in the figure4 ,volta afterward changed this to the form of a cup battery1 the plates of metal being immersed in a series of cups in a mixture of sulphuric acid and water4 ,there have been various improvements from time to time1 but the arrangement is in all the different batteries essentially the same4 ,although ,volta accomplished so much he did not arrive at the truth in full4 ,his contact theory1 as it is called1 so long received as the true theory1 gradually gave way to the true explanation1 viz41 that the electricity produced is owing to chemical action4 #cif4 ,difference ,between ,frictional and ,voltaic ,electricity4--,the electricity produced by the friction of the electrical machine is more intense than that of the voltaic battery4 ,voltaic electricity1 on the other hand1 is much more abundant1 and is more continuous and lasting4 ,as it is therefore more steady and more easily controlled than frictional electricity1 it is used in the working of the ,telegraph4 ,,chapter ,,xvi4 ,,magnetism4 #cig4 ,loadstones4--,it was discovered many centuries ago that a certain ore of iron has the property of attracting pieces of common iron or of steel4 ,the fact was probably considered at first as a mere curiosity1 and the world were slow to find out its value4 ,it is not till quite recently that it has been discovered that in magnetism we have one of the great forces of the earth2 and even now we know but little probably of the real extent and variety of its action4 ,new and important discoveries are yet undoubtedly to be made in regard to the agency and the laws of this mysterious power1 and its connections with the other grand forces of nature4 ,the terms magnet and magnetism come from the fact that the loadstone was first found near ,magnesia1 an ancient city in ,asia ,minor4 ,this ore appears in considerable masses in the iron mines of ,sweden and ,norway1 and also in different parts of ,arabia1 ,china1 and ,siam4 ,it has occasionally been found in small quantities in ,england and in this country4 #cih4 ,attraction of ,magnetism4--,the attraction of the magnet and iron for each other is exhibited in many different ways4 ,if a magnet be brought near to a heap of iron-filings or needles1 it will have a quantity of them adhering to it as you raise it up4 ,in the toy fishes of children there is fastened in the head a bit of iron1 which occasions the following of the fishes after the magnet4 ,in this case you can see very plainly that the nearer the magnet and the iron are to each other the stronger is the attraction4 ,indeed1 the attractive influence is governed by the same law in regard to distance as the common attraction of matter is1 viz41 it is inversely as the square of the distance4 ,the attraction also is mutual here1 the iron attracting the magnet as much as the magnet does the iron4 ,illustration3 ,fig4 #bgi4 ,illustration3 ,fig4 #bhj4 #cii4 ,poles of the ,magnet4--,every magnet has two poles4 ,it is about these poles where the chief power resides4 ,for this reason1 if a magnet be rolled in iron-filings1 these are collected about the ends1 as represented in ,fig4 #bgi4 ,there is a diminution of attraction from the ends to the middle line1 which is called the neutral line4 ,these poles are called north and south poles1 because if a magnet be suspended1 or be supported upon a pivot1 so that it can revolve1 it will take a north and south direction1 one of its ends invariably pointing toward the north4 ,in ,fig4 #bhj is represented a magnet supported upon a pivot1 ,c4 ,illustration3 ,fig4 #bha4 #djj4 ,magnetism by ,induction4--,the magnet in exerting its attraction really temporarily makes a magnet of what it attracts4 ,actual contact is not necessary to this result4 ,thus if a large key be only brought very near to a powerful magnet it will support small keys1 as represented in ,fig4 #bha4 ,when the key is removed away from the magnet the keys attached to it fall4 ,you see the analogy to the induction of electricity noticed in #cgi4 ,as in the induction of electricity1 so here the two ends of the body in which the influence is induced are in opposite states4 ,if the end of the magnet1 to which the first key is near or attached1 be the north pole1 the end of the key next to the magnet is the south pole1 and its farther end is the north pole4 ,the same is the case with the small key attached to the end of the large one4 ,and so if a nail should hang from the small key1 and a needle from that1 both of these would have the same polarities4 ,but all this would be reversed if the large key were attached to the south pole of the magnet4 ,in this case the upper end of each of these articles would be the north pole1 and its lower end the south pole4 #dja4 ,attraction and ,repulsion in ,magnets4--,you have seen in induction that in magnets like poles repel while unlike attract4 ,but this law can be more strikingly illustrated4 ,if a magnet be placed on a pivot1 as in ,fig4 #bhj1 and another magnet be brought near it1 attraction or repulsion will be manifested according to the mode of presentation4 ,if a north pole be presented to a north pole1 or a south to a south1 repulsion will be the result4 ,but if a north pole be presented to a south1 or a south to a north1 then attraction will be manifested4 ,illustration3 ,fig4 #bhb4 #djb4 ,magnetic ,curves4--,the polarity of magnetism causes a very singular arrangement of iron filings when gently agitated upon a sheet of paper over a magnet1 as represented in ,figure #bhb4 ,the curves which you see have been supposed by some to be occasioned by the escape of some fluid or influence from the magnet in these particular directions4 ,but they are owing entirely to the fact that each bit of filing is polarized by the bit next preceding it in the row reckoning from the magnet outward1 the nearest one in each row deriving its magnetic state from the magnet itself4 ,this being so1 as the chief power resides in the ends of the magnet1 it is easy to see how such a disposition of the lines of magnetic filings is effected4 ,these curves may be beautifully and curiously varied by having several magnets variously arranged under the paper4 #djc4 ,artificial ,magnets4--,the power residing in the loadstone can be communicated readily1 as you have seen1 to iron and steel4 ,though soft iron takes the magnetic influence more readily than steel1 it does not retain it as steel does1 and the latter is therefore used in making artificial magnets4 ,when a magnet imparts its magnetic influence it loses none of its own power1 whether it be an original loadstone or an artificial magnet4 ,there1 are many ways of imparting magnetism permanently to steel1 but ,i will notice only two of them4 ,if you wish to magnetize a bar or needle pass one pole of a magnet from one end of it to the other a considerable number of times1 always in the same direction4 ,a more effectual way is to take two magnets1 and1 placing the south pole of one and the north pole of the other in contact over the middle of the bar or needle1 draw them slowly and steadily apart toward the opposite ends4 ,this process must be repeated several times4 ,illustration3 ,fig4 #bhc4 ,illustration3 ,fig4 #bhd4 #djd4 ,horseshoe ,magnets4--,one of the most common forms of the magnet is the horseshoe magnet1 ,fig4 #bhc4 ,there is a piece of soft iron attached to the end of this1 held there by attraction4 ,this is called the armature4 ,so long as it is suffered to remain there it is a magnet having its two poles1 the north pole being attached to the south pole - of the magnet which holds it1 while the reverse is the case with its south pole4 ,the object of the armature is to preserve the power of the instrument4 ,indeed it is found that the exertion of the magnet's power not only preserves but actually increases it4 ,if you attach1 therefore1 to a magnet an armature having a hook1 as seen in ,fig4 #bhd1 you can add to the weight gradually from day to day1 and so considerably augment the power of the magnet4 #dje4 ,magnetic ,needle4--,the magnetic needle is a very small magnet fixed upon a pivot4 ,as it points north and south it is of great use to the mariner4 ,the mariner's compass is a round box with such a needle balanced in it1 and having a card on which is drawn a circle divided into thirty-two parts1 as seen in ,fig4 #bhe4 ,the original compass was a rude affair1 consisting of a slip of loadstone laid upon a piece of cork floating in water4 ,the date and place of its first use are unknown4 ,illustration3 ,fig4 #bhe4 #djf4 ,declination of the ,needle4--,the declination of the needle is its deviation from a north and south line4 ,it is in comparatively few parts of the earth's surface that there is no deviation from this line to the east or the west4 ,true as needle to the pole has become a proverb1 and when it was first uttered it was supposed to be founded in strict truth2 but modern investigation has shown not only that the needle varies in its pointing in different localities1 but that it varies to some little degree in its variations4 ,the declination of the needle was first observed by ,columbus in his first voyage of discovery1 and it occasioned great alarm among the sailors1 who1 as ,irving states1 thought the laws of nature were changing1 and that the compass was about to lose its mysterious power4 ,notwithstanding these and other observations of a similar character1 no great account was made of the declination of the needle till the middle of the seventeenth century4 ,but since that time extensive records of its declinations at different localities have been made1 and tables and charts have been constructed exhibiting them4 ,these declinations are not constant1 but vary somewhat every day1 from the influence1 it is supposed1 of the sun upon the earth4 #djg4 ,dip of the ,needle4--,it is found that in most parts of the earth1 if a needle be balanced before it is magnetized1 and then be suspended from the same point1 it will not be balanced1 but one end will dip downward4 ,this fact was discovered by ,norman1 a ,london optician1 in #aegf4 ,he found that the dip at ,london was toward the north at an angle of #gb4 ,in pursuing the investigation of this phenomenon it was found that going from the north toward the equator the dip constantly lessened1 until a point was reached where the needle was horizontal4 ,then1 on going south of this1 a reverse dip occurred1 that of the south pole1 and the farther south the needle was carried the greater was the dip4 ,in the north1 ,captain ,ross in #ahcb came to a locality north of ,hudson's ,bay1 in lat4 #gj #e ,n41 long4 #if #de ,w41 where the magnetic needle1 freely suspended1 was in a vertical line4 ,no such locality has yet been discovered toward the south pole4 #djh4 ,the ,earth a ,magnet4--,you can readily see1 from all that has been stated in