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If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this ebook. - -Title: The Pneumatic Despatch Tube System of the Batcheller Pneumatic - Tube Co. - Facts and General Information Relating to Pneumatic Despatch - Tubes - -Author: B. C. Batcheller - -Release Date: December 05, 2020 [EBook #63952] - -Language: English - -Character set encoding: UTF-8 - -Image source(s): https://archive.org/details/cu31924022810943 - -Produced by: deaurider, Brian Wilcox and the Online Distributed - Proofreading Team at https://www.pgdp.net (This file was - produced from images generously made available by The Internet - Archive) - -*** START OF THE PROJECT GUTENBERG EBOOK THE PNEUMATIC DESPATCH TUBE SYSTEM -OF THE BATCHELLER PNEUMATIC TUBE CO. *** - -Transcriber’s Notes: - -The spelling, punctuation and hyphenation are as the original, with the -exception of apparent typographical errors which have been corrected. - -Italic text is denoted _thus_. - -Bold text is denoted =thus=. - - - - -[Illustration: THE MAIN POST-OFFICE, PHILADELPHIA.] - -THE - -PNEUMATIC DESPATCH TUBE SYSTEM - -OF THE - -BATCHELLER PNEUMATIC TUBE CO. - -ALSO - -FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC DESPATCH TUBES - -BY B. C. BATCHELLER, B.Sc. MECHANICAL ENGINEER - - -[Illustration] - - -PHILADELPHIA PRESS OF J. B. LIPPINCOTT COMPANY 1897 - - - - -COPYRIGHT, 1896, - -BY - -B. C. BATCHELLER. - - - - -CONTENTS. - - -CHAPTER I. - -A BRIEF HISTORICAL SKETCH. PAGE - -EARLY RECORDS 9 - -PRACTICAL BEGINNING OF THE ART—THE LONDON PNEUMATIC TELEGRAPH 10 - -THE SIEMENS CIRCUIT SYSTEM 14 - -RECENT IMPROVEMENTS IN THE LONDON SYSTEM 16 - -AN UNDERGROUND PNEUMATIC RAILWAY FOR TRANSPORTATION OF MAIL 19 - -THE BERLIN PNEUMATIC TELEGRAPH 20 - -THE PARIS PNEUMATIC TELEGRAPH 22 - -THE PNEUMATIC TELEGRAPH OF OTHER CITIES 25 - -PNEUMATIC TUBES IN AMERICA 25 - - -CHAPTER II. - -THE PNEUMATIC TRANSIT COMPANY AND THE FIRST PNEUMATIC TUBES FOR THE -TRANSPORTATION OF UNITED STATES MAIL. - -ORGANIZATION 28 - -AIM AND OBJECT OF THE COMPANY 28 - -THE CLAY-LIEB PATENTS 30 - -FRANCHISES AND FIRST GOVERNMENT CONTRACT 33 - -SEARCH FOR TUBES 34 - -METHOD OF MANUFACTURING TUBES 35 - -LAYING AND OPENING THE TUBES FOR TRAFFIC 37 - -DESCRIPTION OF THE TUBES, METHOD OF LAYING, ETC. 38 - -THE AIR-COMPRESSOR—METHOD OF CIRCULATING THE AIR 40 - -TERMINAL APPARATUS 42 - -THE SENDER 43 - -SUB-POST-OFFICE RECEIVER 44 - -MAIN POST-OFFICE RECEIVER 47 - -THE CARRIER 50 - -OPERATION OF THE TUBES 52 - -BENEFITS OF THE SYSTEM 54 - - -CHAPTER III. - -THE SYSTEM AND APPARATUS OF THE BATCHELLER PNEUMATIC TUBE COMPANY. - -GENERAL ARRANGEMENT AND ADAPTABILITY OF THE SYSTEM 57 - -THE SIZE OF TUBES 64 - -SYSTEM OF VERY LARGE TUBES 65 - -GENERAL ARRANGEMENT OF APPARATUS IN THE STATIONS—TWO-STATION, -TWO-COMPRESSOR LINE 69 - -TWO-STATION, ONE-COMPRESSOR LINE 72 - -THREE- TO EIGHT-STATION LINE 74 - -SENDING APPARATUS 79 - -SENDING TIME-LOCK 84 - -INTERMEDIATE STATION TIME-LOCK 88 - -ELECTRO-PNEUMATIC CIRCUIT-CLOSER 91 - -THE OPEN RECEIVER 94 - -THE CLOSED RECEIVER 99 - -THE INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS 106 - -CARRIERS 115 - -AIR SUPPLY 117 - -FANS 117 - -BLOWERS 117 - -AIR COMPRESSORS 118 - -THE TUBE, LINE CONSTRUCTION, ETC. 122 - - -CHAPTER IV. - -FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC TUBES. - -DEFINITIONS 124 - -INTERMITTENT AND CONSTANT AIR-CURRENT 125 - -LAWS GOVERNING THE FLOW OF AIR IN LONG TUBES 126 - -LAW OF PRESSURE 128 - -USES OF PRESSURE CURVES 130 - -LAW OF VELOCITY 130 - -CHARACTERISTICS OF THE VELOCITY CURVE 132 - -USES OF VELOCITY CURVES 133 - -QUANTITY OF AIR USED 134 - -TEMPERATURE OF THE AIR 135 - -HORSE-POWER 136 - -EFFICIENCY 137 - -PRESSURE AND EXHAUST SYSTEMS 138 - -LAWS EXPRESSED IN MATHEMATICAL FORMULÆ 141 - -MOISTURE IN THE TUBES 142 - -LOCATION OF OBSTRUCTIONS IN TUBES 143 - - - - -PREFACE. - - -I have been prompted to prepare this book by the frequent inquiries -made regarding the details of our system of pneumatic tubes. These -inquiries have come from people interested in our company, from -others interested in companies that have purchased the right to use -our apparatus, from people desirous of becoming interested in a -pneumatic-tube business, from would-be purchasers of pneumatic tubes, -and from people interested in pneumatic tubes from a scientific, -engineering, or mechanical point of view. This book is not intended to -be a treatise on pneumatic tubes. 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 time. The second chapter -contains a description of the postal tubes in Philadelphia, and the -third chapter describes our system in detail. Following this is a short -chapter explaining the theory of pneumatic tubes, or the theory of -the flow of air in long pipes, stating the more interesting facts and -relations in as plain and simple a manner as possible. Mathematical -formulæ have been purposely avoided. - -Several plates showing the Philadelphia postal line have been kindly -loaned to me by the Engineers’ Club of Philadelphia. They formerly -appeared in a paper read by Mr. A. Falkenau before that club. I -have also to thank the and Drill Co., the B. F. Sturtevant Co., -the Wilbraham-Baker Blower Co., and J. B. Stewart for the use of -electrotypes of their machines. - -B. C. B. - -October 6, 1896. - - - - -THE - -BATCHELLER PNEUMATIC TUBE SYSTEM. - - - - -CHAPTER I. - -A BRIEF HISTORICAL SKETCH. - - -=Early Records.=—The earliest reference to pneumatic transmission of -which we find any record is a paper presented to the Royal Society of -London, by Denis Papin, in the year 1667, entitled “Double Pneumatic -Pump.” His plan was to exhaust the air from a long metal tube by two -large cylinders. The tube was to contain a piston, to which a carriage -was attached by means of a cord. The “American Cyclopædia” goes on to -say, “More than a century elapsed before any further effort in this -direction was made. Paucbrouke’s ‘Dictionnaire Encyclopédique des -Amusements des Sciences’ (1792) gives a description of a machine by -M. Van Estin, 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 -length, and having many curves. This plan seems, however, to have been -more an amusement than an attempt to introduce an industrial scheme. -With more regard to practical results, Medhurst, an engineer of London, -published a pamphlet on the subject in 1810. He proposed to move small -carriages on rails in air-tight tubes or tunnels, by compressed air -behind, or by creating a partial vacuum in front. In 1812 he published -another pamphlet; but the plan was not put into successful operation, -principally from insufficient means of exhaustion. About 1832 he -proposed to connect the carriage inside such a tube with a passenger -carriage running on the top of the tube; and, although the latter -project has never been commercially successful, it was the first to -be practically attempted. More than a score of patents were taken out -on the Continent and in England and America, none of which met with -any practical success. Returning to the original idea of Denis Papin, -inventors attempted to accomplish pneumatic transmission by moving the -load inside the tube, and in course of time achieved success. In France -MM. Jarroux and Taisseau presented a project for atmospheric telegraphy -before the Academy of Sciences, and they were succeeded in the same -direction by MM. Brochet and Ardor.” - - -=Practical Beginning of the Art. The London Pneumatic -Telegraph.=—London has the honor of being the first city to have -a practical system of pneumatic telegraphy. The first tubes were -installed by the Electric and International Telegraph Company, the work -being planned and carried out by their engineer, Mr. Josiah Latimer -Clark, in 1853 and 1854. The first tube to be laid was one and one-half -inches in diameter, and extended from the central station, Founder’s -Court, Lothbury, to the Stock Exchange, Throgmorton Street, a distance -of two hundred and twenty yards. The tube was operated intermittently -by connecting it to a vacuum chamber at the central station. Carriers -were sent only in one direction. A steam-pump was used to maintain the -vacuum. Much experience was gained from the use of this first tube. In -1858 some improvements were made by Mr. C. F. Varley, and I can best -describe them by quoting from the discussion of Mr. Carl Siemens’s -paper on “Pneumatic Despatch Tubes: The Circuit System” before the -Institution of Civil Engineers, as recorded in the minutes of that -society. “Later, about the year 1858, when a pipe two and one-fourth -inches internal diameter was extended from Telegraph Street to Mincing -Lane, thirteen hundred and forty yards in length, the traffic was so -considerable that it was found desirable to have the power of sending -messages in both directions. To effect that a smaller pipe, one and -one-half inches in internal diameter, was laid between Telegraph -Street and Mincing Lane, with a view to carrying the vacuum to the -latter station, so as to take messages in the opposite direction. This -smaller pipe was found to so wiredraw the current that the pipe would -not work, the leakage past the carrier being too considerable; 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 -Lane. This chamber was constructed of timber, fourteen feet by twelve -feet broad and ten feet high, and was covered with lead. It was not -strong enough to withstand the pressure; for one day, a carrier having -stuck half-way, and when there was a higher vacuum than usual,—viz., -twenty-three inches of mercury,—it collapsed with a loud report. At -the time the landlord of the house happened to be dining in the next -room, and he suddenly found himself, his table, dinner, and the door, -which was wrenched off its hinges, precipitated into the room amongst -the _débris_ of the chamber. The windows were forced inwards, and those -on the opposite side of Mincing Lane and Leadenhall Street were drawn -outwards. The damage was considerable. This accident put an end, for a -time, to the attempt to send telegraph messages by means of a vacuum -conveyed through this smaller pipe. About that time he (Mr. Varley) -became the engineer-in-chief of the Electric Telegraph Company, and -got permission from the directors to introduce a new system,—viz., -compressed air,—though many persons contended that it would be -impossible to blow messages through a pipe, because all attempts to -blow air through long pipes had utterly failed; while others said -that, if messages were sent, they would go much slower than with the -vacuum.... In his (Mr. Varley’s) apparatus, for he was the first to -introduce compressed air, the reverse was found to be the case, and for -this reason: the tube did not consume power until a message was about -to be forwarded; and in a tube thirteen hundred yards in length, and -two and one-fourth inches in diameter, 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 tube, -consequently the carrier did not start until after fifteen seconds -had elapsed. When a message was sent by compressed air, it was sent -from the end at which the power was applied, and the carrier started -at once, thus gaining a start of fifteen seconds; now, inasmuch as -the air in the tube had to be compressed, it started at a very high -velocity, and when it reached the other end the compressed air in -expanding gave it a higher velocity. The result was, in thirteen -hundred and forty yards, from Telegraph Street to Mincing Lane, -the carriers were drawn by vacuum, on an average, in from sixty to -seventy seconds, and were propelled by compressed air in about fifty -or fifty-five seconds, the difference of pressure in each case being -nearly equal.” - -The first one and one-half inch tubes laid under the direction of Mr. -Clark were of iron with screwed joints. They gave much trouble from -roughness upon the interior, which wore the carriers very rapidly, and -from water that was drawn in through leaky joints. When the extensions -were made in 1858 and afterwards, two and one-fourth inch lead tubes -were used with plumber’s joints made over a heated mandrel, which made -the joints very smooth upon the interior. The carriers were of gutta -percha in the form of a cylinder closed at one end and fitted with a -cap at the other. The outside was covered with felt or drugget. - -When a carrier was to be despatched, a signal was sent to an attendant -at the pump end of the tube, who closed that end and connected the tube -to an exhausted chamber by opening a valve. As soon as the carrier -arrived, he closed the valve and opened the tube, which allowed the -carrier to drop out. Mr. 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 button. He also improved the -carriers by doing away with the cap and using in its place an elastic -band to hold the messages in place. - -We have seen that Mr. Clark designed the first tube used in connection -with the telegraph, and that it was a single tube, operated in one -direction only by vacuum, being operated only when there were messages -to send. This was extended and improved by Mr. Varley, who increased -the diameter of the tubes from one and one-half inches to two and -one-quarter inches, and operated them in both directions, using vacuum -for sending in one direction and compressed air for sending in the -other. The air current was maintained in the tubes only when messages -were sent. - -Great credit is due to Sir Rowland Hill, who, in 1855, had a proposed -method of conveying mails in the city of London, through nine- and -thirteen-inch tubes, thoroughly investigated. 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 system. - - -=The Siemens Circuit System.=—The next progressive step was made -by Siemens Brothers, of Berlin, who proposed a new system called -the “circuit system,” in which two tubes were used, the up tube -being connected to the down tube at the distant end. The air was -compressed into one end of the circuit and exhausted from the other, -and, furthermore, it was kept in constant circulation. Carriers were -despatched by inserting them into the air-current without stopping it, -in one direction in one tube or in the opposite direction in the other. -Another feature of the Siemens system was the placing of three or more -stations on one double line of tubes. 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 carrier. This system -is described in detail in a paper read by Mr. Carl Siemens before the -Institution of Civil Engineers, London, November 14, 1871, Vol. XXXIII. -of the Proceedings. 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 -tube. One of the tube sections was open at both ends, and was used for -despatching carriers. A carrier was placed in it, then it was swung -into line with the main tube, when the air-current passing through -swept the carrier along. The other tube section contained a perforated -screen in one end and was used to receive carriers. When it was in line -with the main tube and a carrier arrived, the carrier was stopped by -striking the screen, then the tube section was swung to one side and -the carrier pushed out with a rod. A by-pass was provided for the air -around the apparatus so that its flow was not checked when the tube -section was swung. When a carrier was despatched to an intermediate -station, a signal was sent, and then the section of tube containing -the screen was interposed in the line of the tube to stop the carrier -upon its arrival. The carriers used by Mr. Siemens were made of -gutta-percha, papier maché, or tin, closed at one end and fitted -with a cover at the other. They were covered with felt, drugget, or -leather. The front ends of the carriers were provided with thick disks -of drugget or leather fitting the tube loosely, and the opposite ends -were surrounded with pieces of the same material attached to them like -the leather of an ordinary lifting pump. - -In 1869 Messrs. Siemens Bros. received an order from the British -government to install an experimental line of tubes between the central -telegraph station and the general post-office. This was completed in -1870, and after a half-year’s test it was extended to Fleet Street, -and finally to Charing Cross. The tubes were of iron, three inches -in diameter, with flanged and bolted joints. It was found, after -some experience, that there was no advantage in the circuit, so the -up and down tubes were separated at Charing Cross Station and worked -independently. - - -=Recent Improvements in the London System.=—In 1870 Mr. J. W. Wilmot -designed a double sluice-valve by means of which carriers could be -despatched continuously without stopping the flow of air in the tubes. -Mr. Wilmot further increased the working capacity of pneumatic tubes -when, in 1880, he invented an intermediate automatic signaller, by -means of which a carrier signals the passage of a given point on its -journey, showing that the section of the tube traversed is clear, thus -allowing a second carrier to be despatched before the first has reached -its destination. - -[Illustration: FIG. 2. - -DIAGRAM ILLUSTRATING THE PNEUMATIC TUBE SYSTEM LONDON RADIAL SYSTEM. - -LONDON RADIAL SYSTEM.] - -From this beginning the English system developed into what has been -termed a “radial system;” that is to say, one principal and several -minor central pumping stations have been established, and from these -radiate tubes to numerous sub-stations (see Fig. 2). Some of the -stations are connected with double lines for sending in opposite -directions. The out-going tube from the pumping station is worked by -compressed air, and the incoming tube by exhaustion. Other stations -are connected by single tubes, and they are operated alternately by -compression and exhaustion. Intermediate stations are located on some -of the lines. For the central station the Varley valves were found -too expensive and troublesome to keep in order, so they were replaced -by the Wilmot double sluice-valves, which are operated manually. In -recent years the sluice-valves have been in turn replaced by what are -termed D-boxes, a simpler form of apparatus. At the sub-stations the -tube terminates in a box into which the carriers drop. As the system -has been gradually extended, tubes two and three-sixteenths inches -inside diameter have been used for short lines, and three-inch tubes -for long lines. The tubes are of lead laid inside a cast-iron pipe -which serves as a shield, protecting them from injury. They are laid -in twenty-nine foot sections, the joints being made by soldering over -a steel mandrel, which is afterwards drawn out by a chain. The joints -in the cast-iron protecting pipe are made by caulking with yarn and -lead. “Electric signals are used between the central and sub-stations, -consisting of a single stroke bell and indicator, giving notice of -the arrival and departure of carriers, and to answer the necessary -questions required in working. Where there are intermediate stations -the tubes are worked on the block system, as if it were a railway. -Experience shows that, where great exactness of manipulation cannot -be obtained, it is necessary to allow only one train in each section -of a tube, whether worked by vacuum or pressure. But where there is -no intermediate station, and where the tube can be carefully worked, -carriers may be allowed to follow one another at short intervals in -a tube worked by vacuum, although it is not perfectly safe to do so -in one worked by pressure. In working by pressure it has been found -that, notwithstanding a fair interval may be allowed, carriers are -apt to overtake one another, for no two carriers travel in the same -times, because of differences in fit, unless they are placed end to -end. If signalling be neglected and a carrier happens to stick fast, -being followed by several others, a block will ensue which it will be -difficult to clear, while the single carrier could readily have been -dislodged.” (_Proceedings Institute of Civil Engineers, London_, Vol. -XLIII. p. 61.) - -No changes have been made in the carriers from those used in the early -experiments which have already been described. - -The London system has grown until it now includes no less than -forty-two stations and thirty-four miles of tubes. Similar systems -have been established in connection with the telegraph in Liverpool, -Manchester, Birmingham, Glasgow, Dublin, and New Castle. The tubes -give a cheaper and more rapid means of despatching telegrams between -sub-stations and central stations than transmission by telegraph, and -local telegrams can be delivered in the sender’s handwriting. - - -=An Underground Pneumatic Railway for Transportation of Mail.=—Before -describing the systems used in the cities on the Continent of Europe, -we must notice a very large pneumatic tube, or more properly called a -pneumatic tunnel railway, constructed in London for the transportation -of mail from one of the railway stations. The first railway of this -type was constructed in 1863 by the Pneumatic Despatch Company of -London, and extended from Euston to the district post-office in -Eversholt Street, a distance of about eighteen hundred feet. The -tunnel was flat on the bottom, having a D-shaped cross-section two -feet eight inches by two feet eight inches. The carriers or carriages -were cradle-like boxes fitting the tunnel, and they moved at a speed -of seventeen miles per hour, carrying fifteen mails daily. In 1872 -two similar but larger tunnels were built from Euston Station to the -general post-office, a distance of fourteen thousand two hundred and -four feet, or two and three-quarters miles. One was for the up traffic, -and the other for the down. The tunnels were four and a half feet -wide by four feet high, the straight part being built of cast iron -and the bends of brick. The line was operated by a fan twenty-two -feet in diameter, which forced the air into one tunnel and exhausted -it from the other, producing a vacuum of ten inches of water, or six -ounces per square inch. The carriages occupied twelve minutes in -traversing the tunnels, and there was one gradient of one to fourteen. -The carriages were ten feet four inches long and weighed twenty-two -hundredweight. “The system was able to transport over the whole line, -allowing for delays, an average of a ton per minute.” The system was -used to transport the mails in bulk, but it was found to be slow and -unsatisfactory, and was very soon abandoned. - - -=The Berlin Pneumatic Telegraph.