,transcriber's ,notes3 ,the spelling1 punctuation and hyphenation are as the original1 with the exception of apparent typographical errors which have been corrected4 ,italic text is denoted thus4 ,bold text is denoted thus4 ,illustration3 ,,the ,,main ,,post-,,office1 ,,philadelphia4 ,,the ,,pneumatic ,,despatch ,,tube ,,system ,,of ,,the ,,batcheller ,,pneumatic ,,tube ,,co4 ,,also ,,facts ,,and ,,general ,,information ,,relating ,,to ,,pneumatic ,,despatch ,,tubes ,,by ,b4 ,c4 ,,batcheller1 ,b4,sc4 ,,mechanical ,,engineer ,illustration ,,philadelphia ,,press ,,of ,j4 ,b4 ,,lippincott ,,company #ahig ,,copyright1 #ahif1 ,,by ,b4 ,c4 ,,batcheller4 ,,contents4 ,,chapter ,i4 ,a ,,brief ,,historical ,,sketch4 ,,page ,,early ,,records #i ,,practical ,,beginning ,,of ,,the ,,art--,,the ,,london ,,pneumatic ,,telegraph #aj ,,the ,,siemens ,,circuit ,,system #ad ,,recent ,,improvements ,,in ,,the ,,london ,,system #af ,,an ,,underground ,,pneumatic ,,railway ,,for ,,transportation ,,of ,,mail #ai ,,the ,,berlin ,,pneumatic ,,telegraph #bj ,,the ,,paris ,,pneumatic ,,telegraph #bb ,,the ,,pneumatic ,,telegraph ,,of ,,other ,,cities #be ,,pneumatic ,,tubes ,,in ,,america #be ,,chapter ,,ii4 ,,the ,,pneumatic ,,transit ,,company ,,and ,,the ,,first ,,pneumatic ,,tubes ,,for ,,the ,,transportation ,,of ,,united ,,states ,,mail4 ,,organization #bh ,,aim ,,and ,,object ,,of ,,the ,,company #bh ,,the ,,clay-,,lieb ,,patents #cj ,,franchises ,,and ,,first ,,government ,,contract #cc ,,search ,,for ,,tubes #cd ,,method ,,of ,,manufacturing ,,tubes #ce ,,laying ,,and ,,opening ,,the ,,tubes ,,for ,,traffic #cg ,,description ,,of ,,the ,,tubes1 ,,method ,,of ,,laying1 ,,etc4 #ch ,,the ,,air-,,compressor--,,method ,,of ,,circulating ,,the ,,air #dj ,,terminal ,,apparatus #db ,,the ,,sender #dc ,,sub-,,post-,,office ,,receiver #dd ,,main ,,post-,,office ,,receiver #dg ,,the ,,carrier #ej ,,operation ,,of ,,the ,,tubes #eb ,,benefits ,,of ,,the ,,system #ed ,,chapter ,,iii4 ,,the ,,system ,,and ,,apparatus ,,of ,,the ,,batcheller ,,pneumatic ,,tube ,,company4 ,,general ,,arrangement ,,and ,,adaptability ,,of ,,the ,,system #eg ,,the ,,size ,,of ,,tubes #fd ,,system ,,of ,,very ,,large ,,tubes #fe ,,general ,,arrangement ,,of ,,apparatus ,,in ,,the ,,stations--,,two-,,station1 ,,two-,,compressor ,,line #fi ,,two-,,station1 ,,one-,,compressor ,,line #gb ,,three- ,,to ,,eight-,,station ,,line #gd ,,sending ,,apparatus #gi ,,sending ,,time-,,lock #hd ,,intermediate ,,station ,,time-,,lock #hh ,,electro-,,pneumatic ,,circuit-,,closer #ia ,,the ,,open ,,receiver #id ,,the ,,closed ,,receiver #ii ,,the ,,intermediate ,,station ,,receiving ,,and ,,transfer ,,apparatus #ajf ,,carriers #aae ,,air ,,supply #aag ,,fans #aag ,,blowers #aag ,,air ,,compressors #aah ,,the ,,tube1 ,,line ,,construction1 ,,etc4 #abb ,,chapter ,,iv4 ,,facts ,,and ,,general ,,information ,,relating ,,to ,,pneumatic ,,tubes4 ,,definitions #abd ,,intermittent ,,and ,,constant ,,air-,,current #abe ,,laws ,,governing ,,the ,,flow ,,of ,,air ,,in ,,long ,,tubes #abf ,,law ,,of ,,pressure #abh ,,uses ,,of ,,pressure ,,curves #acj ,,law ,,of ,,velocity #acj ,,characteristics ,,of ,,the ,,velocity ,,curve #acb ,,uses ,,of ,,velocity ,,curves #acc ,,quantity ,,of ,,air ,,used #acd ,,temperature ,,of ,,the ,,air #ace ,,horse-,,power #acf ,,efficiency #acg ,,pressure ,,and ,,exhaust ,,systems #ach ,,laws ,,expressed ,,in ,,mathematical ,,formul #ada ,,moisture ,,in ,,the ,,tubes #adb ,,location ,,of ,,obstructions ,,in ,,tubes #adc ,,preface4 ,i have been prompted to prepare this book by the frequent inquiries made regarding the details of our system of pneumatic tubes4 ,these inquiries have come from people interested in our company1 from others interested in companies that have purchased the right to use our apparatus1 from people desirous of becoming interested in a pneumatic-tube business1 from would-be purchasers of pneumatic tubes1 and from people interested in pneumatic tubes from a scientific1 engineering1 or mechanical point of view4 ,this book is not intended to be a treatise on pneumatic tubes4 ,in the first chapter ,i have given a brief sketch of what has been done in the application of pneumatic tubes from the earliest records to the present time4 ,the second chapter contains a description of the postal tubes in ,philadelphia1 and the third chapter describes our system in detail4 ,following this is a short chapter explaining the theory of pneumatic tubes1 or the theory of the flow of air in long pipes1 stating the more interesting facts and relations in as plain and simple a manner as possible4 ,mathematical formul have been purposely avoided4 ,several plates showing the ,philadelphia postal line have been kindly loaned to me by the ,engineers' ,club of ,philadelphia4 ,they formerly appeared in a paper read by ,mr4 ,a4 ,falkenau before that club4 ,i have also to thank the and ,drill ,co41 the ,b4 ,f4 ,sturtevant ,co41 the ,wilbraham-,baker ,blower ,co41 and ,j4 ,b4 ,stewart for the use of electrotypes of their machines4 ,b4 ,c4 ,b4 ,october #f1 #ahif4 ,,the ,,batcheller ,,pneumatic ,,tube ,,system4 ,,chapter ,i4 ,a ,,brief ,,historical ,,sketch4 ,early ,records4--,the earliest reference to pneumatic transmission of which we find any record is a paper presented to the ,royal ,society of ,london1 by ,denis ,papin1 in the year #affg1 entitled 8,double ,pneumatic ,pump40 ,his plan was to exhaust the air from a long metal tube by two large cylinders4 ,the tube was to contain a piston1 to which a carriage was attached by means of a cord4 ,the 8,american ,cyclopdia0 goes on to say1 8,more than a century elapsed before any further effort in this direction was made4 ,paucbrouke's #8,dictionnaire ,encyclopdique des ,amusements des ,sciences' 7#agib7 gives a description of a machine by ,m4 ,van ,estin1 by means of which a hollow ball holding a small package was propelled by a blast of air through a tube several hundred feet in length1 and having many curves4 ,this plan seems1 however1 to have been more an amusement than an attempt to introduce an industrial scheme4 ,with more regard to practical results1 ,medhurst1 an engineer of ,london1 published a pamphlet on the subject in #ahaj4 ,he proposed to move small carriages on rails in air-tight tubes or tunnels1 by compressed air behind1 or by creating a partial vacuum in front4 ,in #ahab he published another pamphlet2 but the plan was not put into successful operation1 principally from insufficient means of exhaustion4 ,about #ahcb he proposed to connect the carriage inside such a tube with a passenger carriage running on the top of the tube2 and1 although the latter project has never been commercially successful1 it was the first to be practically attempted4 ,more than a score of patents were taken out on the ,continent and in ,england and ,america1 none of which met with any practical success4 ,returning to the original idea of ,denis ,papin1 inventors attempted to accomplish pneumatic transmission by moving the load inside the tube1 and in course of time achieved success4 ,in ,france ,,mm4 ,jarroux and ,taisseau presented a project for atmospheric telegraphy before the ,academy of ,sciences1 and they were succeeded in the same direction by ,,mm4 ,brochet and ,ardor40 ,practical ,beginning of the ,art4 ,the ,london ,pneumatic ,telegraph4--,london has the honor of being the first city to have a practical system of pneumatic telegraphy4 ,the first tubes were installed by the ,electric and ,international ,telegraph ,company1 the work being planned and carried out by their engineer1 ,mr4 ,josiah ,latimer ,clark1 in #ahec and #ahed4 ,the first tube to be laid was one and one-half inches in diameter1 and extended from the central station1 ,founder's ,court1 ,lothbury1 to the ,stock ,exchange1 ,throgmorton ,street1 a distance of two hundred and twenty yards4 ,the tube was operated intermittently by connecting it to a vacuum chamber at the central station4 ,carriers were sent only in one direction4 ,a steam-pump was used to maintain the vacuum4 ,much experience was gained from the use of this first tube4 ,in #aheh some improvements were made by ,mr4 ,c4 ,f4 ,varley1 and ,i can best describe them by quoting from the discussion of ,mr4 ,carl ,siemens's paper on 8,pneumatic ,despatch ,tubes3 ,the ,circuit ,system0 before the ,institution of ,civil ,engineers1 as recorded in the minutes of that society4 8,later1 about the year #aheh1 when a pipe two and one-fourth inches internal diameter was extended from ,telegraph ,street to ,mincing ,lane1 thirteen hundred and forty yards in length1 the traffic was so considerable that it was found desirable to have the power of sending messages in both directions4 ,to effect that a smaller pipe1 one and one-half inches in internal diameter1 was laid between ,telegraph ,street and ,mincing ,lane1 with a view to carrying the vacuum to the latter station1 so as to take messages in the opposite direction4 ,this smaller pipe was found to so wiredraw the current that the pipe would not work1 the leakage past the carrier being too considerable2 and accordingly a large chamber was built in the basement floor or kitchen at the corner of ,mincing ,lane and ,leadenhall ,street to collect power or vacuum for bringing the messages from ,telegraph ,street to ,mincing ,lane4 ,this chamber was constructed of timber1 fourteen feet by twelve feet broad and ten feet high1 and was covered with lead4 ,it was not strong enough to withstand the pressure2 for one day1 a carrier having stuck half-way1 and when there was a higher vacuum than usual1--viz41 twenty-three inches of mercury1--it collapsed with a loud report4 ,at the time the landlord of the house happened to be dining in the next room1 and he suddenly found himself1 his table1 dinner1 and the door1 which was wrenched off its hinges1 precipitated into the room amongst the dbris of the chamber4 ,the windows were forced inwards1 and those on the opposite side of ,mincing ,lane and ,leadenhall ,street were drawn outwards4 ,the damage was considerable4 ,this accident put an end1 for a time1 to the attempt to send telegraph messages by means of a vacuum conveyed through this smaller pipe4 ,about that time he 7,mr4 ,varley7 became the engineer-in-chief of the ,electric ,telegraph ,company1 and got permission from the directors to introduce a new system1--viz41 compressed air1--though many persons contended that it would be impossible to blow messages through a pipe1 because all attempts to blow air through long pipes had utterly failed2 while others said that1 if messages were sent1 they would go much slower than with the vacuum4444 ,in his 7,mr4 ,varley's7 apparatus1 for he was the first to introduce compressed air1 the reverse was found to be the case1 and for this reason3 the tube did not consume power until a message was about to be forwarded2 and in a tube thirteen hundred yards in length1 and two and one-fourth inches in diameter1 fifteen seconds elapsed before the vacuum was felt at the distant end after communication had been established with the exhausted chamber at the engine end of the tube1 consequently the carrier did not start until after fifteen seconds had elapsed4 ,when a message was sent by compressed air1 it was sent from the end at which the power was applied1 and the carrier started at once1 thus gaining a start of fifteen seconds2 now1 inasmuch as the air in the tube had to be compressed1 it started at a very high velocity1 and when it reached the other end the compressed air in expanding gave it a higher velocity4 ,the result was1 in thirteen hundred and forty yards1 from ,telegraph ,street to ,mincing ,lane1 the carriers were drawn by vacuum1 on an average1 in from sixty to seventy seconds1 and were propelled by compressed air in about fifty or fifty-five seconds1 the difference of pressure in each case being nearly equal40 ,the first one and one-half inch tubes laid under the direction of ,mr4 ,clark were of iron with screwed joints4 ,they gave much trouble from roughness upon the interior1 which wore the carriers very rapidly1 and from water that was drawn in through leaky joints4 ,when the extensions were made in #aheh and afterwards1 two and one-fourth inch lead tubes were used with plumber's joints made over a heated mandrel1 which made the joints very smooth upon the interior4 ,the carriers were of gutta percha in the form of a cylinder closed at one end and fitted with a cap at the other4 ,the outside was covered with felt or drugget4 ,when a carrier was to be despatched1 a signal was sent to an attendant at the pump end of the tube1 who closed that end and connected the tube to an exhausted chamber by opening a valve4 ,as soon as the carrier arrived1 he closed the valve and opened the tube1 which allowed the carrier to drop out4 ,mr4 ,varley improved the method of operating the valves by making the air pressure do the work by means of cylinders and pistons when the attendant pressed a button4 ,he also improved the carriers by doing away with the cap and using in its place an elastic band to hold the messages in place4 ,we have seen that ,mr4 ,clark designed the first tube used in connection with the telegraph1 and that it was a single tube1 operated in one direction only by vacuum1 being operated only when there were messages to send4 ,this was extended and improved by ,mr4 ,varley1 who increased the diameter of the tubes from one and one-half inches to two and one-quarter inches1 and operated them in both directions1 using vacuum for sending in one direction and compressed air for sending in the other4 ,the air current was maintained in the tubes only when messages were sent4 ,great credit is due to ,sir ,rowland ,hill1 who1 in #ahee1 had a proposed method of conveying mails in the city of ,london1 through nine- and thirteen-inch tubes1 thoroughly investigated4 ,it was decided at this time that the saving in time over that consumed by mail carts would not warrant the expense of installing such a system4 ,the ,siemens ,circuit ,system4--,the next progressive step was made by ,siemens ,brothers1 of ,berlin1 who proposed a new system called the 8circuit system10 in which two tubes were used1 the up tube being connected to the down tube at the distant end4 ,the air was compressed into one end of the circuit and exhausted from the other1 and1 furthermore1 it was kept in constant circulation4 ,carriers were despatched by inserting them into the air-current without stopping it1 in one direction in one tube or in the opposite direction in the other4 ,another feature of the ,siemens system was the placing of three or more stations on one double line of tubes4 ,carriers could be stopped at an intermediate station by inserting in the tube an obstructing screen which the air would pass but which would stop a carrier4 ,this system is described in detail in a paper read by ,mr4 ,carl ,siemens before the ,institution of ,civil ,engineers1 ,london1 ,november #ad1 #ahga1 ,vol4 ,,xxxiii4 of the ,proceedings4 ,the ,siemens apparatus for sending and receiving carriers consisted of two short sections of tube attached to a rocking frame so that either could be swung by hand into line with the main tube4 ,one of the tube sections was open at both ends1 and was used for despatching carriers4 ,a carrier was placed in it1 then it was swung into line with the main tube1 when the air-current passing through swept the carrier along4 ,the other tube section contained a perforated screen in one end and was used to receive carriers4 ,when it was in line with the main tube and a carrier arrived1 the carrier was stopped by striking the screen1 then the tube section was swung to one side and the carrier pushed out with a rod4 ,a by-pass was provided for the air around the apparatus so that its flow was not checked when the tube section was swung4 ,when a carrier was despatched to an intermediate station1 a signal was sent1 and then the section of tube containing the screen was interposed in the line of the tube to stop the carrier upon its arrival4 ,the carriers used by ,mr4 ,siemens were made of gutta-percha1 papier mach1 or tin1 closed at one end and fitted with a cover at the other4 ,they were covered with felt1 drugget1 or leather4 ,the front ends of the carriers were provided with thick disks of drugget or leather fitting the tube loosely1 and the opposite ends were surrounded with pieces of the same material attached to them like the leather of an ordinary lifting pump4 ,in #ahfi ,messrs4 ,siemens ,bros4 received an order from the ,british government to install an experimental line of tubes between the central telegraph station and the general post-office4 ,this was completed in #ahgj1 and after a half-year's test it was extended to ,fleet ,street1 and finally to ,charing ,cross4 ,the tubes were of iron1 three inches in diameter1 with flanged and bolted joints4 ,it was found1 after some experience1 that there was no advantage in the circuit1 so the up and down tubes were separated at ,charing ,cross ,station and worked independently4 ,recent ,improvements in the ,london ,system4--,in #ahgj ,mr4 ,j4 ,w4 ,wilmot designed a double sluice-valve by means of which carriers could be despatched continuously without stopping the flow of air in the tubes4 ,mr4 ,wilmot further increased the working capacity of pneumatic tubes when1 in #ahhj1 he invented an intermediate automatic signaller1 by means of which a carrier signals the passage of a given point on its journey1 showing that the section of the tube traversed is clear1 thus allowing a second carrier to be despatched before the first has reached its destination4 ,illustration3 ,,fig4 #b4 ,,diagram ,,illustrating ,,the ,,pneumatic ,,tube ,,system ,,london ,,radial ,,system4 ,,london ,,radial ,,system4 ,from this beginning the ,english system developed into what has been termed a 8radial system20 that is to say1 one principal and several minor central pumping stations have been established1 and from these radiate tubes to numerous sub-stations 7see ,fig4 #b74 ,some of the stations are connected with double lines for sending in opposite directions4 ,the out-going tube from the pumping station is worked by compressed air1 and the incoming tube by exhaustion4 ,other stations are connected by single tubes1 and they are operated alternately by compression and exhaustion4 ,intermediate stations are located on some of the lines4 ,for the central station the ,varley valves were found too expensive and troublesome to keep in order1 so they were replaced by the ,wilmot double sluice-valves1 which are operated manually4 ,in recent years the sluice-valves have been in turn replaced by what are termed ,d-boxes1 a simpler form of apparatus4 ,at the sub-stations the tube terminates in a box into which the carriers drop4 ,as the system has been gradually extended1 tubes two and three-sixteenths inches inside diameter have been used for short lines1 and three-inch tubes for long lines4 ,the tubes are of lead laid inside a cast-iron pipe which serves as a shield1 protecting them from injury4 ,they are laid in twenty-nine foot sections1 the joints being made by soldering over a steel mandrel1 which is afterwards drawn out by a chain4 ,the joints in the cast-iron protecting pipe are made by caulking with yarn and lead4 8,electric signals are used between the central and sub-stations1 consisting of a single stroke bell and indicator1 giving notice of the arrival and departure of carriers1 and to answer the necessary questions required in working4 ,where there are intermediate stations the tubes are worked on the block system1 as if it were a railway4 ,experience shows that1 where great exactness of manipulation cannot be obtained1 it is necessary to allow only one train in each section of a tube1 whether worked by vacuum or pressure4 ,but where there is no intermediate station1 and where the tube can be carefully worked1 carriers may be allowed to follow one another at short intervals in a tube worked by vacuum1 although it is not perfectly safe to do so in one worked by pressure4 ,in working by pressure it has been found that1 notwithstanding a fair interval may be allowed1 carriers are apt to overtake one another1 for no two carriers travel in the same times1 because of differences in fit1 unless they are placed end to end4 ,if signalling be neglected and a carrier happens to stick fast1 being followed by several others1 a block will ensue which it will be difficult to clear1 while the single carrier could readily have been dislodged40 7,proceedings ,institute of ,civil ,engineers1 ,london1 ,vol4 ,,xliii4 p4 #fa47 ,no changes have been made in the carriers from those used in the early experiments which have already been described4 ,the ,london system has grown until it now includes no less than forty-two stations and thirty-four miles of tubes4 ,similar systems have been established in connection with the telegraph in ,liverpool1 ,manchester1 ,birmingham1 ,glasgow1 ,dublin1 and ,new ,castle4 ,the tubes give a cheaper and more rapid means of despatching telegrams between sub-stations and central stations than transmission by telegraph1 and local telegrams can be delivered in the sender's handwriting4 ,an ,underground ,pneumatic ,railway for ,transportation of ,mail4--,before describing the systems used in the cities on the ,continent of ,europe1 we must notice a very large pneumatic tube1 or more properly called a pneumatic tunnel railway1 constructed in ,london for the transportation of mail from one of the railway stations4 ,the first railway of this type was constructed in #ahfc by the ,pneumatic ,despatch ,company of ,london1 and extended from ,euston to the district post-office in ,eversholt ,street1 a distance of about eighteen hundred feet4 ,the tunnel was flat on the bottom1 having a ,d-shaped cross-section two feet eight inches by two feet eight inches4 ,the carriers or carriages were cradle-like boxes fitting the tunnel1 and they moved at a speed of seventeen miles per hour1 carrying fifteen mails daily4 ,in #ahgb two similar but larger tunnels were built from ,euston ,station to the general post-office1 a distance of fourteen thousand two hundred and four feet1 or two and three-quarters miles4 ,one was for the up traffic1 and the other for the down4 ,the tunnels were four and a half feet wide by four feet high1 the straight part being built of cast iron and the bends of brick4 ,the line was operated by a fan twenty-two feet in diameter1 which forced the air into one tunnel and exhausted it from the other1 producing a vacuum of ten inches of water1 or six ounces per square inch4 ,the carriages occupied twelve minutes in traversing the tunnels1 and there was one gradient of one to fourteen4 ,the carriages were ten feet four inches long and weighed twenty-two hundredweight4 8,the system was able to transport over the whole line1 allowing for delays1 an average of a ton per minute40 ,the system was used to transport the mails in bulk1 but it was found to be slow and unsatisfactory1 and was very soon abandoned4 ,the ,berlin ,pneumatic ,telegraph4--,in #ahfc the ,prussian government applied to the firm of ,siemens and ,halske1 of ,berlin1 for a proposition to establish a system of pneumatic tubes in that city