regard to the magnetic needle1 that the earth is a magnet1 or has that covered up in it which in some way acts as such4 ,the dip of the needle shows that the two poles of this magnet are somewhere near the north and south poles of the earth4 ,the locality which ,captain ,ross found must be near the north pole of the magnet in that quarter of the world4 ,the vertical position of the needle there is analogous to the straight lines of iron filings which you see in ,fig4 #bhb1 near the poles of the magnet2 and it is easy also to trace the analogy between the dip of the needle at different distances from what is called the magnetic equator of the earth1 where the needle is horizontal1 and the curves which you see extending from pole to pole4 ,the different declinations of the needle and the different intensities of the magnetic force in different localities corresponding in latitude show that the magnet in the earth1 if there be one1 is irregular in shape1 or in some way has its power varied much in differed parts of the earth's crust4 #dji4 ,the ,earth as a ,magnetizer4--,as the earth is really a magnet1 it might be expected to impart magnetism by induction as other magnets do4 ,and this is found to be the fact4 ,if you hold a bar of soft iron in the direction of the dip of the needle it becomes a magnet1 its lower end being the north pole1 and its upper the south4 ,that this is so can be ascertained by bringing a small magnetic needle near each end4 ,no effect of this kind is produced when the bar is held horizontally east and west4 ,lightning-rods1 pokers1 upright iron bars in fences1 etc41 are often found to be magnetized because they have continued so long nearly in the required position for magnetization4 ,when a bar of iron has been magnetized in the manner indicated1 its magnetism may sometimes be fixed by giving it a stroke with a hammer4 ,it is a curious but inexplicable fact that this vibration of the particles of the iron should have this effect4 ,but though such vibration helps to impart magnetism1 it is not at all favorable to its retention1 for magnets are always injured by blows or falls1 or indeed any rude treatment4 ,for this reason care is requisite in removing an armature from a magnet4 ,if pulled off abruptly the power of the magnet is lessened4 #daj4 ,magnetism in ,other ,substances besides ,iron4--,it was formerly supposed that magnetism was confined to ferruginous substances1 but this has been found not to be true4 ,various minerals are magnetic1 especially when they have been heated1 also some of the precious stones1 and even silica1 which enters so largely into some of the rocks of the earth4 ,and it is supposed by some that future investigations will show that the influence of magnetism is as extensive in the earth as that of electricity4 #daa4 ,in what ,magnetism is ,like ,electricity4--,magnetism is like electricity in several particulars3 #a4 ,its power is on the surface of bodies4 #b4 ,it is of two kinds1 north and south1 or boreal and austral1 comparing with the positive and negative electricities4 #c4 ,the same rule of attraction and repulsion applies to both2 viz41 like repel and unlike attract4 #d4 ,as electricity can be communicated by induction1 so can magnetism4 #dab4 ,in what ,magnetism is ,unlike ,electricity4--,the circumstances in which magnetism is unlike electricity are chiefly these3 #a4 ,the obvious manifestations of magnetism are to a great extent confined to one class of substances1 the ferruginous1 and to but a portion of them2 while electricity makes its manifestations in connection with all kinds of substances4 #b4 ,magnetism is never transferred as electricity is from one body to another1 but a body gains rather than loses in imparting magnetic power to other bodies4 #c4 ,the two magnetisms1 the boreal and austral1 can not be obtained separately as the two electricities can4 ,if a magnet be broken in two1 each piece will have in it the two magnetisms and the two poles as the whole did4 ,this is in entire contrast with the electrical experiment noticed in the last of #cgi4 #d4 ,there are no non-conductors to interrupt magnetic influence4 ,if in the experiments in #cgi a plate of glass or resin were interposed between ,a and ,b1 the influence would cease1 but it would have no effect on the induction of magnetism if interposed between a magnet and a bit of steel or iron4 #dac4 ,electro-,magnetism4--,though electricity and magnetism differ so much from each other1 yet they have intimate relations1 and it is now the general opinion among scientific men that they are merely different modes of the same power4 ,magnetism can produce electricity1 and electricity can produce magnetism4 ,the first discovery of facts revealing this connection was made by ,professor ,oersted of ,copenhagen in #ahai4 ,since that time electro-magnetism1 or the production of magnetism by electricity1 has been a prominent subject of observation and experiment4 ,oersted's first observation was that a current of electricity passing over a wire near a magnetic needle affected the position of the needle4 ,he found also that iron filings would adhere to a wire over which a current of electricity is passing1 just as they do to a magnet1 dropping off1 however1 as soon as the current ceases to pass4 ,such facts led to a great variety of investigations and arrangements of apparatus by ,oersted and others4 ,illustration3 ,fig4 #bhf4 #dad4 ,electro-,magnets4--,the most powerful electro-magnets are made by bending a thick cylindrical bar of soft iron into the form of a horseshoe1 ,a ,b1 ,fig4 #bhf1 and coiling around it a copper wire4 ,the wire must be insulated by being wound with some non-conducting material1 as silk1 so that the electric current may pass through the whole length of the wire4 ,with the instrument thus prepared1 if the two ends of the wire be connected with the poles of a voltaic battery which is in action1 the bar will be magnetized1 and will hold up a heavy weight so long as the electric current is passing through the wire4 ,whenever the current is cut off by disconnecting the wires the weight will fall4 ,illustration3 ,fig4 #bhg4 ,electro-magnets have been made in this way having such power as to sustain a weight of four thousand pounds4 ,in ,fig4 #bhg we have represented an apparatus which exhibits electro-magnetism very prettily4 ,the soft iron1 you see1 is in two pieces which when put together form a ring1 d1 and each piece has a handle4 ,if the pieces be put together with the coil1 c1 in the position represented1 on connecting the wires ,p and ,n with a battery in action1 the adhesion is so strong as to resist a great force2 but as soon as the connection is broken the pieces come apart at once4 ,illustration3 ,fig4 #bhh4 #dae4 ,electric ,telegraph4--,the most remarkable and useful application of electro-magnetism we have in the electric telegraph4 ,as before stated1 voltaic electricity is used4 ,this is generated at the place from which the message is sent1 and passes over the wire to the place where the message is received4 ,there it acts upon soft iron by passing through coiled wire1 producing the modified power called electro-magnetism4 ,i will make all this plain to you by describing the machine used in ,morse's ,telegraph1 ,fig4 #bhh4 ,w ,w are the wires which connect with the station from which the message is to be received1 and these connect with the copper wire coiled round the horseshoe of soft iron1 m m4 ,above the magnet is a lever1 a l1 which works on a fulcrum at d4 ,one end of this lever has a steel point1 s1 attached to it4 ,at c is an arrangement of wheel-work1 the object of which is to pass along regularly a slip of paper1 p1 in the direction of the arrows4 ,observe now how the apparatus works4 ,when the electric current passes through the coiled copper wire it makes a magnet of the iron1 m m4 ,the lever1 a l1 is therefore attracted at the end1 a1 downward4 ,of course the end1 l1 moves upward1 bringing the steel point1 s1 against the paper1 where it makes a mark4 ,the length of this mark depends upon the length of time the electricity is allowed to pass along the coiled wire1 for the moment that it is shut off m m ceases to be magnetic1 the keeper1 a1 being no longer attracted1 moves upward1 and the other end1 l1 of the lever moves downward1 taking the point1 s1 from the paper4 ,illustration3 ,fig4 #bhi4 ,in order to make the marks on the paper of different lengths1 there is a contrivance for regulating the length of time that the current shall pass through the coiled wire4 ,this contrivance1 called the signal key is represented in ,fig4 #bhi4 ,n and ,p are two strips of brass connected with the two wires ,r and ,m1 of which ,m comes from the battery4 ,the end of the strip ,n is raised a little above the end of ,p4 ,so long as they do not touch the circuit is not complete1 and no electricity passes4 ,but if the operator press ,n down upon ,p1 the circuit is established1 and the electricity passes to the station with which he is in communication1 and there acts upon the apparatus seen in ,fig4 #bhg4 ,now the longer the finger presses down ,n upon ,p the longer will be the mark on the paper at the distant station4 ,an operator then at ,new ,york1 for example1 controls by this key the length of the marks made on the paper in ,new ,haven or any other place with which he is communicating4 ,you can see then very readily how a telegraphic alphabet can be constructed by combinations of marks of different lengths agreed upon to represent different letters and numerals4 ,i give the alphabet used in connection with ,morse's ,telegraph3 ,a -- ---- ,b ---- -- -- -- ,c -- -- -- ,d ---- -- -- ,e -- ,f -- ---- -- ,g ---- ---- -- ,h -- -- -- -- ,i -- -- ,j ---- -- ---- -- ,k ---- -- ---- ,l -------- ,m ---- ---- ,n ---- -- ,o -- -- ,p -- -- -- -- -- ,q -- -- ---- -- ,r -- -- -- ,s -- -- -- ,t ---- ,u -- -- ---- ,v -- -- -- ---- ,w -- ---- ---- ,x -- ---- -- -- ,y -- -- -- -- ,z -- -- -- -- ,numerals4 #a -- ---- ---- -- #b -- -- ---- -- -- #c -- -- -- ---- -- #d -- -- -- -- ---- #e ---- ---- ---- #f -- -- -- -- -- -- #g ---- ---- -- -- #h ---- -- -- -- -- #i ---- -- -- ---- #j --------------- ,one of the most singular and interesting things in the operation of the telegraph remains to be noticed4 ,in order to have the electricity work it is necessary to have the connection between the poles of the battery at the point where the effect is to be produced4 ,you see this in the experiments represented in ,figs4 #bhf and #bhg4 ,the same is true of the electro-magnet of the telegraph4 ,this being so1 it was thought at first that it was necessary to have two wires connecting two communicating stations2 but it was found that only one wire was needed1 the earth itself answering the same purpose as another wire4 ,to make the communication