=—In 1863 the Prussian government -applied to the firm of Siemens and Halske, of Berlin, for a proposition -to establish a system of pneumatic tubes in that city for the -transmission of telegraph messages. A proposition was accordingly -submitted, and the work was completed in 1865. This first line -consisted of two parallel wrought-iron tubes, two and one-half inches -in diameter, one tube being used exclusively to send in one direction, -and the other in the opposite direction. They extended from the -telegraph station to the Exchange, requiring a total length of five -thousand six hundred and seventy feet of tube. The two tubes were -looped together at the Exchange, and a continuous current of air was -made to circulate in them by a double-acting steam air-pump, located -at the telegraph station. Air was compressed into one end of the tube -and exhausted from the other. With nine inches of mercury pressure and -vacuum the passage was made in ninety-five seconds to the Exchange, -and seventy-five seconds from the Exchange. It was similar to the -line established in London by the same firm some years later, which we -have already described, except that there was no intermediate station. -After the line had been in use for a year and a half, the Prussian -government had it extended, first, from the telegraph station to the -Potsdam gate, with an intermediate station at the Brandenburg gate. -After these preliminary experiments, further extensions were made until -a net-work of tubes extended over the city of Berlin, including no -less than thirty-eight stations and over twenty-eight miles of tubes; -but in laying down this net-work a departure was made from the Siemens -system. Air was no longer kept constantly circulating, but power was -stored up in large tanks, some being exhausted and others filled with -compressed air, which was used when required to send messages, usually -at intervals of five or fifteen minutes. The exhausted tanks were -permanently connected with the closed tubes, which were opened when -required for use. The tanks containing compressed air were connected -to the tubes when messages were sent. The internal diameter of the -tubes was 2.559 inches. They were laid in circuits, including several -stations in a circuit, and the carriers travelled only in one direction -around the circuit. Some outlying stations were connected by a single -tube with central pumping-stations, these single tubes being worked in -both directions. Years of experience have shown the disadvantages of -this circuit-system, and it has gradually been changed to the radial -system, such as is used in London, until now nearly all the stations -are grouped around the central pumping-stations, to which they are -connected directly by radiating tubes. The Siemens apparatus has been -replaced by simpler and less expensive valves and receiving-boxes, the -latest form of which was designed and patented by Mr. Josef Wildemann. - -[Illustration: FIG. 3. - -DIAGRAM OF PART OF PARIS PNEUMATIC TUBE SYSTEM. - -PARIS CIRCUIT SYSTEM.] - -=The Paris Pneumatic Telegraph.=—We will now glance at the system used -in Paris, which has some novel features. In 1865 it was decided to -establish a system, and the first experimental line, from Place de la -Bourse to the Grand Hôtel, on the Boulevard des Capuciens, was laid in -1866. Instead of using a steam-engine to drive an air-compressor or -exhaust-pump, air was compressed in tanks by displacement with water -from the city mains. In 1867 this line was extended to Rue de Grennelle -St. Germain, with an intermediate station at the Rue Boissy d’Anglais, -and another line with stations at Rue Jean Jacques Rousseau, Hôtel du -Louvre in the Rue de Rivoli, the Rue des Saints Pères, and terminating -in the central station. In 1868 the system was changed to a polygonal -or circuit system by removing the station in the Rue de Rivoli to the -Place du Théâtre Français and connecting the latter directly with the -Bourse. Other changes and extensions were made in 1870 and 1871, until -three polygons or circuits were formed, with five or six stations -in each circuit, and several outlying stations were connected by -independent tubes. In the middle of the year 1875 seventeen stations -had been connected and plans were made for twenty-one more. Instead of -maintaining an air-current around each circuit by machinery located at -one of the stations on the circuit, at least three of the five or six -stations comprised in the circuit have means of supplying compressed -air or of exhausting it, and each side of the polygon, or section of -the circuit between two stations, is operated independently of the -rest of the circuit (see Fig. 3). Carriers are sent on from station -to station around the circuit, 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 moving. The carriers are made up -in trains of from six to ten, with a piston behind them that fits the -tube closely and forces them ahead. Each carrier is addressed by means -of a label for its destined station. Trains are despatched around the -circuits at stated times, usually at fifteen-minute intervals. As they -arrive at the various stations, carriers are taken out and others put -in, and the trains sent on their way. The carriers consist of iron -cylinders, closed at one end, with a leather case that slides over -them and closes the open end. They weigh, when filled with thirty-five -messages, twelve and one-half ounces, and they will travel about -twelve hundred miles before the leather cover is so worn that it must -be thrown away. The pistons are made of a wooden cone, covered with -iron, and having a “cup-leather” upon the rear end that fits the tube -closely. The sending and receiving apparatus consists of sections -of tube closed at one end, having a door on the side, through which -carriers are inserted or despatched. A peculiar form of fork is used -for picking them out. The air is controlled by valves opened and closed -by hand. - -Several methods are used to compress and exhaust the air. 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 sewer, or in -having the air compressed by allowing water from the city mains to flow -into the tanks and displace the air. Water jets have also been used, -operating similar to a steam-injector. At some of the stations water -turbines drive the air-pumps, and at others steam-engines are used. - -The tubes of the Paris system are of wrought iron, in lengths of from -fifteen to twenty feet, the joints being made with flanges and bolts. -The interior diameter is 2.559 inches with a maximum variation to 2.519 -inches. The bends are made with a radius of from six to one hundred -and fifty feet. Water frequently gives trouble by accumulating in the -tubes. Traps are placed at low points to drain it off. - -The speed of the trains of carriers in the Paris tubes is from fifteen -to twenty-three miles per hour, and the average time that elapses from -the receipt of a message until its delivery is from forty to forty-five -minutes. - - -=The Pneumatic Telegraph of other Cities.=—A system similar to the -one just described is used in Vienna. It differs some in details of -apparatus, but the carriers are despatched around circuits in trains, -stopping at each station, where some carriers are removed and others -inserted. Brussels also is not without its system of pneumatic tubes -for the transmission of telegrams. - - -=Pneumatic Tubes in America.=—Turning our attention now to our own -country, we cannot pass without mention some experiments of Alfred -E. Beach with pneumatic railways, made nearly thirty years ago. His -first experiment upon a large scale was made at the American Institute -Fair held in New York City in 1867. Here he had constructed a circular -wooden tube, one hundred and seven feet long and six feet in diameter. -A car that would seat ten people ran upon a track laid down inside the -tube, and was propelled by a helix fan ten feet in diameter, making -two hundred revolutions per minute. He next tried his railway upon -a practical scale, constructing an eight-foot tunnel for two hundred -feet under Broadway, starting at the corner of Warren Street. A car -was propelled by a large rotary blower located in the basement of a -building near by. The blower was kept constantly running, and the car -was sent alternately in one direction and then the other by changing -valves at the blower. Few people know that this experimental line still -exists under Broadway as Mr. Beach left it. - -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 desk. Seamless brass tubes -are usually used, and, since the tubes are short, the air is either -compressed or exhausted by means of positive rotary blowers. At the -outlying stations the tubes are usually open to the atmosphere, while -at the central station simple forms of valves are used for sending and -receiving. An outgoing and a return tube are always used, and the air -is kept in constant circulation. The carriers are of metal with felt -packing rings and open on the side. These cash-carrying systems have -come into use during the past twenty-five years. - -The Western Union Telegraph Company uses small tubes to transmit its -messages to a considerable extent in some of our large cities. In -1876 four lines were laid in New York City from the main office on -Broadway: two to the branch office at No. 14 Broad Street, one to 134 -Pearl Street, and one to the Cotton Exchange. Since then this company -has laid a double line about two miles in length under Broadway to its -up-town office. It also uses tubes to send messages from the receiving -desks to the operating rooms within the buildings. - -Many of our large hotels use pneumatic tubes to transmit messages to -the different floors and offices of the buildings, taking the place of -messenger, or bell boys, who formerly did this service. - -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 Europe, -none of the tubes are larger than three inches internal diameter; -also that in all the systems, except in Paris where the carriers are -despatched in trains, the carriers are so light and move so slowly that -they can be stopped by allowing them to come in contact with some solid -object, such as a box into which the carriers drop. Very few of the -tubes are more than two miles in length, and most of them are less than -one mile. A speed of more than thirty miles per hour has seldom been -attempted, and usually it is much less than this. - - - - -CHAPTER II. - -THE PNEUMATIC TRANSIT COMPANY AND THE FIRST PNEUMATIC TUBES FOR THE -TRANSPORTATION OF UNITED STATES MAIL. - - -=Organization.=—Early in the year 1892 several Philadelphia gentlemen -organized a corporation and obtained a charter in the State of -New Jersey to construct, lay, and operate pneumatic tubes for the -transmission of United States mail, packages, merchandise, messages, -etc., within the States of New Jersey and Pennsylvania. The corporation -was styled the Pneumatic Transit Company. Mr. William J. Kelly was -elected president, and the company is still under his management. - -[Illustration: WM. J. KELLY, - -President of the Pneumatic Transit Co.] - -=Aim and Object of the Company.=—When the Pneumatic Transit Company -was formed, it was the aim and object of its promoters to construct -an extensive system of underground tubes in the City of Philadelphia -which would serve, first, for the rapid transmission of mail, second, -for the quick delivery of merchandise from the large retail stores, -third, for the transmission of telegrams or messages within the city -limits, and, fourth, to conduct a general local express business with -greater speed than can be done in any other manner. To accomplish -this result sub-stations were to be located six or eight blocks apart -throughout a large portion of the city, and a central station was to be -established in the centre of the business section. Stations were -also to be established in the more important retail stores and large -office buildings, and all of the stations were to be connected by tubes -forming one large system. - -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 matter. The sub-post-offices would be divided into groups, all -of the offices in one group being connected to the same line, which -would terminate at the main post-office. Most of the business would -be between the main and individual sub-offices; in addition to this -there would be some local mail sent between the sub-offices which, for -offices in the same group, could be despatched directly without passing -through the main office. The advantages to be gained by the use of -these tubes over the present wagon service are very apparent. It places -all the sub-post-offices in almost instant communication with the main -office and with each other. - -It was a part of the general plan to lay tubes from the main -post-office to the railway stations, thereby hastening the despatch and -receipt of mails to and from the trains. - -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 city. Of course it would not be practicable -to lay a tube to each house, but with a station not more than four or -five blocks away, the parcels would be sent through the tube to the -nearest station, and then delivered by messengers to the houses with a -minimum loss of time. Ladies could do their shopping and find their -purchases at home when they returned. - -The same tubes used for parcel delivery would also be used for a -district messenger service. With numerous public stations in convenient -locations, all the advantages of the European system would be realized -in the quick despatch of letters and telegrams. Every one knows how -much time is consumed by district messenger-boys in the delivery of -messages, especially when they have to go long distances, and no -argument is required to show that this time would be very much reduced -by the use of pneumatic tubes, besides prompt delivery would be made -much more certain. - -The tubes of this system were to be six or eight inches in diameter, -with a few small tubes in localities where the message service is very -heavy. - -Without going more into detail, such were in brief the plans of the -promoters of this new company; but before launching such an enterprise, -involving a large amount of capital, there were many engineering and -mechanical problems to be solved. It was not simply a question of -obtaining tubes and laying them in the streets, but ways and means -for operating them must be devised. Up to this time only small tubes -had been used for the transmission of telegrams, messages, cash, and -other light objects. Now it was proposed to transmit heavy and bulky -material. There was no experience for a guide. - - -=The Clay-Lieb Patents.=—The Pneumatic Transit Company at this time -turned to the Electro-Pneumatic Transit Company, of New Jersey, a -national company that had been in existence since 1886, and which -claimed to own valuable patents, for the ways and means to carry out -its new enterprise. The patents were those of Henry Clay and Charles -A. Lieb, and the rights to use them in the State of Pennsylvania -were procured by the Pneumatic Transit Company, under a contract -entered into between the two companies. 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 tubes, with -electrically-operated switches at the junctions of the branches with -the main lines. 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 operate. Up to the present time only single- or double-line tubes -have been used, without branches. In the European systems, frequently -several stations are located along a line, but the carriers must stop -at each station, be examined, and if they are destined for another -station, they must be redespatched. 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 store. It is plain to be seen -that, if several of these stations could be connected by branches to -a main tube, a large amount of tubing would be saved—a most desirable -result. The advantages of such a system would be still greater for long -lines of tube laid under the pavements, extending to stations located -in different parts of a large city. It was such a result that the -patents of Clay and Lieb aimed to accomplish. - -In order to demonstrate the practicability of the system, the -Electro-Pneumatic Transit Company had constructed in the basement of -the Mills Building, on Broad Street, New York, a short line of small -brass tubing, about two or three inches in diameter, with one branch, -thus connecting three stations together. The tube was very short, -probably not more than two hundred feet in length. The air-pressure -required was very slight, probably not more than an ounce or two, being -supplied by a small blower run by an electric motor. - -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 branch. This switch was moved by an electro-magnet, -or solenoid, that could be excited by pressing a button at the station -from which the carrier was sent. When the carrier passed into the -branch tube it set the switch back into its normal position, so that a -second carrier, following the first, would pass along the main tube, -unless the switch was again moved by pressing the button at the sending -station. - -This tube in the Mills Building worked well, but it was of a size -only suited to the transmission of cash in a store or other similar -service. It could not be said, because this tube worked well, that -a larger and longer tube with numerous branches would work equally -well. In fact, there are several reasons why such a tube would not -operate satisfactorily. The method of operating the switches was -impracticable. 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 tube. -Again, suppose that we have just despatched a carrier destined for a -station on the main line beyond the junction, and that we wish to -despatch the second carrier to be switched off into the branch tube, -we must wait at least four minutes, until the first carrier has passed -the junction, before we can press the button and set the switch for -the second carrier which may be on its way. How are we to know when -the first carrier has passed the junction, and when the second will -arrive there, in order that we may throw the switch at the proper time? -Must we hold our watch and time each carrier? It is plain that this is -not practical. I take this as an illustration of the impracticability -of the Clay-Lieb System as constructed in the Mills Building when -extended to practical dimensions. I will not describe the mechanism and -details of the system, which are ingenious, but will say in passing -that the automatic sluice-gates, 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 inch, 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 inch. For further information the reader is referred -to the patents of Clay and Lieb. - - -=Franchises and First Government Contract.=—In the spring of 1892 -an ordinance was passed by Common and Select Councils, and signed -by the Mayor of the City of Philadelphia, permitting the Pneumatic -Transit Company to lay pneumatic tubes in the streets of that city. -At the time this franchise was granted negotiations were in progress -with the post-office department, in Washington, for the construction -of a six-inch pneumatic tube, connecting the East Chestnut Street -sub-post-office, at Third and Chestnut Streets, with the main -post-office, at Ninth and Chestnut Streets, for the transmission of -mail. This sub-post-office was selected because more mail passes -through it daily than any other sub-office in the city, it being -located near the centre of the banking district. Negotiations were -delayed by various causes, so that the contract with the government was -not signed until October, 1892. - -[Illustration: FIG. 4. - -[Illustration: FIG. 5. - -SIX-INCH PNEUMATIC TUBES IN PROCESS OF BORING AT THE SHOP OF A. -FALKENAU, PHILADELPHIA, PA.] - -=Search for Tubes.=—It was at this time that the writer was first -employed by the Pneumatic Transit Company, as engineer, to superintend -the construction of this line. The company commenced at once to -carry out its contract with the United States government, both the -post-office department and the company being very desirous of having -the work completed before winter. The time was very short for such -an undertaking, but wrought-iron tubes had already been ordered of a -well-known firm who manufacture pipe and tubing of all kinds. After -waiting four or five weeks the first lot of tubes were finished, -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 intended. The next thing that suggested itself was -seamless drawn brass tubes. While they would be very expensive, the -process of manufacture makes them eminently suited for the purpose, -but they could not be obtained in time. A city ordinance prohibits -the opening of the streets of Philadelphia during the winter months -except in extreme cases. Accurate tubes must be had, and had quickly. -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 purpose. Inquiry was made -at nearly all the machine-shops in the city, to ascertain how many -boring-machines could be put upon this work of boring nearly six -thousand feet of six-inch pipe. It was found impossible to get the work -done in time, if it was to be done in the usual manner of boring with -a rigid bar. At last a man was found in Mr. A. Falkenau, engineer and -machinist, who was prepared to contract for the construction of twelve -special boring-machines and to bore all the tubing required. Suffice it -to say, that the machines were built, and about six thousand feet of -tubes were bored, between November 8 and December 31. - -[Illustration: FIG. 6. - -_PIPE BORING APPARATUS._] - -=Method of Manufacturing Tubes.=—The process of boring was novel in -some respects, and might be termed reaming rather than boring. Figs. -4 and 5 show the interior of the shop and the twelve machines. Fig. 6 -is a drawing of one of the machines. A long flexible bar rotated the -cutter-head, which was pulled through the tube, in distinction from -being pushed. In order to give the feeding motion, a screw was attached -to the cutter-head and extended through the tube in advance of it. 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 began. Since it -was not necessary that the tubes should be perfectly straight, a method -of this kind was permissible, in which the cutters could be allowed to -follow the cored axis of the tube. Air from a Sturtevant blower was -forced through the tubes during the process of boring, for the double -purpose of clearing the chips from the cutters and keeping them cool. -After the tubes were reamed, each piece had to be placed in a lathe, -have a counter-bore turned in the bottom of the bell, and have the -other end squared off and turned for a short distance on the outside to -fit the counter-bore of the next section. - - -=Laying and Opening the Tubes for Traffic.=—The first tubes were laid -about the middle of November, but December 1 came before the work was -completed and special permission had to be obtained from the city -to carry on the work after that date. All work was suspended during -the holidays in order not to interfere with the holiday trade of the -stores on Chestnut Street. Severe frosts prevailed at that season, so -that when the work was begun again, after the holidays, 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 tubes. Several times the -trench was filled with snow by unusually heavy storms. Notwithstanding -all these delays and annoyances, the work was pushed forward, when -a less determined company would have given it up, and was finally -completed. A formal opening took place on February 17, 1893, when Hon. -John Wanamaker, then Postmaster-General, sent through the tube the -first carrier, containing a Bible wrapped in the American flag. - -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 circumstances. The tubes -have been in successful operation from the opening until the present -time, a period of nearly four years, and the repairs that have been -made during that time have not required its stoppage for more than a -few hours. - -In the summer of 1895 the sub-post-office was removed from Chestnut -Street to the basement of the Bourse (see Fig. 7). This required the -abandonment of a few feet of tube on Chestnut Street and the laying of -a slightly greater amount on Fourth Street, thus increasing the total -length of the tubes a little. Wrought-iron tube, coated with some -alloy, probably composed largely of tin or zinc, was used for this -extension. The wrought-iron tube is not as good as the bored cast iron. - -[Illustration: FIG. 7. - -BOURSE BUILDING, PHILADELPHIA.] - -[Illustration: FIG. 8. - -PNEUMATIC TUBES SUSPENDED IN THE BASEMENT OF THE MAIN POST-OFFICE.] - -=Description of the Tubes, Method of Laying, etc.