for the transmission of telegraph messages4 ,a proposition was accordingly submitted1 and the work was completed in #ahfe4 ,this first line consisted of two parallel wrought-iron tubes1 two and one-half inches in diameter1 one tube being used exclusively to send in one direction1 and the other in the opposite direction4 ,they extended from the telegraph station to the ,exchange1 requiring a total length of five thousand six hundred and seventy feet of tube4 ,the two tubes were looped together at the ,exchange1 and a continuous current of air was made to circulate in them by a double-acting steam air-pump1 located at the telegraph station4 ,air was compressed into one end of the tube and exhausted from the other4 ,with nine inches of mercury pressure and vacuum the passage was made in ninety-five seconds to the ,exchange1 and seventy-five seconds from the ,exchange4 ,it was similar to the line established in ,london by the same firm some years later1 which we have already described1 except that there was no intermediate station4 ,after the line had been in use for a year and a half1 the ,prussian government had it extended1 first1 from the telegraph station to the ,potsdam gate1 with an intermediate station at the ,brandenburg gate4 ,after these preliminary experiments1 further extensions were made until a net-work of tubes extended over the city of ,berlin1 including no less than thirty-eight stations and over twenty-eight miles of tubes2 but in laying down this net-work a departure was made from the ,siemens system4 ,air was no longer kept constantly circulating1 but power was stored up in large tanks1 some being exhausted and others filled with compressed air1 which was used when required to send messages1 usually at intervals of five or fifteen minutes4 ,the exhausted tanks were permanently connected with the closed tubes1 which were opened when required for use4 ,the tanks containing compressed air were connected to the tubes when messages were sent4 ,the internal diameter of the tubes was #b4eei inches4 ,they were laid in circuits1 including several stations in a circuit1 and the carriers travelled only in one direction around the circuit4 ,some outlying stations were connected by a single tube with central pumping-stations1 these single tubes being worked in both directions4 ,years of experience have shown the disadvantages of this circuit-system1 and it has gradually been changed to the radial system1 such as is used in ,london1 until now nearly all the stations are grouped around the central pumping-stations1 to which they are connected directly by radiating tubes4 ,the ,siemens apparatus has been replaced by simpler and less expensive valves and receiving-boxes1 the latest form of which was designed and patented by ,mr4 ,josef ,wildemann4 ,illustration3 ,,fig4 #c4 ,,diagram ,,of ,,part ,,of ,,paris ,,pneumatic ,,tube ,,system4 ,,paris ,,circuit ,,system4 ,the ,paris ,pneumatic ,telegraph4--,we will now glance at the system used in ,paris1 which has some novel features4 ,in #ahfe it was decided to establish a system1 and the first experimental line1 from ,place de la ,bourse to the ,grand ,htel1 on the ,boulevard des ,capuciens1 was laid in #ahff4 ,instead of using a steam-engine to drive an air-compressor or exhaust-pump1 air was compressed in tanks by displacement with water from the city mains4 ,in #ahfg this line was extended to ,rue de ,grennelle ,st4 ,germain1 with an intermediate station at the ,rue ,boissy d',anglais1 and another line with stations at ,rue ,jean ,jacques ,rousseau1 ,htel du ,louvre in the ,rue de ,rivoli1 the ,rue des ,saints ,pres1 and terminating in the central station4 ,in #ahfh the system was changed to a polygonal or circuit system by removing the station in the ,rue de ,rivoli to the ,place du ,thtre ,franais and connecting the latter directly with the ,bourse4 ,other changes and extensions were made in #ahgj and #ahga1 until three polygons or circuits were formed1 with five or six stations in each circuit1 and several outlying stations were connected by independent tubes4 ,in the middle of the year #ahge seventeen stations had been connected and plans were made for twenty-one more4 ,instead of maintaining an air-current around each circuit by machinery located at one of the stations on the circuit1 at least three of the five or six stations comprised in the circuit have means of supplying compressed air or of exhausting it1 and each side of the polygon1 or section of the circuit between two stations1 is operated independently of the rest of the circuit 7see ,fig4 #c74 ,carriers are sent on from station to station around the circuit1 either by compressed air from the last station from which they were sent or by means of exhaustion at the next station towards which they are moving4 ,the carriers are made up in trains of from six to ten1 with a piston behind them that fits the tube closely and forces them ahead4 ,each carrier is addressed by means of a label for its destined station4 ,trains are despatched around the circuits at stated times1 usually at fifteen-minute intervals4 ,as they arrive at the various stations1 carriers are taken out and others put in1 and the trains sent on their way4 ,the carriers consist of iron cylinders1 closed at one end1 with a leather case that slides over them and closes the open end4 ,they weigh1 when filled with thirty-five messages1 twelve and one-half ounces1 and they will travel about twelve hundred miles before the leather cover is so worn that it must be thrown away4 ,the pistons are made of a wooden cone1 covered with iron1 and having a 8cup-leather0 upon the rear end that fits the tube closely4 ,the sending and receiving apparatus consists of sections of tube closed at one end1 having a door on the side1 through which carriers are inserted or despatched4 ,a peculiar form of fork is used for picking them out4 ,the air is controlled by valves opened and closed by hand4 ,several methods are used to compress and exhaust the air4 ,the most novel method consists in having tanks in which a partial vacuum is produced by allowing water to flow out of them into the sewer1 or in having the air compressed by allowing water from the city mains to flow into the tanks and displace the air4 ,water jets have also been used1 operating similar to a steam-injector4 ,at some of the stations water turbines drive the air-pumps1 and at others steam-engines are used4 ,the tubes of the ,paris system are of wrought iron1 in lengths of from fifteen to twenty feet1 the joints being made with flanges and bolts4 ,the interior diameter is #b4eei inches with a maximum variation to #b4eai inches4 ,the bends are made with a radius of from six to one hundred and fifty feet4 ,water frequently gives trouble by accumulating in the tubes4 ,traps are placed at low points to drain it off4 ,the speed of the trains of carriers in the ,paris tubes is from fifteen to twenty-three miles per hour1 and the average time that elapses from the receipt of a message until its delivery is from forty to forty-five minutes4 ,the ,pneumatic ,telegraph of other ,cities4--,a system similar to the one just described is used in ,vienna4 ,it differs some in details of apparatus1 but the carriers are despatched around circuits in trains1 stopping at each station1 where some carriers are removed and others inserted4 ,brussels also is not without its system of pneumatic tubes for the transmission of telegrams4 ,pneumatic ,tubes in ,america4--,turning our attention now to our own country1 we cannot pass without mention some experiments of ,alfred ,e4 ,beach with pneumatic railways1 made nearly thirty years ago4 ,his first experiment upon a large scale was made at the ,american ,institute ,fair held in ,new ,york ,city in #ahfg4 ,here he had constructed a circular wooden tube1 one hundred and seven feet long and six feet in diameter4 ,a car that would seat ten people ran upon a track laid down inside the tube1 and was propelled by a helix fan ten feet in diameter1 making two hundred revolutions per minute4 ,he next tried his railway upon a practical scale1 constructing an eight-foot tunnel for two hundred feet under ,broadway1 starting at the corner of ,warren ,street4 ,a car was propelled by a large rotary blower located in the basement of a building near by4 ,the blower was kept constantly running1 and the car was sent alternately in one direction and then the other by changing valves at the blower4 ,few people know that this experimental line still exists under ,broadway as ,mr4 ,beach left it4 ,the most extensive use of small pneumatic tubes in this country has been in our large retail department stores for despatching cash to and from a centrally located cashier's desk4 ,seamless brass tubes are usually used1 and1 since the tubes are short1 the air is either compressed or exhausted by means of positive rotary blowers4 ,at the outlying stations the tubes are usually open to the atmosphere1 while at the central station simple forms of valves are used for sending and receiving4 ,an outgoing and a return tube are always used1 and the air is kept in constant circulation4 ,the carriers are of metal with felt packing rings and open on the side4 ,these cash-carrying systems have come into use during the past twenty-five years4 ,the ,western ,union ,telegraph ,company uses small tubes to transmit its messages to a considerable extent in some of our large cities4 ,in #ahgf four lines were laid in ,new ,york ,city from the main office on ,broadway3 two to the branch office at ,no4 #ad ,broad ,street1 one to #acd ,pearl ,street1 and one to the ,cotton ,exchange4 ,since then this company has laid a double line about two miles in length under ,broadway to its up-town office4 ,it also uses tubes to send messages from the receiving desks to the operating rooms within the buildings4 ,many of our large hotels use pneumatic tubes to transmit messages to the different floors and offices of the buildings1 taking the place of messenger1 or bell boys1 who formerly did this service4 ,we call especial attention to the fact that in all the systems that we have mentioned which are in use both in this country and in ,europe1 none of the tubes are larger than three inches internal diameter2 also that in all the systems1 except in ,paris where the carriers are despatched in trains1 the carriers are so light and move so slowly that they can be stopped by allowing them to come in contact with some solid object1 such as a box into which the carriers drop4 ,very few of the tubes are more than two miles in length1 and most of them are less than one mile4 ,a speed of more than thirty miles per hour has seldom been attempted1 and usually it is much less than this4 ,,chapter ,,ii4 ,,the ,,pneumatic ,,transit ,,company ,,and ,,the ,,first ,,pneumatic ,,tubes ,,for ,,the ,,transportation ,,of ,,united ,,states ,,mail4 ,organization4--,early in the year #ahib several ,philadelphia gentlemen organized a corporation and obtained a charter in the ,state of ,new ,jersey to construct1 lay1 and operate pneumatic tubes for the transmission of ,united ,states mail1 packages1 merchandise1 messages1 etc41 within the ,states of ,new ,jersey and ,pennsylvania4 ,the corporation was styled the ,pneumatic ,transit ,company4 ,mr4 ,william ,j4 ,kelly was elected president1 and the company is still under his management4 ,illustration3 ,,wm4 ,j4 ,,kelly1 ,president of the ,pneumatic ,transit ,co4 ,aim and ,object of the ,company4--,when the ,pneumatic ,transit ,company was formed1 it was the aim and object of its promoters to construct an extensive system of underground tubes in the ,city of ,philadelphia which would serve1 first1 for the rapid transmission of mail1 second1 for the quick delivery of merchandise from the large retail stores1 third1 for the transmission of telegrams or messages within the city limits1 and1 fourth1 to conduct a general local express business with greater speed than can be done in any other manner4 ,to accomplish this result sub-stations were to be located six or eight blocks apart throughout a large portion of the city1 and a central station was to be established in the centre of the business section4 ,stations were also to be established in the more important retail stores and large office buildings1 and all of the stations were to be connected by tubes forming one large system4 ,for the transmission of mail it was planned to connect the main post-office with the sub-post-offices by tubes of a size large enough to carry all of the first-class and most of the other classes of mail matter4 ,the sub-post-offices would be divided into groups1 all of the offices in one group being connected to the same line1 which would terminate at the main post-office4 ,most of the business would be between the main and individual sub-offices2 in addition to this there would be some local mail sent between the sub-offices which1 for offices in the same group1 could be despatched directly without passing through the main office4 ,the advantages to be gained by the use of these tubes over the present wagon service are very apparent4 ,it places all the sub-post-offices in almost instant communication with the main office and with each other4 ,it was a part of the general plan to lay tubes from the main post-office to the railway stations1 thereby hastening the despatch and receipt of mails to and from the trains4 ,it was expected that the bulk of the business would consist in the delivery of parcels from the retail stores to the private houses in the residence sections of the city4 ,of course it would not be practicable to lay a tube to each house1 but with a station not more than four or five blocks away1 the parcels would be sent through the tube to the nearest station1 and then delivered by messengers to the houses with a minimum loss of time4 ,ladies could do their shopping and find their purchases at home when they returned4 ,the same tubes used for parcel delivery would also be used for a district messenger service4 ,with numerous public stations in convenient locations1 all the advantages of the ,european system would be realized in the quick despatch of letters and telegrams4 ,every one knows how much time is consumed by district messenger-boys in the delivery of messages1 especially when they have to go long distances1 and no argument is required to show that this time would be very much reduced by the use of pneumatic tubes1 besides prompt delivery would be made much more certain4 ,the tubes of this system were to be six or eight inches in diameter1 with a few small tubes in localities where the message service is very heavy4 ,without going more into detail1 such were in brief the plans of the promoters of this new company2 but before launching such an enterprise1 involving a large amount of capital1 there were many engineering and mechanical problems to be solved4 ,it was not simply a question of obtaining tubes and laying them in the streets1 but ways and means for operating them must be devised4 ,up to this time only small tubes had been used for the transmission of telegrams1 messages1 cash1 and other light objects4 ,now it was proposed to transmit heavy and bulky material4 ,there was no experience for a guide4 ,the ,clay-,lieb ,patents4--,the ,pneumatic ,transit ,company at this time turned to the ,electro-,pneumatic ,transit ,company1 of ,new ,jersey1 a national company that had been in existence since #ahhf1 and which claimed to own valuable patents1 for the ways and means to carry out its new enterprise4 ,the patents were those of ,henry ,clay and ,charles ,a4 ,lieb1 and the rights to use them in the ,state of ,pennsylvania were procured by the ,pneumatic ,transit ,company1 under a contract entered into between the two companies4 ,the patents claimed to cover a practical working system by which a large number of stations could be connected to a system of main and branch tubes1 with electrically-operated switches at the junctions of the branches with the main lines4 ,any person who gives the subject a little thought will at once see the advantages of such a system if it could be made to operate4 ,up to the present time only single- or double-line tubes have been used1 without branches4 ,in the ,european systems1 frequently several stations are located along a line1 but the carriers must stop at each station1 be examined1 and if they are destined for another station1 they must be redespatched4 ,the cash systems used in many of our large stores have independent tubes running from the central cashier's desk to each station about the store4 ,it is plain to be seen that1 if several of these stations could be connected by branches to a main tube1 a large amount of tubing would be saved--a most desirable result4 ,the advantages of such a system would be still greater for long lines of tube laid under the pavements1 extending to stations located in different parts of a large city4 ,it was such a result that the patents of ,clay and ,lieb aimed to accomplish4 ,in order to demonstrate the practicability of the system1 the ,electro-,pneumatic ,transit ,company had constructed in the basement of the ,mills ,building1 on ,broad ,street1 ,new ,york1 a short line of small brass tubing1 about two or three inches in diameter1 with one branch1 thus connecting three stations together4 ,the tube was very short1 probably not more than two hundred feet in length4 ,the air-pressure required was very slight1 probably not more than an ounce or two1 being supplied by a small blower run by an electric motor4 ,at the junction of the branch and the main tube was located a switch that could be moved across the main tube and so deflect the approaching carrier into the branch4 ,this switch was moved by an electro-magnet1 or solenoid1 that could be excited by pressing a button at the station from which the carrier was sent4 ,when the carrier passed into the branch tube it set the switch back into its normal position1 so that a second carrier1 following the first1 would pass along the main tube1 unless the switch was again moved by pressing the button at the sending station4 ,this tube in the ,mills ,building worked well1 but it was of a size only suited to the transmission of cash in a store or other similar service4 ,it could not be said1 because this tube worked well1 that a larger and longer tube with numerous branches would work equally well4 ,in fact1 there are several reasons why such a tube would not operate satisfactorily4 ,the method of operating the switches was impracticable4 ,suppose the branch tube had been located two miles away from the sending station and that it would take a carrier four minutes to travel from the sending station to the junction of the branch tube4 ,again1 suppose that we have just despatched a carrier destined for a station on the main line beyond the junction1 and that we wish to despatch the second carrier to be switched off into the branch tube1 we must wait at least four minutes1 until the first carrier has passed the junction1 before we can press the button and set the switch for the second carrier which may be on its way4 ,how are we to know when the first carrier has passed the junction1 and when the second will arrive there1 in order that we may throw the switch at the proper time8 ,must we hold our watch and time each carrier8 ,it is plain that this is not practical4 ,i take this as an illustration of the impracticability of the ,clay-,lieb ,system as constructed in the ,mills ,building when extended to practical dimensions4 ,i will not describe the mechanism and details of the system1 which are ingenious1 but will say in passing that the automatic sluice-gates1 which work very well in a three-inch tube with carriers weighing an ounce or two and air-pressures of only a few ounces per square inch1 would be useless and could not be made to operate in a six-inch tube with carriers weighing from eight to twenty-five pounds and an air-pressure of from five to twenty-five pounds per square inch4 ,for further information the reader is referred to the patents of ,clay and ,lieb4 ,franchises and ,first ,government ,contract4--,in the spring of #ahib an ordinance was passed by ,common and ,select ,councils1 and signed by the ,mayor of the ,city of ,philadelphia1 permitting the ,pneumatic ,transit ,company to lay pneumatic tubes in the streets of that city4 ,at the time this franchise was granted negotiations were in progress with the post-office department1 in ,washington1 for the construction of a six-inch pneumatic tube1 connecting the ,east ,chestnut ,street sub-post-office1 at ,third and ,chestnut ,streets1 with the main post-office1 at ,ninth and ,chestnut ,streets1 for the transmission of mail4 ,this sub-post-office was selected because more mail passes through it daily than any other sub-office in the city1 it being located near the centre of the banking district4 ,negotiations were delayed by various causes1 so that the contract with the government was not signed until ,october1 #ahib4 ,illustration3 ,,fig4 #d4 ,illustration3 ,,fig4 #e4 ,,six-,,inch ,,pneumatic ,,tubes ,,in ,,process ,,of ,,boring ,,at ,,the ,,shop ,,of ,a4 ,,falkenau1 ,,philadelphia1 ,,pa4 ,search for ,tubes4--,it was at this time that the writer was first employed by the ,pneumatic ,transit ,company1 as engineer1 to superintend the construction of this line4 ,the company commenced at once to carry out its contract with the ,united ,states government1 both the post-office department and the company being very desirous of having the work completed before winter4 ,the time was very short for such an undertaking1 but wrought-iron tubes had already been ordered of a well-known firm who manufacture pipe and tubing of all kinds4 ,after waiting four or five weeks the first lot of tubes were finished1 but upon inspection it was found that they were not sufficiently accurate and smooth on the interior to permit of their being used for the purpose intended4 ,the next thing that suggested itself was seamless drawn brass tubes4 ,while they would be very expensive1 the process of manufacture makes them eminently suited for the purpose1 but they could not be obtained in time4 ,a city ordinance prohibits the opening of the streets of ,philadelphia during the winter months except in extreme cases4 ,accurate tubes must be had1 and had quickly4 ,it then occurred to the writer that it might be possible to bore a sufficient quantity of ordinary cast-iron water-pipe and fit the ends accurately together to answer our purpose4 ,inquiry was made at nearly all the machine-shops in the city1 to ascertain how many boring-machines could be put upon this work of boring nearly six thousand feet of six-inch pipe4 ,it was found impossible to get the work done in time1 if it was to be done in the usual manner of boring with a rigid bar4 ,at last a man was found in ,mr4 ,a4 ,falkenau1 engineer and machinist1 who was prepared to contract for the construction of twelve special boring-machines and to bore all the tubing required4 ,suffice it to say1 that the machines were built1 and about six thousand feet of tubes were bored1 between ,november #h and ,december #ca4 ,illustration3 ,,fig4 #f4 ,,pipe ,,boring ,,apparatus4 ,method of ,manufacturing ,tubes4--,the