through the earth effectual1 there is at each station a plate of metal1 having a surface of several square feet1 buried in the ground1 with a wire running up to the machine4 ,,questions4 ,teachers differ much in their plans of conducting recitations4 ,some are very minute in their questions2 while others go to the other extreme1 and merely name the topics1 the pupils being expected to give in full what is said upon them4 ,neither of these plans should be adopted exclusively1 but the mode of recitation should be much varied from time to time4 ,this variety is somewhat aimed at in the questions which ,i have prepared1 though in no case are the questions as minute as they should occasionally be made by the teacher4 ,the numbers refer to the pages4 ,it would be well to have the pupils draw many of the figures upon the blackboard1 and then recite from them4 ,by drawing the simplest figures first sufficient skill may be acquired to enable the pupil to draw those which are quite difficult4 ,,chapter ,i4 #ac4 ,what is said of the distinction between matter and spirit8 ,what of ,bishop ,berkeley's ideas8 ,what of ,hume's8 #ad4 ,what is the origin of the word spirit8 ,what is the relation of the senses to the spirit8 ,what is said of the effects of matter on the senses8 ,what are the forms of matter8 #ae4 ,illustrate the difference between elastic and non-elastic fluids4 ,what is said of the union of the particles of a solid8 ,give the difference noted between different solids4 ,how does a liquid differ from a solid8 ,give particularly what is said of water4 #af4 ,what is said of the particles of gaseous substances8 ,what of the atmosphere8 ,what of the vapor in it8 ,what is said of the entering of liquids and gases into interstices8 ,what of the mingling of gases with liquids8 ,give the illustration in regard to fishes4 #ag4 ,what is said of the solution of solids in liquids8 ,what of the evaporation of water in the air8 ,illustrate the influence of heat on the forms of matter4 ,what is said of the thermometer8 ,what of mercury1 water1 and iron in relation to the liquid state8 ,what is said of our knowledge of matter8 #ah4 ,what was the supposition of ,newton about the composition of matter8 ,what is said of the changes of matter8 ,what are the imponderable agents1 and why are they so called8 ,,chapter ,,ii4 #ai4 ,what is said of variety in the properties of matter8 ,what of the divisibility of matter8 #bj4 ,what is said of gold-leaf1 and of the wire of gold-lace8 ,what of the soap-bubble8 ,what of the thread of the silk-worm1 and of the web of the spider8 ,what of solution of blue vitriol8 ,what of odors8 #ba4 ,what is said of the dust of the puff-ball8 ,what of pollen8 ,what of the dust rubbed from a moth's wing8 ,what of guano8 ,what of the glazing of visiting-cards8 ,what of the minuteness of some animals8 #bb4 ,what is said of the ,deity in relation to minute animals8 ,what is said of substances called porous8 ,what of those in which there are no pores apparent8 ,what proof is there that all substances have spaces in them8 #bc4 ,what is said of the amount of space in gases and vapors8 ,give the statement in regard to steam4 ,what is said of solutions of solids in fluids8 ,what of evaporation8 ,what of the diffusion of odors in the air8 #bd4 ,what is the relation of heat to space in matter8 ,upon what do density and rarity depend8 ,explain tenacity4 ,what is said of this property in gases and liquids8 #be4 ,how is the comparative tenacity of substances ascertained8 ,give the comparative tenacity of various substances4 ,what animal substances have great tenacity8 ,what is said of the value of tenacious substances8 #bf4 ,what is said of hardness8 ,what of flexibility and brittleness8 #bg4 ,give examples of flexible and brittle steel4 ,explain the actual difference between them4 ,explain the tempering of steel4 #bh4 ,what is said of the annealing of glass8 ,what of ,prince ,rupert's drops8 ,what metals are the most malleable8 ,what the most ductile8 #bi4 ,what is said of the ductility of melted glass8 ,what of the change of position of particles in making plates and wires of metals8 ,what of welding8 ,what is said of compressibility8 ,what of the incompressiblity of liquids8 #cj4 ,what influence has heat on the bulk of liquids8 ,illustrate by the thermometer4 ,what is said of the compressibility of gases8 ,how does elasticity operate in the case of the ,india-rubber ball8 #ca4 ,give the illustration in regard to jumping4 ,state the experiment with the ivory ball4 ,what is said of the movements of particles on each other in elastic substances8 ,what of the degrees of elasticity in different substances8 #cb4 ,what is the definition of elasticity8 ,what is said of the usefulness of the variety of properties in matter8 ,,chapter ,,iii4 #cc4 ,what is meant by extension as a property of matter8 ,illustrate the fact that it is an essential quality4 ,what is said of it in reference to air8 #cd4 ,does matter ever penetrate matter8 ,give the illustration represented in ,fig4 #f4 ,give that represented in ,fig4 #g4 #ce4 ,state the arrangement of the diving-bell4 ,give the comparison between bullets and needles in relation to penetration4 ,what is said of solution8 ,what of odors8 #cf4 ,what is the property of matter called inertia8 ,give illustrations of it4 ,illustrate the fact that matter has no power to stop its own motion4 ,what is the reason that the popular notion is that matter is more inclined to rest than to motion8 #cg4 ,what is said of perpetual motion8 ,why is it not true of divisibility that it is an essential property of matter8 ,what is said of weight8 ,,chapter ,,iv4 #ch4 ,what is said of the nature of attraction8 ,what was ,newton's idea of it8 #ci4 ,what is said of attraction in solids8 ,what of its different modes of action8 ,what is the difference in attraction in the case of steel and of water8 ,what is said of the freeness with which particles of a fluid move among each other8 #dj4 ,explain ,fig4 #i4 ,explain ,fig4 #aj4 #da4 ,give the difference between mercury and water in regard to the globular form4 ,what if said of drops of water on leaves8 #db4 ,what is said of oil in reference to attraction8 ,describe and explain the manufacture of shot4 ,what is said of the globular form of the earth and the heavenly bodies8 #dc4 ,what is said of crystallization8 ,state the examples cited4 ,what is said of the crystallization of water8 ,give and explain the example of sudden crystallization4 #dd4 ,what is said of frost-work8 ,what of snow8 #de4 ,what is stated in regard to the snow-crystals of the arctic regions8 ,what is said of order in nature8 ,why can you not make the surfaces of broken glass adhere8 #df4 ,explain the cementing of glass4 ,what is said of the adhesion of pieces of ,india-rubber8 ,describe and explain the experiment with bullets and with balls of lead4 ,how may silver and gold be made to adhere to iron8 ,what is said of the adhesion of tin and lead8 ,what of the adhesion of panes of glass8 #dg4 ,upon what does the strength of adhesion depend8 ,illustrate the agency of heat in promoting adhesion4 ,give familiar examples of attraction between solids and liquids4 ,explain the experiment represented in ,fig4 #ae4 #dh4 ,what is said of stems in stagnant water8 ,explain ,figs4 #af1 #ag1 and #ah4 #di4 ,explain ,fig4 #ai4 ,explain the rise of fluids in tubes by ,fig4 #bj4 #ej4 ,what is meant by capillary attraction8 ,give familiar examples of the rising of liquids in interstices4 #ea4 ,describe and explain the process of getting out millstones4 ,how does a blotter differ from writing-paper8 ,,chapter ,v4 #ea4 ,what is the attraction of cohesion8 ,give examples of attraction between masses or portions of matter4 #eb4 ,explain the falling of a stone to the ground4 ,illustrate the fact that attraction is mutual4 ,give the illustration of the ship and boat in full4 #ec4 ,illustrate the proportion between the mutual motions of the attracting bodies4 ,give the calculation in regard to the motion of the earth in attracting smaller bodies4 #ed4 ,what is said of the universality of attraction8 ,explain the tides4 ,what is said of the attraction of the moon for the land8 ,what is the difference between the attraction of cohesion and the attraction of gravitation8 ,why is the word gravitation thus used8 ,what is terrestrial gravitation8 #ee4 ,explain ,fig4 #bb4 ,explain ,fig4 #bc4 ,what is said of substances suspended in different parts of the earth8 #ef4 ,explain ,fig4 #bd4 ,what is said of plumb-lines8 #eg4 ,what is weight8 ,give the comparison in regard to muscular force4 ,what is said of scales and weights8 ,what of using springs in weighing8 #eh4 ,what would be the effect on weight if the density of the earth were increased8 ,in what ways would this be perceived8 ,what is said of the variation of weight with distance8 #ei4 ,what is said of the difference of weight on mountains and in valleys8 ,what of weight in the moon8 ,what of it in the sun8 ,what is said of the different modes of attraction8 #fj4 ,show why attraction of cohesion seems to be different from gravitation4 ,show now that it is really not different4 ,what is said of the experiment with the two bullets mentioned in #ff8 ,what of the adhesion of liquids to solid substances8 #fa4 ,what is said of the various results of attraction8 ,explain fully why you can pour water from a pitcher easier than from a tumbler4 #fb4 ,explain the operation of the quick movement by which you prevent water from running down the side of a tumbler in pouring it out4 ,what is said of dropping from a vial8 ,how is the size of drops limited8 ,what is said of the movements of drops on window panes8 #fc4 ,why are the drops of different liquids different in size8 ,give the illustration about chalk4 ,give that about dust4 ,explain ,figs4 #bg and #bh4 #fd4 ,explain ,fig4 #bi4 ,what is said of the difference in size between water and land animals8 #fe4 ,give the illustration in regard to trees4 ,give that in regard to mountains4 ,what is said of the mountains of the moon8 ,what of those of ,jupiter8 ,give the illustrations in #ic of transgression of the principles which have been elucidated4 #ff4 ,what is the difference between the attraction treated of in natural philosophy and chemical attraction8 ,,chapter ,,vi4 #fg4 ,show what we mean by the centre of gravity by ,figs4 #cj1 #ca1 and #cb4 #fh4 ,give the definition of centre of gravity1 and explain it4 ,what is shown by ,fig4 #cc8 #fi4 ,how can we find the centre of gravity of a body8 ,what is said of scales and steelyards8 #gj4 ,state what is represented by ,fig4 #ch4 ,illustrate the fact that the centre of gravity seeks always the lowest point4 #ga4 ,give the illustrations of the rocking-horse1 the swing1 etc4 ,what is said of the ,laggan stones8 ,why does an egg lie on its