=—This Chestnut -Street line consists of two tubes, one for despatching carriers from -and the other to the main post-office. The distance between the two -stations is two thousand nine hundred and seventy-four feet, requiring -five thousand nine hundred and forty-eight feet of tube. The inside -diameter of the tube is six and one-eighth inches, and it was made in -sections each about eleven feet long, with “bells” cast upon one end, -in order to join the sections with lead and oakum, calked in the usual -manner of making joints in water- and gas-pipes, with this exception, -that at the bottom of the bell a counter-bore was turned to receive -the finished end of the next section. By thus machining the ends of -each section of tube and having them fit accurately together, male and -female, a practically continuous tube was formed with no shoulders -upon the interior to obstruct the smooth passage of the carriers. -Joints made in this way possess another great advantage over flanged -and bolted joints, in that they are slightly yielding without -leaking, and so allow for expansion and contraction due to changes of -temperature. Each joint takes care of the expansion and contraction of -its section, which is very slight, but if all were added together would -amount to a very large movement. Another advantage of the “bell” joint -is that it permits slight bends to be made in the line of tube without -requiring special bent sections. Where short bends had to be made, at -street corners, in entering buildings, and other similar places, brass -tubes were used, bent to a radius of not less than six feet, or about -twelve times the diameter of the tube. (One of the brass bends may be -seen in Fig. 10.) The bends were made of seamless tubing, bent to the -desired form and radius in a hydraulic machine. To prevent them from -being flattened in the process of bending, they were filled with resin, -which was afterwards melted out. Flanges were screwed and soldered -to the ends of the bent brass tubes, and they were bolted to special -flanged sections of the iron tube. - -The tubes were laid in the trench and supported by having the ground -firmly tamped about them. Usually one tube was laid above the other, -with an iron bracket between, but frequently this arrangement had to -be departed from in order to avoid obstructions, such as gas- and -water-pipes, sewers, man-holes, etc. The depth of the tubes below the -pavement varied from two to six feet, and in one place, in order to -pass under a sewer, the extreme depth of thirteen feet was reached. At -the street crossings it was frequently difficult to find sufficient -space to lay the tubes. At the intersection of Fifth and Chestnut -Streets a six-inch water-main had to be cut and a bend put in. A -seven-strand electric cable, used for telephoning and signalling, -was laid upon the top of one of the tubes, protected by a strip of -“vulcanized wood,” grooved to fit over the cable. The cable and -protecting strip of wood were fastened to the tube by wrought-iron -straps and bolts. - -The tubes enter the main post-office on the Chestnut Street side, -through one of the windows, and are suspended from the ceiling along -the corridor in the basement for a distance of nearly two hundred -feet. Fig. 8 shows the tubes thus suspended. They terminate upon the -ground floor about the centre of the building, and near the cancelling -machines. - -[Illustration: FIG. 9. - -DUPLEX AIR-COMPRESSOR IN THE BASEMENT OF THE MAIN POST-OFFICE.] - -[Illustration: FIG. 10. - -TANKS AND TUBE IN THE BASEMENT OF THE MAIN POST-OFFICE.] - -=Air-Compressor—Method of Circulating the Air.=—The current of air -that operates the tubes is supplied by a duplex air-compressor -located in the basement of the main post-office. This machine is -shown in Fig. 9, and requires no detailed description, as it does not -differ materially from air-compressors used for other purposes. The -stroke is twenty-four inches, the diameter of the steam-cylinders -ten inches, and the air-cylinders eighteen inches. The air-cylinders -are double acting, with poppet-valves, and have a closed suction. -The speed of the machine varies slightly, being controlled by a -pressure-regulator that maintains a practically constant pressure -in the tank that feeds the tube. The engines develop a little over -thirty horse-power under normal conditions. The pressure of the air -as it leaves the compressor is usually six or seven pounds per square -inch. Compressing the air heats it to about 156° F., but this is not -sufficient to require water-jackets about the air-cylinders. From the -compressor the air flows to a tank, shown on the right in -Fig. 10, where any oil or dirt contained in the air is deposited. The -principal purpose of the tank is, however, to form a cushion to reduce -the pulsations in the air caused by the periodic discharge from the -cylinders of the compressors, and make the current in the tube more -steady. 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-office. At the sub-post-office, after flowing through -the receiving and sending apparatus, it enters the return tube and -flows back to the main office, passing through the receiving apparatus -there and then to a tank in the basement,—the left tank in Fig. 10. The -air-compressor draws its supply from this tank, so that the air is used -over and over again. This return tank has an opening to the atmosphere, -which allows air to enter and make up for any leakage or escape at the -sending and receiving apparatus, thereby maintaining the atmospheric -pressure in the discharge end of the tube and in the suction of the -compressor. The tank serves to catch any moisture and dirt that come -out of the tube. Fig. 11 is a diagram showing the direction and course -of the air-current. It will be noticed that both the out-going and -return tube are operated by _pressure_, in distinction from _exhaust_. -The air is forced around the circuit by the air-compressor. There is no -exhausting from the return tube. The pressure of the air when it enters -the tube at the main post-office is, say, seven pounds per square inch; -when it arrives at the sub-post-office the pressure is about three and -three-quarters pounds, and when it gets back to the main office and -enters the return tank, the pressure is zero or atmospheric. Thus it -will be seen that the pressure becomes less and less as the air flows -along the tube. This is not the pressure that moves the carriers, but -the pressure of the air in the tube, a pressure that exists when there -are no carriers in the tube. It is the pressure that would be indicated -if you should drill a hole into the tube and attach a gauge. - -[Illustration: FIG. 11.] - -=Terminal Apparatus.=—When the construction of this line was begun, it -was the intention of the Pneumatic Transit Company to use the apparatus -of the Electro-Pneumatic Transit Company, at both stations, for sending -and receiving carriers, and so-called working-drawings were obtained -for this purpose. The sending apparatus was constructed according to -the designs furnished, but, upon examination of the drawings of the -receiving apparatus, it was so apparent that it would not work as -intended that it was never constructed. - -The writer was asked to design an automatic receiver to stop the -carriers without shock upon their arrival at the stations, and to -deliver them upon a table without appreciable escape of air,—something -that would answer the requirements of the present plant. - -[Illustration: FIG. 12. - -_TRANSMITTER.—PHILA._ - -SENDING APPARATUS.] - - -=The Sender.=—The sending apparatus is for the purpose of enabling -the operator to place a carrier in the tube without allowing the air -to escape. In other words, it is a means of despatching carriers. The -apparatus for this purpose, already referred to, is simply a valve. A -side view and section of it are shown in Fig. 12. Fig. 15 is a view of -the apparatus in the main post-office. The sending apparatus is seen -on the left. Fig. 13 is a view of the sub-post-office apparatus, and -here a man may be seen in the act of despatching a carrier. Referring -to the section, Fig. 12, it will be seen that the sending apparatus -consists of a short section of tube supported on trunnions and enclosed -in a circular box. Normally this short section of tube stands in line -with the main tube, and the air-current passes directly through it. -It is shown in this position in the figure. When a carrier is to be -despatched, this short section of tube is rotated by a handle until -one end comes into coincidence with an opening in the side of the box. -In this position the air flows through the box around the movable -tube. A carrier can then be placed in the short section of tube and be -rotated by the handle into line with the main tube. The carrier will -then be carried along with the current of air. A circular plate covers -the opening in the box where the carrier is inserted when the sending -apparatus is closed. - -At the sub-post-office this sending apparatus is placed in a horizontal -position, but its operation is the same. - -[Illustration: FIG. 13. - -RECEIVING AND SENDING APPARATUS IN THE SUB-POST-OFFICE.] - -[Illustration: FIG. 14. - -_APPARATUS AT SUB-STATION—PHILA._] - -[Illustration: FIG. 15. - -TERMINALS OF THE TUBE IN THE MAIN POST-OFFICE.] - -=Sub-Post-Office Receiver.=—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 inch. With such a pressure we -cannot open the tube to allow the carriers to come out. 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 atmosphere. Furthermore, -this chamber must act as an air-cushion to check the momentum of the -carriers. Fig. 13 shows the sub-post-office apparatus when a carrier -is being delivered from the receiving apparatus, or, as we will name -it for convenience, the receiver. Fig. 14 is a drawing of the same -apparatus, partly in section, that shows more clearly its method -of operation. This drawing shows the sending apparatus in a -different position from Fig. 13, but that is immaterial. The receiver -consists of a movable section of tube, about twice the length of a -carrier, closed at one end, supported upon trunnions, and normally -in a position to form a continuation of the main tube from which the -carriers are received. When a carrier arrives it runs directly into -the receiver, which being closed at the end forms an air-cushion -that stops the carrier without shock or injury. 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 -apparatus, as shown in Fig. 14. At the closed end of the receiving -chamber, or air-cushion, is a relief valve, normally held closed by -a spring. As the carrier compresses the air in front of it, this -valve opens and allows some of the air to escape, which prevents the -carrier from rebounding into the tube. Under the outer end of the -receiving chamber is a vertical cylinder, E, Fig. 14, supported upon -the base-plate containing a piston. The piston of this cylinder is -connected by a piston- and connecting-rod to the receiving chamber. -When air is admitted to the cylinder under the piston, the latter rises -and tilts the receiving chamber to an angle of about forty degrees, -which allows the carrier to slide out. The receiving chamber carries -a circular plate, C, that covers the end of the main tube when it is -tilted. A small piston slide-valve, F, located near the trunnion of -the receiving chamber, controls the admission and discharge of air to -and from the cylinder E, upon the arrival of a carrier. When a carrier -arrives and compresses the air in the air-cushion or receiving -chamber, a small portion of this compressed air is forced through pipe -G, to a small cylinder containing a piston and located just above the -piston slide-valve F. The increased pressure acting on the piston moves -it downward, and it in turn moves the slide-valve F. 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 platform, K. -This platform is hinged at one end, and supported at the angle seen -in the figure by a counterweight. When a carrier rests upon it, the -weight of the carrier is sufficient to bear it down into a horizontal -position; in this position the carrier rolls off on to a table or -shelf. The platform, K, is connected by rods, bell-cranks, etc., to the -piston slide-valve, so that when it swings downward by the weight of -a carrier, the slide-valve is moved upward into its normal position, -and this causes the receiving chamber to tilt back into a horizontal -position ready to receive the next carrier. 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 seconds. Nothing -could operate in a more satisfactory manner. - - -=Main Post-Office Receiver.=—At the main post-office we have a -receiver of a different type. It will be remembered that the pressure -in the return tube at the main post-office is nearly down to zero or -atmospheric, so that we can open the tube to allow the carriers to pass -out without noise or an annoying blast of air. Figs. 15 and 16 show -the main-office apparatus, and Fig. 17 is a drawing of the same. Here -the receiver consists of a section of tube closed by a sluice-gate, -located at B, Fig. 17. The air-current passes out through slots in the -tube into a branch pipe leading to the return tank in the basement. -These slots are located about four feet back of the sluice-gate, so -that the portion of the tube between the slots and the sluice-gate -forms an air-cushion to check the momentum of the carriers. The -sluice-gate is raised and lowered by a piston moving in a cylinder -located just above the gate. The movement of this piston is controlled -by a piston slide-valve in a manner similar to the apparatus at the -sub-post-office. Air for operating the piston is conveyed through the -pipe D, Fig. 17, from the pipe leading from the air-compressor to the -sending apparatus. This air is at about seven pounds pressure per -square inch. - -[Illustration: FIG. 16. - -RECEIVING APPARATUS AT THE MAIN POST-OFFICE.] - -[Illustration: FIG. 17. - -_APPARATUS AT THE MAIN OFFICE—PHILA._] - -When a carrier arrives, after passing the slots that allow the -air-current to flow into the branch pipe, it compresses the air in -front of it against the gate. This compression checks its momentum, -and it comes gradually to rest. The air compressed between the -carrier and the sluice-gate operates to move the piston slide-valve, -thereby admitting air to the gate cylinder under the piston, which -rises, carrying with it the sluice-gate. The tube is now open to the -atmosphere, and there is just sufficient pressure in the tube to push -the carrier out on to a table arranged to receive it. As the carrier -passes out of the tube it lifts a finger out of its path. This finger -is located at E, Fig. 17, and when it is lifted by the passing carrier -it moves the piston slide-valve, and the sluice-gate is closed. A valve -is located in the branch-pipe that conducts the air to the return tank -in the basement. If the pressure in the tube is not sufficient to -push the carrier out on to the table, this valve is partially closed, -thereby increasing the pressure to a desired amount. - -[Illustration: FIG. 18. - -CARRIER.] - -[Illustration: FIG. 19. - -CARRIER.] - -=The Carrier.=—We have frequently spoken of the carrier, which contains -the mail and other parcels that are transported from one office to -the other. In Fig. 13, showing the sub-post-office apparatus, we see -one of these carriers being despatched by the attendant and another -being delivered from the tube. In Fig. 15 several carriers may be -seen standing on the floor. Fig. 18 shows a carrier with the lid -open, ready to receive a charge of mail, and Fig. 19 shows the same -closed, ready for despatching. The construction of the carrier is -shown by the drawing, Fig. 20. The body of the carrier is steel, -about one-thirty-second of an inch in thickness. It is made from a -flat sheet, bent into a cylinder, riveted, and soldered. The -length outside is eighteen inches, and the inside diameter is five and -one-quarter inches. The front end is made of a convex disk of steel, -stamped in the desired form, and secured to the body of the carrier -by rivets, with the convex side inward. It is necessary to have a -buffer upon the front end of the carrier to protect it from blows -that it might receive, and this buffer is made by filling the concave -side of the front head with felt, held in place by a disk of leather -and a central bolt. The leather disk is made of two pieces, riveted -together, with a steel washer between. The steel washer is attached -to the head of the bolt. The carrier is supported in the tube on two -bearing-rings, located on the body of the carrier a short distance from -each end. 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 wedged. This is a very important feature in the -construction of carriers, but does not appear to have been utilized in -other systems. - -[Illustration: FIG. 20. - -_MAIL CARRIER.—PHILA._] - -The bearing-rings are made of fibrous woven material, especially -prepared, and held in place by being clamped between two metal rings, -one of which is riveted to the body of the carrier. Of course these -rings wear out and have to be replaced occasionally, but their usual -life is about one thousand miles. The rear end of the carrier is closed -by a hinged lid and secured by a special lock. The lock consists of -three radial bolts that pass through the body of the carrier and the -rim of the lid. These bolts are thrown by three cams, attached to a -short shaft that passes through the lid and has a handle or lever -attached to it upon the outside of the lid. 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 position, it -projects beyond the periphery of the lid, and in this position the -carrier will not enter the tube. When the lid is closed and locked, -the lever lies across the lid in the position shown in Fig. 19, and -when the carrier is in the tube it cannot become unlocked, for the -lever cannot swing around without coming in contact with the wall of -the tube. This insures against the possibility of the carriers opening -during transit through the tube. The empty carriers weigh about nine -pounds, and when filled with mail, from twelve to fifteen pounds. They -have a capacity for two hundred ordinary letters, packed in the usual -manner. - - -=Operation of the Tubes.=—The tubes are kept in constant operation -during the day, and six days of the week. The air-compressor is started -at nine o’clock in the morning and runs until seven in the evening, -except during the noon hour, the air flowing in a constant steady -current through the tubes. When a carrier is placed in the tube it is -carried along in the current without appreciably affecting the load on -the compressor. Carriers may be despatched at six-second intervals, -and when they are despatched thus frequently at each office, there will -be eighteen carriers in the tube at the same time. If ten carriers per -minute are despatched from each office, and each carrier contains two -hundred letters, the tube has a carrying capacity of two hundred and -forty thousand letters per hour, which is far beyond the requirements -of this office. About five hundred carriers a day are despatched -from each office. This varies considerably on different days and at -different seasons of the year. Experience has taught that a certain -period of time should elapse between the despatching of carriers, in -order that they may not come in contact with each other, and that the -receivers may have time to act. With the present plant this period is -made about six seconds. In order to make it impossible for carriers -to be despatched more frequently than this, time-locks are attached -to the sending apparatus. One of these locks may be seen in Fig. 13, -connected to the handle of the sending apparatus. It is so arranged -that when a carrier is despatched a weight is raised and allowed to -fall, carrying with it a piston in a cylinder filled with oil. While -the weight is rising and falling the sending apparatus is locked, but -becomes unlocked when the weight is all the way down. A by-pass in the -cylinder permits the oil to flow from one side of the piston to the -other, and the size of this by-pass can be regulated, thus determining -the time that the weight shall take in descending. This makes a simple -and effective time-lock that does not get out of order. - -The time required for a carrier to travel from the main to the -sub-post-office is sixty seconds, and from the sub- to the main -post-office, fifty-five seconds. This difference of time in going and -returning is due to the expansion of the air in the tube, as will -be explained more fully in another place. The distance between the -offices being two thousand nine hundred and seventy-four feet, gives -an average speed of about fifty-two feet per second, or 35.27 miles -per hour. Of course the speed can be increased by increasing the -air-pressure, but this speed is found in practice to be ample for all -requirements. In order to give some idea of the energy possessed by -one of these carriers travelling at this speed, it may be said that -if the end of the tube were left open and turned upward, an emerging -carrier would rise about forty feet into the air. It is easy to imagine -how apparatus, depending for its operation upon impact with a moving -carrier, would be soon destroyed, as well as the carriers themselves. -This is why receiving apparatus used with small tubes and light -carriers cannot be applied to large tubes with heavy carriers. - -No serious trouble has ever been experienced from carriers getting -wedged in any part of these tubes. - - -=Benefits of the System.=—The advantages to the post-office department -by the adoption of this system have been numerous, and the post-office -officials who are familiar with the operation of the tubes frequently -speak in high terms of their usefulness. Formerly the mail was -transported from one office to the other by a wagon making a trip -every half-hour. Considerable time has been saved by the greater speed -of transit, but even more time is gained by keeping the mail moving -instead of allowing it to accumulate and then despatching it in bulk. -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 office, -and sometimes more quickly. Let us take an example, first, with the old -wagon service. Suppose that you drop a letter in the sub-post-office; -it lies there, say, fifteen minutes waiting for the departure of the -next wagon; it is put into a pouch with hundreds of other letters, and -ten minutes are consumed in transporting it to the main office. When -it arrives there the pouch is thrown on the floor at the entrance of -the building; in a few minutes, more or less, a clerk takes the pouch, -throws it on a truck and wheels it around to the cancelling machines, -where it may lie for five or ten minutes more before being opened, -and then perhaps five minutes will elapse before your letter reaches -the cancelling machine. 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 cancelled. Now with the pneumatic tube service -forty minutes of this time will be saved; 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 machines. 