process of boring was novel in some respects1 and might be termed reaming rather than boring4 ,figs4 #d and #e show the interior of the shop and the twelve machines4 ,fig4 #f is a drawing of one of the machines4 ,a long flexible bar rotated the cutter-head1 which was pulled through the tube1 in distinction from being pushed4 ,in order to give the feeding motion1 a screw was attached to the cutter-head and extended through the tube in advance of it4 ,the feed-screw was drawn forward by a nut attached to a hand-wheel located at the opposite end of the tube from which the boring began4 ,since it was not necessary that the tubes should be perfectly straight1 a method of this kind was permissible1 in which the cutters could be allowed to follow the cored axis of the tube4 ,air from a ,sturtevant blower was forced through the tubes during the process of boring1 for the double purpose of clearing the chips from the cutters and keeping them cool4 ,after the tubes were reamed1 each piece had to be placed in a lathe1 have a counter-bore turned in the bottom of the bell1 and have the other end squared off and turned for a short distance on the outside to fit the counter-bore of the next section4 ,laying and ,opening the ,tubes for ,traffic4--,the first tubes were laid about the middle of ,november1 but ,december #a came before the work was completed and special permission had to be obtained from the city to carry on the work after that date4 ,all work was suspended during the holidays in order not to interfere with the holiday trade of the stores on ,chestnut ,street4 ,severe frosts prevailed at that season1 so that when the work was begun again1 after the holidays1 bonfires had to be built in the streets to thaw out the ground in order to take up the paving-stones and dig the trench for the tubes4 ,several times the trench was filled with snow by unusually heavy storms4 ,notwithstanding all these delays and annoyances1 the work was pushed forward1 when a less determined company would have given it up1 and was finally completed4 ,a formal opening took place on ,february #ag1 #ahic1 when ,hon4 ,john ,wanamaker1 then ,postmaster-,general1 sent through the tube the first carrier1 containing a ,bible wrapped in the ,american flag4 ,it was certainly a credit to the ,pneumatic ,transit ,company and its managers that they were able to complete this first line of tubes so quickly and successfully under such trying circumstances4 ,the tubes have been in successful operation from the opening until the present time1 a period of nearly four years1 and the repairs that have been made during that time have not required its stoppage for more than a few hours4 ,in the summer of #ahie the sub-post-office was removed from ,chestnut ,street to the basement of the ,bourse 7see ,fig4 #g74 ,this required the abandonment of a few feet of tube on ,chestnut ,street and the laying of a slightly greater amount on ,fourth ,street1 thus increasing the total length of the tubes a little4 ,wrought-iron tube1 coated with some alloy1 probably composed largely of tin or zinc1 was used for this extension4 ,the wrought-iron tube is not as good as the bored cast iron4 ,illustration3 ,,fig4 #g4 ,,bourse ,,building1 ,,philadelphia4 ,illustration3 ,,fig4 #h4 ,,pneumatic ,,tubes ,,suspended ,,in ,,the ,,basement ,,of ,,the ,,main ,,post-,,office4 ,description of the ,tubes1 ,method of ,laying1 etc4--,this ,chestnut ,street line consists of two tubes1 one for despatching carriers from and the other to the main post-office4 ,the distance between the two stations is two thousand nine hundred and seventy-four feet1 requiring five thousand nine hundred and forty-eight feet of tube4 ,the inside diameter of the tube is six and one-eighth inches1 and it was made in sections each about eleven feet long1 with 8bells0 cast upon one end1 in order to join the sections with lead and oakum1 calked in the usual manner of making joints in water- and gas-pipes1 with this exception1 that at the bottom of the bell a counter-bore was turned to receive the finished end of the next section4 ,by thus machining the ends of each section of tube and having them fit accurately together1 male and female1 a practically continuous tube was formed with no shoulders upon the interior to obstruct the smooth passage of the carriers4 ,joints made in this way possess another great advantage over flanged and bolted joints1 in that they are slightly yielding without leaking1 and so allow for expansion and contraction due to changes of temperature4 ,each joint takes care of the expansion and contraction of its section1 which is very slight1 but if all were added together would amount to a very large movement4 ,another advantage of the 8bell0 joint is that it permits slight bends to be made in the line of tube without requiring special bent sections4 ,where short bends had to be made1 at street corners1 in entering buildings1 and other similar places1 brass tubes were used1 bent to a radius of not less than six feet1 or about twelve times the diameter of the tube4 7,one of the brass bends may be seen in ,fig4 #aj47 ,the bends were made of seamless tubing1 bent to the desired form and radius in a hydraulic machine4 ,to prevent them from being flattened in the process of bending1 they were filled with resin1 which was afterwards melted out4 ,flanges were screwed and soldered to the ends of the bent brass tubes1 and they were bolted to special flanged sections of the iron tube4 ,the tubes were laid in the trench and supported by having the ground firmly tamped about them4 ,usually one tube was laid above the other1 with an iron bracket between1 but frequently this arrangement had to be departed from in order to avoid obstructions1 such as gas- and water-pipes1 sewers1 man-holes1 etc4 ,the depth of the tubes below the pavement varied from two to six feet1 and in one place1 in order to pass under a sewer1 the extreme depth of thirteen feet was reached4 ,at the street crossings it was frequently difficult to find sufficient space to lay the tubes4 ,at the intersection of ,fifth and ,chestnut ,streets a six-inch water-main had to be cut and a bend put in4 ,a seven-strand electric cable1 used for telephoning and signalling1 was laid upon the top of one of the tubes1 protected by a strip of 8vulcanized wood10 grooved to fit over the cable4 ,the cable and protecting strip of wood were fastened to the tube by wrought-iron straps and bolts4 ,the tubes enter the main post-office on the ,chestnut ,street side1 through one of the windows1 and are suspended from the ceiling along the corridor in the basement for a distance of nearly two hundred feet4 ,fig4 #h shows the tubes thus suspended4 ,they terminate upon the ground floor about the centre of the building1 and near the cancelling machines4 ,illustration3 ,,fig4 #i4 ,,duplex ,,air-,,compressor ,,in ,,the ,,basement ,,of ,,the ,,main ,,post-,,office4 ,illustration3 ,,fig4 #aj4 ,,tanks ,,and ,,tube ,,in ,,the ,,basement ,,of ,,the ,,main ,,post-,,office4 ,air-,compressor--,method of ,circulating the ,air4--,the current of air that operates the tubes is supplied by a duplex air-compressor located in the basement of the main post-office4 ,this machine is shown in ,fig4 #i1 and requires no detailed description1 as it does not differ materially from air-compressors used for other purposes4 ,the stroke is twenty-four inches1 the diameter of the steam-cylinders ten inches1 and the air-cylinders eighteen inches4 ,the air-cylinders are double acting1 with poppet-valves1 and have a closed suction4 ,the speed of the machine varies slightly1 being controlled by a pressure-regulator that maintains a practically constant pressure in the tank that feeds the tube4 ,the engines develop a little over thirty horse-power under normal conditions4 ,the pressure of the air as it leaves the compressor is usually six or seven pounds per square inch4 ,compressing the air heats it to about #aef ,f41 but this is not sufficient to require water-jackets about the air-cylinders4 ,from the compressor the air flows to a tank1 shown on the right in ,fig4 #aj1 where any oil or dirt contained in the air is deposited4 ,the principal purpose of the tank is1 however1 to form a cushion to reduce the pulsations in the air caused by the periodic discharge from the cylinders of the compressors1 and make the current in the tube more steady4 ,from this tank the air flows to the sending apparatus on the ground floor of the post-office and thence through the outgoing tube to the sub-post-office4 ,at the sub-post-office1 after flowing through the receiving and sending apparatus1 it enters the return tube and flows back to the main office1 passing through the receiving apparatus there and then to a tank in the basement1--the left tank in ,fig4 #aj4 ,the air-compressor draws its supply from this tank1 so that the air is used over and over again4 ,this return tank has an opening to the atmosphere1 which allows air to enter and make up for any leakage or escape at the sending and receiving apparatus1 thereby maintaining the atmospheric pressure in the discharge end of the tube and in the suction of the compressor4 ,the tank serves to catch any moisture and dirt that come out of the tube4 ,fig4 #aa is a diagram showing the direction and course of the air-current4 ,it will be noticed that both the out-going and return tube are operated by pressure1 in distinction from exhaust4 ,the air is forced around the circuit by the air-compressor4 ,there is no exhausting from the return tube4 ,the pressure of the air when it enters the tube at the main post-office is1 say1 seven pounds per square inch2 when it arrives at the sub-post-office the pressure is about three and three-quarters pounds1 and when it gets back to the main office and enters the return tank1 the pressure is zero or atmospheric4 ,thus it will be seen that the pressure becomes less and less as the air flows along the tube4 ,this is not the pressure that moves the carriers1 but the pressure of the air in the tube1 a pressure that exists when there are no carriers in the tube4 ,it is the pressure that would be indicated if you should drill a hole into the tube and attach a gauge4 ,illustration3 ,,fig4 #aa4 ,terminal ,apparatus4--,when the construction of this line was begun1 it was the intention of the ,pneumatic ,transit ,company to use the apparatus of the ,electro-,pneumatic ,transit ,company1 at both stations1 for sending and receiving carriers1 and so-called working-drawings were obtained for this purpose4 ,the sending apparatus was constructed according to the designs furnished1 but1 upon examination of the drawings of the receiving apparatus1 it was so apparent that it would not work as intended that it was never constructed4 ,the writer was asked to design an automatic receiver to stop the carriers without shock upon their arrival at the stations1 and to deliver them upon a table without appreciable escape of air1--something that would answer the requirements of the present plant4 ,illustration3 ,,fig4 #ab4 ,,transmitter4--,,phila4 ,,sending ,,apparatus4 ,the ,sender4--,the sending apparatus is for the purpose of enabling the operator to place a carrier in the tube without allowing the air to escape4 ,in other words1 it is a means of despatching carriers4 ,the apparatus for this purpose1 already referred to1 is simply a valve4 ,a side view and section of it are shown in ,fig4 #ab4 ,fig4 #ae is a view of the apparatus in the main post-office4 ,the sending apparatus is seen on the left4 ,fig4 #ac is a view of the sub-post-office apparatus1 and here a man may be seen in the act of despatching a carrier4 ,referring to the section1 ,fig4 #ab1 it will be seen that the sending apparatus consists of a short section of tube supported on trunnions and enclosed in a circular box4 ,normally this short section of tube stands in line with the main tube1 and the air-current passes directly through it4 ,it is shown in this position in the figure4 ,when a carrier is to be despatched1 this short section of tube is rotated by a handle until one end comes into coincidence with an opening in the side of the box4 ,in this position the air flows through the box around the movable tube4 ,a carrier can then be placed in the short section of tube and be rotated by the handle into line with the main tube4 ,the carrier will then be carried along with the current of air4 ,a circular plate covers the opening in the box where the carrier is inserted when the sending apparatus is closed4 ,at the sub-post-office this sending apparatus is placed in a horizontal position1 but its operation is the same4 ,illustration3 ,,fig4 #ac4 ,,receiving ,,and ,,sending ,,apparatus ,,in ,,the ,,sub-,,post-,,office4 ,illustration3 ,,fig4 #ad4 ,,apparatus ,,at ,,sub-,,station--,,phila4 ,illustration3 ,,fig4 #ae4 ,,terminals ,,of ,,the ,,tube ,,in ,,the ,,main ,,post-,,office4 ,sub-,post-,office ,receiver4--,we have already explained that the air-pressure in the tube at the sub-post-office is about three and three-quarters pounds per square inch4 ,with such a pressure we cannot open the tube to allow the carriers to come out4 ,they must be received in a chamber that can be closed to the tube after the arrival of a carrier and then opened to the atmosphere4 ,furthermore1 this chamber must act as an air-cushion to check the momentum of the carriers4 ,fig4 #ac shows the sub-post-office apparatus when a carrier is being delivered from the receiving apparatus1 or1 as we will name it for convenience1 the receiver4 ,fig4 #ad is a drawing of the same apparatus1 partly in section1 that shows more clearly its method of operation4 ,this drawing shows the sending apparatus in a different position from ,fig4 #ac1 but that is immaterial4 ,the receiver consists of a movable section of tube1 about twice the length of a carrier1 closed at one end1 supported upon trunnions1 and normally in a position to form a continuation of the main tube from which the carriers are received4 ,when a carrier arrives it runs directly into the receiver1 which being closed at the end forms an air-cushion that stops the carrier without shock or injury4 ,just before reaching the receiving chamber the current of air passes out through slots in the walls of the tube into a jacket that conducts it to the sending apparatus1 as shown in ,fig4 #ad4 ,at the closed end of the receiving chamber1 or air-cushion1 is a relief valve1 normally held closed by a spring4 ,as the carrier compresses the air in front of it1 this valve opens and allows some of the air to escape1 which prevents the carrier from rebounding into the tube4 ,under the outer end of the receiving chamber is a vertical cylinder1 ,e1 ,fig4 #ad1 supported upon the base-plate containing a piston4 ,the piston of this cylinder is connected by a piston- and connecting-rod to the receiving chamber4 ,when air is admitted to the cylinder under the piston1 the latter rises and tilts the receiving chamber to an angle of about forty degrees1 which allows the carrier to slide out4 ,the receiving chamber carries a circular plate1 ,c1 that covers the end of the main tube when it is tilted4 ,a small piston slide-valve1 ,f1 located near the trunnion of the receiving chamber1 controls the admission and discharge of air to and from the cylinder ,e1 upon the arrival of a carrier4 ,when a carrier arrives and compresses the air in the air-cushion or receiving chamber1 a small portion of this compressed air is forced through pipe ,g1 to a small cylinder containing a piston and located just above the piston slide-valve ,f4 ,the increased pressure acting on the piston moves it downward1 and it in turn moves the slide-valve ,f4 ,thus it will be seen that the stopping of the carrier causes the receiving chamber to be tilted and the carrier slides out on to an inclined platform1 ,k4 ,this platform is hinged at one end1 and supported at the angle seen in the figure by a counterweight4 ,when a carrier rests upon it1 the weight of the carrier is sufficient to bear it down into a horizontal position2 in this position the carrier rolls off on to a table or shelf4 ,the platform1 ,k1 is connected by rods1 bell-cranks1 etc41 to the piston slide-valve1 so that when it swings downward by the weight of a carrier1 the slide-valve is moved upward into its normal position1 and this causes the receiving chamber to tilt back into a horizontal position ready to receive the next carrier4 ,the time that elapses from the arrival of a carrier until the receiving chamber has returned to its horizontal position is not more than three or four seconds4 ,nothing could operate in a more satisfactory manner4 ,main ,post-,office ,receiver4--,at the main post-office we have a receiver of a different type4 ,it will be remembered that the pressure in the return tube at the main post-office is nearly down to zero or atmospheric1 so that we can open the tube to allow the carriers to pass out without noise or an annoying blast of air4 ,figs4 #ae and #af show the main-office apparatus1 and ,fig4 #ag is a drawing of the same4 ,here the receiver consists of a section of tube closed by a sluice-gate1 located at ,b1 ,fig4 #ag4 ,the air-current passes out through slots in the tube into a branch pipe leading to the return tank in the basement4 ,these slots are located about four feet back of the sluice-gate1 so that the portion of the tube between the slots and the sluice-gate forms an air-cushion to check the momentum of the carriers4 ,the sluice-gate is raised and lowered by a piston moving in a cylinder located just above the gate4 ,the movement of this piston is controlled by a piston slide-valve in a manner similar to the apparatus at the sub-post-office4 ,air for operating the piston is conveyed through the pipe ,d1 ,fig4 #ag1 from the pipe leading from the air-compressor to the sending apparatus4 ,this air is at about seven pounds pressure per square inch4 ,illustration3 ,,fig4 #af4 ,,receiving ,,apparatus ,,at ,,the ,,main ,,post-,,office4 ,illustration3 ,,fig4 #ag4 ,,apparatus ,,at ,,the ,,main ,,office--,,phila4 ,when a carrier arrives1 after passing the slots that allow the air-current to flow into the branch pipe1 it compresses the air in front of it against the gate4 ,this compression checks its momentum1 and it comes gradually to rest4 ,the air compressed between the carrier and the sluice-gate operates to move the piston slide-valve1 thereby admitting air to the gate cylinder under the piston1 which rises1 carrying with it the sluice-gate4 ,the tube is now open to the atmosphere1 and there is just sufficient pressure in the tube to push the carrier out on to a table arranged to receive it4 ,as the carrier passes out of the tube it lifts a finger out of its path4 ,this finger is located at ,e1 ,fig4 #ag1 and when it is lifted by the passing carrier it moves the piston slide-valve1 and the sluice-gate is closed4 ,a valve is located in the branch-pipe that conducts the air to the return tank in the basement4 ,if the pressure in the tube is not sufficient to push the carrier out on to the table1 this valve is partially closed1 thereby increasing the pressure to a desired amount4 ,illustration3 ,,fig4 #ah4 ,,carrier4 ,illustration3 ,,fig4 #ai4 ,,carrier4 ,the ,carrier4--,we have frequently spoken of the carrier1 which contains the mail and other parcels that are transported from one office to the other4 ,in ,fig4 #ac1 showing the sub-post-office apparatus1 we see one of these carriers being despatched by the attendant and another being delivered from the tube4 ,in ,fig4 #ae several carriers may be seen standing on the floor4 ,fig4 #ah shows a carrier with the lid open1 ready to receive a charge of mail1 and ,fig4 #ai shows the same closed1 ready for despatching4 ,the construction of the carrier is shown by the drawing1 ,fig4 #bj4 ,the body of the carrier is steel1 about one-thirty-second of an inch in thickness4 ,it is made from a flat sheet1 bent into a cylinder1 riveted1 and soldered4 ,the length outside is eighteen inches1 and the inside diameter is five and one-quarter inches4 ,the front end is made of a convex disk of steel1 stamped in the desired form1 and secured to the body of the carrier by rivets1 with the convex side inward4 ,it is necessary to have a buffer upon the front end of the carrier to protect it from blows that it might receive1 and this buffer is made by filling the concave side of the front head with felt1 held in place by a disk of leather and a central bolt4 ,the leather disk is made of two pieces1 riveted together1 with a steel washer between4 ,the steel washer is attached to the head of the bolt4 ,the carrier is supported in the tube on two bearing-rings1 located on the body of the carrier a short distance from each end4 ,the location of these rings is so chosen that it permits a carrier of maximum length to pass through a bend in the tube of minimum radius without becoming wedged4 ,this is a very important feature in the construction of carriers1 but does not appear to have been utilized in other systems4 ,illustration3 ,,fig4 #bj4 ,,mail ,,carrier4--,,phila4 ,the bearing-rings are made of fibrous woven material1 especially prepared1 and held in place by being clamped between two metal rings1 one of which is riveted to the body of the carrier4 ,of course these rings wear out and have to be replaced occasionally1 but their usual life is about one thousand miles4 ,the rear end of the carrier is closed by a hinged lid and secured by a special lock4 ,the lock consists of three radial bolts that pass through the body of the carrier and the rim of the lid4 ,these bolts are thrown by three cams1 attached to a short shaft that passes through the lid and has a handle or lever attached to it upon the outside of the lid4 ,this cam-shaft is located out of the geometrical centre of the lid in such a position that when the lever or handle is swung around in the unlocked position1 it projects beyond the periphery of the lid1 and in this position the carrier will not enter the tube4 ,when the lid is closed and locked1 the lever lies across the lid in the position shown in ,fig4 #ai1 and when the carrier is in the tube it cannot become unlocked1 for the lever cannot swing around without coming in contact with the wall of the tube4 ,this insures against the possibility of the carriers opening during transit through the tube4 ,the empty carriers weigh about nine pounds1 and when filled with mail1 from twelve to fifteen pounds4 ,they have a capacity for two hundred ordinary letters1 packed in the usual manner4 ,operation of the ,tubes4--,the tubes are kept in constant operation during the day1 and six days of the week4 ,the air-compressor is started at nine o'clock in the morning and runs until seven in the evening1 except during the noon hour1 the air flowing in a constant steady current through the tubes4 ,when a carrier is placed in the tube it is carried along in the current without appreciably affecting the load on the compressor4 ,carriers may be despatched at six-second intervals1 and when they are despatched thus frequently at each office1 there will be eighteen carriers in the tube at the same time4 ,if ten carriers per minute are despatched from each office1 and each carrier contains two hundred letters1 