side8 #gb4 ,give the illustrations from toys in #aja4 ,give the illustrations in #ajb4 #gc4 ,upon what two things does the stability of a body depend8 #gd4 ,what is said of the stability of bodies whose shapes are represented in ,figs4 #dh1 #di1 and #ej8 ,what of that of a round ball8 ,why is the pyramid the firmest of all structures8 #ge4 ,what is the relation of upright position to stability8 ,what is stated of the tower of ,pisa8 #gf4 ,give the familiar illustrations in #aje4 ,what is said of the support of the centre of gravity in animals8 #gg4 ,what is said of the skill exercised in walking8 ,what of the mode of walking in a child8 ,what of the motions of the centre of gravity in walking8 #gh4 ,what is said of the walking of a man with wooden legs8 ,illustrate the management of the centre of gravity in different attitudes4 ,describe and explain the way in which one rises from a chair4 #gi4 ,state and explain the wager case4 ,what is said of unstable equilibrium8 ,give illustrations4 ,,chapter ,,vii4 #hj4 ,what is said of the phenomena treated of in ,hydrostatics8 ,what are the two characteristics of liquids8 ,what makes a liquid have a level surface8 ,give the explanation4 ,give the comparison of the shot4 #ha4 ,what is said of water as a mirror8 ,show that the surface of a liquid is not strictly level4 ,if the earth had no elevations of land why would it have a perfectly globular covering of water8 #hb4 ,what is a so-called perfectly level surface8 ,what is the variation per mile from a real level8 ,describe the spirit-level4 ,give the comparison between a trough and a river4 #hc4 ,what is said of the declivity of rivers8 ,how have some rivers been made8 ,what is stated in regard to the ,river ,danube8 #hd4 ,what is stated about the ,lake of ,geneva8 ,describe the arrangement of canal locks4 #he4 ,how are canals used for working machinery8 ,give various illustrations of the tendency of water to be on a level4 #hf4 ,describe the arrangement represented in ,fig4 #ga1 and give the explanation4 #hg4 ,describe a foolish man's plan for perpetual motion1 and give the reason of its failure4 ,what is said of ancient and modern aqueducts8 #hh4 ,explain the operation of springs and ,artesian wells4 #hi4 ,whence comes the name ,artesian8 ,what is stated of a well in ,paris8 ,what of the situation of ,london8 ,why is the pressure of a liquid in proportion to its depth8 ,give the illustrations of this mentioned in #abb4 #ij4 ,explain ,fig4 #ge4 #ia4 ,what is said about the construction of dams and brewers' vats8 ,explain the lateral pressure of liquids4 ,show the difference between a liquid and a solid in this respect4 #ib4 ,show how the earth's attraction causes the lateral pressure by ,figs4 #gg and #gh4 ,give the view presented in #abd4 #ic4 ,what is said of the proposed ship canal between the ,mediterranean and the ,red ,sea8 ,show that pressure in liquids is equal in all directions4 #id4 ,give the illustrations in #abf4 ,show that the upward pressure in a liquid is as the depth1 and that this is produced by gravitation4 #ie4 ,state the experiment represented in ,fig4 #hb4 ,give the experiment with the tube and ,india-rubber4 #if4 ,state the examples given of great effects produced by small quantities of a fluid4 ,explain these effects by ,fig4 #hc4 #ig4 ,explain ,fig4 #hd4 #ih4 ,what is the ,hydrostatic ,paradox1 and why is it so called8 ,describe and explain the ,hydrostatic ,bellows4 #ii4 ,describe and explain ,bramah's ,hydrostatic ,press4 ,,chapter ,,viii4 #ajj4 ,define specific gravity4 #aja4 ,what is the most obvious way of ascertaining the specific gravities of different liquids8 ,explain the sinking of heavy substances in water4 ,explain the rising of light substances in water4 ,explain what is illustrated in ,fig4 #hg4 #ajb4 ,explain ,fig4 #hh4 ,explain ,fig4 #hi4 #ajc4 ,give the illustrations in #ach3 lifting a stone2 raising a bucket2 and raising the arm in a bath4 ,relate the anecdote of ,archimedes4 ,what is said of boats and life-boats8 ,what of estimating the weight of the load in a canal-boat8 #ajd4 ,what is said of the specific gravity of birds8 ,of insects8 ,of fishes8 ,what of the specific gravity of the human body1 and of the prevention of drowning8 #aje4 ,give the reasons why so many are drowned that might easily be saved8 #ajf4 ,what is stated about children in ,china8 ,why does the body of a drowned person sink8 ,why does it after a while rise8 ,what is said about wading in rivers8 #ajg4 ,explain the manner in which the specific gravity of a solid may be ascertained8 ,give the experiment of weighing water4 ,what is stated of ,archimedes and the crown8 #ajh4 ,describe and explain the hydrometer4 ,relate the anecdote of the ,chinese4 ,what is said of the selling of milk in ,switzerland8 #aji4 ,what is said of the centre of gravity in floating bodies8 ,give the illustrations4 ,,chapter ,,ix4 #aaj4 ,what does pneumatics teach8 ,how can you show that air is material8 ,how that it has weight8 ,what is its weight compared with that of water8 #aaa4 ,what is said of the air's being attracted by the earth8 ,explain why some things rise and others fall in air4 ,how thick is the earth's air-covering8 #aab4 ,how is the height of the atmosphere ascertained8 ,at what rate does the earth move round the sun8 ,how does it carry along the air with it8 ,state the influence which gravitation has upon the density of the air at different heights4 #aac4 ,give the comparison of air to wool4 ,what is said of hydrogen and balloons8 ,in what are gases and liquids alike1 and what are the results of the similarity8 ,what is the amount of pressure of the atmosphere on each square inch of surface8 ,give the calculations in regard to this pressure4 #aad4 ,show why the great pressure of the air does not produce destructive effects4 ,describe the air-pump4 #aae4 ,explain by ,fig4 #ie the plan and working of the air-pump4 #aaf4 ,state some of the experiments with the air-pump4 ,how can you prove that air1 like water1 presses equally in all directions8 ,state the comparison about the fish4 #aag4 ,what is said of the ,magdeburg hemispheres8 ,give the experiment with mercury4 ,explain the operation of the boy's sucker4 #aah4 ,give the statements about sucker-like arrangements in animals4 ,state the experiment of the bladder and weight4 ,give the experiment with the ,india-rubber bag4 #aai4 ,state the experiment with the egg4 ,explain the operation of the hydrostatic balloon4 #abj4 ,explain the operation of the ,cartesian image4 ,what is said of the presence of air in various substances8 #aba4 ,what is said of the elasticity of air8 ,describe and explain the condenser4 #abb4 ,describe and explain the gasometer4 ,show how the air-gun operates4 ,explain the pop-gun4 #abc4 ,explain the operation of gunpowder4 ,explain that of steam4 ,what is said of retardation by condensed air in gunnery8 #abd4 ,describe and explain what is represented in ,fig4 #aac4 ,explain the collection of gases in the pneumatic trough4 #abe4 ,explain the experiment represented in ,fig4 #aae4 ,what is said of tapping a barrel8 ,what causes the gurgling sound when a liquid is poured from a bottle8 ,how high a column of water will the pressure of the atmosphere sustain8 ,how do you find from this the pressure of the air on every square inch of surface8 ,how high a column of mercury will the atmosphere sustain8 #abf4 ,explain the barometer4 ,relate the incident given by ,dr4 ,arnot4 #abg4 ,why would not a water-barometer answer8 ,what is said of the barometer as a measurer of heights8 ,how is the boiling point influenced by the amount of the air's pressure8 ,give the experiment with ether4 #abh4 ,state the experiment with the flask4 ,what would happen to liquids if the atmosphere were removed from the earth8 ,explain the operation of the syphon by ,fig4 #aag4 #abi4 ,explain what is represented by ,fig4 #aah4 #acj4 ,explain the uses of the syphon4 ,explain the operation of the ,cup of ,tantalus4 #aca4 ,how are intermitting springs accounted for8 ,explain the operation of the common pump4 #acb4 ,why does the water rise in the pump8 ,how is sucking done8 ,explain the forcing pump4 #acc4 ,explain the fire-engine4 ,,chapter ,x4 #acd4 ,what is said of the universality of motion8 ,what of attraction as a cause of motion8 ,what of heat8 ,what of chemical agencies8 ,what of life8 #ace4 ,what is meant by saying that action and reaction are equal8 ,state the illustrations of this truth which are given4 ,describe ,barker's mill4 #acf4 ,give the comparisons to the operation of a spring1 of firing of a cannon1 and the throwing of stones from the crater of a volcano4 ,what is said of the jumping of a man from the ground8 #acg4 ,what is said of the reaction in the case of a hopping bird8 ,illustrate the inertia of matter as shown in the communication of motion4 ,give the illustrations of the fact that time is required to communicate motion to bodies4 #ach4 ,give illustrations of inertia as shown in the disposition of motion to continue4 #aci4 ,describe and explain the equestrian feat represented in ,fig4 #abg4 ,what is said of skill in jumping from a moving carriage8 ,relate the case in court which is stated4 #adj4 ,what is said of the course of bodies thrown into the air8 ,what of a man falling from a mast-head8 #ada4 ,what is said of the atmosphere as revolving with the earth8 ,what rapid motions are we subjected to when we speak of ourselves as at rest8 ,why are we insensible to these motions8 #adb4 ,follow out in full the comparison of the steamboat4 ,what is the difference between absolute and relative motion8 ,what is said of absolute rest8 #adc4 ,illustrate the truth that all the motions which are apparent to the eye are slight differences in the common absolute motions4 ,what are the obstacles to motion8 ,how is the motion of a stone thrown upward destroyed8 ,what causes and what opposes its descent8 #add4 ,state and explain the experiment with the lead and feather4 ,explain the operation of the water-hammer4 ,show the relation of bulk to the resistance of liquids and gases4 #ade4 ,illustrate the relation of bulk to the motion of solids1 produced by moving gases and liquids4 #adf4 ,what is said of the opposition of gravitation to water and air in moving solids8 ,what difference does the presence of obstacles make in the relation of force to velocity8 #adg4 ,state the law of the relation of force to velocity1 and illustrate by ,fig4 #acf4 ,what are some of the practical applications of this law8 ,what is said of the relation of shape to velocity8 ,what is said of the shape of fishes8 #adh4 ,what is said of the shapes of boats8 ,what of the management of the webbed feet of water-fowls8 ,what of the wings of birds8 ,what is said of friction as an obstacle to motion8 ,what of it as a cause to motion8 ,illustrate fully in