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 use. People -frequently post letters requesting that they be sent through the tube; -of course they would be sent in that way if the request was not made, -but it shows that the public recognize the better service. Formerly -mail was collected from the street boxes in the banking section of the -city and the collectors carried it to the main office. After the tubes -were installed this mail was carried to the sub-post-office to be sent -through the tube, 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 machines,—a double saving in time besides making -their labor much lighter and enabling them to do more useful work. - -Since the sub-office has been established in the Bourse, it has been -made a distributing as well as receiving office. At least two more -deliveries of mail are made each day in the Bourse building than in any -other office building in the city. - -All letters mailed in the sub-office with a special delivery stamp are -despatched through the tube immediately. - -It is now nearly four years since the system was put into operation. -During that time more than thirty-five million letters have been -transported, and all the repairs to the system have not required it -to be stopped for more than a few hours. During the first year the -Pneumatic Transit Company operated the tubes at their own expense, -agreeing at the end of that time to take them out if the government so -requested. Since the first year the government has paid the running -expenses. - -Such is the history of the first United States pneumatic postal system. -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 mail, in -this or any other country, so far as the writer knows. - - - - -CHAPTER III. - -THE SYSTEM AND APPARATUS OF THE BATCHELLER PNEUMATIC TUBE COMPANY. - - -=General Arrangement and Adaptability of the System.=—The experience -gained in the construction and operation of the Philadelphia -post-office tubes has naturally suggested improvements that can be -made in future construction, and, furthermore, it has taught us what -the requirements will be of an extensive system of tubes laid in the -streets of our cities, both for the transmission of mail and for -a general commercial business. Since the Philadelphia post-office -tube was completed, 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 other. It is the purpose of the present chapter to -describe this system. - -While the Pneumatic Transit Company has ample field in the State of -Pennsylvania to carry out the work which it has mapped out, a field -broad enough to yield a good profit for the capital invested, there -is no reason why the system should be limited to one State. So, in -order to obtain a broader charter, covering all places where pneumatic -service may be needed, a new corporation was formed and styled the -BATCHELLER PNEUMATIC TUBE COMPANY. - -It is impossible to lay down a rigid system equally well adapted to -all places and purposes. We must accommodate ourselves somewhat to -circumstances. For example, the post-office department may require -one size of tube, arranged to operate in a particular way, while the -requirements of a parcel delivery business would be utterly different. -The geographical location of the stations will have much to do with the -general arrangement; also the condition of the streets. Some of the -streets of our large cities are so filled with water- and gas-pipes, -electrical conduits, sewers, steam-pipes, etc., etc., that it is almost -impossible to find space for pneumatic tubes, especially of large -diameter. Railway or water facilities have much to do with the location -of a central pumping station, on account of the coal supply. All of -these and many other things have to be taken into consideration in -planning a system for any locality. - -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 cities. This would be in many respects a unique -plant. Two air-compressors would be used, one at each office. - -In order to give a general idea how a large number of stations can be -connected into one system, the diagram Fig. 21 has been drawn. - -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 line. -Their method was to use branch tubes leading off from the main line -with switches at the junctions. They deflected the air-current into -the branch by placing an automatic closed valve in the main line just -beyond the junction, returning the air from the branch to the main line -just beyond the valve. The carriers were to open and close this valve -automatically as they passed. - -The branch and switch system has many attractions for the inventor, -and upon first thought it would seem the most feasible solution of -the problem. It has been the dream of more than one inventor, as the -records of the patent-office show, but no one has succeeded in working -it out. The current of air cannot be divided; carriers passing from the -branch into the main line must not collide with other carriers running -in the main line; a certain minimum distance must always be maintained -between the carriers in the same tube; 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 -carriers; expense of manufacture prohibits the use of any but round -smooth tubes up to eight inches in diameter, 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 line; the carrier is -free to rotate in a round tube about its longitudinal axis, therefore, -its individuality must be indicated by some symmetrical marking about -this axis, if it is to be automatic in its operation; 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 motion, for -mechanism having inertia could not be moved during the short time that -the electric circuit would be closed; only the simplest attachments -can be made to the carrier, for constructional reasons and because of -the rough usage that they receive. These and numerous other reasons -make the problem most difficult. We have not attempted to solve it by -the use of branch tubes and electrically operated switches, but have -adopted the simpler and equally effective method of carrying the main -line through each of the stations that it unites. In our system each -carrier has an individuality determining the station at which it will -be discharged from the tube. By a simple attachment to, the front end -of the carrier, consisting of a circular metal disk, 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 -tube. 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 tube. - -[Illustration: FIG. 21. - -A DIAGRAM SHOWING VARIOUS METHODS OF CONNECTING THE STATIONS OF A LARGE -SYSTEM WITH PNEUMATIC TUBES.] - -Referring now to the diagram, Fig. 21, we have here an imaginary system -which we will suppose to be located in some large city. The two large -squares I and II indicate central pumping stations, and the small -squares A, B, C, D, etc., indicate receiving and sending stations. Some -of the stations, such as A, B, C, D, E, F, and Y, which do a large -amount of business and may be supposed to be large retail stores, are -connected directly with the central station by double tubes, one for -sending and the other for receiving carriers. Two smaller stores, such -as G and H, may be located on the same line. At I, J, K, and L we have -four stations, all connected by the same double line of tubes. These -stations we will imagine to be located in the residence section of -the city. Carriers containing parcels of merchandise or other matter -destined for private residences would be sent from the stores A, B, C, -etc., to the central station I, where they would be transferred to the -line 2 and be adjusted to stop at the station nearest the residence to -which the parcels were addressed. From this station the parcels would -be delivered by messengers to the residences. If a carrier is to be -sent from the central station I to station K, it will be so adjusted -before it is put into the tube that it will pass stations I and J, but -be discharged automatically from the tube when it arrives at station -K. In a similar manner carriers can be despatched from station L to -station I or from station J to station L. In passing through the -central station the carriers are manually transferred from one line to -another. - -In another part of the city we may have another central pumping -station, II; and the two central stations may be connected by a double -trunk line, 3. Again, we have lines radiating from this central -station, as shown by station Y. There will be some localities where it -will be an advantage to arrange the stations upon a loop, as shown in -circuit 4, where stations S, T, U, V, W, and X are connected together -in this way. Or we can combine the two arrangements of loop and direct -line, as shown in circuit 5. Stations O and R are on the double line, -but from O a loop is formed including stations N, M, P, and Q. Here it -is supposed that the stations O and R do a much larger business with -the central station II than the stations N, M, P, and Q, this being -the principal reason for placing them on the double line. All carriers -must be returned to the station from which they were sent, or others -to replace them, otherwise there will be an accumulation of carriers -at some of the stations. It is like a railway: there must be as many -trains despatched in one direction as the other, each day. Station O -can receive a carrier from the central station and return it directly, -but when station N receives one it must be returned via M, P, Q, O, -and R, a much longer route than that by which it was received. This -disadvantage is compensated, when stations N, M, P, and Q do only a -small amount of business, by the less cost of laying a single line. -If a carrier is to be sent from M to N, it must go via P, Q, and O, -being manually transferred at O from the “down” to the “up” line. P -can send directly to Q, but Q must send to P via O, N, and M. R can -send directly to O and O to R. Similarly in circuit 4 the carriers must -all travel around the loop in the same direction, shown by the arrows. -Station S can receive carriers directly from the central station, but -they must return via U, W, X, V, and T. - -Again, we may have a double-loop line, as indicated in the diagram -by circuit 6. Here five stations, _a_, _b_, _c_, _d_, and _e_, are -connected by a loop consisting of two lines of tube, in which the air -circulates in one direction in one line and in the opposite direction -in the other. Here _b_ can send directly to _c_, _c_ directly to -_b_, and _e_ to _b_ via _d_ and _c_, or via central and _a_. This -is an arrangement that would be used where there is a large amount -of business between the stations on the loop. As stated before, the -best arrangement for any particular locality depends entirely upon -circumstances. - -=Size of Tubes.=—The pneumatic-tube system that we are describing -is not limited to any particular size of tube. The size is usually -determined by the number and size of packages to be transported. -A small tube, two or three inches in diameter, is best suited for -telegrams and messages; mail, parcels, etc., require a six- or -eight-inch tube, while mail pouches and bulky material, a thirty-six -inch or possibly larger tube. We divide tubes into three classes, -according to their size, naming them small, large, and very large -tubes. By small tubes we mean those not larger than three or possibly -four inches in diameter. Large tubes are those having a diameter more -than four inches and not more than eight inches. Very large tubes -include all that are more than eight inches. This classification is -for convenience, but it has a deeper significance. For example, in -the transportation of mail, it must either be handled in bulk, that -is, in pouches, or in broken-bulk, that is, loose or tied up in small -packages. There are many advantages in transporting it in broken-bulk, -in fact, there are very few places where it could be handled in any -other way. For this service six- or eight-inch tubes—not larger—are -best suited. The carriers are light enough to be easily handled; they -are not so large in capacity as to make it necessary to wait for an -accumulation of mail to fill them; they can be delivered from the tube -on to tables at any point in the building where the mail is wanted, -for cancelling, distribution, or pouching, thus rendering a very rapid -service; the mail is kept moving in an almost constant stream, keeping -the postal employees more uniformly employed; special carriers can be -despatched with “special delivery” letters. In other words, the most -rapid service can be rendered by this size of tube. - -If a larger than eight-inch tube is to be used for mail service, -it should be not less than thirty-six inches. Carriers larger than -eight inches cannot be handled: they are too heavy. They are also too -heavy to slide through the tube, hence, must be mounted upon wheels. -It is not practical to make a carrier on wheels less than eighteen -or twenty-four inches, and the carrier must be at least twenty-four -inches to contain a large mail-pouch. Now, if we are going to despatch -mail-pouches through a pneumatic tube we must send more than one in a -carrier, otherwise the service will be too slow. Such large carriers -could not be despatched oftener than once or at most twice in a minute. -Suppose we were to transport the mail from a railway station to a main -post-office. A train arrives with, say, sixty pouches. If only one -pouch could be put into a carrier and the carriers could be despatched -at half-minute intervals, it would take thirty minutes to despatch -all the pouches. Now, suppose we make the tube thirty-six inches. -The carriers will be eight feet long and will contain from twelve to -fifteen pouches. Five carriers would contain the entire train-load of -mail, and they could be despatched in four or five minutes. - -[Illustration: FIG. 22. - -CROSS-SECTION OF A 36-INCH TUBE.] - -[Illustration: FIG. 23. - -CARRIER FOR A 36-INCH TUBE.] - -=System of Very Large Tubes.=—The cross-section of a thirty-six-inch -tube is shown in Fig. 22. It is built flat on the bottom and sides, -with an arched top. The floor is of concrete containing creosoted -ties; the side walls and top are of brick, plastered with cement upon -the interior. The two tubes may be built one above the other or -side by side, depending upon the condition of the streets, but one -common separating wall will serve for both. The carriers, one of which -is shown in Fig. 23, run on two rails laid close to the sides of the -tube. At curves a guard-rail is placed upon the side wall, making it -impossible for a carrier to leave the track. The carriers are made -of hard wood with an iron frame, and are as light as consistent with -the service required of them. They are open on top. Their outside -dimensions are thirty-four inches by thirty-four inches by eight feet. -The sending and receiving apparatus for these very large tubes have to -be specially designed for each particular station, so no attempt will -be made here to describe them. The air-pressure required depends upon -the length of the line. If it were not more than six or eight ounces -a fan would be used to maintain the air-current, but for pressures -above this, up to a pound or two per square inch, some form of positive -blower would be used. - -At the stations considerable floor space or “yard room” would be -required for side tracks, switches, etc. Usually the basement of a -building would have to be utilized for the termination of such a tube. -There are but few places in our large cities where the streets are so -free from pipes, sewers, conduits, etc., that it would be practicable -to build a thirty-six-inch pneumatic tube. When the service can be -rendered by an eight-inch tube, the cost of installation favors its -adoption. Steep grades cannot be ascended by these very large tubes, -while the eight-inch tubes can be placed vertically. We do not say that -there is no use for eighteen- and twenty-four-inch tubes, but the -demand for them would be in special cases and we will not discuss them -here. For ordinary mail and parcel service we recommend the use of -six- and eight-inch tubes. An eight-inch carrier is twenty-four inches -long, about seven inches inside diameter, and will contain five hundred -ordinary letters. It weighs about thirteen pounds empty, and one can -be despatched every six to ten seconds. We estimate that eighty per -cent. of all the parcels delivered from a large retail department store -could be wrapped up to go into these carriers. The minimum radius of -curvature of an eight-inch tube is eight feet. - - -=General Arrangement of Apparatus in the Stations. Two-Station, -Two-Compressor Line.=—We will now proceed to a description of our -system in detail. Figs. 24, 25, and 26 are diagrams showing how the -tubes, air-compressor, tanks, sending and receiving apparatus are -connected together at the stations. These diagrams are drawn to -represent an eight-inch tube, but essentially the same arrangement -would be used for smaller tubes. - -Fig. 24 represents a line of two stations with an air-compressor at -each station. Such an arrangement is proposed for the line of postal -tubes over the New York and Brooklyn bridge, or for any two stations -located a very long distance apart, say six or eight miles. - -[Illustration: FIG. 24. - -DIAGRAM OF A TWO-STATION, TWO-COMPRESSOR LINE.] - -Referring to the diagram, we have at station A an air-compressor, _c_, -which draws its supply of air from the tank _e_, and delivers it, -compressed to the necessary pressure, into the tank _d_. From the tank -_d_ the air flows to the sending apparatus, _a_, and thence through -the tube _f_ to the station B. Upon arrival at B it flows through the -receiving apparatus _m_, and then by the pipe _l_ to the tank _j_. A -second air-compressor, _o_, is located at station B, and it draws its -supply of air from the tank _j_. The tank _j_ has an opening to the -atmosphere, _i_, through which air can enter when the air-compressor -draws more than is supplied from the pipe _l_. 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 B. Stations A and B are similar in -their arrangements. At B the air-compressor _o_ delivers its compressed -air to the tank _k_, from which it flows to the sending apparatus _n_, -and thence through the tube _g_ back to station A. Upon its arrival at -A it passes through the receiving apparatus and enters the tank _e_, -which is open to the atmosphere at _h_. The tanks _d_ and _k_ serve -as separators to remove from the air any dirt and oil coming from the -compressors, and they form a cushion, deadening, to some extent, the -pulsations of the compressors and making the current of air in the -tubes more steady and uniform. The tanks _e_ and _j_ form traps to -catch any moisture, oil, or dirt coming out of the tubes. - -Carriers are placed in the tubes and despatched by means of the sending -apparatus _a_ and _n_. They are received from the tubes and delivered -on to tables by means of the receiving apparatus _b_ and _m_. It will -be seen that the arrangement is such that the air flows through one -tube and returns through the other, the same air being used over and -over again. Any air that escapes at the sending and receiving apparatus -is replaced by an equal amount entering the tanks _e_ and _j_ from the -atmosphere. By thus keeping the same air circulating in the tubes we -prevent an accumulation of moisture in the tubes. - -The air is at its maximum pressure in the tanks _d_ and _k_. The -pressure falls gradually as it flows along the tubes and is down to -atmospheric when it enters the tanks _e_ and _j_. The pressure at the -receivers, _b_ and _m_, is just sufficient to push the carriers out on -to the tables. The construction of the sending and receiving apparatus -will be described in another place. - -[Illustration: FIG. 25. - -DIAGRAM OF A TWO-STATION, ONE-COMPRESSOR LINE.] - -=Two-Station, One-Compressor Line.=—Fig. 25 is a diagram showing two -stations, A and B, connected by a double line of tubes, both operated -by one air-compressor located at station A. This is the arrangement -used in the Philadelphia post-office line, and is the arrangement that -will ordinarily be used for all two-station lines except where unusual -conditions require something different. Station A is arranged precisely -like station A in Fig. 24, so it need not be described again. The -air flows from the sending apparatus _a_ through the tube _f_ to the -receiving apparatus _p_ at station B. From the receiver _p_ it flows -through the pipe _l_ to the sending apparatus _n_ and thence through -the tube _g_ back to station A. The receiver _p_ at station B is what -we will call a closed receiver,—_i.e._, it delivers the carrier from -the tube on to the table without opening the tube to the atmosphere. -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 -atmospheric. The air-pressure is at a maximum in the tank _d_. It falls -gradually along the tube _f_, and when the air arrives at the receiver -_p_, at station B, the pressure has fallen nearly to one-half its -maximum amount in the tank _d_. 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 A. - -The entire line of tube, going and returning, is operated by air at a -pressure above the atmospheric. There is no exhausting in the return -tube. It is distinctly a _pressure_ system. - -[Illustration: FIG. 26. - -DIAGRAM OF A THREE- TO EIGHT-STATION LINE.] - -=Three- to Eight-Station Line.=—Thus far we have described only -two-station lines. In Fig. 26 we have a diagram of three stations -connected together by a double line of tubes, and the arrangement -would be similar if it were extended to four, five, six, seven, or -eight stations. The stations are called A, B, and H. Station A is -arranged exactly the same as stations A in Figs. 24 and 25, therefore, -needs no description. Station B, being an intermediate station, is -quite differently arranged from any of the preceding. From station -A the air flows through the tube _f_ to station B, where it enters -the automatic receiving and transferring apparatus, _s_. From this -it flows through the tube _f_{´}_ to the sending apparatus _r_, and -thence through the tube _f_{´´}_ to the next station, which may be -another intermediate station, C, or the terminal station H. Station H -is arranged like station B, Fig. 25. The air from the tube _f_{´´}_ -enters the receiver _p_, and is then returned, through the pipe _l_, to -the sending apparatus _n_. From the sending apparatus _n_ it continues -on its return journey through the tube _g_ to the intermediate station -B, where it enters the receiver and transfer apparatus _t_, then passes -to the sending apparatus _q_, and through the tube _g_{´´}_ back to -the receiver _b_ at station A. Thus we have followed the air-current -out through one tube and back through the other. The current is kept -circulating by the compressor located at station A. The pressure is -at a maximum in the tank _d_, and falls gradually as the air flows -along the tube until it returns to the tank _e_, when the pressure has -fallen to atmospheric. A carrier is despatched from station A, and -after passing through the tube _f_ arrives at station B, where it stops -momentarily in the automatic receiver and transfer apparatus _s_. If -the carrier is intended for station B, and was properly adjusted when -it was despatched at A, it will be discharged from the apparatus _s_ -on to the table _u_. But if it were intended for some other station -and were so adjusted, after the delay of two or three seconds in -the apparatus _s_, it will be automatically transferred to the tube -_f_{´},_ pass through the sending apparatus _r_, and go on its journey -through tube _f_{´´}_ to the next station. If it is not discharged from -the tube at any of the intermediate stations, it will finally arrive -at the terminal station H and there stop. Just how the carriers are -adjusted and the details of the receiving and transfer apparatus will -be described hereafter. Carriers arrive at station B from H, or other -stations on the line, through tube _g_, in the apparatus _t_, which -either discharges them on to the table _u_ or sends them on through -the tube _g_{´}_ and _g_{´´}_ to station A. Carriers are despatched -from station B to station A by means of the sending apparatus _q_, and -from station B to other stations along the line, C, D, E, F, G, and -H, by means of the sending apparatus _r_. Thus, from B carriers can -be sent and received in either direction. 