the tube has a carrying capacity of two hundred and forty thousand letters per hour1 which is far beyond the requirements of this office4 ,about five hundred carriers a day are despatched from each office4 ,this varies considerably on different days and at different seasons of the year4 ,experience has taught that a certain period of time should elapse between the despatching of carriers1 in order that they may not come in contact with each other1 and that the receivers may have time to act4 ,with the present plant this period is made about six seconds4 ,in order to make it impossible for carriers to be despatched more frequently than this1 time-locks are attached to the sending apparatus4 ,one of these locks may be seen in ,fig4 #ac1 connected to the handle of the sending apparatus4 ,it is so arranged that when a carrier is despatched a weight is raised and allowed to fall1 carrying with it a piston in a cylinder filled with oil4 ,while the weight is rising and falling the sending apparatus is locked1 but becomes unlocked when the weight is all the way down4 ,a by-pass in the cylinder permits the oil to flow from one side of the piston to the other1 and the size of this by-pass can be regulated1 thus determining the time that the weight shall take in descending4 ,this makes a simple and effective time-lock that does not get out of order4 ,the time required for a carrier to travel from the main to the sub-post-office is sixty seconds1 and from the sub- to the main post-office1 fifty-five seconds4 ,this difference of time in going and returning is due to the expansion of the air in the tube1 as will be explained more fully in another place4 ,the distance between the offices being two thousand nine hundred and seventy-four feet1 gives an average speed of about fifty-two feet per second1 or #ce4bg miles per hour4 ,of course the speed can be increased by increasing the air-pressure1 but this speed is found in practice to be ample for all requirements4 ,in order to give some idea of the energy possessed by one of these carriers travelling at this speed1 it may be said that if the end of the tube were left open and turned upward1 an emerging carrier would rise about forty feet into the air4 ,it is easy to imagine how apparatus1 depending for its operation upon impact with a moving carrier1 would be soon destroyed1 as well as the carriers themselves4 ,this is why receiving apparatus used with small tubes and light carriers cannot be applied to large tubes with heavy carriers4 ,no serious trouble has ever been experienced from carriers getting wedged in any part of these tubes4 ,benefits of the ,system4--,the advantages to the post-office department by the adoption of this system have been numerous1 and the post-office officials who are familiar with the operation of the tubes frequently speak in high terms of their usefulness4 ,formerly the mail was transported from one office to the other by a wagon making a trip every half-hour4 ,considerable time has been saved by the greater speed of transit1 but even more time is gained by keeping the mail moving instead of allowing it to accumulate and then despatching it in bulk4 ,with the pneumatic system a letter posted in the sub-post-office will reach its destination just as quickly as if posted at the main office1 and sometimes more quickly4 ,let us take an example1 first1 with the old wagon service4 ,suppose that you drop a letter in the sub-post-office2 it lies there1 say1 fifteen minutes waiting for the departure of the next wagon2 it is put into a pouch with hundreds of other letters1 and ten minutes are consumed in transporting it to the main office4 ,when it arrives there the pouch is thrown on the floor at the entrance of the building2 in a few minutes1 more or less1 a clerk takes the pouch1 throws it on a truck and wheels it around to the cancelling machines1 where it may lie for five or ten minutes more before being opened1 and then perhaps five minutes will elapse before your letter reaches the cancelling machine4 ,it would not be unusual for three-quarters of an hour to elapse from the time you dropped your letter in the office until it was cancelled4 ,now with the pneumatic tube service forty minutes of this time will be saved2 for immediately after you drop your letter in the office it will be despatched through the tube and delivered on the table in front of the cancelling machines4 ,soon after the tubes were installed the postmaster's attention was called to an instance where letters from the sub-office were sent through the tube and were despatched to ,new ,york ,city one train earlier than they could have been had the old wagon service been in use4 ,people frequently post letters requesting that they be sent through the tube2 of course they would be sent in that way if the request was not made1 but it shows that the public recognize the better service4 ,formerly mail was collected from the street boxes in the banking section of the city and the collectors carried it to the main office4 ,after the tubes were installed this mail was carried to the sub-post-office to be sent through the tube1 and the time formerly occupied in walking to the main office was then utilized in having the men face up the letters ready for the cancelling machines1--a double saving in time besides making their labor much lighter and enabling them to do more useful work4 ,since the sub-office has been established in the ,bourse1 it has been made a distributing as well as receiving office4 ,at least two more deliveries of mail are made each day in the ,bourse building than in any other office building in the city4 ,all letters mailed in the sub-office with a special delivery stamp are despatched through the tube immediately4 ,it is now nearly four years since the system was put into operation4 ,during that time more than thirty-five million letters have been transported1 and all the repairs to the system have not required it to be stopped for more than a few hours4 ,during the first year the ,pneumatic ,transit ,company operated the tubes at their own expense1 agreeing at the end of that time to take them out if the government so requested4 ,since the first year the government has paid the running expenses4 ,such is the history of the first ,united ,states pneumatic postal system4 ,such is the history of the first pneumatic tubes of sufficient size to carry all the first class and most of the lower classes of mail1 in this or any other country1 so far as the writer knows4 ,,chapter ,,iii4 ,,the ,,system ,,and ,,apparatus ,,of ,,the ,,batcheller ,,pneumatic ,,tube ,,company4 ,general ,arrangement and ,adaptability of the ,system4--,the experience gained in the construction and operation of the ,philadelphia post-office tubes has naturally suggested improvements that can be made in future construction1 and1 furthermore1 it has taught us what the requirements will be of an extensive system of tubes laid in the streets of our cities1 both for the transmission of mail and for a general commercial business4 ,since the ,philadelphia post-office tube was completed1 we have been busily engaged in working out all the details of a system of many stations so connected together that carriers can be despatched in the most direct manner possible from any station to any other4 ,it is the purpose of the present chapter to describe this system4 ,while the ,pneumatic ,transit ,company has ample field in the ,state of ,pennsylvania to carry out the work which it has mapped out1 a field broad enough to yield a good profit for the capital invested1 there is no reason why the system should be limited to one ,state4 ,so1 in order to obtain a broader charter1 covering all places where pneumatic service may be needed1 a new corporation was formed and styled the ,,batcheller ,,pneumatic ,,tube ,,company4 ,it is impossible to lay down a rigid system equally well adapted to all places and purposes4 ,we must accommodate ourselves somewhat to circumstances4 ,for example1 the post-office department may require one size of tube1 arranged to operate in a particular way1 while the requirements of a parcel delivery business would be utterly different4 ,the geographical location of the stations will have much to do with the general arrangement2 also the condition of the streets4 ,some of the streets of our large cities are so filled with water- and gas-pipes1 electrical conduits1 sewers1 steam-pipes1 etc41 etc41 that it is almost impossible to find space for pneumatic tubes1 especially of large diameter4 ,railway or water facilities have much to do with the location of a central pumping station1 on account of the coal supply4 ,all of these and many other things have to be taken into consideration in planning a system for any locality4 ,we have an example of a peculiar location and conditions in a proposed line of tubes over the ,new ,york and ,brooklyn bridge connecting the main post-offices of those cities4 ,this would be in many respects a unique plant4 ,two air-compressors would be used1 one at each office4 ,in order to give a general idea how a large number of stations can be connected into one system1 the diagram ,fig4 #ba has been drawn4 ,we have already referred to the attempts of ,clay and ,lieb to devise means whereby several stations could be located along a main line and carriers be sent from any station to any other through the main line4 ,their method was to use branch tubes leading off from the main line with switches at the junctions4 ,they deflected the air-current into the branch by placing an automatic closed valve in the main line just beyond the junction1 returning the air from the branch to the main line just beyond the valve4 ,the carriers were to open and close this valve automatically as they passed4 ,the branch and switch system has many attractions for the inventor1 and upon first thought it would seem the most feasible solution of the problem4 ,it has been the dream of more than one inventor1 as the records of the patent-office show1 but no one has succeeded in working it out4 ,the current of air cannot be divided2 carriers passing from the branch into the main line must not collide with other carriers running in the main line2 a certain minimum distance must always be maintained between the carriers in the same tube2 when a carrier is despatched it must go directly to the station for which it is intended without further attention from the sender and it must not interfere with other carriers2 expense of manufacture prohibits the use of any but round smooth tubes up to eight inches in diameter1 hence projections cannot be placed upon the carrier to give it an individuality and cause it to operate a switch at any particular point along the line2 the carrier is free to rotate in a round tube about its longitudinal axis1 therefore1 its individuality must be indicated by some symmetrical marking about this axis1 if it is to be automatic in its operation2 the speed of the carrier is so high that electrical contacts placed in distinctive positions on the carriers cannot be used while it is in motion1 for mechanism having inertia could not be moved during the short time that the electric circuit would be closed2 only the simplest attachments can be made to the carrier1 for constructional reasons and because of the rough usage that they receive4 ,these and numerous other reasons make the problem most difficult4 ,we have not attempted to solve it by the use of branch tubes and electrically operated switches1 but have adopted the simpler and equally effective method of carrying the main line through each of the stations that it unites4 ,in our system each carrier has an individuality determining the station at which it will be discharged from the tube4 ,by a simple attachment to1 the front end of the carrier1 consisting of a circular metal disk1 the sender so marks the carrier that it will pass all stations until it arrives at the station for which it was destined and will there pass out of the tube4 ,in addition to this a method has been devised whereby carriers can be inserted into the tube without the possibility of collision with carriers already running in the tube4 ,illustration3 ,,fig4 #ba4 ,a ,,diagram ,,showing ,,various ,,methods ,,of ,,connecting ,,the ,,stations ,,of ,a ,,large ,,system ,,with ,,pneumatic ,,tubes4 ,referring now to the diagram1 ,fig4 #ba1 we have here an imaginary system which we will suppose to be located in some large city4 ,the two large squares ,i and ,,ii indicate central pumping stations1 and the small squares ,a1 ,b1 ,c1 ,d1 etc41 indicate receiving and sending stations4 ,some of the stations1 such as ,a1 ,b1 ,c1 ,d1 ,e1 ,f1 and ,y1 which do a large amount of business and may be supposed to be large retail stores1 are connected directly with the central station by double tubes1 one for sending and the other for receiving carriers4 ,two smaller stores1 such as ,g and ,h1 may be located on the same line4 ,at ,i1 ,j1 ,k1 and ,l we have four stations1 all connected by the same double line of tubes4 ,these stations we will imagine to be located in the residence section of the city4 ,carriers containing parcels of merchandise or other matter destined for private residences would be sent from the stores ,a1 ,b1 ,c1 etc41 to the central station ,i1 where they would be transferred to the line #b and be adjusted to stop at the station nearest the residence to which the parcels were addressed4 ,from this station the parcels would be delivered by messengers to the residences4 ,if a carrier is to be sent from the central station ,i to station ,k1 it will be so adjusted before it is put into the tube that it will pass stations ,i and ,j1 but be discharged automatically from the tube when it arrives at station ,k4 ,in a similar manner carriers can be despatched from station ,l to station ,i or from station ,j to station ,l4 ,in passing through the central station the carriers are manually transferred from one line to another4 ,in another part of the city we may have another central pumping station1 ,,ii2 and the two central stations may be connected by a double trunk line1 #c4 ,again1 we have lines radiating from this central station1 as shown by station ,y4 ,there will be some localities where it will be an advantage to arrange the stations upon a loop1 as shown in circuit #d1 where stations ,s1 ,t1 ,u1 ,v1 ,w1 and ,x are connected together in this way4 ,or we can combine the two arrangements of loop and direct line1 as shown in circuit #e4 ,stations ,o and ,r are on the double line1 but from ,o a loop is formed including stations ,n1 ,m1 ,p1 and ,q4 ,here it is supposed that the stations ,o and ,r do a much larger business with the central station ,,ii than the stations ,n1 ,m1 ,p1 and ,q1 this being the principal reason for placing them on the double line4 ,all carriers must be returned to the station from which they were sent1 or others to replace them1 otherwise there will be an accumulation of carriers at some of the stations4 ,it is like a railway3 there must be as many trains despatched in one direction as the other1 each day4 ,station ,o can receive a carrier from the central station and return it directly1 but when station ,n receives one it must be returned via ,m1 ,p1 ,q1 ,o1 and ,r1 a much longer route than that by which it was received4 ,this disadvantage is compensated1 when stations ,n1 ,m1 ,p1 and ,q do only a small amount of business1 by the less cost of laying a single line4 ,if a carrier is to be sent from ,m to ,n1 it must go via ,p1 ,q1 and ,o1 being manually transferred at ,o from the 8down0 to the 8up0 line4 ,p can send directly to ,q1 but ,q must send to ,p via ,o1 ,n1 and ,m4 ,r can send directly to ,o and ,o to ,r4 ,similarly in circuit #d the carriers must all travel around the loop in the same direction1 shown by the arrows4 ,station ,s can receive carriers directly from the central station1 but they must return via ,u1 ,w1 ,x1 ,v1 and ,t4 ,again1 we may have a double-loop line1 as indicated in the diagram by circuit #f4 ,here five stations1 a1 b1 c1 d1 and e1 are connected by a loop consisting of two lines of tube1 in which the air circulates in one direction in one line and in the opposite direction in the other4 ,here b can send directly to c1 c directly to b1 and e to b via d and c1 or via central and a4 ,this is an arrangement that would be used where there is a large amount of business between the stations on the loop4 ,as stated before1 the best arrangement for any particular locality depends entirely upon circumstances4 ,size of ,tubes4--,the pneumatic-tube system that we are describing is not limited to any particular size of tube4 ,the size is usually determined by the number and size of packages to be transported4 ,a small tube1 two or three inches in diameter1 is best suited for telegrams and messages2 mail1 parcels1 etc41 require a six- or eight-inch tube1 while mail pouches and bulky material1 a thirty-six inch or possibly larger tube4 ,we divide tubes into three classes1 according to their size1 naming them small1 large1 and very large tubes4 ,by small tubes we mean those not larger than three or possibly four inches in diameter4 ,large tubes are those having a diameter more than four inches and not more than eight inches4 ,very large tubes include all that are more than eight inches4 ,this classification is for convenience1 but it has a deeper significance4 ,for example1 in the transportation of mail1 it must either be handled in bulk1 that is1 in pouches1 or in broken-bulk1 that is1 loose or tied up in small packages4 ,there are many advantages in transporting it in broken-bulk1 in fact1 there are very few places where it could be handled in any other way4 ,for this service six- or eight-inch tubes--not larger--are best suited4 ,the carriers are light enough to be easily handled2 they are not so large in capacity as to make it necessary to wait for an accumulation of mail to fill them2 they can be delivered from the tube on to tables at any point in the building where the mail is wanted1 for cancelling1 distribution1 or pouching1 thus rendering a very rapid service2 the mail is kept moving in an almost constant stream1 keeping the postal employees more uniformly employed2 special carriers can be despatched with 8special delivery0 letters4 ,in other words1 the most rapid service can be rendered by this size of tube4 ,if a larger than eight-inch tube is to be used for mail service1 it should be not less than thirty-six inches4 ,carriers larger than eight inches cannot be handled3 they are too heavy4 ,they are also too heavy to slide through the tube1 hence1 must be mounted upon wheels4 ,it is not practical to make a carrier on wheels less than eighteen or twenty-four inches1 and the carrier must be at least twenty-four inches to contain a large mail-pouch4 ,now1 if we are going to despatch mail-pouches through a pneumatic tube we must send more than one in a carrier1 otherwise the service will be too slow4 ,such large carriers could not be despatched oftener than once or at most twice in a minute4 ,suppose we were to transport the mail from a railway station to a main post-office4 ,a train arrives with1 say1 sixty pouches4 ,if only one pouch could be put into a carrier and the carriers could be despatched at half-minute intervals1 it would take thirty minutes to despatch all the pouches4 ,now1 suppose we make the tube thirty-six inches4 ,the carriers will be eight feet long and will contain from twelve to fifteen pouches4 ,five carriers would contain the entire train-load of mail1 and they could be despatched in four or five minutes4 ,illustration3 ,,fig4 #bb4 ,,cross-,,section ,,of ,a #cf-;,,inch ,,tube4 ,illustration3 ,,fig4 #bc4 ,,carrier ,,for ,a #cf-;,,inch ,,tube4 ,system of ,very ,large ,tubes4--,the cross-section of a thirty-six-inch tube is shown in ,fig4 #bb4 ,it is built flat on the bottom and sides1 with an arched top4 ,the floor is of concrete containing creosoted ties2 the side walls and top are of brick1 plastered with cement upon the interior4 ,the two tubes may be built one above the other or side by side1 depending upon the condition of the streets1 but one common separating wall will serve for both4 ,the carriers1 one of which is shown in ,fig4 #bc1 run on two rails laid close to the sides of the tube4 ,at curves a guard-rail is placed upon the side wall1 making it impossible for a carrier to leave the track4 ,the carriers are made of hard wood with an iron frame1 and are as light as consistent with the service required of them4 ,they are open on top4 ,their outside dimensions are thirty-four inches by thirty-four inches by eight feet4 ,the sending and receiving apparatus for these very large tubes have to be specially designed for each particular station1 so no attempt will be made here to describe them4 ,the air-pressure required depends upon the length of the line4 ,if it were not more than six or eight ounces a fan would be used to maintain the air-current1 but for pressures above this1 up to a pound or two per square inch1 some form of positive blower would be used4 ,at the stations considerable floor space or 8yard room0 would be required for side tracks1 switches1 etc4 ,usually the basement of a building would have to be utilized for the termination of such a tube4 ,there are but few places in our large cities where the streets are so free from pipes1 sewers1 conduits1 etc41 that it would be practicable to build a thirty-six-inch pneumatic tube4 ,when the service can be rendered by an eight-inch tube1 the cost of installation favors its adoption4 ,steep grades cannot be ascended by these very large tubes1 while the eight-inch tubes can be placed vertically4 ,we do not say that there is no use for eighteen- and twenty-four-inch tubes1 but the demand for them would be in special cases and we will not discuss them here4 ,for ordinary mail and parcel service we recommend the use of six- and eight-inch tubes4 ,an eight-inch carrier is twenty-four inches long1 about seven inches inside diameter1 and will contain five hundred ordinary letters4 ,it weighs about thirteen pounds empty1 and one can be despatched every six to ten seconds4 ,we estimate that eighty per cent4 of all the parcels delivered from a large retail department store could be wrapped up to go into these carriers4 ,the minimum radius of curvature of an eight-inch tube is eight feet4 ,general ,arrangement of ,apparatus in the ,stations4 ,two-,station1 ,two-,compressor ,line4--,we will now proceed to a description of our system in detail4 ,figs4 #bd1 #be1 and #bf are diagrams showing how the tubes1 air-compressor1 tanks1 sending and receiving apparatus are connected together at the stations4 ,these diagrams are drawn to represent an eight-inch tube1 but essentially the same arrangement would be used for smaller tubes4 ,fig4 #bd represents a line of two stations with an air-compressor at each station4 ,such an arrangement is proposed for the line of postal tubes over the ,new ,york and ,brooklyn bridge1 or for any two stations located a very long distance apart1 say six or eight miles4 ,illustration3 ,,fig4 #bd4 ,,diagram ,,of ,a ,,two-,,station1 ,,two-,,compressor ,,line4 ,referring to the diagram1 we have at station ,a an