the case of the wheel4 #adi4 ,what is said of the friction of liquids in tubes8 ,what is the effect of sudden turns in pipes8 ,what is the arrangement of arteries in the heads of grazing animals8 ,illustrate the difference of friction in small and large pipes by ,fig4 #aca4 #aej4 ,what is said of the effect of friction in brooks and rivers8 ,in what part of a stream does the water move most rapidly8 ,explain the formation and breaking of the crest of waves rolling over a beach4 ,what is said of the velocity of rivers as affected by friction8 ,explain the formation of waves4 #aea4 ,what is it that really advances in the forward movement of a wave8 ,give the comparison mentioned4 ,what is said of the height of waves8 #aeb4 ,what is momentum8 ,upon what two things does it depend8 ,illustrate this dependence4 ,explain ,fig4 #acc4 #aec4 ,give the illustration of the musket-ball and cannon-ball4 ,give that of the plank4 ,that of the candle4 ,that of the air4 #aed4 ,what is said of the expression1 quantity of motion8 ,under what circumstances may a single impulse produce a great velocity8 ,what examples have we of this8 ,how is it with the motions that we see around us8 ,what is said of the fall of bodies to the earth8 #aee4 ,give examples from muscular action4 ,give that of the arrow4 ,give that of gunpowder4 ,what is said of the arrest of great velocities8 ,give the illustrations in regard to cannon-balls4 #aef4 ,state and explain the feat of the anvil4 ,give examples from common efforts and labors4 #aeg4 ,explain the communication of motion in the case of elastic bodies by ,figs4 #acc and #acd4 ,what is said of the reflection of motion8 #aeh4 ,what is said of the uniformity of motion8 ,what of its uniformity in velocity8 ,state by what means we calculate as to time4 #aei4 ,what is said of the sun-dial8 ,what of the hour-glass8 ,what of ,galileo and the pendulums8 ,explain the operation of the pendulum4 #afj4 ,explain ,fig4 #acg4 ,explain the operation of the gridiron pendulum by ,fig4 #ach4 #afa4 ,what is said of the disposition of motion to be straight8 ,why is motion never straight1 so far as we know8 ,how can we make motion very nearly straight8 ,give the illustration of the bullet in full4 #afb4 ,give the illustration represented in ,fig4 #ada4 ,what is compound motion8 ,illustrate straight compound motion4 #afc4 ,explain ,fig4 #adc4 #afd4 ,explain what is represented ,figs4 #add1 #ade1 and #adf4 #afe4 ,explain ,fig4 #adg4 ,how is curved motion produced8 ,give the illustration of the ball and string4 ,what are centrifugal and centripetal forces8 #aff4 ,what are these two forces in the revolution of the earth around the sun8 ,give various illustrations of the operation of centrifugal force4 #afg4 ,what is said of the formation of bends in rivers8 #afh4 ,show how eddies and whirlpools are formed4 ,how is the centrifugal force used in the art of pottery4 ,how in making window-glass8 #afi4 ,describe and explain the operation of the steam-governor4 #agj4 ,what is said of the agency of the centrifugal force in shaping the earth8 #aga4 ,explain the operation of the apparatus represented in ,fig4 #aed4 ,what are the forces which act on a projectile8 ,what is said of balls thrown horizontally from cannon with different velocities8 #agb4 ,show by ,fig4 #aee why a ball dropped from the mouth of a cannon will fall to the ground in the same time that one fired from it will4 ,by ,what two forces is a falling body acted upon8 #agc4 ,explain ,fig4 #aef4 ,what is the course of a ball dropped from a railway car or from a mast-head8 ,give the comparison between the cannon-ball and the moon4 #agd4 ,what is said of the velocities of the heavenly bodies8 ,,chapter ,,xi4 #agd4 ,what are the ,mechanical ,powers8 ,why is the term power not strictly proper8 #age4 ,explain the terms power1 weight1 and fulcrum4 ,what is said of the use of the lever8 ,what is the lever of the first kind8 ,what is said of its force8 #agf4 ,what is said of scales8 ,what of steelyards8 #agg4 ,give examples of the first kind of lever4 ,show by ,fig4 #aei that there is no gain of power in this lever4 #agh4 ,give the illustration of the see-saw4 ,what is said of ,archimedes's lever8 #agi4 ,state the analogy between this lever and the ,hydrostatic ,bellows1 ,bramah's ,press1 etc4 ,what is the lever of the second kind8 ,apply the rule of equilibrium to it4 ,show how the common wheel-barrow is a lever of this kind4 #ahj4 ,give other examples of the second kind of lever4 ,what is lever of the third kind8 ,how does this differ from the other two kinds8 ,apply the rule of equilibrium to it4 #aha4 ,give examples of the third kind of lever4 ,show how it acts at a mechanical disadvantage in the different examples mentioned4 ,state in full what is said of muscular action4 #ahb4 ,explain by the figures the operation of compound levers4 #ahc4 ,state the comparison between the lever and the wheel and axle4 ,what is said of the common windlass8 #ahd4 ,describe and explain the capstan4 ,what are its chief uses8 ,what is said of the fusee of a watch8 #ahe4 ,describe the arrangement of the fixed pulley4 ,what are its uses8 #ahf4 ,describe the arrangement of the movable pulley4 ,show how the relation of the power to the weight is estimated in the case of compound pulleys4 #ahg4 ,explain the mechanical advantage of the inclined plane4 ,give examples of it4 #ahh4 ,what is said of roads8 ,give the comparison of the wedge to the inclined plane4 ,how is the power of the wedge estimated8 ,give examples of the wedge4 #ahi4 ,what is said of the screw8 ,show by ,fig4 #ahj how the force of the screw is estimated4 ,what are some of the uses of the screw8 #aij4 ,give the estimate of the power of the screw and lever as used together4 ,how can you show that there are really but three mechanical powers8 ,what is said of these as composing tools and machinery8 ,what is said of friction in machinery8 #aia4 ,what is the first advantage of the mechanical powers which is mentioned8 ,give the illustrations4 ,what is the second advantage8 ,give the illustrations4 #aib4 ,what is the third advantage8 ,give examples4 ,how is the velocity of motion in machinery usually varied8 ,what is the fourth advantage8 ,mention examples4 ,describe the instrument called a ,lewis4 #aic4 ,what is said of the title by which ,aristotle distinguished man from other animals8 ,,chapter ,,xii4 #aid4 ,what is sound8 ,what relation has sound to rapidity of vibration8 ,mention cases in which the vibration of sounding bodies is manifest to the sight and touch4 ,what is said of wind instruments8 #aie4 ,state the analogy of a sounding body to a pendulum4 ,describe the process by which the sensation of sound is produced4 ,where does the vibration caused by the sounding body stop in the ear8 ,what is transmitted from thence to the brain8 #aif4 ,give examples of the transmission of sound through various substances8 ,state the experiment by which it is shown that sound is not transmitted through a vacuum4 ,what is said of sound at great heights8 #aig4 ,how far has the sound of a volcano been heard8 ,if the same sound were made in space at that distance from the earth why could not the inhabitants hear it8 ,what is the cause of the noise of bodies passing through the air8 ,why do the heavenly bodies1 moving so rapidly1 produce no sound8 ,cite examples showing the different velocities of sound in different media4 #aih4 ,what is said of the uniformity of the velocity of sound8 ,show how we can measure distances by sound as compared with light in velocity4 ,upon what does the loudness of sound depend8 ,illustrate this point4 #aii4 ,what is said of the diffusion of sound8 ,what of its reflection8 ,what of echoes8 #bjj4 ,what is said of multiplied and mingled reflections of sound8 ,explain the operation of whispering galleries by ,fig4 #ahg4 #bja4 ,explain the operation of the speaking-trumpet4 ,give other examples of the concentration of sonorous vibrations4 #bjb4 ,what is the difference between a musical sound and a noise8 ,what is said of the exact regularity of musical vibrations8 ,how are different notes produced in stringed instruments8 ,upon what does the note depend in wind-instruments8 #bjc4 ,explain the operation of the organ-pipe represented in ,fig4 #aij4 ,what is said of the notes of bells and of musical glasses8 ,explain the mechanism of the human voice4 #bjd4 ,what is harmony8 ,upon what does it depend8 ,between what two notes of the scale is there the greatest harmony8 ,what note next to the octave harmonizes best with the fundamental note8 ,and what note next8 ,show why the second note1 in contrast with the octave1 is so discordant with the fundamental note4 #bje4 ,state the proportions between the numbers of the vibrations in the different notes4 ,if you know the number of vibrations of the fundamental note in a second1 how may you determine the number of vibrations in the other notes8 ,what is said of the number of notes in the diatonic scale8 ,what of the proportionate lengths of strings for different notes8 ,what is said of tuning instruments8 #bjf4 ,what is meant by saying that a note is too sharp or too flat8 ,state in full what is said about the mysteries of sound and hearing4 ,,chapter ,,xii4 #bjg4 ,state the experiment of the three vessels1 and the inference from it4 #bjh4 ,what other facts sustain this inference8 ,how did ,sir ,humphrey ,davy prove that there is heat in ice8 ,what are the two theories of heat8 ,what is the chief source of heat for the earth8 ,what is said of the heat of the sun itself8 #bji4 ,what is said of the universal influence of the heat of the sun in the earth8 ,what of the heat supplied from within the earth itself8 ,what of electricity as a source of heat8 #baj4 ,what is said of chemical action as a source of heat8 ,give examples of the production of heat by mechanical action4 ,what is said of the relations of heat and light8 #baa4 ,show the expansive influence of heat by describing the experiment represented in ,fig4 #aib4 ,give familiar examples of this expansion4 #bab4 ,how can you loosen a stopper stuck fast in a bottle8 ,give the anecdote about the ,persia4 ,give the statement about the building in ,paris4 #bac4 ,what is said of the expansion of liquids by heat8 ,how may the influence of this expansion upon specific gravity be shown8 #bad4 ,what is said of thermometers8 ,what of the invention of the thermometer8 #bae4 ,state the plan of ,fahrenheit's thermometer4 ,give the plans of other thermometers4 #baf4 ,why is ,fahrenheit's thermometer1 on the whole1 the best8 ,what is said of the expansion of gases by heat8 ,state experiments in illustration4 #bag4 ,what is said of balloons8 ,what of the influence of heat on the atmosphere8 ,give examples of this influence4 #bah4 ,why1 in heating apartments1 do we have the heat created or