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 -apparatus, an automatic lock is attached to each sending apparatus. -Just outside the station B, say three hundred feet on each side, are -located manholes, and in these manholes boxes are attached to the tube -containing an electric circuit-closing apparatus, so arranged that -when a carrier passes it will close an electric circuit leading to the -sending apparatus in the station. These manholes and circuit-closers -are shown and located on the diagram at _v_ and _w_. Wires _x_ and _y_ -lead from them to the sending apparatus _r_ and _q_. When a carrier -from station A passes the box _v_, it closes the electric circuit _x_, -which sets a time-lock on the sending apparatus _r_, holding this -apparatus locked, so that it is impossible to despatch a carrier for, -say, twelve seconds, a sufficient time for the carrier coming from the -station A to pass station B and get three hundred feet beyond it. After -the twelve seconds have elapsed the sending apparatus is unlocked and -a carrier can then be despatched. In a similar manner a carrier coming -from station H, in passing the box _w_, closes the electric circuit _y_ -and locks the sending apparatus _q_ for a sufficient length of time to -let the carrier pass the station. This resembles, in some respects, -the “block system” as used on railroads. A “block” about six hundred -feet in length, depending upon the speed of the carriers, is made at -each intermediate station with the station in the centre of the block. -Whenever a carrier enters this “block” the sending apparatus at the -station is locked, and a carrier cannot be inserted into the tube to -collide with the one which is passing. It will be noted that a carrier -in passing out of the “block” does not unlock the sending apparatus; -this is done automatically at a definite time after the carrier entered -the block. The unlocking is entirely independent of the carrier after -it has entered the block, and the reason it is so arranged is this: -suppose that a second carrier enters the “block” before the first one -leaves it; if the first carrier unlocked the apparatus when it left the -“block,” then it would be unlocked with the second carrier in the block -and a collision might occur, but by arranging it as we have done, if a -second carrier enters the “block” before the first has passed out, the -sending apparatus remains locked for a period of time beginning with -the arrival of the first carrier in the “block” and ending, say, twelve -seconds after the arrival of the last carrier, which is sufficient time -for the last carrier to pass out of the block. Of course, if a carrier -becomes wedged in the tube a collision may occur, but this very seldom -if ever happens. The details of the locking apparatus will be described -in another place. - -If stations A, B, ... and H were arranged on a loop, as shown in -circuit 6, Fig. 21, then station H, Fig. 26, would be at the central, -or station A. If it were a single loop, like circuit 4, Fig. 21, -then there would be only one sending apparatus and one receiving and -transferring apparatus at the intermediate stations. - -A telephone circuit will include all stations, in order to give orders -to the station attendants and to signal to the central station in case -of an accident, when it might be necessary to stop the air-compressor. -The telephone wires, in the form of a lead-covered cable, are laid in -the same trench with the tubes and fastened to them. - -[Illustration: FIG. 27. - -SENDING APPARATUS.] - -[Illustration: FIG. 28. - -SENDING APPARATUS.—LONGITUDINAL SECTION.] - -=The Sending Apparatus.=—We have, in the preceding pages, frequently -spoken of the sending apparatus, and have described it as mechanism by -which carriers are inserted into the tube. In the Philadelphia postal -line this apparatus consisted of a large valve, operated by hand. For -an eight-inch tube such a valve would be too large and heavy to be -manually operated. Furthermore, that type of apparatus is not suited -to an intermediate station, where carriers have to pass through it. To -meet all of these requirements we have designed an apparatus, of which -Fig. 27 is a side elevation, Fig. 28 a longitudinal section, and Fig. -29 a cross-section. Referring to the longitudinal section, Fig. 28, -the sending apparatus is shown inserted into the line of a pneumatic -tube, A, A. We have a movable section of tube, B, that can be swung -about the large bolt, G, at the top, into and out of line with the main -tube, A, A. When the section of tube B is being swung to one side, the -air-current has a by-pass through the slots E and F and the U-shaped -pipe D. The joints at the ends of the movable section B are packed with -specially-formed leathers. Referring to the cross-section, Fig. 29, -when the movable section of tube B is swung out of line with the main -tube, another and similar tube, C, takes its place. The two movable -tubes, B and C, are made in one piece, so that they must always move -together. They are connected together at each end by plates, M, that -serve not only as connecting-plates, but covers for the ends of the -main-line tube while the tubes B and C are being moved. The tubes B -and C swing between four plates or wings, L, that extend out on each -side of the apparatus. They serve as guards, and, at certain positions -of the swinging tubes, prevent the air from escaping. - -We will, for convenience, call the system of swinging tubes B and C, -with their supports, etc., the swing-frame or simply the frame. This -frame is moved or swung from one position to the other by means of a -cylinder and piston, H, placed in an inclined position under it. A lug, -N, is cast on the tube B, to which the connecting-rod, O, is attached. -The cross-head, P, slides upon an inclined guide, Q. On top of the -cylinder is placed a controlling valve, made in the form of a piston -slide-valve. The piston in the cylinder H is moved by the pressure of -the air taken from the main tube through the pipe I. The apparatus is -operated by a hand-lever, K. When this lever is pulled, it moves the -sliding-head R, and this, through the spring S, moves the controlling -valve, if the valve is not locked. If it is locked, pulling the lever -simply compresses the spring S. 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 atmosphere, and compressed air from the main tube -flows through the pipe I, the passages T and U, to the cylinder H, -under the piston, causing the piston to move up the inclined cylinder -and swing the frame until the tube C is in line with the main tube. -Carriers are despatched by placing them in the tube C, then pulling -the lever K, and swinging the frame until the tube C is in line with -the main tube. The carrier is then taken up and carried along by the -current of air in the main tube. - -[Illustration: FIG. 29. - -SENDING APPARATUS.—CROSS-SECTION.] - -Replacing the hand-lever K in its original position returns the frame -to its normal position. - -[Illustration: FIG. 30. - -SENDING TIME-LOCK.] - -=Sending Time-Lock.=—In any system of large pneumatic tubes a short -time should elapse between the despatching of carriers, in order that -they may not collide in the tube, and to give the receiving apparatus -at the stations time to act. To insure the impossibility of having -carriers despatched too rapidly, we place on the sending apparatus a -time-lock that will automatically lock it for a determined length of -time after each carrier is despatched, the time-lock being adjustable -for any desired time. The time-lock, W, is shown attached to the -sending apparatus in Fig. 27. When the swing-frame is swung to despatch -a carrier, it pulls up the rod X by means of a link and bell-crank, Y, -thereby locking the controlling valve of the cylinder H and starting -the time-lock W, which will unlock the controlling valve after the -required time has elapsed. The details of this time-lock are shown -in Fig. 30. It consists of a long vertical cylinder, A, containing a -piston, B, and a spiral spring, C, that tends to force the piston to -the bottom of the cylinder. The cylinder is filled with oil, and holes, -D, in the piston allow the oil to pass freely through it when it is -moved upward in the cylinder. When the piston moves downward an annular -collar, E, forming a valve, closes the holes in the piston and prevents -the oil from passing through. Extending from one side of the piston -to the other is a by-pass, F, in the wall of the cylinder. When the -piston moves downward the displaced oil is forced to flow through this -by-pass. A small cock, G, is arranged in the by-pass to throttle the -stream of oil flowing through it. The opening in this cock, or the -amount of throttling, is indicated on the outside by an index and dial, -Z (see Fig. 27). When the piston B is raised and allowed to descend by -the force of the spring C, it forces the oil through the by-pass F and -the cock G. If the latter is wide open the piston will descend quickly, -but if it is nearly closed the piston will descend very slowly. In -other words, the time of descent can be regulated by opening and -closing the cock G. The reading on the dial Z can be made seconds of -time that elapse while the piston is descending. - -Above the cylinder is a cross-head, H, that moves up and down between -vertical guides. This cross-head is moved by the rod X, also shown -in Fig. 27, that receives its motion from the swinging frame of the -sending apparatus. A piston rod, I, attached to the piston in the -cylinder, extends up through the travelling cross-head but is not -attached to it. On the piston-rod are two enlargements, J and K, one -made a solid part of it, the other formed by two nuts. The travelling -cross-head H carries a pawl, L, that engages under the shoulder formed -by the nuts K. This pawl is kept against the piston-rod by the spring -M. The enlargement, J, on the piston-rod forms a shoulder that bears -against the bell-crank, N, that connects with the bolt, O, which locks -the controlling valve. In the present down position of the piston and -piston-rod, the enlargement J, by pressing against the bell-crank N, -holds the bolt O in an unlocked position. When a carrier is despatched -the cross-head H is lifted by the rod X, and carries with it the piston -and piston-rod, compressing the spring C. This upward movement of the -piston-rod allows the bolt O to be thrown by a spring, not shown in the -figure, and so lock the controlling valve of the sending apparatus. -As the cross-head continues its upward movement, the pawl L comes in -contact with the end of the screw P and disengages the piston-rod. This -allows the piston to descend as rapidly as the oil can pass through the -by-pass and cock G. When the piston has reached nearly to the bottom of -the cylinder, the shoulder J, on the piston-rod, engages the bell-crank -N and withdraws the bolt O, thereby unlocking the controlling valve. -The time that the sending apparatus is locked depends upon the time -required for the piston to descend. While the sending apparatus is -locked against the sending of another carrier, 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 -expires. This time is usually not more than ten seconds. Not only may -the second carrier be placed in the tube C, Fig. 29, ready to be sent, -but the handle K may be pulled and fastened in the notch _a_, thereby -compressing the spring S, which, as soon as the controlling valve is -unlocked, will move the valve and automatically despatch the carrier. -The controlling valve is locked by the passage of a bolt through the -hole _b_, in a block carried on the end of the valve stem, when it -returns to the normal position shown in the figure. Usually little or -no time will be lost in thus locking the sending apparatus, for the -small amount of time that the apparatus is locked will be needed in -handling the carriers. - -=Intermediate Station Time-Lock.=—We have another time-lock attached to -the sending apparatus that has been already referred to in describing -the “block system” used at intermediate stations; a time-lock to -prevent carriers being inserted into the tube at intermediate stations -while another carrier is passing that station. This time-lock is shown -in Fig. 27 at W´, and is shown in detail by a sectional drawing, Fig. -31. - -When a carrier closes an electric circuit in passing one of the boxes -located in a manhole about three hundred feet from an intermediate -station, it indicates its approach to the station by exciting the -electro-magnet A, Fig. 31. This magnet pulls down its armature B and -raises the small piston valve C, which admits compressed air to a small -chamber, D. The air is supplied to this chamber from the main tube -through the pipe E. In one end of this chamber is fitted a piston, F, -held to one end of its stroke by a spring, G. When compressed air is -admitted to the chamber D, this piston is moved to the left, and by -such movement throws the controlling valve of the sending apparatus -into its normal position (shown in Fig. 29) and holds it there. This -forms a positive lock, and, no matter in what position the sending -apparatus may be, it puts the tube B, Fig. 29, into line with the main -tube so that the approaching carrier can pass through the apparatus. -The piston-rod H, Fig. 31, is connected to the finger _d_, Fig. 29, -and by rocking this finger moves the controlling valve, or prevents it -being moved by the handle K. - -[Illustration: FIG. 31. - -INTERMEDIATE STATION TIME-LOCK.] - -Returning now to Fig. 31, we have on the top of the chamber D, in -addition to the electro-magnet A and its armature B, a differential -cylinder and piston, K, L, whose function is to close the valve C -when the chamber D is filled with air. The piston K is smaller than -the piston L, and sustains a constant air-pressure, supplied through -the small pipe M M, from the pipe E, which leads to the main tube. -When the chamber D becomes filled from the pipe E through the valve -C, the pressure in the chamber moves the piston L upward against the -pressure on the piston K, because of the greater area of the piston -L. This movement of the differential piston raises the lever I, which -passes through a slot in the stem of the differential piston, and thus -closes the valve C. The air in the chamber now gradually escapes to -the atmosphere through a small orifice Q; in fact it has been escaping -here all the time while the chamber was being filled, 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 filling. -Now, however, that all supply to the chamber is shut off, the air in -the chamber is gradually being discharged through the orifice. When -nearly all the air has escaped, the piston F will return to its normal -position, shown in the figure, and unlock the controlling valve. -The time required for the air to escape from the chamber, D, is the -time that the sending apparatus will be locked, and this time can be -regulated by varying the size of the orifice Q. The opening of the -orifice, or the time that the sending apparatus is locked, is indicated -by an index and dial, P. - -This locking mechanism is secured to a bracket on the side of the -large cylinder H, Fig. 27, in a position where it can be easily -inspected. The moving parts of the electro-magnetic valve—for such is -the valve C, with the magnet A, Fig. 31—are made very light, in order -that they may respond easily and quickly to the closing of the electric -circuit. - -It is a disadvantage to have stations too numerous upon the same line, -especially if they do a large amount of business, for each station -will delay the sending of carriers from the others more or less, and -the interference will be greatest during the busiest hours of the -day. This condition is inherent in any system of large tubes where -carriers have to be run a certain minimum distance apart, and cannot be -overcome by any mechanism. But the disadvantage is greatly overshadowed -by the advantage of being able to connect several stations by one -line, instead of having to run independent lines from each station to -the central, especially when the business of the individual stations -is not sufficient to occupy a separate tube all the time. It makes -it possible to have stations where otherwise the business would not -warrant the cost of installation and expense of operation. We recommend -the establishing of not more than eight stations on a line, and usually -a smaller number than this, depending, of course, upon the amount of -business to be done at each station. - -[Illustration: FIG. 32. - -ELECTRO-PNEUMATIC CIRCUIT-CLOSER.] - -=The Electro-Pneumatic Circuit-Closer.=—There is one piece of -mechanism used in connection with the sending apparatus that we have -yet to describe, and that is the circuit-closing device located in -the manholes in the street. Since the carriers travel at a high -rate of speed, they should not be made to operate any mechanism by -impact with fingers or levers protruding into the tube when it can be -avoided, even though the work to be done is so slight as the closing -of an electric circuit, for the repeated impacts cannot fail to work -injury to the carriers and the mechanism to be operated, no matter how -carefully they are designed. To avoid such impacts, we have designed -the electro-pneumatic circuit-closer, shown by the drawing in Fig. 32. -It is operated by a passing carrier, but pneumatically rather than -mechanically. In the figure we have a pneumatic tube, A, A, in which a -carrier, B, is moving in the direction indicated by the arrow. At two -points, about twenty or thirty feet apart, two small holes are tapped -into the tube and pipes, C and D, are screwed in. These pipes lead to -two chambers in a cast-iron box, F, separated by a diaphragm, E. This -diaphragm is insulated electrically from the box supporting it, and -is connected with the wire G. Just out of contact with the diaphragm -is an insulated screw, H, connected with the wire I. These wires lead -to the time-lock, already described, on the sending apparatus at the -station. When no carrier is passing, the air-pressure is the same on -both sides of the diaphragm, but when a carrier enters that part of the -tube between the two points where the pipes C and D are connected, the -equality of pressure on opposite sides of the diaphragm is destroyed. -There is always a slightly greater pressure in rear of the carrier than -in front of it, equal to the frictional resistance of the carrier in -the tube. It is this difference of pressure in front and in rear of -the carrier that moves it through the tube. When the carrier is in -the position shown in the figure, the same difference of pressure will -exist on opposite sides of the diaphragm, and it will be deflected into -contact with the screw H, thereby closing the electric circuit. When -the carrier has passed, equality of pressure on opposite sides of the -diaphragm is established and the diaphragm takes its normal position, -out of contact with the screw H. This apparatus is easily attached to -the tube, and it contains no mechanism to get out of order. - -[Illustration: FIG. 33. - -OPEN RECEIVER.] - - -=The Open Receiver.=—Wherever the pressure in the tube is down nearly -to atmospheric, we can use an open receiver to discharge the carriers -from the tube. This is a receiver that opens the tube to the atmosphere -and allows the carrier to come out. Such a receiver is used at the -main post-office in the Philadelphia postal-line, and was described in -the last chapter. The present receiver is similar in operation, but -contains some improvements in details. Fig. 33 is a side elevation of -the apparatus, Fig. 34 is a longitudinal section, and Fig. 35 is a -cross-section through the cylinder and valve, showing the sluice-gate. - -[Illustration: FIG. 34. - -OPEN RECEIVER.—LONGITUDINAL SECTION.] - -[Illustration: FIG. 35. - -OPEN RECEIVER.—SLUICE-GATE MECHANISM.] - -Referring to the longitudinal section, the apparatus is attached to -the end of a pneumatic tube, A. The current of air from the tube -A flows through the slots B into a pipe, C, that conducts it to a -tank near the air-compressor. About the centre of the apparatus is a -sluice-gate, E, that is raised and lowered by a piston in a vertical -cylinder, F, located just above the sluice-gate. This piston is moved -by air-pressure taken from some part of the system. When a carrier -arrives from the tube A, it passes over the slots B and runs into the -air-cushion D, where it comes gradually to rest. Checking the -momentum of the carrier compresses the air in front of it considerably, -and this excess of pressure is utilized to move a small slide-valve -that controls the movement of the piston in the cylinder F, 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 table. The -small pipe G conducts a small portion of the air compressed in front -of the retarded carrier to the controlling valve, H, seen in Figs. 33 -and 35. Referring now to the section of the valve and cylinder, Fig. -35, the pipe G enters the top of a small valve-cylinder containing a -hemispherical piston, I, that is held up by a spiral spring, J. This -spring has just sufficient tension to hold the piston I up against -the normal pressure of air in the tube. When a carrier arrives -and compresses the air in the air-cushion, the excess of pressure -forces the piston I down against the spring J, and moves the piston -slide-valve K. This change of position of the slide-valve allows the -air in the cylinder F to escape to the atmosphere through the passage -L, passage P, and pipe M, 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 F. This moves the piston up, carrying with -it the sluice-gate E. - -There is just sufficient pressure in the tube in rear of the carrier -to push the carrier past the gate and on to the table. As the carrier -moves out it raises a finger, Q, Fig. 34, that projects into its -path. Raising this finger extends the spring R, Fig. 33, and rotates -the lever S, bringing the pawl T under the end of the controlling -valve-stem. When the carrier has passed out and the finger Q is free -to descend, the spring R rotates the lever S back to its original -position, and thereby raises the controlling slide-valve, which causes -the sluice-gate to close. By having the upward motion of the finger Q -simply extend the spring R, and the downward motion, by the force of -the spring, move the valve, 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 out. If raising the finger Q moved -the valve, then when the first carrier passed out, the gate would close -down upon the second. Attached to the receiving apparatus and extending -beyond it is a tube, U, cut away upon one side so that the carriers can -roll out of it on to a table, and having in the end a buffer to stop -the carriers if by any accident they come out of the tube with too much -speed. This buffer consists of a piston covered with several layers -of leather and having a stiff spring behind it. The whole apparatus -is supported from the floor upon suitable standards, and, for an -eight-inch tube, occupies a floor-space twelve feet long by two feet -wide, not including the table. - -This is the simplest form of receiving apparatus. Owing to conditions -of pressure already explained, its use is confined principally to the -pumping stations. The only care that it requires is an occasional -cleaning and oiling. - -[Illustration: FIG. 36. - -CLOSED RECEIVER.] - -[Illustration: FIG. 37. - -CLOSED RECEIVER.