air-compressor1 c1 which draws its supply of air from the tank e1 and delivers it1 compressed to the necessary pressure1 into the tank d4 ,from the tank d the air flows to the sending apparatus1 a1 and thence through the tube f to the station ,b4 ,upon arrival at ,b it flows through the receiving apparatus m1 and then by the pipe l to the tank j4 ,a second air-compressor1 o1 is located at station ,b1 and it draws its supply of air from the tank j4 ,the tank j has an opening to the atmosphere1 i1 through which air can enter when the air-compressor draws more than is supplied from the pipe l4 ,the opening i in the tank j serves as an escape for air when the air-compressor at station ,a is started before that at station ,b4 ,stations ,a and ,b are similar in their arrangements4 ,at ,b the air-compressor o delivers its compressed air to the tank k1 from which it flows to the sending apparatus n1 and thence through the tube g back to station ,a4 ,upon its arrival at ,a it passes through the receiving apparatus and enters the tank e1 which is open to the atmosphere at h4 ,the tanks d and k serve as separators to remove from the air any dirt and oil coming from the compressors1 and they form a cushion1 deadening1 to some extent1 the pulsations of the compressors and making the current of air in the tubes more steady and uniform4 ,the tanks e and j form traps to catch any moisture1 oil1 or dirt coming out of the tubes4 ,carriers are placed in the tubes and despatched by means of the sending apparatus a and n4 ,they are received from the tubes and delivered on to tables by means of the receiving apparatus b and m4 ,it will be seen that the arrangement is such that the air flows through one tube and returns through the other1 the same air being used over and over again4 ,any air that escapes at the sending and receiving apparatus is replaced by an equal amount entering the tanks e and j from the atmosphere4 ,by thus keeping the same air circulating in the tubes we prevent an accumulation of moisture in the tubes4 ,the air is at its maximum pressure in the tanks d and k4 ,the pressure falls gradually as it flows along the tubes and is down to atmospheric when it enters the tanks e and j4 ,the pressure at the receivers1 b and m1 is just sufficient to push the carriers out on to the tables4 ,the construction of the sending and receiving apparatus will be described in another place4 ,illustration3 ,,fig4 #be4 ,,diagram ,,of ,a ,,two-,,station1 ,,one-,,compressor ,,line4 ,two-,station1 ,one-,compressor ,line4--,fig4 #be is a diagram showing two stations1 ,a and ,b1 connected by a double line of tubes1 both operated by one air-compressor located at station ,a4 ,this is the arrangement used in the ,philadelphia post-office line1 and is the arrangement that will ordinarily be used for all two-station lines except where unusual conditions require something different4 ,station ,a is arranged precisely like station ,a in ,fig4 #bd1 so it need not be described again4 ,the air flows from the sending apparatus a through the tube f to the receiving apparatus p at station ,b4 ,from the receiver p it flows through the pipe l to the sending apparatus n and thence through the tube g back to station ,a4 ,the receiver p at station ,b is what we will call a closed receiver1--i4e41 it delivers the carrier from the tube on to the table without opening the tube to the atmosphere4 ,the use of this form of receiver is made necessary by the fact that the air-pressure in the tube at this station is considerably above atmospheric4 ,the air-pressure is at a maximum in the tank d4 ,it falls gradually along the tube f1 and when the air arrives at the receiver p1 at station ,b1 the pressure has fallen nearly to one-half its maximum amount in the tank d4 ,on its return journey through the tube g the pressure continues falling until it reaches the atmospheric pressure when the air enters the tank e at station ,a4 ,the entire line of tube1 going and returning1 is operated by air at a pressure above the atmospheric4 ,there is no exhausting in the return tube4 ,it is distinctly a pressure system4 ,illustration3 ,,fig4 #bf4 ,,diagram ,,of ,a ,,three- ,,to ,,eight-,,station ,,line4 ,three- to ,eight-,station ,line4--,thus far we have described only two-station lines4 ,in ,fig4 #bf we have a diagram of three stations connected together by a double line of tubes1 and the arrangement would be similar if it were extended to four1 five1 six1 seven1 or eight stations4 ,the stations are called ,a1 ,b1 and ,h4 ,station ,a is arranged exactly the same as stations ,a in ,figs4 #bd and #be1 therefore1 needs no description4 ,station ,b1 being an intermediate station1 is quite differently arranged from any of the preceding4 ,from station ,a the air flows through the tube f to station ,b1 where it enters the automatic receiving and transferring apparatus1 s4 ,from this it flows through the tube f to the sending apparatus r1 and thence through the tube f to the next station1 which may be another intermediate station1 ,c1 or the terminal station ,h4 ,station ,h is arranged like station ,b1 ,fig4 #be4 ,the air from the tube f enters the receiver p1 and is then returned1 through the pipe l1 to the sending apparatus n4 ,from the sending apparatus n it continues on its return journey through the tube g to the intermediate station ,b1 where it enters the receiver and transfer apparatus t1 then passes to the sending apparatus q1 and through the tube g back to the receiver b at station ,a4 ,thus we have followed the air-current out through one tube and back through the other4 ,the current is kept circulating by the compressor located at station ,a4 ,the pressure is at a maximum in the tank d1 and falls gradually as the air flows along the tube until it returns to the tank e1 when the pressure has fallen to atmospheric4 ,a carrier is despatched from station ,a1 and after passing through the tube f arrives at station ,b1 where it stops momentarily in the automatic receiver and transfer apparatus s4 ,if the carrier is intended for station ,b1 and was properly adjusted when it was despatched at ,a1 it will be discharged from the apparatus s on to the table u4 ,but if it were intended for some other station and were so adjusted1 after the delay of two or three seconds in the apparatus s1 it will be automatically transferred to the tube f1 pass through the sending apparatus r1 and go on its journey through tube f to the next station4 ,if it is not discharged from the tube at any of the intermediate stations1 it will finally arrive at the terminal station ,h and there stop4 ,just how the carriers are adjusted and the details of the receiving and transfer apparatus will be described hereafter4 ,carriers arrive at station ,b from ,h1 or other stations on the line1 through tube g1 in the apparatus t1 which either discharges them on to the table u or sends them on through the tube g and g to station ,a4 ,carriers are despatched from station ,b to station ,a by means of the sending apparatus q1 and from station ,b to other stations along the line1 ,c1 ,d1 ,e1 ,f1 ,g1 and ,h1 by means of the sending apparatus r4 ,thus1 from ,b carriers can be sent and received in either direction4 ,in order to prevent the possibility of a collision of carriers by attempting to despatch one at station ,b at the instant another is passing through the sending apparatus1 an automatic lock is attached to each sending apparatus4 ,just outside the station ,b1 say three hundred feet on each side1 are located manholes1 and in these manholes boxes are attached to the tube containing an electric circuit-closing apparatus1 so arranged that when a carrier passes it will close an electric circuit leading to the sending apparatus in the station4 ,these manholes and circuit-closers are shown and located on the diagram at v and w4 ,wires x and y lead from them to the sending apparatus r and q4 ,when a carrier from station ,a passes the box v1 it closes the electric circuit x1 which sets a time-lock on the sending apparatus r1 holding this apparatus locked1 so that it is impossible to despatch a carrier for1 say1 twelve seconds1 a sufficient time for the carrier coming from the station ,a to pass station ,b and get three hundred feet beyond it4 ,after the twelve seconds have elapsed the sending apparatus is unlocked and a carrier can then be despatched4 ,in a similar manner a carrier coming from station ,h1 in passing the box w1 closes the electric circuit y and locks the sending apparatus q for a sufficient length of time to let the carrier pass the station4 ,this resembles1 in some respects1 the 8block system0 as used on railroads4 ,a 8block0 about six hundred feet in length1 depending upon the speed of the carriers1 is made at each intermediate station with the station in the centre of the block4 ,whenever a carrier enters this 8block0 the sending apparatus at the station is locked1 and a carrier cannot be inserted into the tube to collide with the one which is passing4 ,it will be noted that a carrier in passing out of the 8block0 does not unlock the sending apparatus2 this is done automatically at a definite time after the carrier entered the block4 ,the unlocking is entirely independent of the carrier after it has entered the block1 and the reason it is so arranged is this3 suppose that a second carrier enters the 8block0 before the first one leaves it2 if the first carrier unlocked the apparatus when it left the 8block10 then it would be unlocked with the second carrier in the block and a collision might occur1 but by arranging it as we have done1 if a second carrier enters the 8block0 before the first has passed out1 the sending apparatus remains locked for a period of time beginning with the arrival of the first carrier in the 8block0 and ending1 say1 twelve seconds after the arrival of the last carrier1 which is sufficient time for the last carrier to pass out of the block4 ,of course1 if a carrier becomes wedged in the tube a collision may occur1 but this very seldom if ever happens4 ,the details of the locking apparatus will be described in another place4 ,if stations ,a1 ,b1 444 and ,h were arranged on a loop1 as shown in circuit #f1 ,fig4 #ba1 then station ,h1 ,fig4 #bf1 would be at the central1 or station ,a4 ,if it were a single loop1 like circuit #d1 ,fig4 #ba1 then there would be only one sending apparatus and one receiving and transferring apparatus at the intermediate stations4 ,a telephone circuit will include all stations1 in order to give orders to the station attendants and to signal to the central station in case of an accident1 when it might be necessary to stop the air-compressor4 ,the telephone wires1 in the form of a lead-covered cable1 are laid in the same trench with the tubes and fastened to them4 ,illustration3 ,,fig4 #bg4 ,,sending ,,apparatus4 ,illustration3 ,,fig4 #bh4 ,,sending ,,apparatus4--,,longitudinal ,,section4 ,the ,sending ,apparatus4--,we have1 in the preceding pages1 frequently spoken of the sending apparatus1 and have described it as mechanism by which carriers are inserted into the tube4 ,in the ,philadelphia postal line this apparatus consisted of a large valve1 operated by hand4 ,for an eight-inch tube such a valve would be too large and heavy to be manually operated4 ,furthermore1 that type of apparatus is not suited to an intermediate station1 where carriers have to pass through it4 ,to meet all of these requirements we have designed an apparatus1 of which ,fig4 #bg is a side elevation1 ,fig4 #bh a longitudinal section1 and ,fig4 #bi a cross-section4 ,referring to the longitudinal section1 ,fig4 #bh1 the sending apparatus is shown inserted into the line of a pneumatic tube1 ,a1 ,a4 ,we have a movable section of tube1 ,b1 that can be swung about the large bolt1 ,g1 at the top1 into and out of line with the main tube1 ,a1 ,a4 ,when the section of tube ,b is being swung to one side1 the air-current has a by-pass through the slots ,e and ,f and the ,u-shaped pipe ,d4 ,the joints at the ends of the movable section ,b are packed with specially-formed leathers4 ,referring to the cross-section1 ,fig4 #bi1 when the movable section of tube ,b is swung out of line with the main tube1 another and similar tube1 ,c1 takes its place4 ,the two movable tubes1 ,b and ,c1 are made in one piece1 so that they must always move together4 ,they are connected together at each end by plates1 ,m1 that serve not only as connecting-plates1 but covers for the ends of the main-line tube while the tubes ,b and ,c are being moved4 ,the tubes ,b and ,c swing between four plates or wings1 ,l1 that extend out on each side of the apparatus4 ,they serve as guards1 and1 at certain positions of the swinging tubes1 prevent the air from escaping4 ,we will1 for convenience1 call the system of swinging tubes ,b and ,c1 with their supports1 etc41 the swing-frame or simply the frame4 ,this frame is moved or swung from one position to the other by means of a cylinder and piston1 ,h1 placed in an inclined position under it4 ,a lug1 ,n1 is cast on the tube ,b1 to which the connecting-rod1 ,o1 is attached4 ,the cross-head1 ,p1 slides upon an inclined guide1 ,q4 ,on top of the cylinder is placed a controlling valve1 made in the form of a piston slide-valve4 ,the piston in the cylinder ,h is moved by the pressure of the air taken from the main tube through the pipe ,i4 ,the apparatus is operated by a hand-lever1 ,k4 ,when this lever is pulled1 it moves the sliding-head ,r1 and this1 through the spring ,s1 moves the controlling valve1 if the valve is not locked4 ,if it is locked1 pulling the lever simply compresses the spring ,s4 ,when the controlling valve is moved to the right the air in the cylinder ,h escapes through the passage ,v and the port ,j to the atmosphere1 and compressed air from the main tube flows through the pipe ,i1 the passages ,t and ,u1 to the cylinder ,h1 under the piston1 causing the piston to move up the inclined cylinder and swing the frame until the tube ,c is in line with the main tube4 ,carriers are despatched by placing them in the tube ,c1 then pulling the lever ,k1 and swinging the frame until the tube ,c is in line with the main tube4 ,the carrier is then taken up and carried along by the current of air in the main tube4 ,illustration3 ,,fig4 #bi4 ,,sending ,,apparatus4--,,cross-,,section4 ,replacing the hand-lever ,k in its original position returns the frame to its normal position4 ,illustration3 ,,fig4 #cj4 ,,sending ,,time-,,lock4 ,sending ,time-,lock4--,in any system of large pneumatic tubes a short time should elapse between the despatching of carriers1 in order that they may not collide in the tube1 and to give the receiving apparatus at the stations time to act4 ,to insure the impossibility of having carriers despatched too rapidly1 we place on the sending apparatus a time-lock that will automatically lock it for a determined length of time after each carrier is despatched1 the time-lock being adjustable for any desired time4 ,the time-lock1 ,w1 is shown attached to the sending apparatus in ,fig4 #bg4 ,when the swing-frame is swung to despatch a carrier1 it pulls up the rod ,x by means of a link and bell-crank1 ,y1 thereby locking the controlling valve of the cylinder ,h and starting the time-lock ,w1 which will unlock the controlling valve after the required time has elapsed4 ,the details of this time-lock are shown in ,fig4 #cj4 ,it consists of a long vertical cylinder1 ,a1 containing a piston1 ,b1 and a spiral spring1 ,c1 that tends to force the piston to the bottom of the cylinder4 ,the cylinder is filled with oil1 and holes1 ,d1 in the piston allow the oil to pass freely through it when it is moved upward in the cylinder4 ,when the piston moves downward an annular collar1 ,e1 forming a valve1 closes the holes in the piston and prevents the oil from passing through4 ,extending from one side of the piston to the other is a by-pass1 ,f1 in the wall of the cylinder4 ,when the piston moves downward the displaced oil is forced to flow through this by-pass4 ,a small cock1 ,g1 is arranged in the by-pass to throttle the stream of oil flowing through it4 ,the opening in this cock1 or the amount of throttling1 is indicated on the outside by an index and dial1 ,z 7see ,fig4 #bg74 ,when the piston ,b is raised and allowed to descend by the force of the spring ,c1 it forces the oil through the by-pass ,f and the cock ,g4 ,if the latter is wide open the piston will descend quickly1 but if it is nearly closed the piston will descend very slowly4 ,in other words1 the time of descent can be regulated by opening and closing the cock ,g4 ,the reading on the dial ,z can be made seconds of time that elapse while the piston is descending4 ,above the cylinder is a cross-head1 ,h1 that moves up and down between vertical guides4 ,this cross-head is moved by the rod ,x1 also shown in ,fig4 #bg1 that receives its motion from the swinging frame of the sending apparatus4 ,a piston rod1 ,i1 attached to the piston in the cylinder1 extends up through the travelling cross-head but is not attached to it4 ,on the piston-rod are two enlargements1 ,j and ,k1 one made a solid part of it1 the other formed by two nuts4 ,the travelling cross-head ,h carries a pawl1 ,l1 that engages under the shoulder formed by the nuts ,k4 ,this pawl is kept against the piston-rod by the spring ,m4 ,the enlargement1 ,j1 on the piston-rod forms a shoulder that bears against the bell-crank1 ,n1 that connects with the bolt1 ,o1 which locks the controlling valve4 ,in the present down position of the piston and piston-rod1 the enlargement ,j1 by pressing against the bell-crank ,n1 holds the bolt ,o in an unlocked position4 ,when a carrier is despatched the cross-head ,h is lifted by the rod ,x1 and carries with it the piston and piston-rod1 compressing the spring ,c4 ,this upward movement of the piston-rod allows the bolt ,o to be thrown by a spring1 not shown in the figure1 and so lock the controlling valve of the sending apparatus4 ,as the cross-head continues its upward movement1 the pawl ,l comes in contact with the end of the screw ,p and disengages the piston-rod4 ,this allows the piston to descend as rapidly as the oil can pass through the by-pass and cock ,g4 ,when the piston has reached nearly to the bottom of the cylinder1 the shoulder ,j1 on the piston-rod1 engages the bell-crank ,n and withdraws the bolt ,o1 thereby unlocking the controlling valve4 ,the time that the sending apparatus is locked depends upon the time required for the piston to descend4 ,while the sending apparatus is locked against the sending of another carrier1 it is not so locked that the swing-frame cannot be returned to its normal position and another carrier inserted ready to be sent as soon as the necessary time expires4 ,this time is usually not more than ten seconds4 ,not only may the second carrier be placed in the tube ,c1 ,fig4 #bi1 ready to be sent1 but the handle ,k may be pulled and fastened in the notch a1 thereby compressing the spring ,s1 which1 as soon as the controlling valve is unlocked1 will move the valve and automatically despatch the carrier4 ,the controlling valve is locked by the passage of a bolt through the hole b1 in a block carried on the end of the valve stem1 when it returns to the normal position shown in the figure4 ,usually little or no time will be lost in thus locking the sending apparatus1 for the small amount of time that the apparatus is locked will be needed in handling the carriers4 ,intermediate ,station ,time-,lock4--,we have another time-lock attached to the sending apparatus that has been already referred to in describing the 8block system0 used at intermediate stations2 a time-lock to prevent carriers being inserted into the tube at intermediate stations while another carrier is passing that station4 ,this time-lock is shown in ,fig4 #bg at ,w1 and is shown in detail by a sectional drawing1 ,fig4 #ca4 ,when a carrier closes an electric circuit in passing one of the boxes located in a manhole about three hundred feet from an intermediate station1 it indicates its approach to the station by exciting the electro-magnet ,a1 ,fig4 #ca4 ,this magnet pulls down its armature ,b and raises the small piston valve ,c1 which admits compressed air to a small chamber1 ,d4 ,the air is supplied to this chamber from the main tube through the pipe ,e4 ,in one end of this chamber is fitted a piston1 ,f1 held to one end of its stroke by a spring1 ,g4 ,when compressed air is admitted to the chamber ,d1 this piston is moved to the left1 and by such movement throws the controlling valve of the sending apparatus into its normal position 7shown in ,fig4 #bi7 and holds it there4 ,this forms a positive lock1 and1 no matter in what position the sending apparatus may be1 it puts the tube ,b1 ,fig4 #bi1 into line with the main tube so that the approaching carrier can pass through the apparatus4 ,the piston-rod ,h1 ,fig4 #ca1 is connected to the finger d1 ,fig4 #bi1 and by rocking this finger moves the controlling valve1 or prevents it being moved by the handle ,k4 ,illustration3 ,,fig4 #ca4 ,,intermediate ,,station ,,time-,,lock4 ,returning now to ,fig4 #ca1 we have on the top of the chamber ,d1 in addition to the electro-magnet ,a and its armature ,b1 a differential cylinder and piston1 ,k1 ,l1 whose function is to close the valve ,c when the chamber ,d is filled with air4 ,the piston ,k is smaller than the piston ,l1 and sustains a constant air-pressure1 supplied through the small pipe ,m ,m1 from the pipe ,e1 which leads to the main tube4 ,when the chamber ,d becomes filled from the pipe ,e through the valve ,c1 the pressure in the chamber moves the piston ,l upward against the pressure on the piston ,k1 because of the greater area of the piston ,l4 ,this movement of the differential piston raises the lever ,i1 which passes through a slot in the stem of the differential piston1 and thus closes the valve ,c4 ,the air in the chamber now gradually escapes to the atmosphere through a small orifice ,q2 in fact it has been escaping here all the time while the chamber was being filled1 but the opening through the valve ,c is so many times larger than the orifice ,q that the escape of air was not sufficient to prevent the chamber from filling4 ,now1 however1 that all supply to the chamber is shut off1 the air in the chamber is gradually being discharged through the orifice4 ,when nearly all the air has escaped1 the piston ,f will return to its normal position1 shown in the figure1 and unlock the controlling valve4 ,the time required for the air to escape from the chamber1 ,d1 is the time that the sending apparatus will be locked1 and this time can be regulated by varying the size of the orifice ,q4 ,the opening of the orifice1 or the time that the sending apparatus is locked1 is indicated by an index and dial1 ,p4 ,this locking mechanism is secured to a bracket on the side of the large cylinder ,h1 ,fig4 #bg1 in a position where it can be easily inspected4 ,the moving parts of the electro-magnetic valve--for such is the valve ,c1 with the magnet ,a1 ,fig4 #ca--;are made very light1 in order that they may respond easily and quickly to the closing of the electric circuit4 ,it is a disadvantage to have stations too numerous upon the same line1 