introduced at as low a place as possible8 ,explain the draught of a chimney4 ,why does a stove-pipe generally draw better than a chimney8 #bai4 ,state the experiment with the candle and the door4 ,what is the explanation of the occurrence of wind8 ,explain the land-breeze4 #bbj4 ,explain the sea-breeze4 ,how are winds affected by the rotation of the earth8 #bba4 ,show by ,fig4 #bja why the prevailing winds at the equator are northeast and southeast4 #bbb4 ,mention the melting points of various substances4 ,what is said about the natural state of water and other substances8 ,what are the two modes of changing a liquid into vapor8 #bbc4 ,what is said of the rapidity of evaporation8 ,what of the solution of water in air8 ,what influence has heat upon the capability of air to dissolve water8 ,what phenomena illustrate this8 ,how is it supposed that water rises in air8 ,what fact is in opposition to this supposition8 #bbd4 ,what becomes of the water that rises in the air8 ,what is said of the formation of fog and of clouds8 ,mention the different shapes of clouds and their names4 #bbf4 ,what is said of the influences that give shape to clouds8 #bbh4 ,state how rain is produced1 and explain ,fig4 #bjh4 ,how are snow and hail formed8 ,what is said of vaporization8 #bbi4 ,what influence has pressure upon the formation of vapor8 ,give the experiment with ether in illustration4 ,describe the experiment represented in ,fig4 #bji4 ,what is said of ,papin's digester8 #bcj4 ,what is said of steam8 ,in what consists the power of the steam-engine8 ,how is the expansive force of the steam in the boiler estimated8 ,describe the working of the engine by ,fig4 #baj4 #bca4 ,what is the difference between high and low pressure engines8 ,what is said of the communication of heat8 ,how many modes of communication are there1 and what are they8 ,what is the mode called convection8 #bcb4 ,give examples of convection4 ,what is the conduction of heat8 #bcc4 ,state the experiment represented in ,fig4 #baa4 ,state the experiment represented in ,fig4 #bab4 ,what is said of non-conductors of heat8 ,give the examples cited4 #bcd4 ,explain ,davy's safety-lamp4 #bce4 ,give what is stated in the note about ,stephenson and ,davy4 ,what is said of the influence of density on the conduction of heat8 ,give the illustration about melting snow4 #bcf4 ,state the experiments which show that liquids are poor conductors of heat4 ,what is said of air as a non-conductor of heat8 ,what is said of double windows8 #bcg4 ,what is said of arrangements of the walls of buildings8 ,what of an arrangements for preventing the spreading of fires in blocks8 #bch4 ,how are animals in very cold regions protected from the cold8 ,what is it in their coverings that affords the protection8 ,what is said of the coverings of quadrupeds that are natives of warm climates8 ,what of the elephants whose remains are found in ,siberia8 #bci4 ,what changes take place in the coverings of animals carried from a cold to a warm climate1 and the reverse8 ,why has man no covering against the cold8 ,explain the object of clothing4 ,what is said of articles of clothing8 ,what of loose clothing8 ,what of coatings of straw put on trees8 ,what of bricks compared with stones8 #bdj4 ,what is said of cocoons8 ,what of buds of plants in winter8 ,what of snow as a protection of plants8 #bda4 ,state the arrangement of snow observed in the arctic regions4 ,state in full what is said of the influence of the conduction of heat upon sensation4 #bdb4 ,what is meant by the radiation of heat8 ,give examples of it4 ,what is said of the connection of heat and light in the rays of the sun8 ,what is said of heat from a common fire8 #bdc4 ,what is said of the relation between absorption and radiation8 ,what of the reflection of heat8 ,state the experiment with the mirrors and the thermometer and flask4 #bdd4 ,explain the experiment with the ice4 ,give the experiment with phosphorus4 ,give the experiment represented in ,fig4 #bah4 #bde4 ,explain the formation of dew4 ,state the analogy of the tumbler4 ,what is said of the circumstances that influence the deposition of dew and frost8 #bdf4 ,what is said of different substances in regard to the deposition of dew8 ,what about ,gideon's fleece8 ,what is the dew-point8 ,how can you ascertain in it8 #bdg4 ,what is said of the freezing of mercury8 ,explain the difference between sensible and latent heat4 ,what is said of capacity for heat8 ,state the experiment represented in ,fig4 #bai4 #bdh4 ,what is the relation of heat to density8 ,give the illustrations4 #bdi4 ,what is the reason that the air is so cold on great heights8 ,what is the relation of heat to the forms of substances8 ,what is said of the melting of ice8 ,what of the vaporization of water8 ,state the general conclusion in regard to latent and sensible heat4 #bej4 ,state in full what is said of latent heat in reference to clouds4 ,explain the operation of freezing mixtures4 #bea4 ,state the examples of the production of cold by evaporation4 #beb4 ,state and explain the experiment represented in ,fig4 #bba4 #bec4 ,give the facts stated in regard to the degree of heat which man can endure4 ,give the reasons why the heat did not produce a greater effect in these cases4 #bed4 ,what effect does heat produce upon the bulk of substances8 ,what is said of water as an exception8 ,describe the process of freezing as illustrated by the diagram4 #bef4 ,what would be the process if the exception did not exist8 ,state what would be the results4 #beg4 ,what would be the consequence if the freezing point were above #cb8 ,what if it were below8 ,what is said of the force of expansion in ice8 ,what are some of the benefits which come from this expansion8 ,,chapter ,,xiv4 #beh4 ,what is ,newton's theory of light8 ,what is the undulatory theory8 ,state the analogies to sound and heat4 ,when is a body luminous8 ,what are the sources of light8 #bei4 ,how may you see that light moves in straight lines8 ,state various familiar recognitions of this fact4 ,illustrate the fact that the intensity of light is inversely as the square of the distance4 #bfj4 ,what is said of the velocity of light in regard to ordinary distances8 ,how long is light coming from the sun to the earth8 ,what is said of the light coming to us from certain stars8 #bfa4 ,give the observation of ,roemer represented in ,fig4 #bbf4 #bfb4 ,what is said of the reflection of light8 ,what of its reflection in relation to seeing8 ,what of the images formed in mirrors8 #bfc4 ,show by ,fig4 #bbh why the image in a mirror seems to be as the same distance behind it that the object is before it4 ,explain by ,fig4 #bbi the operation of the kaleidoscope4 #bfd4 ,explain the operation of a concave mirror by ,fig4 #bcj4 ,explain that of a convex mirror by ,fig4 #bca4 #bfe4 ,what is meant by the refraction of light8 ,illustrate its reflection in passing from a denser into a rarer medium4 ,then from a rarer into a denser4 #bff4 ,how is the refraction in regard to a perpendicular in the two cases8 ,explain dawn and twilight4 ,explain what is represented in ,fig4 #bcd4 #bfg4 ,what are mirages8 ,describe the mirage which occurred at ,ramsgate4 ,describe that seen by ,captain ,scoresby4 ,relate the incident which occurred at ,new ,haven4 #bfh4 ,what is said of mirages in deserts8 #bfi4 ,describe the mirage of the ,french coast4 ,explain what is meant by the visual angle as illustrated by ,fig4 #bcf4 #bgj4 ,explain ,fig4 #bcg4 ,what are lenses8 ,what are the different kinds8 #bga4 ,what is the difference of effect in convex and concave lenses8 ,explain the effect of a convex lens on the visual angle4 ,what is said of microscopes and telescopes8 #bgb4 ,describe and explain the magic lantern4 ,describe and explain the camera obscura4 #bgc4 ,describe the arrangement of a camera for sketching4 ,how is the eye like a camera8 #bgd4 ,describe the arrangement of the parts of the eye as mapped in ,fig4 #bdd4 #bge4 ,show now particularly how the eye is like a camera4 ,what is said of the influence of the cornea on the light8 ,show what is required for distinct vision1 as illustrated in ,fig4 #bde4 ,show why it is that objects brought very near the eye are not seen distinctly4 #bgf4 ,what is said of the microscope8 ,explain the difficulty in the near-sighted4 ,in the far-sighted4 #bgg4 ,how can you show that the images of objects in the retina are inverted8 ,give in full what is said of explanations of the fact that we see objects erect notwithstanding this inversion4 ,explain single vision4 #bgh4 ,by what simple experiment can you show the explanation of single vision to be correct8 ,what is said of squinting8 ,explain the stereoscope4 #bgi4 ,what is said of distinct impressions on the retina8 ,explain the thaumatrope4 #bhj4 ,state in full what is said of the compound nature of light4 ,give the proportions of the colors in it4 ,what is said about there being only three colors8 #bha4 ,what is said of the recomposition of decomposed light8 ,give the illustration of the powder--the circular board--the top4 #bhb4 ,what is said of the colors of substances8 ,what of the variations of these colors in different lights8 ,what of variations with varying positions8 ,what of the colors of clouds8 #bhc4 ,explain the formation of the first rainbow by ,fig4 #bec4 ,explain the formation of the second bow by ,fig4 #bed4 #bhd4 ,what is said of the circumstances under which rainbows are seen8 #bhe4 ,explain in full the formation of the two bows as illustrated by ,fig4 #bee4 ,what is said of the bow as seen by different persons1 and at different moments by the same person8 ,what of rainbow hues in dew-drops and ice-crystals8 #bhf4 ,give the dissection of light as represented in ,fig4 #bef4 ,what is said of ,daguerreotyping8 ,,chapter ,,xv4 #bhg4 ,what is the origin of the term electricity8 ,what is said of attraction and repulsion in electricity8 #bhh4 ,what is the supposed explanation of electrical repulsion8 ,explain the difference between resinous and vitreous electricity4 ,what is said of the two supposed electrical fluids8 ,detail the illustrations of attraction which are stated4 #bhi4 ,state the theory of ,franklin4 ,explain the use of the terms positive and negative4 ,illustrate the fact that the kind of electricity excited depends on what a substance is rubbed with4 #bij4 ,what is said of the incorrectness of the terms vitreous and resinous8 ,what is said of conductors and non-conductors8 ,why are non-conductors called insulators8 #bia4 ,what marked difference is there between heat and electricity8 ,state the experiment represented in ,fig4 #beh4 ,what is said of electrics and non-electrics8 #bib4 ,what is said of equilibrium in electricity1 and of its disturbance8 ,give in full what is said of the universality of electricity4 #bic4 ,state what is said