—LONGITUDINAL SECTION.] - -=The Closed Receiver.=—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 atmosphere, 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 carrier. -This apparatus is similar to the receiver used in the sub-post-office -of the Philadelphia postal line, but contains several modifications -and improvements tending towards simplification. Fig. 36 shows it in -elevation, and Fig. 37 in longitudinal section. As in the open receiver -just described, the air from the tube A is deflected through slots B -into a branch pipe, C, that conducts it from the receiving apparatus -to the sending apparatus and return tube. The carriers arrive from the -tube A, pass over the slots B, where the air makes its exit, and run -into an air-cushion, D. This air-cushion is a tube about twice the -length of the carrier, closed at one end, and supported upon trunnions. -When the carrier has been brought to rest, 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 out; the tube then returns to its original -position. If the end of the air-cushion was closed perfectly tight the -carrier, after coming to rest, would rebound and might be caught in the -joint between the stationary and movable parts of the apparatus, when -the air-cushion tube tilted. To prevent the rebounding of the carrier a -relief-valve, E, has been placed in the head of the air-cushion tube. -It is held closed against the normal pressure in the tube by a spiral -spring, 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 G, down the pedestal H, to the atmosphere. When -the air-cushion or receiving tube D is tilted to discharge a carrier, -the circular plate I covers the end of the main tube. In order to -prevent carriers sticking in the receiving tube when it is tilted, -and to insure their prompt discharge, the pipe J is provided. In the -tilted position of the receiving tube, the end of this pipe coincides -with the end of the main tube, from which it receives air to hasten -the discharge of the carrier. A check-valve, K, prevents the air from -flowing backward in this pipe when a carrier is being received in the -air-cushion chamber. The opening of this check-valve can be adjusted by -a screw, thereby regulating the speed of ejection of the carrier. - -[Illustration: FIG. 38. - -INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.] - -[Illustration: FIG. 39. - -INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.—VERTICAL -SECTION.] - -The carrier is discharged down a chute, L, which has a buffer at the -bottom, and from the chute it rolls off on to a table. The buffer is -made similar to the buffer in the open receiver already described. -The cylinder and piston M, that operate to tilt the receiving tube D, -are supported upon the base of the apparatus under the closed end of -the receiving tube. The cross-head of the piston- and connecting-rods -travels between guides that are made a part of the upper cylinder -head. The movement of the piston in the cylinder M is controlled by -a piston slide-valve exactly similar to the one shown in Fig. 35. -The slide-valve is moved, in the same manner, by the air compressed -ahead of the carrier when it is brought to rest in the air-cushion -D. The air is conducted from the air-cushion to the controlling -slide-valve through a small pipe, N, Fig. 36. This pipe leads to one -of the trunnions, where it has a joint to allow for the tilting of -the receiving tube. When the carrier is discharged from the receiving -tube, it raises a finger, O, Fig. 37, located just outside the tube. -Raising this finger pulls the rod P, Fig. 36, extends the spring -Q, turns the lever R, and catches the pawl S, under the end of the -controlling valve stem. When the carrier has passed down the chute and -allowed the finger O to drop down, the spring Q turns the lever R back -to its original position and moves the controlling valve. This causes -the receiving tube to return to a horizontal position, where it is -ready to receive the next carrier. - -At first this apparatus may seem a little cumbersome, but nothing could -work better. It is certain in its action and almost noiseless. Carriers -are received, discharged, and the receiving tube returned to its normal -position in four seconds, and it can be done in less time if necessary. - - -=The Intermediate Station Receiving and Transfer Apparatus.=—One other -form of receiving apparatus remains to be described, 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 station. A side elevation of the apparatus is shown in -Fig. 38 and a vertical section in Fig. 39. The tubes are led into an -intermediate station, carried upward, and then, with a bend of one -hundred and eighty degrees, are connected to the top of the receiving -and transfer apparatus, as shown in the diagram, Fig. 26. The object of -this arrangement will be seen as we describe the apparatus. Referring -to the sectional drawing, Fig. 39, the connection of the tube A is seen -at the top. As in the other receivers, the current of air arriving -from the tube A is deflected through slots, B, into a passage, C, made -in the frame of the apparatus. From this passage it enters the tube D -through the slots E. The tube D leads to the sending apparatus and -on to the next station, as seen in Fig. 26. The carriers are received -in a closed section of tube F, which forms an air-cushion, similar to -the closed receiver last described. This receiving tube F is made a -part of what we might term a wheel. This wheel fits accurately into a -circular casing and is supported by two trunnions or axles, upon which -it revolves. The wheel has a broad flat rim, G, that covers the end of -the tube at H when the wheel is revolved, and, in the normal position -in which it is shown in the figure, covers the interior openings I, J, -K, and L, in the casing. 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 casing. From the bottom of the receiving -tube F a passage, M, leads past a check-valve, N, to the tube D. When -a carrier arrives from the tube A, it descends into the receiving tube -F, compressing the air in front of it. This compressed air begins to -escape through the passage M, but the high velocity of it closes the -check-valve N as much as possible. A stop on the stem of the valve -prevents it being closed entirely. The small opening past the valve -allows some of the air to pass, thereby preventing the carrier from -rebounding on the air-cushion. As soon as the carrier has come to rest, -the check-valve N, by its own weight, opens wide, and the carrier, by -its weight, settles gradually down to the bottom of the receiving tube. -The wheel containing the receiving tube and the carrier will then be -revolved by the cylinder and piston O, which is operated by compressed -air taken from the tube through the pipe P. If the carrier is for this -station, the wheel will rotate through an angle of forty-five degrees -and discharge the carrier through the opening J, down the chute Q, from -which it will roll on to a table arranged to receive it. If, however, -the carrier is intended for some other station, the wheel will rotate -through an angle of ninety degrees and discharge the carrier through -the opening K into the tube D, and it will go on its way to the next -station. This selection of carriers is brought about in a comparatively -simple manner. At the bottom of the receiving tube F there are two -vertical needles, R and S, shown upon a larger scale in Fig. 40. 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 -apparatus. Wires _a_ and _b_ make connection with the needles through -metal plugs that form a guide for the needles, and through the springs -U and V. Directly below the needle R is an insulated spring clip, W, -held by two bolts and connected to the wire _e_. - -[Illustration: FIG. 40. - -A DETAIL OF THE INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.] - -The end of a carrier is represented at T. As the carrier settles down -to the bottom of the receiving tube, it comes in contact with the -ends of the needles and presses them down, they being supported by -two springs U and V. As the needle R is moved down, it makes contact -with the spring clip W, located just below it, and closes an electric -circuit that includes the electro-magnet X, Figs. 38 and 39, on the -valve of the rotating cylinder O. When this electro-magnet is excited -it attracts its armature and moves the piston slide-valve Y, that -admits air to the top of the piston in the cylinder O, and allows the -air under the piston to escape to the atmosphere. The piston moves -downward and revolves the wheel by means of a connecting rod. - -Upon the end of the carrier T is placed a thin circular metal disk, -_f_, which may be copper, brass, tin-plate or any metal that is not -easily oxidized. The diameter of this disk of metal determines the -station at which the carrier will be discharged from the tube. Disks of -various diameters, that may be attached to the carrier, are represented -by dashed lines, _g_, in Fig. 40. When the carrier comes in contact -with the two needles R and S, if the circular metal disk on the front -end of the carrier has a diameter sufficient to span the space between -the two needles, in the position in which it is held, then an electric -circuit, made by the wires _a_ and _b_, will be closed through the -needles and the metal disk on the carrier. The metal disk makes a -short-circuit from one needle to the other. If the metal disk is not -large enough to span the distance between the two needles, then the -electric circuit remains broken. - -Returning again to Fig. 39, we have the opening J, where the carriers -are discharged, closed by a sluice-gate. This gate is opened and -closed by a piston moving in a cylinder, _h_, shown in Fig. 38. A -piston slide-valve, _i_, similar in all respects to the valve on the -cylinder O, controls the movement of the piston in this cylinder and -the sluice-gate to which it is attached. The slide-valve is moved in -one direction, that opens the sluice-gate, by an electro-magnet in the -circuit of the wires _a_ and _b_, Fig. 40. - -When the electric circuit made by these wires is closed by a disk -on the front end of a carrier, short-circuiting the two needles, -the valve is moved by the electro-magnet in the circuit, and the -sluice-gate is opened. As the wheel, including the receiving tube and -carrier, revolves, a lug, _j_, Fig. 38, on the outside of the wheel -comes in contact with the open sluice-gate and the wheel can rotate -no farther. A blast of air through the valve L, Fig. 39, assisted by -gravity, pushes the carrier out of the receiving tube, through the -opening J and down the chute Q, on to the receiving table. - -Had the disk on the front end of the carrier been too small to span -the distance between the two needles, the circuit would not have been -closed, the sluice-gate would not have been opened, no obstruction -would have been placed in the path of the lug _j_, on the wheel, and -the wheel would have continued its rotation through ninety degrees -until the receiving tube F came in line with the tube D. During the -latter part of the rotation, a pin on the wheel engages a lever, _k_, -Fig. 38, and turns a valve, _l_, Fig. 39, stopping the flow of air -through the passage C, compelling it to take another route through -the passage _m_, and the receiving tube F, taking with it the carrier -into the tube D. When the carrier leaves the receiving tube and passes -through either of the openings J or K, it engages one of the fingers, -_n_ or _o_, that lie in its path. These fingers are connected by rods -and levers to the valves on the rotating and sluice-gate cylinders. -The ejected carrier pushes these fingers to one side, and after it has -passed the fingers return, by the force of a spring, to their former -position and move the valves, causing the sluice-gate to close and the -wheel to rotate backward into its normal position ready to receive -the next carrier. The connection between the fingers and the valves is -similar to the mechanism on the open and closed receivers, so need not -be described in detail here. - -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 -L, which can be opened or closed by a hand-wheel, _p_. Before the -wheel and receiving tube can be rotated, the needles must be withdrawn -from the receiving tube, and this is accomplished by a small cylinder -and piston, _q_, shown in Fig. 40. The needles and their encasement -are attached to a cross-head, _r_, on the end of a hollow piston-rod, -_s_. When air is admitted to the top of the piston in the rotating -cylinder O, Fig. 39, it is also admitted through the pipe _t_, Fig. -38, to the cylinder and upper side of the piston _q_, Fig. 40. This -moves the piston _q_ down against the force of a spring, _u_, and -withdraws the needles from the receiving tube. This takes place after -the needles have served their purpose and before the wheel is rotated. -The piston _q_ has much less inertia than the wheel, therefore it -moves much quicker. When the wheel begins to rotate it closes a valve, -_v_, in the pipe _t_, Fig. 38, confining the air in the cylinder _q_, -and preventing the needles from being raised by the spring _u_ before -the wheel returns to its normal position. If by any accident the -needles should be raised, no serious harm would result, for their ends -would simply bear against the face of the wheel. If this took place -constantly, grooves might be worn in the face of the wheel; for this -reason the valve _v_ is provided. - -In order to facilitate the inspection of the needles and electric -contact springs W, they are contained in a cylindrical brass case, -_w_, that is held in place beneath the receiving tube by two bolts. By -removing the nuts from these bolts the entire mechanism can be removed, -examined, and cleaned. It also gives easy access to the receiving tube. -The receiving tube is long enough to receive two carriers, if it should -ever happen that two arrive at the same time. - -[Illustration: FIG. 41. - -DIAGRAM OF CONTACT-DISKS AND NEEDLES.] - -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 -carriers, a diagram, Fig. 41, has been made, in which the needles at -the bottom of the receiving tubes of the apparatus at six intermediate -stations are represented at A, B, C, D, E, and F. Six disks of -different sizes are represented at _a_, _b_, _c_, _d_, _e_, and _f_. -The needles are placed farthest apart at station A and nearer together -at each succeeding station until we arrive at station F, where they -are nearest together. If we wish to send a carrier to station A from -the central, we place the largest disk, _a_, upon the front end of it. -When it arrives at station A, it closes the electric circuit between -the needles and is discharged from the tube. Should we wish to send a -carrier to station D, then we place the disk _d_ upon the front end of -it. When the carrier arrives at the station A, the disk is not large -enough to span the needles; therefore the sluice-gate is not opened -and the carrier is sent on in the tube. When it arrives at stations -B and C, the same thing occurs again, but when it reaches station D, -the needles are sufficiently close together so that the disk makes an -electric circuit between them, and the carrier is discharged from the -tube, as was intended when despatched. Since the carriers always travel -in the same direction in a tube, the first station at which they arrive -where the needles are near enough together to have both touch the disk, -will be the station at which the carrier was intended to stop. Carriers -can be despatched from any station, but if we wish to send from say D -to A, they must either travel around a loop or be sent through a return -tube in which the needles are arranged in the reverse order. If no disk -is placed on the carrier, it will go to the last station on the line. - -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 line. We have worked out two other systems which are entirely -mechanical in their operation, not using electric circuits and -electro-magnets to move the valves. While such a mechanical system has -some advantages over the present combined mechanical and electrical -system, yet there is one great advantage in the latter, and that is the -simplicity of the attachment made to the carrier. A round flat disk of -tin-plate is attached to the front end; it is something that is not -in the way; it does not prevent standing the carriers on end in racks -to fill them; it is not easily injured, and only those who have had -experience can realize the rough usage that the carriers receive; it -is quickly and easily attached to the carrier, and it is so cheap that -when bent it can be thrown away. - - -=Carriers.=—The carriers are similar in all respects to those used in -the Philadelphia postal-line, that have been described in the preceding -chapter and illustrated in Figs. 18, 19, and 20. When there are -intermediate stations upon the lines, means are provided for attaching -disks to the front end of the carriers. The disks have a central stem -that secures them to the bolt in the centre of the head, and are so -arranged that they can be quickly attached or removed. - -Many experiments have been made to find the best material for -bearing-rings, but thus far nothing better than a specially-prepared -woven fabric has been found. These rings will run about a thousand -miles, when they become so reduced in diameter that they have to be -replaced by new ones. - -The most essential elements of a carrier are strength, lightness, and -security of the contents. Aluminum has frequently been proposed as a -suitable material for the bodies of carriers, but for the same weight -steel is much stronger, especially in thin rolled sheets, and for this -reason it has been used. - -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 carriers. We believe that the one which has been -adopted fulfils all requirements in a satisfactory manner. - -Some experiments have been made with carriers that open on the side, -but structurally they are weak and unsuited to stand the blows that -carriers frequently receive. They are not so easily and quickly filled -and emptied as those that open on the end. These remarks apply to -carriers for large tubes. In small tubes for the transportation of cash -in retail stores, carriers with side openings are found convenient. - -When United States mail is sent through tubes not used exclusively for -postal service, carriers with special locks can be used, so that they -can be opened only by post-office employees. - - -=Air Supply.=—This completes the description of the special apparatus -used in this system, but we have yet to say something regarding the -machines that supply the air. In Paris the water from the city mains -has been used to compress or exhaust the air used in small tubes, -but to operate large tubes in most of our cities steam is the only -available power. Except in isolated cases, an independent steam plant -will be erected to supply the air for a system of tubes. This plant -should be designed with a view to obtaining the maximum economy in coal -consumption, labor, water, cartage, and incidental expenses. 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 plants. - -Three types of blowing machines are used,—viz., centrifugal fans, -positive blowers, and air-compressors. - - -=Fans.=—Very large tubes of moderate length can be operated by ordinary -centrifugal fans. These fans are capable of supplying air under a -pressure not exceeding ten or twelve ounces per square inch with very -good efficiency. They are the simplest and most inexpensive of all -blowing-machines. - - -=Blowers.=—When tubes have a length and diameter that require a -pressure from one to four pounds per square inch, some form of positive -blower of the Root type can be used with economy. Their construction -is familiar to nearly every one at all interested in machinery, so we -need give no space to their description here. - -[Illustration: FIG. 42. - -THE STURTEVANT STEEL PRESSURE BLOWER.] - -[Illustration: FIG. 43. - -ROOT’S POSITIVE PRESSURE BLOWER.] - -[Illustration: FIG. 44. - -SECTION OF ROOT’S TRUE CIRCLE BLOWER.] - -[Illustration: FIG. 45. - -THE GREEN BLOWER.] - -[Illustration: FIG. 46. - -SECTION OF THE GREEN BLOWER.] - -=Air-Compressors.=—By far the greater number of our tubes require an -air-pressure of more than five pounds per square inch. For such air -supply we recommend some form of air-compressor, and usually this is -driven by a steam-engine, which forms a part of the compressor. In -making our selection we should bear in mind the conditions under which -the compressor will run. Usually it must be kept in constant operation -at least ten hours per day, and frequently for a much longer period. -This makes it important that the compressor be substantially built -and supported upon a solid and firm foundation. The bearings should -be broad, of good wearing material that has a low coefficient of -friction, and provided at all times with ample lubrication. If poppet -valves are used in the air-cylinders, and they are most common, the -speed in revolutions per minute should not be high. Duplex are better -than single cylinder compressors, because they deliver the air in a -more steady stream,—the pulsations are less. For constant running, -economy of steam is an important item; therefore some good type of -cut-off valve should be provided. The air-cylinders should not be -water-jacketed unless the pressure is above twenty-five pounds per -square inch. It is better to use the air as warm as possible, for it -will soon be cooled after entering the tube. A speed-governor should -be provided with compressors which are to run at constant speed, but -usually they will be run to maintain a constant pressure in the tank, -and to this end a good and reliable form of pressure-governor should -be provided, together with some reliable safety device to stop the -engine when the speed exceeds a safe limit. But most important of all -is to have the valves of the air-cylinders large in area; otherwise -the efficiency of the machine will be very low. With machines working -under eighty pounds pressure, a difference in pressure of one pound -on opposite sides of the valves has but little effect, but when -the machine is only compressing to five or ten pounds, one pound -is a very large proportion of the total pressure and reduces the -efficiency. Besides these few suggestions, only the requirements of -good engineering need be demanded. In Figs. 42, 43, 44, 45, 46, and -47 we show a fan, two blowers, and an air-compressor suited to the -requirements of pneumatic-tube service that can be found in the market, -and that are built by responsible concerns. We believe they are all -good of their kind, but do not recommend any particular make. - - -=The Tube, Line Construction, etc.