especially if they do a large amount of business1 for each station will delay the sending of carriers from the others more or less1 and the interference will be greatest during the busiest hours of the day4 ,this condition is inherent in any system of large tubes where carriers have to be run a certain minimum distance apart1 and cannot be overcome by any mechanism4 ,but the disadvantage is greatly overshadowed by the advantage of being able to connect several stations by one line1 instead of having to run independent lines from each station to the central1 especially when the business of the individual stations is not sufficient to occupy a separate tube all the time4 ,it makes it possible to have stations where otherwise the business would not warrant the cost of installation and expense of operation4 ,we recommend the establishing of not more than eight stations on a line1 and usually a smaller number than this1 depending1 of course1 upon the amount of business to be done at each station4 ,illustration3 ,,fig4 #cb4 ,,electro-,,pneumatic ,,circuit-,,closer4 ,the ,electro-,pneumatic ,circuit-,closer4--,there is one piece of mechanism used in connection with the sending apparatus that we have yet to describe1 and that is the circuit-closing device located in the manholes in the street4 ,since the carriers travel at a high rate of speed1 they should not be made to operate any mechanism by impact with fingers or levers protruding into the tube when it can be avoided1 even though the work to be done is so slight as the closing of an electric circuit1 for the repeated impacts cannot fail to work injury to the carriers and the mechanism to be operated1 no matter how carefully they are designed4 ,to avoid such impacts1 we have designed the electro-pneumatic circuit-closer1 shown by the drawing in ,fig4 #cb4 ,it is operated by a passing carrier1 but pneumatically rather than mechanically4 ,in the figure we have a pneumatic tube1 ,a1 ,a1 in which a carrier1 ,b1 is moving in the direction indicated by the arrow4 ,at two points1 about twenty or thirty feet apart1 two small holes are tapped into the tube and pipes1 ,c and ,d1 are screwed in4 ,these pipes lead to two chambers in a cast-iron box1 ,f1 separated by a diaphragm1 ,e4 ,this diaphragm is insulated electrically from the box supporting it1 and is connected with the wire ,g4 ,just out of contact with the diaphragm is an insulated screw1 ,h1 connected with the wire ,i4 ,these wires lead to the time-lock1 already described1 on the sending apparatus at the station4 ,when no carrier is passing1 the air-pressure is the same on both sides of the diaphragm1 but when a carrier enters that part of the tube between the two points where the pipes ,c and ,d are connected1 the equality of pressure on opposite sides of the diaphragm is destroyed4 ,there is always a slightly greater pressure in rear of the carrier than in front of it1 equal to the frictional resistance of the carrier in the tube4 ,it is this difference of pressure in front and in rear of the carrier that moves it through the tube4 ,when the carrier is in the position shown in the figure1 the same difference of pressure will exist on opposite sides of the diaphragm1 and it will be deflected into contact with the screw ,h1 thereby closing the electric circuit4 ,when the carrier has passed1 equality of pressure on opposite sides of the diaphragm is established and the diaphragm takes its normal position1 out of contact with the screw ,h4 ,this apparatus is easily attached to the tube1 and it contains no mechanism to get out of order4 ,illustration3 ,,fig4 #cc4 ,,open ,,receiver4 ,the ,open ,receiver4--,wherever the pressure in the tube is down nearly to atmospheric1 we can use an open receiver to discharge the carriers from the tube4 ,this is a receiver that opens the tube to the atmosphere and allows the carrier to come out4 ,such a receiver is used at the main post-office in the ,philadelphia postal-line1 and was described in the last chapter4 ,the present receiver is similar in operation1 but contains some improvements in details4 ,fig4 #cc is a side elevation of the apparatus1 ,fig4 #cd is a longitudinal section1 and ,fig4 #ce is a cross-section through the cylinder and valve1 showing the sluice-gate4 ,illustration3 ,,fig4 #cd4 ,,open ,,receiver4--,,longitudinal ,,section4 ,illustration3 ,,fig4 #ce4 ,,open ,,receiver4--,,sluice-,,gate ,,mechanism4 ,referring to the longitudinal section1 the apparatus is attached to the end of a pneumatic tube1 ,a4 ,the current of air from the tube ,a flows through the slots ,b into a pipe1 ,c1 that conducts it to a tank near the air-compressor4 ,about the centre of the apparatus is a sluice-gate1 ,e1 that is raised and lowered by a piston in a vertical cylinder1 ,f1 located just above the sluice-gate4 ,this piston is moved by air-pressure taken from some part of the system4 ,when a carrier arrives from the tube ,a1 it passes over the slots ,b and runs into the air-cushion ,d1 where it comes gradually to rest4 ,checking the momentum of the carrier compresses the air in front of it considerably1 and this excess of pressure is utilized to move a small slide-valve that controls the movement of the piston in the cylinder ,f1 so that as soon as the carrier has come to rest the sluice-gate rises and allows the carrier to be pushed out with a low velocity on to a table4 ,the small pipe ,g conducts a small portion of the air compressed in front of the retarded carrier to the controlling valve1 ,h1 seen in ,figs4 #cc and #ce4 ,referring now to the section of the valve and cylinder1 ,fig4 #ce1 the pipe ,g enters the top of a small valve-cylinder containing a hemispherical piston1 ,i1 that is held up by a spiral spring1 ,j4 ,this spring has just sufficient tension to hold the piston ,i up against the normal pressure of air in the tube4 ,when a carrier arrives and compresses the air in the air-cushion1 the excess of pressure forces the piston ,i down against the spring ,j1 and moves the piston slide-valve ,k4 ,this change of position of the slide-valve allows the air in the cylinder ,f to escape to the atmosphere through the passage ,l1 passage ,p1 and pipe ,m1 while compressed air from some part of the main tube enters through the port ,n and passage ,o to the under side of the piston in the cylinder ,f4 ,this moves the piston up1 carrying with it the sluice-gate ,e4 ,there is just sufficient pressure in the tube in rear of the carrier to push the carrier past the gate and on to the table4 ,as the carrier moves out it raises a finger1 ,q1 ,fig4 #cd1 that projects into its path4 ,raising this finger extends the spring ,r1 ,fig4 #cc1 and rotates the lever ,s1 bringing the pawl ,t under the end of the controlling valve-stem4 ,when the carrier has passed out and the finger ,q is free to descend1 the spring ,r rotates the lever ,s back to its original position1 and thereby raises the controlling slide-valve1 which causes the sluice-gate to close4 ,by having the upward motion of the finger ,q simply extend the spring ,r1 and the downward motion1 by the force of the spring1 move the valve1 we are enabled to have several carriers pass out of the tube together without having the sluice-gate close until the last carrier has passed out4 ,if raising the finger ,q moved the valve1 then when the first carrier passed out1 the gate would close down upon the second4 ,attached to the receiving apparatus and extending beyond it is a tube1 ,u1 cut away upon one side so that the carriers can roll out of it on to a table1 and having in the end a buffer to stop the carriers if by any accident they come out of the tube with too much speed4 ,this buffer consists of a piston covered with several layers of leather and having a stiff spring behind it4 ,the whole apparatus is supported from the floor upon suitable standards1 and1 for an eight-inch tube1 occupies a floor-space twelve feet long by two feet wide1 not including the table4 ,this is the simplest form of receiving apparatus4 ,owing to conditions of pressure already explained1 its use is confined principally to the pumping stations4 ,the only care that it requires is an occasional cleaning and oiling4 ,illustration3 ,,fig4 #cf4 ,,closed ,,receiver4 ,illustration3 ,,fig4 #cg4 ,,closed ,,receiver4--,,longitudinal ,,section4 ,the ,closed ,receiver4--,next we will turn our attention to the closed receiving apparatus used at all terminal stations where the pressure in the tube is considerably above the pressure of the atmosphere1 so much so that the tube cannot be opened to allow the carrier to pass out without an annoying blast of air and a high velocity of the carrier4 ,this apparatus is similar to the receiver used in the sub-post-office of the ,philadelphia postal line1 but contains several modifications and improvements tending towards simplification4 ,fig4 #cf shows it in elevation1 and ,fig4 #cg in longitudinal section4 ,as in the open receiver just described1 the air from the tube ,a is deflected through slots ,b into a branch pipe1 ,c1 that conducts it from the receiving apparatus to the sending apparatus and return tube4 ,the carriers arrive from the tube ,a1 pass over the slots ,b1 where the air makes its exit1 and run into an air-cushion1 ,d4 ,this air-cushion is a tube about twice the length of the carrier1 closed at one end1 and supported upon trunnions4 ,when the carrier has been brought to rest1 this closed section of tube is tilted by the movement of a piston in a cylinder to an angle that allows the carrier to slide out2 the tube then returns to its original position4 ,if the end of the air-cushion was closed perfectly tight the carrier1 after coming to rest1 would rebound and might be caught in the joint between the stationary and movable parts of the apparatus1 when the air-cushion tube tilted4 ,to prevent the rebounding of the carrier a relief-valve1 ,e1 has been placed in the head of the air-cushion tube4 ,it is held closed against the normal pressure in the tube by a spiral spring1 but the excessive pressure created by checking the momentum of the carrier opens the valve and allows a little air to escape through the passage ,f and pipe ,g1 down the pedestal ,h1 to the atmosphere4 ,when the air-cushion or receiving tube ,d is tilted to discharge a carrier1 the circular plate ,i covers the end of the main tube4 ,in order to prevent carriers sticking in the receiving tube when it is tilted1 and to insure their prompt discharge1 the pipe ,j is provided4 ,in the tilted position of the receiving tube1 the end of this pipe coincides with the end of the main tube1 from which it receives air to hasten the discharge of the carrier4 ,a check-valve1 ,k1 prevents the air from flowing backward in this pipe when a carrier is being received in the air-cushion chamber4 ,the opening of this check-valve can be adjusted by a screw1 thereby regulating the speed of ejection of the carrier4 ,illustration3 ,,fig4 #ch4 ,,intermediate ,,station ,,receiving ,,and ,,transfer ,,apparatus4 ,illustration3 ,,fig4 #ci4 ,,intermediate ,,station ,,receiving ,,and ,,transfer ,,apparatus4--,,vertical ,,section4 ,the carrier is discharged down a chute1 ,l1 which has a buffer at the bottom1 and from the chute it rolls off on to a table4 ,the buffer is made similar to the buffer in the open receiver already described4 ,the cylinder and piston ,m1 that operate to tilt the receiving tube ,d1 are supported upon the base of the apparatus under the closed end of the receiving tube4 ,the cross-head of the piston- and connecting-rods travels between guides that are made a part of the upper cylinder head4 ,the movement of the piston in the cylinder ,m is controlled by a piston slide-valve exactly similar to the one shown in ,fig4 #ce4 ,the slide-valve is moved1 in the same manner1 by the air compressed ahead of the carrier when it is brought to rest in the air-cushion ,d4 ,the air is conducted from the air-cushion to the controlling slide-valve through a small pipe1 ,n1 ,fig4 #cf4 ,this pipe leads to one of the trunnions1 where it has a joint to allow for the tilting of the receiving tube4 ,when the carrier is discharged from the receiving tube1 it raises a finger1 ,o1 ,fig4 #cg1 located just outside the tube4 ,raising this finger pulls the rod ,p1 ,fig4 #cf1 extends the spring ,q1 turns the lever ,r1 and catches the pawl ,s1 under the end of the controlling valve stem4 ,when the carrier has passed down the chute and allowed the finger ,o to drop down1 the spring ,q turns the lever ,r back to its original position and moves the controlling valve4 ,this causes the receiving tube to return to a horizontal position1 where it is ready to receive the next carrier4 ,at first this apparatus may seem a little cumbersome1 but nothing could work better4 ,it is certain in its action and almost noiseless4 ,carriers are received1 discharged1 and the receiving tube returned to its normal position in four seconds1 and it can be done in less time if necessary4 ,the ,intermediate ,station ,receiving and ,transfer ,apparatus4--,one other form of receiving apparatus remains to be described1 and this is the apparatus used at intermediate stations to intercept all carriers intended for that station and to send the others on through the tube to the next station4 ,a side elevation of the apparatus is shown in ,fig4 #ch and a vertical section in ,fig4 #ci4 ,the tubes are led into an intermediate station1 carried upward1 and then1 with a bend of one hundred and eighty degrees1 are connected to the top of the receiving and transfer apparatus1 as shown in the diagram1 ,fig4 #bf4 ,the object of this arrangement will be seen as we describe the apparatus4 ,referring to the sectional drawing1 ,fig4 #ci1 the connection of the tube ,a is seen at the top4 ,as in the other receivers1 the current of air arriving from the tube ,a is deflected through slots1 ,b1 into a passage1 ,c1 made in the frame of the apparatus4 ,from this passage it enters the tube ,d through the slots ,e4 ,the tube ,d leads to the sending apparatus and on to the next station1 as seen in ,fig4 #bf4 ,the carriers are received in a closed section of tube ,f1 which forms an air-cushion1 similar to the closed receiver last described4 ,this receiving tube ,f is made a part of what we might term a wheel4 ,this wheel fits accurately into a circular casing and is supported by two trunnions or axles1 upon which it revolves4 ,the wheel has a broad flat rim1 ,g1 that covers the end of the tube at ,h when the wheel is revolved1 and1 in the normal position in which it is shown in the figure1 covers the interior openings ,i1 ,j1 ,k1 and ,l1 in the casing4 ,leather packing is provided around each of the openings to prevent the escape of air between the face of the wheel and the interior face of the casing4 ,from the bottom of the receiving tube ,f a passage1 ,m1 leads past a check-valve1 ,n1 to the tube ,d4 ,when a carrier arrives from the tube ,a1 it descends into the receiving tube ,f1 compressing the air in front of it4 ,this compressed air begins to escape through the passage ,m1 but the high velocity of it closes the check-valve ,n as much as possible4 ,a stop on the stem of the valve prevents it being closed entirely4 ,the small opening past the valve allows some of the air to pass1 thereby preventing the carrier from rebounding on the air-cushion4 ,as soon as the carrier has come to rest1 the check-valve ,n1 by its own weight1 opens wide1 and the carrier1 by its weight1 settles gradually down to the bottom of the receiving tube4 ,the wheel containing the receiving tube and the carrier will then be revolved by the cylinder and piston ,o1 which is operated by compressed air taken from the tube through the pipe ,p4 ,if the carrier is for this station1 the wheel will rotate through an angle of forty-five degrees and discharge the carrier through the opening ,j1 down the chute ,q1 from which it will roll on to a table arranged to receive it4 ,if1 however1 the carrier is intended for some other station1 the wheel will rotate through an angle of ninety degrees and discharge the carrier through the opening ,k into the tube ,d1 and it will go on its way to the next station4 ,this selection of carriers is brought about in a comparatively simple manner4 ,at the bottom of the receiving tube ,f there are two vertical needles1 ,r and ,s1 shown upon a larger scale in ,fig4 #dj4 ,the needles ,r and ,s are contained in tubes having an insulating lining which keeps them out of electrical contact with the frame of the apparatus4 ,wires a and b make connection with the needles through metal plugs that form a guide for the needles1 and through the springs ,u and ,v4 ,directly below the needle ,r is an insulated spring clip1 ,w1 held by two bolts and connected to the wire e4 ,illustration3 ,,fig4 #dj4 ,a ,,detail ,,of ,,the ,,intermediate ,,station ,,receiving ,,and ,,transfer ,,apparatus4 ,the end of a carrier is represented at ,t4 ,as the carrier settles down to the bottom of the receiving tube1 it comes in contact with the ends of the needles and presses them down1 they being supported by two springs ,u and ,v4 ,as the needle ,r is moved down1 it makes contact with the spring clip ,w1 located just below it1 and closes an electric circuit that includes the electro-magnet ,x1 ,figs4 #ch and #ci1 on the valve of the rotating cylinder ,o4 ,when this electro-magnet is excited it attracts its armature and moves the piston slide-valve ,y1 that admits air to the top of the piston in the cylinder ,o1 and allows the air under the piston to escape to the atmosphere4 ,the piston moves downward and revolves the wheel by means of a connecting rod4 ,upon the end of the carrier ,t is placed a thin circular metal disk1 f1 which may be copper1 brass1 tin-plate or any metal that is not easily oxidized4 ,the diameter of this disk of metal determines the station at which the carrier will be discharged from the tube4 ,disks of various diameters1 that may be attached to the carrier1 are represented by dashed lines1 g1 in ,fig4 #dj4 ,when the carrier comes in contact with the two needles ,r and ,s1 if the circular metal disk on the front end of the carrier has a diameter sufficient to span the space between the two needles1 in the position in which it is held1 then an electric circuit1 made by the wires a and b1 will be closed through the needles and the metal disk on the carrier4 ,the metal disk makes a short-circuit from one needle to the other4 ,if the metal disk is not large enough to span the distance between the two needles1 then the electric circuit remains broken4 ,returning again to ,fig4 #ci1 we have the opening ,j1 where the carriers are discharged1 closed by a sluice-gate4 ,this gate is opened and closed by a piston moving in a cylinder1 h1 shown in ,fig4 #ch4 ,a piston slide-valve1 i1 similar in all respects to the valve on the cylinder ,o1 controls the movement of the piston in this cylinder and the sluice-gate to which it is attached4 ,the slide-valve is moved in one direction1 that opens the sluice-gate1 by an electro-magnet in the circuit of the wires a and b1 ,fig4 #dj4 ,when the electric circuit made by these wires is closed by a disk on the front end of a carrier1 short-circuiting the two needles1 the valve is moved by the electro-magnet in the circuit1 and the sluice-gate is opened4 ,as the wheel1 including the receiving tube and carrier1 revolves1 a lug1 j1 ,fig4 #ch1 on the outside of the wheel comes in contact with the open sluice-gate and the wheel can rotate no farther4 ,a blast of air through the valve ,l1 ,fig4 #ci1 assisted by gravity1 pushes the carrier out of the receiving tube1 through the opening ,j and down the chute ,q1 on to the receiving table4 ,had the disk on the front end of the carrier been too small to span the distance between the two needles1 the circuit would not have been closed1 the sluice-gate would not have been opened1 no obstruction would have been placed in the path of the lug j1 on the wheel1 and the wheel would have continued its rotation through ninety degrees until the receiving tube ,f came in line with the tube ,d4 ,during the latter part of the rotation1 a pin on the wheel engages a lever1 k1 ,fig4 #ch1 and turns a valve1 l1 ,fig4 #ci1 stopping the flow of air through the passage ,c1 compelling it to take another route through the passage m1 and the receiving tube ,f1 taking with it the carrier into the tube ,d4 ,when the carrier leaves the receiving tube and passes through either of the openings ,j or ,k1 it engages one of the fingers1 n or o1 that lie in its path4 ,these fingers are connected by rods and levers to the valves on the rotating and sluice-gate cylinders4 ,the ejected carrier pushes these fingers to one side1 and after it has passed the fingers return1 by the force of a spring1 to their former position and move the valves1 causing the sluice-gate to close and the wheel to rotate backward into its normal position ready to receive the next carrier4 ,the connection between the fingers and the valves is similar to the mechanism on the open and closed receivers1 so need not be described in detail here4 ,the speed with which the carriers are ejected from the receiving tube through the opening ,j and down the chute ,q is regulated by the valve ,l1 which can be opened or closed by a hand-wheel1 p4 ,before the wheel and receiving tube can be rotated1 the needles must be withdrawn from the receiving tube1 and this is accomplished by a small cylinder and piston1 q1 shown in ,fig4 #dj4 ,the needles and their encasement are attached to a cross-head1 r1 on the end of a hollow piston-rod1 s4 ,when air is admitted to the top of the piston in the rotating cylinder ,o1 ,fig4 #ci1 it is also admitted through the pipe t1 ,fig4 #ch1 to the cylinder and upper side of the piston q1 ,fig4 #dj4 ,this moves the piston q down against the force of a spring1 u1 and withdraws the needles from the receiving tube4 ,this takes place after the needles have served their purpose and before the wheel is rotated4 ,the piston q has much less inertia than the wheel1 therefore it moves much quicker4 ,when the wheel begins to rotate it closes a valve1 v1 in the pipe t1 ,fig4 #ch1 confining the air in the cylinder q1 and preventing the needles from being raised by the spring u before the wheel returns to its normal position4 ,if by any accident the needles should be raised1 no serious harm would result1 for their ends would simply bear against the face of the wheel4 ,if this took place constantly1 grooves might be worn in the face of the wheel2 for this reason the valve v is provided4 ,in order to facilitate the inspection of the needles and electric contact springs ,w1 they are contained in a cylindrical brass case1 w1 that is held in place beneath the receiving tube by two bolts4 ,by removing the nuts from these bolts the entire mechanism can be removed1 examined1 and cleaned4 ,it also gives easy access to the receiving tube4 ,the receiving tube is long enough to receive two carriers1 if it should ever happen that two arrive at the same time4 ,illustration3 ,,fig4 #da4 ,,diagram ,,of ,,contact-,,disks ,,and ,,needles4 ,to show how the apparatus at the various stations is arranged to correspond with the disks of various sizes attached to the