of induction1 as illustrated by the experiment represented in ,fig4 #bei4 #bid4 ,state the experiment represented in ,fig4 #bfj4 ,describe the arrangement and operation of the electrical machine represented in ,fig4 #bfa4 #bie4 ,describe the cylinder machine4 ,state the experiment with the pith balls4 ,state that with the head of hair4 #bif4 ,state the experiment with the tissue-paper4 ,state that with the dancing figures4 ,state that with the bells4 #big4 ,describe the experiment with the tin-foil4 ,describe the insulating stool and the operation of it4 #bih4 ,what is said of the escape of electricity from points8 ,describe the apparatus represented in ,fig4 #bfi1 and the operation of it4 ,what is said of the discharge of electricity from a point in a dark room8 ,describe the ,leyden jar4 #bii4 ,explain the operation of the ,leyden jar4 ,what would be the effect of connecting the inside foil with the outer by a strip of foil8 ,give in full the experiment represented in ,fig4 #bgb1 and the explanation4 #cjj4 ,what is said of the discharge of the jar8 ,how can a large number of persons take a shock from it together8 #cja4 ,explain the effect of moisture upon the charged jar4 ,state the experiment represented in ,fig4 #bgd4 #cjb4 ,what is the electrical battery8 ,what is said of the light produced by electricity8 #cjc4 ,to what is the report of electricity owing8 ,what is said of mechanical injuries caused by electricity8 ,what of the heat caused by it8 ,what effects may be produced by this heat8 #cjd4 ,what was the discovery of ,franklin1 and how did he make it8 #cje4 ,relate the accident which occurred at ,st4 ,petersburg4 ,what is said of lightning-rods8 #cjf4 ,what was ,sulzer's experiment8 ,what were ,galvani's observations8 #cjg4 ,what is said of the pile of ,volta8 ,what of his cup battery and of other batteries8 #cjh4 ,what difference is there between frictional and voltaic electricity8 ,,chapter ,,xvi4 #cjh4 ,what are loadstones8 ,where do they abound8 ,what is said of discoveries in magnetism8 ,whence come the terms magnetism and magnet8 #cji4 ,what is said of the attraction of magnetism8 ,what law is there in regard to it8 ,what is said of the poles of a magnet8 ,what of magnetism by induction8 #caj4 ,what is said of attraction and repulsion in magnets8 ,explain the formation of the curves of iron filings in the experiment represented in ,fig4 #bhb4 #caa4 ,how may artificial magnets be made8 ,what is said of the horseshoe magnet and its armature8 #cab4 ,what is said of the magnetic needle and the mariner's compass8 #cac4 ,what is the declination of the needle8 ,when was it first observed8 ,what is said of observations after this8 ,what is said of the dip of the needle8 #cad4 ,what is said of the earth as a magnet8 ,what of it as a magnetizer8 #cae4 ,what is said of fixing magnetism8 ,what of impairing it8 ,in what other substances besides iron does magnetism exist8 ,in what is magnetism like electricity8 ,in what is it unlike it8 #caf4 ,what relation has magnetism to electricity8 ,what were the original observations of ,oersted in regard to it8 #cag4 ,describe the manner of making the most powerful electro-magnets4 ,describe the experiment represented in ,fig4 #bhg4 #cah4 ,show the application of electro-magnetism in the electric telegraph4 #cai4 ,what is the contrivance called the signal key8 ,how is the alphabet of ,morse's ,telegraph constructed8 #cbj4 ,what is said of the communication through the earth in telegraphing8 ,,index4 ,the numbers refer to the paragraphs ,action and reaction equal1 #ahb ,air1 compressibility of1 #aeb ,air1 elasticity of1 #afa ,air1 density of1 dependent on pressure1 #aeh ,air1 pressure of1 as affecting the boiling point1 #aga1 #cbg ,air1 currents in1 from heat1 #bha ,air1 solution of water in1 #bhg ,air attracted by the earth1 #adh ,air in interstices of substances1 #afj ,air a non-conductor of heat1 #cjb1 #cjd ,air-pump1 #aee ,air-guns1 #afd ,animals1 coverings suited to climate1 #cje ,archimedes1 his lever1 #bbi ,archimedes1 his discovery in the bath1 #ach ,archimedes and the crown1 #adc ,artesian wells1 #abj ,aqueducts1 #aai ,atmosphere1 pressure of1 #aed1 #afh ,atmosphere1 how it moves with the earth1 #ahh ,atomic theory1 #ae ,attraction1 nature of1 #ec ,attraction1 capillary1 #gb ,attraction between masses1 #gc ,attraction between solids and liquids1 #fi ,attraction1 ,newton's idea of1 #ed ,attraction1 universality of1 #gg ,attraction1 variety in results of1 #hg ,attraction1 opposition between modes of1 #hh ,attraction1 proportion of the mutual motions of1 #gf ,balloons1 #adi1 #bhj ,balloon1 hydrostatic1 #aei ,barometer1 #afi ,barker's mill1 #ahb ,battery1 electrical1 #chg ,berkeley1 his ideas of matter1 #b ,boats1 why they float1 #aci ,boiling point as affected by the pressure of the air1 #aga ,brittleness1 #bi ,buds in winter1 #cjh ,bulk1 relation of1 to resistance of liquids and gases1 #aic ,camera obscura1 #ceb ,canals1 #aag ,capillary attraction1 #gb1 #hf ,cartesian image1 #aei ,centre of gravity defined1 #if ,centre of gravity1 support of1 in animals1 #ajf ,centre of gravity1 movements of1 in walking1 #ajg ,centre of gravity in relation to attitudes1 #ajh ,centre of gravity in floating bodies1 #ade ,centrifugal force1 #bac ,centripetal force1 #bac ,chimneys1 draught of1 #bhb ,clothing1 object of1 #cjf ,clouds1 how formed1 #bhh ,clouds1 colors of1 #cfd ,clouds1 shapes of1 #bhi ,clouds1 latent heat in1 #cbd ,cocoons1 constructed for winter1 #cjg ,colors of objects1 #cfc ,colors of the clouds1 #cfd ,cohesion1 strength of1 #fh ,cohesion1 what is essential to it1 #ff ,cold1 absence of heat1 #bgj ,cold at great heights1 cause of1 #cbb ,cold from evaporation1 #cbf ,compressibility of substances1 #ce ,compressibility of air1 #aeb ,condenser1 #afb ,conductors of heat1 #bih ,conductors of electricity1 #cge ,conduction of heat1 #big ,conduction1 relation of density to1 #cjj ,conduction in liquids1 #cja ,conduction1 influence of1 on sensation1 #caj ,convection of heat1 #bif ,coverings of animals suited to climate1 #cje ,crystallization1 #fb ,daguerreotype1 #cfi ,dams1 #abb ,davy's safety-lamp1 #bii ,dawn explained1 #cdf ,density1 #bd ,dew1 formation of1 #cae ,dew-drops1 colors in1 #cfg ,dew-point1 #cag ,diatonic scale1 #bfg ,draught of chimneys1 #bhb ,drowning1 how it can often be prevented1 #ada ,earth1 globular form of1 #fa ,earth1 roundness of1 shown1 #aac ,earth1 attraction toward centre of1 #hj ,earth1 thickness of its air-covering1 #aej ,earth1 shape of1 influenced by centrifugal force1 #bah ,earth1 rotation of1 affecting winds1 #bhd ,elasticity1 #ci ,elasticity1 definition of1 #db ,electricity1 origin of the term1 #cgj ,electricity1 attraction and repulsion in1 #cga ,electricity1 vitreous and resinous1 #cgb ,electricity1 galvanic1 #cid ,electricity1 universality of1 #cgh ,electricity1 discharge of1 from points1 #chc ,electricity1 sound of1 #chi ,electricity1 heat produced by1 #cia ,electricity1 mechanical injuries by1 #cij ,electrics and non-electrics1 #cgg ,electrical battery1 #chg ,electro-magnetism1 #dac ,electro-magnets1 #dad ,electric telegraph1 #dae ,equestrian feat1 #ahe ,equilibrium1 stable and unstable1 #aji ,evaporation1 #bhf ,evaporation1 cold produced by1 #cbf ,expansion by heat1 #bgd ,expansion1 exception to it1 #ccj1 #cca ,extension1 a property of matter1 #dd ,eye1 description of1 #cec ,eye1 images in1 inverted1 #cef ,feathers1 how a non-conductor of heat1 #cje ,fire-engine1 #agi ,flexibility1 #bi ,flies1 feet of1 #aeg ,force1 relation of1 to velocity1 #aid ,forcing-pumps1 #agh ,franklin1 his theory of electricity1 #cgc ,franklin1 his discovery1 #cib ,freezing1 process of1 described1 #ccj ,freezing point1 why at #cb1 #ccb ,freezing in the midst of boiling1 #cbg ,freezing mercury1 #cah ,freezing mixtures1 #cbe ,friction1 #aif-aih ,friction in machinery1 #bde ,frost1 #dd ,fur1 how a non-conductor of heat1 #cje ,fur1 how altered by climate1 #cje ,fusee of a watch1 #bch ,galvanism1 #cid ,gases1 movements of the particles in1 #aj ,gases1 space in1 #ba ,gases1 compressibility of1 #ch ,gasometer1 #afc ,glass1 annealing of1 #cb ,glass-making1 centrifugal force used in1 #baf ,governor1 steam1 #bag ,gravitation1 #gi1 #hf ,gravitation1 limiting size1 #ij ,gravitation1 action of1 on solids in a liquid1 #acf ,gravity1 specific1 defined1 #ace ,gridiron pendulum1 #baj ,gunnery1 retardation by condensed air in1 #aff ,hail1 how produced1 #bij ,hardness1 #bh ,harmony1 #bff ,hearing1 mysteries of1 #bfi ,hearing1 trumpet1 #bfb ,heat1 nature of1 #bga ,heat1 sources of1 #bgb ,heat1 relations of1 to forms of matter1 #ac ,heat1 relation of1 to bulk of liquids1 #cg ,heat1 relation of1 to light1 #bgc ,heat producing expansion1 #bgd1 #bge1 #bgi ,heat1 communication of1 #bie ,heat1 convection of1 #bif ,heat1 conduction of1 #big ,heat1 radiation of1 #caa ,heat1 reflection of1 #cad ,heat1 connection of1 with light1 #cab ,heat1 production of1 by electricity1 #cia ,heat1 degree of1 endurable by man1 #cbh ,heat1 latent1 #cai ,heat1 capacity of different substances for1 #cbj ,heat1 relation of1 to density1 #cba ,heat1 relation of1 to forms of substances1 #cbc ,heat and cold1 #bgj ,heights measured by barometer1 #agj ,hume1 his ideas of matter1 #c ,hydrometer1 #add ,hydrostatics1 what it teaches1 #aaj ,hydrostatic balloon1 #aei ,hydrostatic bellows1 #acb ,hydrostatic paradox1 #aca ,hydrostatic press1 #acc ,ice1 formation of1 #cbi ,ice1 force of its expansion1 #ccc ,ice-crystals1 colors in1 #cfg ,icebergs1 centre of gravity in1 #ade ,impenetrability1 #de ,imponderable agents1 #af ,inclined plane1 #bda ,induction in electricity1 #cgi ,induction in magnetism1 #djj ,inertia1 #dh ,inertia shown in the communication of motion1 #ahc ,inertia shown in disposition of motion to continue1 #ahd ,insulating stool1 #chb ,jupiter1 mountains in1 #ib ,kaleidoscope1 #cdc ,laggan stones1 #ajj ,land-breeze1 #bhc ,lenses1 #cdi ,lever of first kind1 #bbd ,lever1 no gain of power in it1 #bbg ,lever of second kind1 #bca ,lever of third kind1 #bcb ,leyden jar1 #chd1 #che ,life-boats1 #aci ,light1 nature of1 #ccd ,light1 sources of1 #cce ,light1 diffusion of1 #ccg ,light1 velocity of1 #cch ,light1 reflection of1 #cdj ,light1 refraction of1 #cde ,light moves in straight lines1 #ccf ,light shown to be compound1 #cfj ,light1 colors in1 #cfj ,light1 recomposition of1 #cfb ,light1 chemistry of1 #cfi ,lightning-rods1 #cic ,liquids1 movability of their particles1 #i1 #eg ,liquids1 incompressibility of1 #cf ,liquids1 attraction in1 #ef ,liquids1 globular shape of drops of1 #eh ,liquids1 cause of level surface of1 #aaa ,liquids1 pressure of1 in proportion to depth1 #aba ,liquids1 lateral pressure of1 #abc ,liquids1 pressure of1 equal in all directions1 #abe ,liquids1 upward pressure as depth1 #abg ,liquids1 in what like gases1 #aec ,liquids1 friction of1 in tubes1 #aig ,liquids1 friction of1 in streams1 #aih ,liquids1 expansion of1 by heat1 #bge ,loadstones1 #cig ,machines not sources of power1 #bbb ,machinery1 friction in1 #bde ,magic lantern1 #cea ,magdeburg hemispheres1 #aef ,magnetism1 origin of term1 #cig ,magnetism1 attraction of1 #cih ,magnetism1 repulsion of1 #dja ,magnet1 poles of1 #cii ,magnets1 artificial1 #djc ,magnets1 horseshoe1 #djd ,magnetic needle1 #dje ,malleability1 #bh ,man a tool-making animal1 #bdg ,mariner's compass1 #dje ,matter distinguished from spirit1 #a ,matter1 effects of1 on senses1 #f ,matter1 forms of1 #g ,matter1 nature of1 #ad ,matter1 constitution of1 #ad ,matter1 variety in properties of1 #ag ,matter1 divisibility of1 #ah1 #ea ,matter1 minute division of1 #ai ,matter1 pores and spaces in1 #bb ,matter1 relation of heat to spaces of1 #bc ,matter1 inertia of1 #dh ,matter1 impenetrability of1 #de ,matter equally inclined to rest and motion1 #ej ,mechanical powers1 real advantages of1 #bdf ,mercury1 freezing of1 #cah ,microscopes1 #cej ,microscopical animals1 #ai ,mirages1 #cdg ,mirrors1 #cdb ,mirrors1 curved1 #cdd ,moth's wing1 #ai ,motion1 causes of1 #aha ,motion1 universality of1 #ahj ,motion1 absolute and relative1 #aia ,motion1 obstacles to1 #aib ,motion1 reflection of1 #bjf ,motion1 compound1 #bab ,motion1 curved1 #bac ,motion in orbits1 #bba ,motion disposed to be straight1 #baa ,motion1 that of falling bodies projectile1 #bbj ,motion and rest1 #ahi ,momentum1 #bja ,mountains1 size limited1 #ib ,nature1 order in1 #fe ,near-sightedness1 #cee ,needle1 magnetic1 #cii ,needle1 declination of1 #djf ,needle1 dip of1 #djg ,noise1 how it differs from musical sound1 #bfc ,non-conductors of heat1 #bih ,non-conductors of electricity1 #cge ,notes1 musical1 how produced1 #bfd ,odors1 minuteness of their particles1 #ai ,order in nature1 #fe ,papin's digester1 #bib ,pendulum1 #bjh-baj ,perpetual motion attempted1 #aah ,persia1 accident to1 #bgd ,pisa1 tower of1 #ajd ,pitchers1 why they have lips1 #hi ,pneumatics1 what it teaches1 #adf ,pneumatic trough1 #afg ,pop-guns1 #afd ,pottery1 centrifugal force used in1 #baf ,powder1 explosive force of1 #afe ,prince ,rupert's drops1 #cc ,projectiles1 #bai1 #bbj1 #bba ,pulleys1 #bci ,pumps1 #agf ,pumps1 forcing1 #agh ,radiation of heat1 #caa ,radiation1 its relation to absorption1 #cac ,rain1 how it is caused1 #bij ,rainbow1 #cfe1 #cff ,rarity1 #bd ,reaction equal to action1 #ahb ,reflection of heat1 #cad ,reflection of light1 #cdj ,reflection of motion1 #bjf ,reflection of sound1 #bfj ,remora1 #aeg ,rest1 merely relative1 #ahi ,rivers1 #aae1 #aaf ,rivers1 bends in1 #bae ,roemer1 observations of1 in regard to the velocity of light1 #cci ,rope-dancers1 skill of1 in managing centre of gravity1 #aji ,safety-lamp1 #bii ,scales1 #ih1 #bbe ,screw1 #bdc ,sea-breeze1 #bhc ,see-saw1 a lever1 #bbh ,sensation1 influence of the conduction of heat on1 #caj ,shape1 relation of1 to velocity1 #aie ,shot1 manufacture of1 #fj ,signal key1 #dae ,silk-worm1 thread of1 #ai ,size limited by gravity1 #ij ,sluice-gates1 #abb ,snow1 how produced1 #bij ,snow1 crystals of1 #fd ,snow a protection to plants1 #cji ,solids1 constitution of1 #h ,solids1 attraction in1 #ci ,solution1 #ab1 #bb ,sound1 what it is1 #bdh ,sound1 sensation of1 how produced1 #beb ,sound1 velocity of1 #bef ,sound1 transmission of1 #bec ,sound1 loudness of1 #beh ,sound1 diffusion of1 #bei ,sound1 reflection of1 #bfj ,sound1 concentration of1 #bfb ,sound1 none produced by the heavenly bodies1 #bee ,speaking-trumpet1 #bfb ,specific gravity defined1 #ace ,specific gravity of animals1 #adj ,specific gravity of the human body1 #ada ,specific gravity of solids1 how ascertained1 #adb ,specific gravity of liquids1 how ascertained1 #add ,spectrum1 #cfj ,spider1 web of1 #ai ,spirit distinguished from matter1 #a ,spirit1 origin of word1 #d ,spirit-level1 #aad ,springs1 #abj ,stability of bodies1 #ajc-aje ,steam transparent and invisible1 #bic ,steam1 expansive force of1 #afe ,steam-engine1 #bid ,steel1 flexible and brittle1 #cj ,steel1 tempering of1 #ca ,steelyards1 #ih1 #bbe ,stereoscope1 #ceh ,suckers1 #aeg ,sucking1 explanation of1 #afg ,suction1 #agg ,sulzer's experiment1 #cid ,sun as a source of heat1 #bgb ,surface1 relation of1 to movability1 #aic ,syphon1 #agb ,tantalus1 cup of1 #agd ,telegraph1 #dae ,telescopes1 #cej ,tenacity1 #be ,tenacity1 comparative1 of substances1 #bf ,tenacious substances1 value of1 #bg ,thermometer1 #cg1 #bgf ,thermometer1 ,fahrenheit's1 #bgg ,tides1 #gh ,tubes1 friction of liquids in1 #aig ,twilight explained1 #cdf ,unison1 #bfh ,up and down1 explanation of1 #ha ,vapor1 influence of pressure upon the formation of1 #bib ,vaporization1 #bia ,velocity1 relation of1 to force1 #aid ,velocity1 relation of1 to shape1 #aie ,velocities1 great1 how produced1 #bjc ,velocities1 great1 how arrested1 #bjd ,vibration of sounding bodies1 #bdi ,vision1 distinct1 what necessary to1 #ced ,vision1 why single1 #ceg ,vision1 why erect1 #cef ,visual angle1 #cdh ,voice1 how produced1 #bfe ,voltaic electricity1 #cid ,volta's pile1 #cie ,walrus1 feet of1 #aeg ,water1 crystallization of1 #fc ,water man's first mirror1 #aab ,waves1 how formed1 #aii ,waves1 height of1 #bjj ,wedge1 #bdb ,weight1 #eb1 #hb-he ,wheel and ,axle1 #bce ,wheel-barrow a lever1 #bca ,whispering galleries1 #bfa ,windlass1 #bcf ,windows1 double1 #cjc ,winds1 #bhc ,winds as affected by the rotation of the earth1 #bhd ,,the ,,end4 ,,footnotes3 #a ,i take the following from ,dr4 ,arnot3 ,the reasons that so many people are drowned in ordinary cases1 who might easily he saved1 are the following3 #a4 ,their believing that continued exertion is necessary to keep the body from sinking1 and hence their generally assuming the position of a swimmer1 in which the face is downward1 and the whole head must be kept out of the water to allow of breathing4 ,now as a man can not retain this position without continued exertion1 he is soon exhausted1 even if a swimmer1 and if not1 the unskillful attempt will scarcely secure for him even a few respirations4 ,the body raised for a moment by exertion above the natural level1 sinks as far below when the exertion ceases2 and the plunge1 by appearing the commencement of a permanent sinking1 terrifies the unpracticed individual1 and renders him an easier victim to his fate4 #b4 ,from a fear that water entering by the ears may drown as if it entered by the nose or mouth1 a wasteful exertion is made to prevent it2 the truth being1 however1 that it can only fill the outer ear1 or as far as the membrane of the drum1 and is therefore of no consequence4 #c4 ,persons unaccustomed to the water1 and in danger of being drowned1 generally attempt in their struggle to keep their hands above the surface1 from feeling as if their hands were tied while held below2 but this act is most hurtful1 because any part of the body kept out of the water1 in addition to the face1 which must be so1 requires an effort to support it which the individual is supposed at the time incompetent to afford4 #d4 ,not having reflected that when a log of wood or a human body is floating upright1 with only a small portion above the surface1 in rough water1 as at sea1 every wave in passing must cover the head for a little time1 but will again leave it projecting in the interval4 ,the practiced swimmer chooses this interval for breathing4 #e4 ,not knowing the importance of keeping the chest as full of air as possible2 the doing which has nearly the same effect as tying a bladder of air to the neck1 and without other effort will cause nearly the whole head to remain above the water4 ,if the chest be once emptied1 and if from the face being under water the person can not inhale again1 the body is then specifically heavier than water1 and will sink4 #b ,this is true except when the tube is so small that capillary attraction exerts considerable influence4 #c ,i was once consulted in regard to a smoking stove4 ,it was an open ,franklin stove1 the pipe of which went through a fire-board into a monstrous chimney4 ,i recommended that a pipe with a knee should extend from the pipe of the stove a little way up the chimney4 ,the expedient was successful1 because but a small body of air1 that in the pipe1 needed to be heated to establish an upward current4 #d ,as in the case of many other inventions1 so here the same idea was originated and put to practical use by more minds than one4 ,george ,stephenson1 who from being a common engine-wright in a colliery rose step by step till he invented the locomotive1 constructed a lamp which illustrated in another way the same principle as the lamp of ,davy does--in other words1 he invented another safety-lamp4 ,but this does not in the least detract from the glory which the invention has given to the name of ,davy1 for each acted independently of the other4 ,in ,davy's case1 it is to be remarked1 there was a long course of scientific reasoning and investigation which led him at length to the invention1 the record of which is exceedingly interesting4 ,no invention or discovery is made without thought1 though accident may suggest the thought2 but here is an invention which1 without any suggestion by accident1 was evolved by laborious and long-continued thought1 proceeding step by step to its conclusion4 #e ,i will mention here a contrivance that ,i once adopted for a small conservatory1 which ,i wished to keep warm from the heat of a room to which it was adjoining4 ,in each space of the window-frames were put two panes of glass1 there being nearly half an inch of space between them4 ,in this way ,i secured all the benefit of double windows with less expense and a less cumbrous arrangement4 ,in mentioning this contrivance now and then1 casually1 ,i have found that a few others have thought of it1 and adopted it with the same success that ,i did4 #f ,we are in entire ignorance of the nature of electricity1 and we use the term fluid merely as a matter of convenience4 ,,transcriber',s ,,note ,italic text is denoted by underscores4 ,bold text is denoted by equal signs4 ,obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources4 ,except for those changes noted below1 all misspellings in the text1 and inconsistent or archaic usage1 have been retained4 ,for example3 farther1 further2 ,india rubber1 ,india-rubber2 every where2 catechetical2 incloses2 tricksy2 pervious2 motal2 enrobes2 subtile4 ,pg #gi1 'gravity made made him' replaced by 'gravity made him'4 ,pg #aeg1 'if non elastic' replaced by 'if non-elastic'4 ,pg #bfd1 'the divergency when' replaced by 'the divergence when'4 ,pg #big1 '#bhc4 ,electricity' replaced by '#chc4 ,electricity'4 ,pg #cba1 the ,questions begin with number '#ac' and has not been changed4 ,pg #cca1 the number '#agd' is used twice and has not been changed4 ,pg #cce1 'with phosporus' replaced by 'with phosphorus'4