=—Up to the present time we have -found no material better suited for the straight parts of pneumatic -tubes than cast iron, machined upon the interior. It gives a smooth and -accurate tube. It can be made in most convenient lengths. It is strong -and not easily deformed. The bell-joint, calked with lead and oakum, -having the tubes fitted together male and female at the bottom of the -bell, is the best joint yet devised for pneumatic tubes. It is slightly -yielding, accommodating itself to slight changes of length of tube -due to changes of temperature, and it allows slight bends to be made -at each joint. The joints are very accurate, presenting no shoulders -to obstruct the passage of carriers. The joints can be made by men -accustomed to laying water- and gas-pipe. The cast iron is so stiff -that it is not distorted in calking, as may be done with wrought-iron -tube. The principal objections to its use are the expense of boring and -the readiness with which it corrodes upon the interior. - -[Illustration: FIG. 47. - -RAND COMPOUND COMPRESSOR OF MODERATE SIZE.] - -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 them, but our experiments thus far have been discouraging. It -may be that some of the new processes of making tubes will give us -what we want, but we have not yet found it. - -Small tubes and the short bends of large tubes are made of brass, it -being the most suitable material. It would be very difficult to bend -iron tubes without involving great expense. 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 inch. - -Where the ground is firm, no other support is needed for the tubes -than to tamp the earth solidly about them. In order to economize space -in the streets, it is customary to lay the tubes one above the other; -and it is very convenient, although not necessary, to separate them by -cast-iron saddle brackets. Such an arrangement has to be frequently -departed from in order to overcome obstructions in the streets and to -get through narrow passages. At all low points in a tube line, traps -are provided to catch any moisture that may accumulate. These traps -are made accessible for frequent inspection by means of man-holes -or otherwise. The tube is usually laid about three feet below the -pavement. This distance has frequently to be varied, but it never -becomes so small as to render the tubes liable to injury from heavy -trucks passing over the pavement. - - - - -CHAPTER IV. - -FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC TUBES. - - -We will now discuss, in an elementary way, the theory of pneumatic -tubes, in order to understand more clearly their _modus operandi_ and -the principles upon which they should be constructed. Let us begin with -the definition of a pneumatic tube. - - -=Definitions.=—A pneumatic tube is a tube containing air. This is -perhaps the broadest and most comprehensive definition that can be -given, but we usually associate with the idea of a pneumatic tube -the use to which it is put. 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 air. This is still a very -broad definition, including all kinds of material for transportation, -for every conceivable purpose. It places no limit upon the dimensions -of the tube nor the manner of its operation. This definition would -include the toy commonly known as a putty-blower, and the pneumatic gun. - -These instruments are not usually pictured in our minds when we hear -or see the term pneumatic tube used. Instead of these, 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 desk. Again narrowing our definition to -conform more nearly with the mental picture presented, 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 tube. This, like all -definitions, is not entirely satisfactory, if we examine it critically, -but it will answer our present purpose. - - -=Intermittent and Constant Air-Current.=—Having thus defined a -pneumatic tube, there are two ways in which we may operate it to -transport our carriers containing mail, packages, or other matter. The -first method consists in storing our compressed air in a suitable tank, -or by exhausting the air from the tank; then, when we wish to despatch -a carrier we place it in the tube and connect the tube with the tank by -opening a valve. As soon as the carrier arrives at the distant end of -the tube the valve is closed and the air soon ceases to flow. When a -long interval of time elapses between the despatching of carriers, this -is the most economical method of operation, 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 carrier. - -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 time. It is analogous to launching boats -in a rapidly flowing stream, allowing them to float down stream and -then withdrawing them. When the boats are in the stream they present -little obstruction to the flow of water and check its speed but very -little. In order to compute the speed with which the boat will pass -from one point to another, we only have to know the speed of the stream -between those points when no boat is in it. The presence of the boat -does not change the speed appreciably. So it is with carriers in a -pneumatic tube: they are carried along with the current of air. The air -flows nearly as rapidly when a carrier is in the tube as when there -is none. The friction of the carrier against the inner surface of the -tube creates a slight drag, but it checks the speed of the air only a -little. Therefore, in order to know the speed with which a carrier will -be transported from one station to another through a pneumatic tube, -we need only to know the velocity with which the air flows through the -tube when no carrier is present. Of course there are special cases of -heavy carriers, or carriers having a large amount of friction from -their packing, or of tubes not laid horizontally, where the resistance -of the carrier must be taken into consideration, but for our present -purpose we will neglect all of these conditions. - - -=Laws Governing the Flow of Air in Long Tubes.=—This leads us to study -the laws governing the flow of air in long tubes, omitting for the -present the presence of a carrier. Since tubes operated intermittently -have become obsolete, we will only consider the case of a constant -current of air, this being what we have to deal with in practice. - -[Illustration: FIG. 48. - -PRESSURE AND VELOCITY CURVES.] - -In order to make our ideas and thoughts as clear as possible let us -represent them by a diagram, Fig. 48. We will suppose that a tank, A, -is kept constantly filled with compressed air at a pressure of ten -pounds per square inch, from some source of supply. We will suppose -that the pressure of the air in this tank never changes, air being -supplied as fast as it flows away. Next, let us assume that a tube -eight inches in diameter inside and one mile long (five thousand two -hundred and eighty feet) is connected to the tank at one end and left -open to the atmosphere at the other. The air will flow in a constant -stream from the tank into the atmosphere, for the reason that air is -being supplied to the tank as fast as it flows away. - - -=Law of Pressure.=—First, let us consider the pressure of the air at -various points in the tube. We will, for convenience, represent the -pressure in the tank by a vertical line, D E, ten units in length, -since the pressure is ten pounds per square inch. Now let us go to a -point on the tube one quarter of a mile (one thousand three hundred -and twenty feet) from the tank, drill a hole in the tube, attach a -pressure-gauge and measure the pressure of the air at this point. -We shall find it to be about 7.91 pounds per square inch; or, 2.09 -pounds below the pressure in the tank. We will represent this on our -diagram by another vertical line, F G, having a length of 7.91 units. -Again let us measure the pressure in the tube at a point one-half a -mile (two thousand six hundred and forty feet) from the tank. Here we -find it to be about 5.61 pounds per square inch, and we represent it -by the vertical line, H I, having 5.61 units of length. We note that -the pressure is 4.39 pounds below the pressure in the tank. We are at -the middle point of the tube and the pressure has fallen to nearly, -but not quite, one-half the pressure in the tank. We will now go to -a point three-quarters of a mile (three thousand nine hundred and -sixty feet) from the tank, and here the pressure is about 3.01 pounds -per square inch. We represent it by the vertical line, J K. Lastly, -we measure the pressure very near the end of the tube, one mile from -the tank, and find it to be about zero, or the same as the pressure of -the atmosphere. All of our measurements have been in pounds above the -atmospheric pressure; to express them in absolute pressure, we should -add to each the pressure of the atmosphere, which is 14.69 pounds, -nearly. - -Now we will draw a smooth curve through the tops of all our vertical -lines, and we have a curve, E, G, I, K, L, representing the pressure in -the tube at every point. It falls gradually from ten pounds to zero, -but it does not fall in exact proportion to the distance from the tank. -Such a fall of pressure would be represented by the straight dash-line, -E, L. The reason why the true pressure-curve is not a straight line, -and lies above a straight line, is because air is an elastic fluid and -expands, becoming larger in volume as the pressure diminishes. The -straight dash-line represents the fall of pressure of an inelastic -fluid, like water, when flowing in the tube. - -The fall of pressure along the tube is analogous to the fall of level -along a flowing stream. In fact, we frequently speak of the descent of -a stream as the “head of water” when it is used for power purposes, -and we mean by this the pressure the water would exert if it were -confined in a pipe. The descent, or change of level, in the bed of a -stream is necessary to keep the water flowing against the friction of -the banks. The descent of the water imparts energy to overcome the -friction. In a similar manner, we must have a fall of pressure along -the pneumatic tube to overcome the friction of the air against the -interior surface of the tube. We find another analogue in the flow of -the electric current along a wire; here there is a fall of potential -necessary to overcome the resistance of the wire. Since power has to be -expended to compress the air and impart to it its pressure, when this -pressure disappears we know that the air must be losing its energy or -doing work, and we look to see what becomes of it. In the present case, -we find that most of this work is expended in overcoming the friction -between the air and the surface of the tube. - - -=Uses of Pressure Curves.=—The pressure curve teaches us many -things. Suppose we were to establish stations on this tube at the -quarter, half, three-quarter, and mile points; we see at once that -intermediate-station or closed receivers, described in the last -chapter, must be used at all of the stations except the mile point at -the end of the tube, 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 out, but at the end of the tube we could use the open -receiver. In designing our sending and receiving apparatus for each -station, we look to this pressure curve to tell us the pressure which -we shall have on the pistons in our cylinders, and are thereby enabled -to make them with proper proportions for the work that they have to do. - - -=Law of Velocity.=—Next let us see what the velocity of the air is -in the tube. Suppose that we have some convenient means of measuring -the velocity of the air at any point, in feet per second or miles -per hour, with some form of anemometer. We will have our measurements -taken at the five points where we measured the pressure,—viz., at the -tank, one-quarter, one-half, three-quarters and one mile from the -tank. We will represent the velocities by a diagram similar to the -one used for pressures. At the tank we find the air entering the tube -with a velocity of 59.5 feet per second (40.6 miles per hour). We draw -the vertical line M N, to represent this. At the quarter mile point -the velocity is sixty-five feet per second (44.4 miles per hour) an -increase in the first quarter of a mile of 5.5 feet per second. We -construct the vertical line O P. At the half-mile point the velocity is -72.4 feet per second (49.4 miles per hour); at the three-quarter mile -point it is eighty-three feet per second (56.8 miles per hour); and at -the end of the tube, one mile from the tank, the air comes out of the -tube with a velocity of 100.4 feet per second (68.5 miles per hour), -about 1.7 times faster than it entered the tube at the tank. Drawing -all the vertical lines to represent these velocities, and drawing a -smooth curve line through the tops of our vertical lines, we have the -curve of velocities, N, P, R, T, V, for all points along the tube. It -is an increasing velocity and increases more rapidly as we approach the -end of the tube. This is shown more clearly by drawing the straight -dashed line N V. - -If the fluid flowing in the tube were inelastic, like water, then the -curve of velocities would be a straight horizontal line, for the water -would not come out of the tube any faster than it went in. But we are -dealing with air, which is an elastic fluid, and, as we stated before, -it expands as the pressure is reduced and becomes larger in volume. -It is this expansion that increases its velocity as it flows along the -tube. It must go faster and faster to make room to expand. 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 time, to prevent -an accumulation of air in the tube, and since it increases in volume as -it flows through the tube, it follows that its velocity must increase. - - -=Characteristics of the Velocity Curve.=—This velocity curve is both -interesting and surprising, if we have not given the subject any -previous thought. It might occur to us that the air expands in volume -in the tube, and we might reason from this fact that the velocity of -the air would increase as it flowed through the tube, but very few of -us would be able to see that the rate of increase of velocity also -increases. That is to say, it gains in velocity more rapidly as it -approaches the open end of the tube. If the velocity were represented -on the diagram by a straight horizontal line, we should know that it -was constant in all parts of the tube, which would be the case if water -were flowing instead of air. If it were represented by a straight -inclined line, like the dashed line N V, then we should know that -the velocity increased as the air flowed along the tube, but that it -increased at a uniform rate. The slope of the line would indicate the -rate of increase. Neither of these suppositions represent correctly -the velocity of the air at all points in the tube; this can only be -done by a curved line such as we have shown. The slope of the curve at -any point represents the rate of increase of velocity of the air at -that point. If the curve is nearly horizontal, then we know that the -velocity does not increase much; but if the curve is steep, then we -know that it is increasing rapidly, the actual velocity being indicated -by the vertical height of the curve above the horizontal line M U. - - -=Use of Velocity Curves.=—Besides being interesting, a knowledge of the -velocity of the air at all points in a tube is of much practical value. -It gives us the time a carrier will take in going from one station to -another. Usually the first questions asked, when it is proposed to lay -a pneumatic tube from station A to station B, are, How quickly can -you send a carrier between these points? How much time can be saved? -These questions are answered by constructing a velocity curve. Since -the velocity changes at every point along a tube, to get the time of -transit between two points we must know the average velocity of the -air between those points. 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 points, and then taking the average of all -these heights; but there is a more exact and easier method by means of -a mathematical formula. As such formula would be out of place here, -we will not give it; suffice it to say, that the average velocity of -the air between the tank and the end of the tube, in the case we have -assumed, is about seventy-three feet per second (49.7 miles per hour), -a little less than one-half the sum of the velocities at the two ends, -and a little more than the velocity at the half-mile point. Knowing -the average velocity, we can tell how long it takes for a particle -of air, and it will be nearly the same for a carrier, to travel from -the tank to the end of the tube, by dividing the distance in feet by -the average velocity in feet per second. This we find to be one minute -12.3 seconds. Since the air moves more rapidly as it approaches the -open end of the tube, 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 end. The last quarter of a -mile will be covered in a little more than fourteen seconds, while -the first quarter will require a little more than twenty-one seconds. -This difference is surprising, and it becomes even more marked in very -long tubes with high initial pressures. 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 line. - -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 station. It gives us the length of the “blocks” in our -“block system.” When we know the velocity and weight of our carriers, -we can compute the energy stored up in them, and from this the length -we need to make our air-cushions so as not to have the air too highly -compressed. It would be impossible to design our apparatus properly if -we did not know the laws that govern the flow of air in the tubes. - - -=Quantity of Air Used.=—The next important fact that we learn from -the velocity curve is the quantity of air that flows through the tube -each second or minute. 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 feet, we have the number of cubic feet of air at -atmospheric pressure discharged from the tube per unit of time. The -same quantity of air must be supplied to the tank in order to maintain -a constant flow in the tube. In the present case that we have assumed, -the tube is eight inches in diameter; therefore the cross-sectional -area is 0.349 square foot. The velocity of the air as it comes out -of the end of the tube is 100.4 feet per second; therefore about -thirty-five cubic feet of air are discharged from the tube each second, -or two thousand one hundred cubic feet per minute. This same amount -must be supplied to the tank A in order to maintain the pressure -constant, but when it is compressed so that it exerts a pressure of -ten pounds per square inch, the two thousand one hundred cubic feet -will only occupy a space of one thousand two hundred and fifty cubic -feet, if its temperature does not change. This leads us to consider the -effect of temperature changes. - - -=Temperature of the Air.=—If the air is allowed to become heated -by compression, as is the case in practice, we have a new set of -conditions. If the air in the tank A is hot,—that is, warmer than the -surrounding atmosphere,—it will by radiation cool somewhat before it -enters the tube, and it will be still further cooled when it expands -in the tube. Again, if its temperature falls below the temperature of -the ground in which the tube is laid, it will absorb heat from the -ground, and this will tend to keep up its temperature; so in practice -we have very complicated relations between the temperature, pressure, -and volume of the air. These relations cannot be exactly expressed by -mathematical formulæ, and we will make no attempt so to express them, -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 feet, so that we can without appreciable error compute the -pressures and velocities as if it were constant. Now, if the air in the -tank A is hot, we must raise the pressure a little above ten pounds -per square inch to obtain the velocities given on our diagram. When -the air cools it contracts in volume, or, if the volume cannot change, -being fixed by the limits of the containing vessel, then the pressure -is reduced, so by raising the pressure in the tank A a little above ten -pounds, we compensate the loss of pressure. - - -=Horse-Power.=—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 tube. I say estimate, because we have to take -into consideration the efficiency of the air-compressor, and that is -not an absolutely fixed quantity: it varies with different types of -machines and with their construction. When working with pressures of -less than ten pounds, the friction of the machine is an important -factor. The area and construction of the valves in the air-cylinders -is another very important factor. If the valves are not large enough -or do not open promptly, our cylinders will not be filled with air -at each stroke; this will reduce the efficiency of the machine. In -practice we go to a manufacturer of air-compressors and tell him how -much air must be compressed per minute, and the pressure to which it -must be compressed, with other conditions, and then he tells us what -size of machine we shall require and the horse-power of the machine -approximately. He is supposed to know the efficiency of his own -machines. We may endeavor to prove his estimate by computations of our -own. To give some idea of the horse-power required to supply the air -needed to operate our eight-inch tube one mile long, 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-power. From the -horse-power of the steam-engine we can easily compute the coal that -will be consumed under boilers of the usual type. - - -=Efficiency.=—It will be noted that most of the power is not used -primarily in moving the carriers, but to move the air through the tube. -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 despatched. 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 air, we should find this so-called efficiency to -be very low. It is analogous to pulling the carrier with a long rope -and dragging the rope on the ground. Much more power would be consumed -by the rope than by the carrier. But a business man would define the -efficiency of a pneumatic tube as the ratio of the cost of transporting -his letters, parcels, etc., to the cost of transporting them with equal -speed in any other way. Defined in this practical manner the efficiency -of a pneumatic tube is high. We do not care what becomes of the power -so long as it accomplishes our purpose. - - -=Pressure and Exhaust Systems.