front of the carriers1 a diagram1 ,fig4 #da1 has been made1 in which the needles at the bottom of the receiving tubes of the apparatus at six intermediate stations are represented at ,a1 ,b1 ,c1 ,d1 ,e1 and ,f4 ,six disks of different sizes are represented at a1 b1 c1 d1 e1 and f4 ,the needles are placed farthest apart at station ,a and nearer together at each succeeding station until we arrive at station ,f1 where they are nearest together4 ,if we wish to send a carrier to station ,a from the central1 we place the largest disk1 a1 upon the front end of it4 ,when it arrives at station ,a1 it closes the electric circuit between the needles and is discharged from the tube4 ,should we wish to send a carrier to station ,d1 then we place the disk d upon the front end of it4 ,when the carrier arrives at the station ,a1 the disk is not large enough to span the needles2 therefore the sluice-gate is not opened and the carrier is sent on in the tube4 ,when it arrives at stations ,b and ,c1 the same thing occurs again1 but when it reaches station ,d1 the needles are sufficiently close together so that the disk makes an electric circuit between them1 and the carrier is discharged from the tube1 as was intended when despatched4 ,since the carriers always travel in the same direction in a tube1 the first station at which they arrive where the needles are near enough together to have both touch the disk1 will be the station at which the carrier was intended to stop4 ,carriers can be despatched from any station1 but if we wish to send from say ,d to ,a1 they must either travel around a loop or be sent through a return tube in which the needles are arranged in the reverse order4 ,if no disk is placed on the carrier1 it will go to the last station on the line4 ,there are other attachments that might be made to the front end of the carriers in order to have them stop at any desired station along a line4 ,we have worked out two other systems which are entirely mechanical in their operation1 not using electric circuits and electro-magnets to move the valves4 ,while such a mechanical system has some advantages over the present combined mechanical and electrical system1 yet there is one great advantage in the latter1 and that is the simplicity of the attachment made to the carrier4 ,a round flat disk of tin-plate is attached to the front end2 it is something that is not in the way2 it does not prevent standing the carriers on end in racks to fill them2 it is not easily injured1 and only those who have had experience can realize the rough usage that the carriers receive2 it is quickly and easily attached to the carrier1 and it is so cheap that when bent it can be thrown away4 ,carriers4--,the carriers are similar in all respects to those used in the ,philadelphia postal-line1 that have been described in the preceding chapter and illustrated in ,figs4 #ah1 #ai1 and #bj4 ,when there are intermediate stations upon the lines1 means are provided for attaching disks to the front end of the carriers4 ,the disks have a central stem that secures them to the bolt in the centre of the head1 and are so arranged that they can be quickly attached or removed4 ,many experiments have been made to find the best material for bearing-rings1 but thus far nothing better than a specially-prepared woven fabric has been found4 ,these rings will run about a thousand miles1 when they become so reduced in diameter that they have to be replaced by new ones4 ,the most essential elements of a carrier are strength1 lightness1 and security of the contents4 ,aluminum has frequently been proposed as a suitable material for the bodies of carriers1 but for the same weight steel is much stronger1 especially in thin rolled sheets1 and for this reason it has been used4 ,one of the most perplexing problems that presented itself in working out the details of the system was to design a secure and reliable lock for the lids of the carriers4 ,we believe that the one which has been adopted fulfils all requirements in a satisfactory manner4 ,some experiments have been made with carriers that open on the side1 but structurally they are weak and unsuited to stand the blows that carriers frequently receive4 ,they are not so easily and quickly filled and emptied as those that open on the end4 ,these remarks apply to carriers for large tubes4 ,in small tubes for the transportation of cash in retail stores1 carriers with side openings are found convenient4 ,when ,united ,states mail is sent through tubes not used exclusively for postal service1 carriers with special locks can be used1 so that they can be opened only by post-office employees4 ,air ,supply4--,this completes the description of the special apparatus used in this system1 but we have yet to say something regarding the machines that supply the air4 ,in ,paris the water from the city mains has been used to compress or exhaust the air used in small tubes1 but to operate large tubes in most of our cities steam is the only available power4 ,except in isolated cases1 an independent steam plant will be erected to supply the air for a system of tubes4 ,this plant should be designed with a view to obtaining the maximum economy in coal consumption1 labor1 water1 cartage1 and incidental expenses4 ,we might say that the same general rules of economy which govern the design and construction of electric-lighting plants should be applied to the plans and construction of air-compressing plants4 ,three types of blowing machines are used1--viz41 centrifugal fans1 positive blowers1 and air-compressors4 ,fans4--,very large tubes of moderate length can be operated by ordinary centrifugal fans4 ,these fans are capable of supplying air under a pressure not exceeding ten or twelve ounces per square inch with very good efficiency4 ,they are the simplest and most inexpensive of all blowing-machines4 ,blowers4--,when tubes have a length and diameter that require a pressure from one to four pounds per square inch1 some form of positive blower of the ,root type can be used with economy4 ,their construction is familiar to nearly every one at all interested in machinery1 so we need give no space to their description here4 ,illustration3 ,,fig4 #db4 ,,the ,,sturtevant ,,steel ,,pressure ,,blower4 ,illustration3 ,,fig4 #dc4 ,,root',s ,,positive ,,pressure ,,blower4 ,illustration3 ,,fig4 #dd4 ,,section ,,of ,,root',s ,,true ,,circle ,,blower4 ,illustration3 ,,fig4 #de4 ,,the ,,green ,,blower4 ,illustration3 ,,fig4 #df4 ,,section ,,of ,,the ,,green ,,blower4 ,air-,compressors4--,by far the greater number of our tubes require an air-pressure of more than five pounds per square inch4 ,for such air supply we recommend some form of air-compressor1 and usually this is driven by a steam-engine1 which forms a part of the compressor4 ,in making our selection we should bear in mind the conditions under which the compressor will run4 ,usually it must be kept in constant operation at least ten hours per day1 and frequently for a much longer period4 ,this makes it important that the compressor be substantially built and supported upon a solid and firm foundation4 ,the bearings should be broad1 of good wearing material that has a low coefficient of friction1 and provided at all times with ample lubrication4 ,if poppet valves are used in the air-cylinders1 and they are most common1 the speed in revolutions per minute should not be high4 ,duplex are better than single cylinder compressors1 because they deliver the air in a more steady stream1--the pulsations are less4 ,for constant running1 economy of steam is an important item2 therefore some good type of cut-off valve should be provided4 ,the air-cylinders should not be water-jacketed unless the pressure is above twenty-five pounds per square inch4 ,it is better to use the air as warm as possible1 for it will soon be cooled after entering the tube4 ,a speed-governor should be provided with compressors which are to run at constant speed1 but usually they will be run to maintain a constant pressure in the tank1 and to this end a good and reliable form of pressure-governor should be provided1 together with some reliable safety device to stop the engine when the speed exceeds a safe limit4 ,but most important of all is to have the valves of the air-cylinders large in area2 otherwise the efficiency of the machine will be very low4 ,with machines working under eighty pounds pressure1 a difference in pressure of one pound on opposite sides of the valves has but little effect1 but when the machine is only compressing to five or ten pounds1 one pound is a very large proportion of the total pressure and reduces the efficiency4 ,besides these few suggestions1 only the requirements of good engineering need be demanded4 ,in ,figs4 #db1 #dc1 #dd1 #de1 #df1 and #dg we show a fan1 two blowers1 and an air-compressor suited to the requirements of pneumatic-tube service that can be found in the market1 and that are built by responsible concerns4 ,we believe they are all good of their kind1 but do not recommend any particular make4 ,the ,tube1 ,line ,construction1 etc4--,up to the present time we have found no material better suited for the straight parts of pneumatic tubes than cast iron1 machined upon the interior4 ,it gives a smooth and accurate tube4 ,it can be made in most convenient lengths4 ,it is strong and not easily deformed4 ,the bell-joint1 calked with lead and oakum1 having the tubes fitted together male and female at the bottom of the bell1 is the best joint yet devised for pneumatic tubes4 ,it is slightly yielding1 accommodating itself to slight changes of length of tube due to changes of temperature1 and it allows slight bends to be made at each joint4 ,the joints are very accurate1 presenting no shoulders to obstruct the passage of carriers4 ,the joints can be made by men accustomed to laying water- and gas-pipe4 ,the cast iron is so stiff that it is not distorted in calking1 as may be done with wrought-iron tube4 ,the principal objections to its use are the expense of boring and the readiness with which it corrodes upon the interior4 ,illustration3 ,,fig4 #dg4 ,,rand ,,compound ,,compressor ,,of ,,moderate ,,size4 ,we are always hoping that wrought-iron or steel tubes will be so much improved in uniformity of dimensions and smoothness of interior that we can use them1 but our experiments thus far have been discouraging4 ,it may be that some of the new processes of making tubes will give us what we want1 but we have not yet found it4 ,small tubes and the short bends of large tubes are made of brass1 it being the most suitable material4 ,it would be very difficult to bend iron tubes without involving great expense4 ,the thickness of the bent portion of an eight-inch tube is usually three-sixteenths of an inch and never less than one-eighth of an inch4 ,where the ground is firm1 no other support is needed for the tubes than to tamp the earth solidly about them4 ,in order to economize space in the streets1 it is customary to lay the tubes one above the other2 and it is very convenient1 although not necessary1 to separate them by cast-iron saddle brackets4 ,such an arrangement has to be frequently departed from in order to overcome obstructions in the streets and to get through narrow passages4 ,at all low points in a tube line1 traps are provided to catch any moisture that may accumulate4 ,these traps are made accessible for frequent inspection by means of man-holes or otherwise4 ,the tube is usually laid about three feet below the pavement4 ,this distance has frequently to be varied1 but it never becomes so small as to render the tubes liable to injury from heavy trucks passing over the pavement4 ,,chapter ,,iv4 ,,facts ,,and ,,general ,,information ,,relating ,,to ,,pneumatic ,,tubes4 ,we will now discuss1 in an elementary way1 the theory of pneumatic tubes1 in order to understand more clearly their modus operandi and the principles upon which they should be constructed4 ,let us begin with the definition of a pneumatic tube4 ,definitions4--,a pneumatic tube is a tube containing air4 ,this is perhaps the broadest and most comprehensive definition that can be given1 but we usually associate with the idea of a pneumatic tube the use to which it is put4 ,if we were to embody this idea in our definition we might define a pneumatic tube as a tube through which material is sent by means of a current of air4 ,this is still a very broad definition1 including all kinds of material for transportation1 for every conceivable purpose4 ,it places no limit upon the dimensions of the tube nor the manner of its operation4 ,this definition would include the toy commonly known as a putty-blower1 and the pneumatic gun4 ,these instruments are not usually pictured in our minds when we hear or see the term pneumatic tube used4 ,instead of these1 we think of the brass tubes that we have seen in the large retail stores in some of our cities for conveying cash from the various counters to the centrally located cashier's desk4 ,again narrowing our definition to conform more nearly with the mental picture presented1 we will define a pneumatic tube as a long tube for the purpose of transporting material in carriers by means of a current of air in the tube4 ,this1 like all definitions1 is not entirely satisfactory1 if we examine it critically1 but it will answer our present purpose4 ,intermittent and ,constant ,air-,current4--,having thus defined a pneumatic tube1 there are two ways in which we may operate it to transport our carriers containing mail1 packages1 or other matter4 ,the first method consists in storing our compressed air in a suitable tank1 or by exhausting the air from the tank2 then1 when we wish to despatch a carrier we place it in the tube and connect the tube with the tank by opening a valve4 ,as soon as the carrier arrives at the distant end of the tube the valve is closed and the air soon ceases to flow4 ,when a long interval of time elapses between the despatching of carriers1 this is the most economical method of operation1 but usually carriers have to be despatched so frequently that a great deal of time would be lost if the air-current had to be started and stopped for each carrier4 ,the second and more usual method of operation consists in maintaining a constant current of air in the tube and in having the carriers inserted and ejected at the ends of the tube without stopping the current of air for any appreciable length of time4 ,it is analogous to launching boats in a rapidly flowing stream1 allowing them to float down stream and then withdrawing them4 ,when the boats are in the stream they present little obstruction to the flow of water and check its speed but very little4 ,in order to compute the speed with which the boat will pass from one point to another1 we only have to know the speed of the stream between those points when no boat is in it4 ,the presence of the boat does not change the speed appreciably4 ,so it is with carriers in a pneumatic tube3 they are carried along with the current of air4 ,the air flows nearly as rapidly when a carrier is in the tube as when there is none4 ,the friction of the carrier against the inner surface of the tube creates a slight drag1 but it checks the speed of the air only a little4 ,therefore1 in order to know the speed with which a carrier will be transported from one station to another through a pneumatic tube1 we need only to know the velocity with which the air flows through the tube when no carrier is present4 ,of course there are special cases of heavy carriers1 or carriers having a large amount of friction from their packing1 or of tubes not laid horizontally1 where the resistance of the carrier must be taken into consideration1 but for our present purpose we will neglect all of these conditions4 ,laws ,governing the ,flow of ,air in ,long ,tubes4--,this leads us to study the laws governing the flow of air in long tubes1 omitting for the present the presence of a carrier4 ,since tubes operated intermittently have become obsolete1 we will only consider the case of a constant current of air1 this being what we have to deal with in practice4 ,illustration3 ,,fig4 #dh4 ,,pressure ,,and ,,velocity ,,curves4 ,in order to make our ideas and thoughts as clear as possible let us represent them by a diagram1 ,fig4 #dh4 ,we will suppose that a tank1 ,a1 is kept constantly filled with compressed air at a pressure of ten pounds per square inch1 from some source of supply4 ,we will suppose that the pressure of the air in this tank never changes1 air being supplied as fast as it flows away4 ,next1 let us assume that a tube eight inches in diameter inside and one mile long 7five thousand two hundred and eighty feet7 is connected to the tank at one end and left open to the atmosphere at the other4 ,the air will flow in a constant stream from the tank into the atmosphere1 for the reason that air is being supplied to the tank as fast as it flows away4 ,law of ,pressure4--,first1 let us consider the pressure of the air at various points in the tube4 ,we will1 for convenience1 represent the pressure in the tank by a vertical line1 ,d ,e1 ten units in length1 since the pressure is ten pounds per square inch4 ,now let us go to a point on the tube one quarter of a mile 7one thousand three hundred and twenty feet7 from the tank1 drill a hole in the tube1 attach a pressure-gauge and measure the pressure of the air at this point4 ,we shall find it to be about #g4ia pounds per square inch2 or1 #b4ji pounds below the pressure in the tank4 ,we will represent this on our diagram by another vertical line1 ,f ,g1 having a length of #g4ia units4 ,again let us measure the pressure in the tube at a point one-half a mile 7two thousand six hundred and forty feet7 from the tank4 ,here we find it to be about #e4fa pounds per square inch1 and we represent it by the vertical line1 ,h ,i1 having #e4fa units of length4 ,we note that the pressure is #d4ci pounds below the pressure in the tank4 ,we are at the middle point of the tube and the pressure has fallen to nearly1 but not quite1 one-half the pressure in the tank4 ,we will now go to a point three-quarters of a mile 7three thousand nine hundred and sixty feet7 from the tank1 and here the pressure is about #c4ja pounds per square inch4 ,we represent it by the vertical line1 ,j ,k4 ,lastly1 we measure the pressure very near the end of the tube1 one mile from the tank1 and find it to be about zero1 or the same as the pressure of the atmosphere4 ,all of our measurements have been in pounds above the atmospheric pressure2 to express them in absolute pressure1 we should add to each the pressure of the atmosphere1 which is #ad4fi pounds1 nearly4 ,now we will draw a smooth curve through the tops of all our vertical lines1 and we have a curve1 ,e1 ,g1 ,i1 ,k1 ,l1 representing the pressure in the tube at every point4 ,it falls gradually from ten pounds to zero1 but it does not fall in exact proportion to the distance from the tank4 ,such a fall of pressure would be represented by the straight dash-line1 ,e1 ,l4 ,the reason why the true pressure-curve is not a straight line1 and lies above a straight line1 is because air is an elastic fluid and expands1 becoming larger in volume as the pressure diminishes4 ,the straight dash-line represents the fall of pressure of an inelastic fluid1 like water1 when flowing in the tube4 ,the fall of pressure along the tube is analogous to the fall of level along a flowing stream4 ,in fact1 we frequently speak of the descent of a stream as the 8head of water0 when it is used for power purposes1 and we mean by this the pressure the water would exert if it were confined in a pipe4 ,the descent1 or change of level1 in the bed of a stream is necessary to keep the water flowing against the friction of the banks4 ,the descent of the water imparts energy to overcome the friction4 ,in a similar manner1 we must have a fall of pressure along the pneumatic tube to overcome the friction of the air against the interior surface of the tube4 ,we find another analogue in the flow of the electric current along a wire2 here there is a fall of potential necessary to overcome the resistance of the wire4 ,since power has to be expended to compress the air and impart to it its pressure1 when this pressure disappears we know that the air must be losing its energy or doing work1 and we look to see what becomes of it4 ,in the present case1 we find that most of this work is expended in overcoming the friction between the air and the surface of the tube4 ,uses of ,pressure ,curves4--,the pressure curve teaches us many things4 ,suppose we were to establish stations on this tube at the quarter1 half1 three-quarter1 and mile points2 we see at once that intermediate-station or closed receivers1 described in the last chapter1 must be used at all of the stations except the mile point at the end of the tube1 because the pressure in the tube is so high above the pressure of the atmosphere that we could not open the tube to let the carriers come out1 but at the end of the tube we could use the open receiver4 ,in designing our sending and receiving apparatus for each station1 we look to this pressure curve to tell us the pressure which we shall have on the pistons in our cylinders1 and are thereby enabled to make them with proper proportions for the work that they have to do4 ,law of ,velocity4--,next let us see what the velocity of the air is in the tube4 ,suppose that we have some convenient means of measuring the velocity of the air at any point1 in feet per second or miles per hour1 with some form of anemometer4 ,we will have our measurements taken at the five points where we measured the pressure1--viz41 at the tank1 one-quarter1 one-half1 three-quarters and one mile from the tank4 ,we will represent the velocities by a diagram similar to the one used for pressures4 ,at the tank we find the air entering the tube with a velocity of #ei4e feet per second 7#dj4f miles per hour74 ,we draw the vertical line ,m ,n1 to represent this4 ,at the quarter mile point the velocity is sixty-five feet per second 7#dd4d miles per hour7 an increase in the first quarter of a mile of #e4e feet per second4 ,we construct the vertical line ,o ,p4 ,at the half-mile point the velocity is #gb4d feet per second 7#di4d miles per hour72 at the three-quarter mile point it is eighty-three feet per second 7#ef4h miles per hour72 and at the end of the tube1 one mile from the tank1 the air comes out of the tube with a velocity of #ajj4d feet per second 7#fh4e miles per hour71 about #a4g times faster than it entered the tube at the tank4 ,drawing all the vertical lines to represent these velocities1 and drawing a smooth curve line through the tops of our vertical lines1 we have the curve of velocities1 ,n1 ,p1 ,r1 ,t1 ,v1 for all points along the tube4 ,it is an increasing velocity and increases more rapidly as we approach the end of the tube4 ,this is shown more clearly by drawing the straight dashed line ,n ,v4 ,if the fluid flowing in the tube were inelastic1 like water1 then the curve of velocities would be a straight horizontal line1 for the water would not come out of the tube any faster than it went in4 ,but we are dealing with air1 which is an elastic fluid1 and1 as we stated before1 it expands as the pressure is reduced and becomes larger in volume4 ,it is this expansion that increases its velocity as it flows along the tube4 ,it must go faster and faster to make room to expand4 ,since the same actual quantity of air in pounds must come out of the tube each minute as enters the tube at the other end in the same time1 to prevent an accumulation of air in the tube1 and since it increases in volume as it flows through the tube1 it follows that its velocity must increase4 ,characteristics of the ,velocity ,curve4--,this velocity curve is both interesting and surprising1 if we have not given the subject any previous thought4 ,it might occur to us that the air expands in volume in the tube1 and we might reason from this fact that the velocity of the air would increase as it flowed through the tube1 but very few of us would be able to see that the rate of increase of velocity also increases4 ,that is to say1 it gains in velocity more rapidly as it approaches the open end of the tube4 ,if the velocity were represented on the diagram by a straight horizontal line1 we should know that it was constant in all parts of the tube1 which would be the case if water were flowing instead of air4 ,if it were represented by a straight inclined line1 like the dashed line ,n ,v1 then we should know that the velocity increased as the air flowed along the tube1 but that it increased at a uniform rate4 ,the slope of the line would indicate the rate of increase4 ,neither of these suppositions represent correctly the velocity of the air at all points in the tube2 this can only be done by a curved line such as we have shown4 ,the slope of the curve at any point represents the rate of increase of velocity of the air at that point4 ,if the curve is nearly horizontal1 then we know that the velocity does not increase much2 but if the curve is steep1 then we know that it is increasing rapidly1 the actual velocity being indicated by the vertical height of the curve above the horizontal line ,m ,u4 ,use of ,velocity ,curves4--,besides being interesting1 a knowledge of the velocity of the air at all points in a tube is of much practical value4 ,it gives us the time a carrier will take in going from one station to another4 ,usually the first questions asked1 when it is proposed to lay a pneumatic tube from station ,a to station ,b1 are1 ,how quickly can you send a carrier between these points8 ,how much time can be saved8 ,these questions are answered by constructing a velocity curve4 ,since the velocity changes at every point along a tube1 to get the time of transit between two points we must know the average velocity of the air between those points4 ,we can find this approximately from our curve by measuring the height of the curve above the horizontal line ,m ,u at a large number of points1 and then taking the average of all these heights2 but there is a more exact and easier method by means of a mathematical formula4 ,as such formula would be out of place here1 we will not give it2 suffice it to say1 that the average velocity of the air between the tank and the end of the tube1 in the case we have assumed1 is about seventy-three feet per second 7#di4g miles per hour71 a little less than one-half the sum of the velocities at the two ends1 and a little more than the velocity at the half-mile point4 ,knowing the average velocity1 we can tell how long it takes for a particle of air1 and it will be nearly the same for a carrier1 to travel from the tank to the end of the tube1 by dividing the distance in feet by the average velocity in feet per second4 ,this we find to be one minute #ab4c seconds4 ,since the air moves more rapidly as it approaches the open end of the tube1 a carrier will consume a greater period of time in going from the tank to the quarter mile point than in going from the three-quarter mile point to the open end4 ,the last quarter of a mile will be covered in a little more than fourteen seconds1 while the first quarter will require a little more than twenty-one seconds4 ,this difference is surprising1 and it becomes even more marked in very long tubes with high initial pressures4 ,this explains why the service between stations located near the end of the tube is more rapid than between stations on other parts of the line4 ,this velocity curve shows us the velocity of the carriers at each station along the line and enables us to regulate our time-locks and to locate the man-holes and circuit-closers connected with each intermediate station4 ,it gives us the length of the 8blocks0 in our 8block system40 ,when we know the velocity and weight of our carriers1 we can compute the energy stored up in them1 and from this the length we need to make our air-cushions so as not to have the air too highly compressed4 ,it would be impossible to design our apparatus properly if we did not know the laws that govern the flow of air in the tubes4 ,quantity of ,air ,used4--,the next important fact that we learn from the velocity curve is the quantity of air that flows through the tube each second or minute4 ,if we multiply the velocity with which the air escapes from the open end of the tube by the area of the end of the tube in square feet1 we have the number of cubic feet of air at atmospheric pressure discharged from the tube per unit of time4 ,the same quantity of air must be supplied to the tank in order to maintain a constant flow in the tube4 ,in the present case that we have assumed1 the tube is eight inches in diameter2 therefore the cross-sectional area is #j4cdi square foot4 ,the velocity of the air as it comes out of the end of the tube is #ajj4d feet per second2 therefore about thirty-five cubic feet of air are discharged from the tube each second1 or two thousand one hundred cubic feet per minute4 ,this same amount must be supplied to the tank ,a in order to maintain the pressure constant1 but when it is compressed so that it exerts a pressure of ten pounds per square inch1 the two thousand one hundred cubic feet will only occupy a space of one thousand two hundred and fifty cubic feet1 if its temperature does not change4 ,this leads us to consider the effect of temperature changes4 ,temperature of the ,air4--,if the air is allowed to become heated by compression1 as is the case in practice1 we have a new set of conditions4 ,if the air in the tank ,a is hot1--that is1 warmer than the surrounding atmosphere1--it will by radiation cool somewhat before it enters the tube1 and it will be still further cooled when it expands in the tube4 ,again1 if its temperature falls below the temperature of the ground in which the tube is laid1 it will absorb heat from the ground1 and this will tend to keep up its temperature2 so in practice we have very complicated relations between the temperature1 pressure1 and volume of the air4 ,these relations cannot be exactly expressed by mathematical formul1 and we will make no attempt so to express them1 but will be content with saying that in practice we find that the temperature of the air in the tubes is nearly constant after the first few hundred feet1 so that we can without appreciable error compute the pressures and velocities as if it were constant4 ,now1 if the air in the tank ,a is hot1 we must raise the pressure a little above ten pounds per square inch to obtain the velocities given on our diagram4 ,when the air cools it contracts in volume1 or1 if the volume cannot change1 being fixed by the limits of the containing vessel1 then the pressure is reduced1 so by raising the pressure in the tank ,a a little above ten pounds1 we compensate the loss of pressure4 ,horse-,power4--,having shown how the quantity of air can be computed we are now in a position to estimate the horse-power of the air-compressor necessary to operate the tube4 ,i say estimate1 because we have to take into consideration the efficiency of the air-compressor1 and that is not an absolutely fixed quantity3 it varies with different types of machines and with their construction4 ,when working with pressures of less than ten pounds1 the friction of the machine is an important factor4 ,the area and construction of the valves in the air-cylinders is another very important factor4 ,if the valves are not large enough or do not open promptly1 our cylinders will not be filled with air at each stroke2 this will reduce the efficiency of the machine4 ,in practice we go to a manufacturer of air-compressors and tell him how much air must be compressed per minute1 and the pressure to which it must be compressed1 with other conditions1 and then he tells us what size of machine we shall require and the horse-power of the machine approximately4 ,he is supposed to know the efficiency of his own machines4 ,we may endeavor to prove his estimate by computations of our own4 ,to give some idea of the horse-power required to supply the air needed to operate our eight-inch tube one mile long1 ,i will say that the steam-engines of the air-compressor will have to develop in the vicinity of one hundred and twenty-five actual horse-power4 ,from the horse-power of the steam-engine we can easily compute the coal that will be consumed under boilers of the usual type4 ,efficiency4--,it will be noted that most of the power is not used primarily in moving the carriers1 but to move the air through the tube4 ,very nearly as much power is used to keep the air flowing in the tube when no carriers are in it as when carriers are being despatched4 ,if we should define the efficiency of a pneumatic tube as the ratio of the power consumed in moving the carriers to the power consumed in moving the carriers and the air1 we should find this so-called efficiency to be very low4 ,it is analogous to pulling the carrier with a long rope and dragging the rope on the ground4 ,much more power would be consumed by the rope than by the carrier4 ,but a business man would define the efficiency of a pneumatic tube as the ratio of the cost of transporting his letters1 parcels1 etc41 to the cost of transporting them with equal speed in any other way4 ,defined in this practical manner the efficiency of a pneumatic tube is high4 ,we do not care what becomes of the power so long as it accomplishes our purpose4 ,pressure and ,exhaust ,systems4--,we have noticed that pneumatic tubes have not always been operated by compression of the air1 but that some of the small tubes used in the telegraph service of ,european cities have been operated by exhausting the air4 ,the two systems are sometimes distinguished by calling one a pressure system and the other an exhaust system4 ,these terms are very misleading1 for an exhaust system is a pressure system4 ,the current of air is kept flowing in a tube by maintaining a difference of pressure at the two ends1 and the result is the same whether we raise the pressure at one end above the atmospheric or lower it at the other below the atmospheric4 ,in either case it is pressure that causes the air to flow4 ,it happens that we are living in an atmosphere of about fifteen pounds pressure per square inch1 and it is very convenient oftentimes in our computations to take the pressure of the atmosphere as our zero1 and reckon all other pressures above and below this4 ,if all our pressure scales read from absolute zero1 the pressure of a perfect vacuum1 then all this confusion would be avoided4 ,we have not used the absolute zero in our diagram1 because all our gauges are graduated with their zero at atmospheric pressure1 and it is customary to speak of pressures above and below the atmospheric4 ,it is very natural to ask the question1 why are tubes sometimes operated by compressing the air and at other times by exhausting it8 ,we answer by saying that it is usually a question of simplicity and convenience that determines which system shall be used4 ,some of the cash systems in the stores use compressed air in the out-going tubes from the cashier's desk and exhaust the air from the return tubes4 ,both ends of the tubes are then left open at the counters1 no sending or receiving apparatus being required there4 ,the carriers are so light1 their velocity so low1 and the air-pressure varies so little from the pressure of the atmosphere that the carriers can be allowed to drop out of the tubes on to the counters1 and they can be despatched by simply placing them at the open end of the tube into which the air is flowing4 ,the currents of air entering and leaving the tubes are not so strong as to cause any special annoyance4 ,at the cashier's desk some simple receiving and sending apparatus has to be used1 but it is better to concentrate all of the apparatus at one point rather than have it distributed about the store1 as would be the case if the double system were not used4 ,in the ,london pneumatic telegraph both methods of operation have been in use4 ,double lines were laid1 and the engines exhausted the air from one tube and forced it into the other4 ,the exhausted tube was therefore used to despatch in one direction1 while the other tube1 operated by the compressed air1 served for despatching in the opposite direction4 ,so far as ,i know1 both work equally well4 ,in operating large tubes1 that is to say1 tubes six or eight inches in diameter1 there is an advantage in using compressed1 rather than exhausted air1 in the construction of the sending and receiving apparatus1 especially when the tubes are very long4 ,with an ample supply of compressed air always at hand1 the air-cushions can be made shorter and more effective in bringing the carriers quickly to rest4 ,with exhausted air the cushions are ineffective1 and consequently must be made very long in order to stop the carrier before it strikes the closed end of the tube4 ,this does not apply to small tubes where the carriers are so light that they can be stopped without injury by allowing them to strike solid buffers4 ,again1 when compressed air is used1 we have a larger difference in pressure between the pressure in the tube and the atmosphere to operate our mechanism by cylinders and pistons4 ,with an exhaust system carriers are not so easily ejected from the tubes of the receiving apparatus2 we could not use the simple form of open receiver4 ,again1 if the tubes are laid in wet ground1 and a leak occurs in any of the joints1 water will be drawn in if air is being exhausted from the tube1 while it will be kept out if compressed air is used4 ,in regard to the question of relative economy of the two systems1 we will say that when long tubes are used1 requiring high pressures1 or1 more strictly speaking1 a large difference of pressure1 to maintain the desired velocity of air-current1 there seems to be some advantage in using an exhaust system4 ,the reason is this3 the friction of the air in the tube1 which absorbs most of the power1 increases as the air becomes heavier and more dense4 ,when the air is exhausted from the tube1 we are using a current of rarefied air1 and this moves through the tube with less friction and1 consequently1 a higher velocity1 for the same difference of pressure1 than the more dense compressed air4 ,but for short tubes that require only a small difference of pressure1 this advantage becomes very small1 and is overbalanced by other advantages of a compressed air system4 ,so1 taking everything into consideration1 there is not so much to be said in favor of an exhaust system4 ,laws ,expressed in ,mathematical ,formul4--,while we have heretofore purposely avoided all complicated mathematical formul1 it may not be out of place here to give a few of the more simple relations that exist between the pressure1 velocity1 length and diameter of the tubes1 etc4 ,in two tubes having the same diameter1 with the same pressures maintained at each end1 but of different lengths1 the mean velocities of the air in the tubes will bear the inverse ratio to the square roots of the lengths of the tubes4 ,this is expressed by the following proportion3 u 3 ,u 33 ,l 3 l u and ,u represent the mean velocities of the air in the two tubes and l and ,l the respective lengths of tubes4 ,a similar but direct ratio exists between the mean velocities and the diameters of the tubes1 thus3 u 3 ,u 33 d 3 ,d ,this relation1 however1 is only approximately true for tubes differing greatly in diameter4 ,the relation of the pressure to other factors is not so simply expressed4 ,for example1 in two tubes of the same length and diameter1 the relation between the pressures at the ends and the mean velocity of the air may be expressed as follows3 7p - p7_/ 7,p - ,p7_/ u 3 ,u 33 -------------------------- 3 ------------------ 7p - p7 7,p - ,p7 where u and ,u are the respective mean velocities1 p#j and ,p#j the respective pressures at the initial ends of the tubes1 and p#a and ,p#a the respective pressures at the final ends of the tubes1--the pressures being measured above absolute zero4 ,there are other relations that can be similarly expressed1 but for them we must refer the reader to a mathematical treatise on the subject4 ,moisture in the ,tubes4--,when pressures of more than five pounds per square inch are used1 it is not unusual to find some moisture on the interior of the tube and upon the outside of the carriers when they come out of the tube4 ,it is seldom more than a slight dampness1 or at most a degree of wetness equal to that seen on the outside of a pitcher of ice-water on a warm day4 ,a slight amount of moisture in the tube is not objectionable1 for it serves as a lubricant to the carriers2 but when it is present in considerable quantity it becomes objectionable and even annoying4 ,this moisture is brought into the tube with the air1 and is deposited upon the walls of the tube when their temperature is sufficiently below that of the atmosphere4 ,the atmosphere always contains more or less moisture in the state of vapor4 ,the capacity of air for water-vapor depends upon its temperature1 being greater the higher the temperature1 but it is a fixed and definite quantity at any given temperature4 ,when the air contains all the water-vapor it can hold at a certain temperature1 it is said to be saturated4 ,if it is not saturated1 we express the amount that it contains in per cent4 of the amount it would contain if it were saturated1 and this is termed the 8relative humidity40 ,for example1 if the air is three-fourths saturated1 we say the 8relative humidity0 is seventy-five2 but if the temperature changes1 the 8relative humidity0 changes also4 ,suppose the temperature to-day is seventy-five degrees ,fahrenheit1 and that the 8relative humidity0 is eighty1 a cubic foot of air then contains #j4jjajg pound of water-vapor4 ,now suppose this air enters a pneumatic tube and is cooled by expansion and contact with the colder walls of the tube to sixty degrees4 ,at this temperature a cubic foot of air can contain only #j4jjjhb pound of water-vapor when it is saturated4 ,now1 each cubic foot of air brought into the tube brings with it #j4jjajg pound of vapor1 and after it is cooled down to sixty degrees it cannot hold it all1 consequently the difference1 or #j4jjjbe pound1 must be deposited in the tube4 ,under these conditions one hundred thousand cubic feet of air will deposit twenty-five pounds of water in the tube4 ,in the system of pneumatic tubes built and operated by the ,batcheller ,pneumatic ,tube ,company1 the presence of a large quantity of moisture in the tube is prevented by using the same air over and over again4 ,a little moisture may be deposited when the tube starts into operation1 but the amount does not increase appreciably1 as very little fresh air is admitted after starting4 ,location of ,obstructions in ,tubes4--,in regard to the removal of obstructions in the tubes1 ,i have had little or no experience2 therefore under this heading ,i am satisfied to quote from the 8,minutes of the ,proceedings of the ,institution of ,civil ,engineers10 ,london1 ,volume ,,xliii4 8,intimately connected with the working of the tubes is the removal of obstructions which occur from time to time1 causing not unfrequently serious inconvenience and delay4 ,the most general cause of obstruction is a stoppage of the train arising from accident to the tube1 to the carriers or piston1 or to the transmitting apparatus4 ,in such cases the delay is generally very brief1 it being for the most part sufficient to reverse the pressure on the train from the next station1 and to drive it back to the point it started from4 ,if one or more of the carriers break in the tube1 reverse pressure is also generally sufficient to remove the obstacle2 but where this fails1 the point of obstruction must be ascertained4 ,this is done by carefully observing the variations of air pressure in the reservoir when placed in connection1 first with a line of known length1 and then with the obstructed tube4 ,by this means the position of the obstruction can be ascertained within one hundred feet4 ,or the tube may be probed with a long rod up to a length of two hundred feet4 ,a very ingenious apparatus1 by ,m4 ,ch4 ,bontemps1 is shown in ,figs4 #di and #ej1 and is employed to ascertain the exact position of the obstruction4 ,it acts by the reflection of sound-waves on a rubber diaphragm4 ,a small metal disk is cemented to the rubber1 and above this is a pointed screw1 ,d4 ,an electric circuit is closed where the points ,c and ,d are brought in contact4 ,to locate an obstruction a pistol is fired into the tube as shown1 and the resulting wave1 traversing the tube at the rate of three hundred and thirty metres a second1 strikes the obstruction and is then reflected against the diaphragm1 which in its turn reflects it to the obstacle1 whence it returns to the diaphragm4 ,by this means indications are marked on the recording cylinder1 and if the interval of time between the first and second indications be recorded1 the distance of the obstacle from the membrane is easily ascertained4 ,the chronograph employed is provided with three points2 the first of these is placed in a circuit1 which is closed by the successive vibrations of the diaphragm2 the second corresponds to an electric regulator1 marking seconds on the cylinder2 and the third subdivides the seconds there marked4 ,fig4 #ej indicates a record thus made4 ,in this case the obstacle is situated at a distance of sixty-two metres1 and the vibration marks thirty-three oscillations per second4 ,the interval occupied by two successive marks from the diaphragm on the paper corresponds to twelve oscillations1 and the distance of the obstruction is then calculated by the following formula3 ,d #j4e #ccj #ab_/cc #fj metres2 so that the distance of the obstacle is recorded within two metres4 ,illustration3 ,,fig4 #di4 ,,obstruction-,,recording ,,apparatus4 ,illustration3 ,,fig4 #ej4 ,,obstruction-,,recording ,,apparatus4 8,amongst the special causes of accident may be mentioned the accidental absence of a piston to the train1 breaking of the piston1 the freezing up of a piston in the tube1 and even forgetting the presence of a train1 which has caused the entire service to be one train late throughout the day4 ,finally1 the tubes themselves are sometimes broken or disturbed during street repairs1 resulting of course in a complete cessation of traffic in the system till the damage is made good40