=—We have noticed that pneumatic tubes -have not always been operated by compression of the air, but that some -of the small tubes used in the telegraph service of European cities -have been operated by exhausting the air. The two systems are sometimes -distinguished by calling one a pressure system and the other an exhaust -system. These terms are very misleading, for an exhaust system is -a pressure system. The current of air is kept flowing in a tube by -maintaining a difference of pressure at the two ends, 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 atmospheric. In either case it is -pressure that causes the air to flow. It happens that we are living in -an atmosphere of about fifteen pounds pressure per square inch, and it -is very convenient oftentimes in our computations to take the pressure -of the atmosphere as our zero, and reckon all other pressures above and -below this. If all our pressure scales read from absolute zero, the -pressure of a perfect vacuum, then all this confusion would be avoided. -We have not used the absolute zero in our diagram, because all our -gauges are graduated with their zero at atmospheric pressure, and it is -customary to speak of pressures above and below the atmospheric. - -It is very natural to ask the question, why are tubes sometimes -operated by compressing the air and at other times by exhausting it? -We answer by saying that it is usually a question of simplicity and -convenience that determines which system shall be used. 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 tubes. Both -ends of the tubes are then left open at the counters, no sending or -receiving apparatus being required there. The carriers are so light, -their velocity so low, 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 counters, and they can be despatched by -simply placing them at the open end of the tube into which the air is -flowing. The currents of air entering and leaving the tubes are not -so strong as to cause any special annoyance. At the cashier’s desk -some simple receiving and sending apparatus has to be used, but it is -better to concentrate all of the apparatus at one point rather than -have it distributed about the store, as would be the case if the double -system were not used. In the London pneumatic telegraph both methods -of operation have been in use. Double lines were laid, and the engines -exhausted the air from one tube and forced it into the other. The -exhausted tube was therefore used to despatch in one direction, while -the other tube, operated by the compressed air, served for despatching -in the opposite direction. So far as I know, both work equally well. - -In operating large tubes, that is to say, tubes six or eight inches -in diameter, there is an advantage in using compressed, rather than -exhausted air, in the construction of the sending and receiving -apparatus, especially when the tubes are very long. With an ample -supply of compressed air always at hand, the air-cushions can be made -shorter and more effective in bringing the carriers quickly to rest. -With exhausted air the cushions are ineffective, and consequently -must be made very long in order to stop the carrier before it strikes -the closed end of the tube. 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 buffers. Again, when compressed air is -used, we have a larger difference in pressure between the pressure in -the tube and the atmosphere to operate our mechanism by cylinders and -pistons. With an exhaust system carriers are not so easily ejected -from the tubes of the receiving apparatus; we could not use the simple -form of open receiver. Again, if the tubes are laid in wet ground, and -a leak occurs in any of the joints, water will be drawn in if air is -being exhausted from the tube, while it will be kept out if compressed -air is used. - -In regard to the question of relative economy of the two systems, we -will say that when long tubes are used, requiring high pressures, or, -more strictly speaking, a large difference of pressure, to maintain the -desired velocity of air-current, there seems to be some advantage in -using an exhaust system. The reason is this: the friction of the air in -the tube, which absorbs most of the power, increases as the air becomes -heavier and more dense. When the air is exhausted from the tube, we -are using a current of rarefied air, and this moves through the tube -with less friction and, consequently, a higher velocity, for the same -difference of pressure, than the more dense compressed air. But for -short tubes that require only a small difference of pressure, this -advantage becomes very small, and is overbalanced by other advantages -of a compressed air system. So, taking everything into consideration, -there is not so much to be said in favor of an exhaust system. - - -=Laws Expressed in Mathematical Formulæ.=—While we have heretofore -purposely avoided all complicated mathematical formulæ, it may not -be out of place here to give a few of the more simple relations that -exist between the pressure, velocity, length and diameter of the tubes, -etc. In two tubes having the same diameter, with the same pressures -maintained at each end, but of different lengths, the mean velocities -of the air in the tubes will bear the inverse ratio to the square -roots of the lengths of the tubes. This is expressed by the following -proportion: - -_u_ : U :: √L : √_l_ - -_u_ and U represent the mean velocities of the air in the two tubes and -_l_ and L the respective lengths of tubes. - -A similar but direct ratio exists between the mean velocities and the -diameters of the tubes, thus: - -_u_ : U :: √_d_ : √D - -This relation, however, is only approximately true for tubes differing -greatly in diameter. - -The relation of the pressure to other factors is not so simply -expressed. For example, in two tubes of the same length and diameter, -the relation between the pressures at the ends and the mean velocity of -the air may be expressed as follows: - - (_pₒ_² - _p₁_²)^³/² (Pₒ² - P₁²)^³/² -_u_ : U :: ————————————— : ————————— - (_pₒ_³ - _p₁_³) (Pₒ³ - P₁³) - -where _u_ and U are the respective mean velocities, _p__{0} and P_{0} -the respective pressures at the initial ends of the tubes, and _p__{1} -and P_{1} the respective pressures at the final ends of the tubes,—the -pressures being measured above absolute zero. - -There are other relations that can be similarly expressed, but for them -we must refer the reader to a mathematical treatise on the subject. - - -=Moisture in the Tubes.=—When pressures of more than five pounds per -square inch are used, 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 tube. It is seldom more than a slight dampness, or at -most a degree of wetness equal to that seen on the outside of a pitcher -of ice-water on a warm day. A slight amount of moisture in the tube is -not objectionable, for it serves as a lubricant to the carriers; but -when it is present in considerable quantity it becomes objectionable -and even annoying. This moisture is brought into the tube with the air, -and is deposited upon the walls of the tube when their temperature -is sufficiently below that of the atmosphere. The atmosphere always -contains more or less moisture in the state of vapor. The capacity of -air for water-vapor depends upon its temperature, being greater the -higher the temperature, but it is a fixed and definite quantity at any -given temperature. When the air contains all the water-vapor it can -hold at a certain temperature, it is said to be saturated. If it is -not saturated, we express the amount that it contains in per cent. of -the amount it would contain if it were saturated, and this is termed -the “relative humidity.” For example, if the air is three-fourths -saturated, we say the “relative humidity” is seventy-five; but if the -temperature changes, the “relative humidity” changes also. Suppose -the temperature to-day is seventy-five degrees Fahrenheit, and that -the “relative humidity” is eighty, a cubic foot of air then contains -0.00107 pound of water-vapor. Now suppose this air enters a pneumatic -tube and is cooled by expansion and contact with the colder walls of -the tube to sixty degrees. At this temperature a cubic foot of air can -contain only 0.00082 pound of water-vapor when it is saturated. Now, -each cubic foot of air brought into the tube brings with it 0.00107 -pound of vapor, and after it is cooled down to sixty degrees it cannot -hold it all, consequently the difference, or 0.00025 pound, must be -deposited in the tube. Under these conditions one hundred thousand -cubic feet of air will deposit twenty-five pounds of water in the tube. - -In the system of pneumatic tubes built and operated by the Batcheller -Pneumatic Tube Company, the presence of a large quantity of moisture -in the tube is prevented by using the same air over and over again. A -little moisture may be deposited when the tube starts into operation, -but the amount does not increase appreciably, as very little fresh air -is admitted after starting. - - -=Location of Obstructions in Tubes.=—In regard to the removal of -obstructions in the tubes, I have had little or no experience; -therefore under this heading I am satisfied to quote from the “Minutes -of the Proceedings of the Institution of Civil Engineers,” London, -Volume XLIII. - -“Intimately connected with the working of the tubes is the removal of -obstructions which occur from time to time, causing not unfrequently -serious inconvenience and delay. The most general cause of obstruction -is a stoppage of the train arising from accident to the tube, to the -carriers or piston, or to the transmitting apparatus. In such cases the -delay is generally very brief, it being for the most part sufficient to -reverse the pressure on the train from the next station, and to drive -it back to the point it started from. If one or more of the carriers -break in the tube, reverse pressure is also generally sufficient to -remove the obstacle; but where this fails, the point of obstruction -must be ascertained. This is done by carefully observing the variations -of air pressure in the reservoir when placed in connection, first -with a line of known length, and then with the obstructed tube. By -this means the position of the obstruction can be ascertained within -one hundred feet. Or the tube may be probed with a long rod up to a -length of two hundred feet. A very ingenious apparatus, by M. Ch. -Bontemps, is shown in Figs. 49 and 50, and is employed to ascertain -the exact position of the obstruction. It acts by the reflection of -sound-waves on a rubber diaphragm. A small metal disk is cemented to -the rubber, and above this is a pointed screw, D. An electric circuit -is closed where the points C and D are brought in contact. To locate an -obstruction a pistol is fired into the tube as shown, and the resulting -wave, traversing the tube at the rate of three hundred and thirty -metres a second, strikes the obstruction and is then reflected against -the diaphragm, which in its turn reflects it to the obstacle, whence it -returns to the diaphragm. By this means indications are marked on the -recording cylinder, and if the interval of time between the first and -second indications be recorded, the distance of the obstacle from the -membrane is easily ascertained. The chronograph employed is provided -with three points; the first of these is placed in a circuit, which -is closed by the successive vibrations of the diaphragm; the second -corresponds to an electric regulator, marking seconds on the cylinder; -and the third subdivides the seconds there marked. Fig. 50 indicates a -record thus made. In this case the obstacle is situated at a distance -of sixty-two metres, and the vibration marks thirty-three oscillations -per second. The interval occupied by two successive marks from the -diaphragm on the paper corresponds to twelve oscillations, and the -distance of the obstruction is then calculated by the following formula: - -D = 0.5 × 330 × 12/33 = 60 metres; - -so that the distance of the obstacle is recorded within two metres. - -[Illustration: FIG. 49. - -OBSTRUCTION-RECORDING APPARATUS.] - -[Illustration: FIG. 50. - -OBSTRUCTION-RECORDING APPARATUS.] - -“Amongst the special causes of accident may be mentioned the accidental -absence of a piston to the train, breaking of the piston, the freezing -up of a piston in the tube, and even forgetting the presence of a -train, which has caused the entire service to be one train late -throughout the day. Finally, the tubes themselves are sometimes broken -or disturbed during street repairs, resulting of course in a complete -cessation of traffic in the system till the damage is made good.” - -*** END OF THE PROJECT GUTENBERG EBOOK THE PNEUMATIC DESPATCH TUBE SYSTEM -OF THE BATCHELLER PNEUMATIC TUBE CO. *** - -***** This file should be named 63952-0.txt or 63952-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/3/9/5/63952/ - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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C. Batcheller. - </title> - <link rel="coverpage" href="images/cover.jpg" /> - <style type="text/css"> - -body { - margin-left: 10%; - margin-right: 10%; -} - - h1 {font-weight: bold; - text-align: center; - clear: both; - page-break-before: always; -} - h2 {font-weight: bold; font-size: 1.1em; - text-align: center; - clear: both; - } - - h2.nobreak {page-break-after: avoid; padding-top: 0;} - - h3 {font-weight: normal; font-size: 1.0em; - text-indent: 1em; - text-align: justify; - margin-bottom: 0em; -} - -/*Run-in headings*/ -h3.inline {display: inline;} -p.inline {display: inline;} -.section-no-break {margin-top: .51em; page-break-before: avoid; - text-align: justify;} - -hr.chap { - width: 33%; - margin-top: 2em; - margin-bottom: 2em; - margin-left: 33.5%; - margin-right: 33.5%; - clear: both;} - -p { - margin-top: .51em; - text-align: justify; - margin-bottom: .49em; - text-indent: 1em; -} - -p.sans { - margin-top: .9em; - text-align: center; - text-indent: 0em; - margin-bottom: .1em; - font-family: sans-serif; - font-weight: bold; -} - -p.indent { - margin-top: 0em; - text-align: left; - margin-bottom: 0em; - text-indent: -2em; margin-left: 2em; -} - -p.largeimg {text-align: right; font-size: .8em;} - -/* Images */ - -div.figcenter { - clear: both; - margin: 2em auto; /* or margin: auto;*/ - text-align: center; - max-width: 100%; /* div no wider than screen, even when screen is narrow */ -} - -.chapter {page-break-before: always;} - -img { max-width: 100%; /* no image to be wider than screen or containing div */ height: auto; /* keep height in proportion to width */ } - -.padtone {padding-top: 1em;} -.padttwo {padding-top: 2em;} -.padltwo {padding-left: 2em;} -.padb2 {padding-bottom: 2em;} - -.vertb {vertical-align: bottom;} - -.sans {font-family:sans-serif, serif; } - -table { - margin-left: auto; - margin-right: auto; -} - -table.my100 {border-collapse: collapse; table-layout: auto; -margin-left: 1%; margin-top: 1em; margin-bottom: 1em; -font-size: 100%;} - - .tdl {text-align: left;} - .tdr {text-align: right;} - .tdc {text-align: center;} - -.center table { margin-left: auto; margin-right: auto;} -.pagenum { /* uncomment the next line for invisible page numbers */ - /* visibility: hidden; */ - position: absolute; - right: 5%; - left: 92%; - font-size: smaller; - text-align: right; -} /* page numbers */ - -.noindent {text-indent: 0;} - -.normal {font-weight: normal;} - -.center {text-align: center;} - -.right {text-align: right;} - -.smcap {font-variant: small-caps;} - -.small {font-size: 90%;} -.smaller {font-size: 80%;} -.smallest {font-size: 70%;} -.large {font-size: 120%;} -.largest {font-size: 150%;} - -.add1em {margin-left: 1em;} -.add2em {margin-left: 2em;} -.add4em {margin-left: 4em;} - -.caption {font-weight: normal; text-align: center;} - -.right {float: right; clear: right; margin-right: -.2em;} - - .tdr {text-align: right; } - .tdc {text-align: center; } - -/* Transcriber's notes */ -.transnote {background-color: #E6E6FA; - color: black; - font-size: smaller; - padding: 0.5em; - margin-bottom: 5em; - font-family: sans-serif, serif; } - -.covernote {visibility: hidden; display: none;} - -@media handheld { - .covernote {visibility: visible; display: block;} - } -@media handheld {p.largeimg {display: none;}} - </style> - </head> -<body> -<pre style='margin-bottom:6em;'>The Project Gutenberg EBook of The Pneumatic Despatch Tube System of the -Batcheller Pneumatic Tube Co., by B. C. Batcheller - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this ebook. - -Title: The Pneumatic Despatch Tube System of the Batcheller Pneumatic - Tube Co. - Facts and General Information Relating to Pneumatic Despatch - Tubes - -Author: B. C. Batcheller - -Release Date: December 05, 2020 [EBook #63952] - -Language: English - -Character set encoding: UTF-8 - -Produced by: deaurider, Brian Wilcox and the Online Distributed - Proofreading Team at https://www.pgdp.net (This file was - produced from images generously made available by The Internet - Archive) - -*** START OF THE PROJECT GUTENBERG EBOOK THE PNEUMATIC DESPATCH TUBE SYSTEM -OF THE BATCHELLER PNEUMATIC TUBE CO. *** -</pre> -<div class="transnote covernote"><p>Transcriber’s note:</p> -<p>The cover image was created by the transcriber and is placed in the public domain.</p></div> - -<div class="figcenter"> -<img src="images/001.jpg" width="600" height="421" alt="" /> -<p class="caption">THE MAIN POST-OFFICE, PHILADELPHIA.</p></div> - -<hr class="chap" /> - -<h1><span class="smaller">The</span><br /> - -<br /><span class="smcap largest"><b>Pneumatic Despatch Tube System</b></span><br /> - -<br /><span class="smallest">of the</span><br /> - -<br />Batcheller Pneumatic Tube Co.</h1> - -<p class="center noindent"><span class="smaller">ALSO</span><br /> - -<br />FACTS AND GENERAL INFORMATION<br /> -RELATING TO PNEUMATIC<br /> -DESPATCH TUBES</p> - -<p class="center noindent smallest">BY</p> - -<h2>B. C. BATCHELLER, B.Sc.</h2> - -<p class="center noindent"><span class="smallest">MECHANICAL ENGINEER</span></p> - -<div class="figcenter"> -<img src="images/i_004.jpg" width="400" height="240" alt="terminal" /> -</div> - -<p class="center noindent"><span class="small">PHILADELPHIA</span><br /> -<span class="large">PRESS OF J. B. LIPPINCOTT COMPANY</span><br /> -1897</p> - -<hr class="chap" /> - -<p class="center noindent smallest padttwo padb2"> -<span class="smcap">Copyright,<br /> -<br />1896,<br /> -<br /> -by<br /> -<br /> -B. C. Batcheller</span>.</p> - -<hr class="chap" /> -<div class="chapter"> -<p><span class="pagenum"><a name="Page_3" id="Page_3">3</a></span></p> - -<h2><a name="CONTENTS" id="CONTENTS">CONTENTS.</a></h2></div> - -<div class="center"> -<table class="my100" border="0" cellpadding="2" cellspacing="0" summary="toc"> -<tr> -<th class="tdc normal padtone" colspan="2">CHAPTER I.</th> -</tr> -<tr> -<td class="tdl"><p class="indent">A BRIEF HISTORICAL SKETCH.</p></td> -<td class="tdr smaller">PAGE</td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Early Records</span></p></td> -<td class="tdr vertb"><a href="#Early_Records">9</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Practical beginning of the Art—The London Pneumatic -Telegraph</span></p></td> -<td class="tdr vertb"><a href="#Practical_Beginning">10</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Siemens Circuit System</span></p></td> -<td class="tdr vertb"><a href="#The_Siemens_Circuit_System">14</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Recent Improvements in the London System</span></p></td> -<td class="tdr vertb"><a href="#Recent_Improvements_in_the_London_System">16</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">An Underground Pneumatic Railway for Transportation -of Mail</span></p></td> <td class="tdr vertb"><a href="#An_Underground_Pneumatic_Railway">19</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Berlin Pneumatic Telegraph</span></p></td> -<td class="tdr vertb"><a href="#The_Berlin_Pneumatic_Telegraph">20</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Paris Pneumatic Telegraph</span></p></td> -<td class="tdr vertb"><a href="#The_Paris_Pneumatic_Telegraph">22</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Pneumatic Telegraph of other Cities</span></p></td> -<td class="tdr vertb"><a href="#The_Pneumatic_Telegraph_of_other_Cities">25</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Pneumatic Tubes in America</span></p></td> -<td class="tdr vertb"><a href="#Pneumatic_Tubes_in_America">25</a></td> -</tr> -<tr> -<td class="tdc padtone" colspan="2">CHAPTER II.</td> -</tr> -<tr> -<td class="tdl" colspan="2"><p class="indent">THE PNEUMATIC TRANSIT COMPANY AND THE FIRST -PNEUMATIC TUBES FOR THE TRANSPORTATION -OF UNITED STATES MAIL.</p></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Organization</span></p></td> -<td class="tdr vertb"><a href="#Organization">28</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Aim and Object of the Company</span></p></td> -<td class="tdr vertb"><a href="#Aim_and_Object_of_the_Company">28</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Clay-Lieb Patents</span></p></td> -<td class="tdr vertb"><a href="#The_Clay_Lieb_Patents">30</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Franchises and First Government Contract</span></p></td> -<td class="tdr vertb"><a href="#Franchises_and_First_Government_Contract">33</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Search for Tubes</span></p></td> -<td class="tdr vertb"><a href="#Search_for_Tubes">34</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Method of Manufacturing Tubes</span></p></td> -<td class="tdr vertb"><a href="#Method_of_Manufacturing_Tubes">35</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Laying and Opening the Tubes for Traffic</span></p></td> -<td class="tdr vertb"><a href="#Laying_and_Opening_the_Tubes_for_Traffic">37</a></td> -</tr><tr> -<td class="tdl padltwo"><span class="pagenum"><a name="Page_4" id="Page_4">4</a></span><p class="indent"><span class="smcap">Description of the Tubes, Method of Laying, etc.</span></p></td> -<td class="tdr vertb"><a href="#Description_of_the_Tubes_Method_of_Laying_etc">38</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Air-Compressor—Method of Circulating the Air</span></p></td> -<td class="tdr vertb"><a href="#Air_Compressor">40</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Terminal Apparatus</span></p></td> -<td class="tdr vertb"><a href="#Terminal_Apparatus">42</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Sender</span></p></td> -<td class="tdr vertb"><a href="#The_Sender">43</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Sub-Post-Office Receiver</span></p></td> -<td class="tdr vertb"><a href="#Sub_Post_Office_Receiver">44</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Main Post-Office Receiver</span></p></td> -<td class="tdr vertb"><a href="#Main_Post_Office_Receiver">47</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Carrier</span></p></td> -<td class="tdr vertb"><a href="#The_Carrier">50</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Operation of the Tubes</span></p></td> -<td class="tdr vertb"><a href="#Operation_of_the_Tubes">52</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Benefits of the System</span></p></td> -<td class="tdr vertb"><a href="#Benefits_of_the_System">54</a></td> -</tr> -<tr> -<td class="tdc padtone" colspan="2">CHAPTER III.</td> -</tr> -<tr> -<td class="tdl" colspan="2"><p class="indent">THE SYSTEM AND APPARATUS OF THE BATCHELLER -PNEUMATIC TUBE COMPANY.</p></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">General Arrangement and Adaptability of the -System</span></p></td> -<td class="tdr vertb"><a href="#General_Arrangement">57</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Size of Tubes</span></p></td> -<td class="tdr vertb"><a href="#Size_of_Tubes">64</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">System of very Large Tubes</span></p></td> -<td class="tdr vertb"><a href="#System_of_Very_Large_Tubes">65</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">General Arrangement of Apparatus in the Stations—Two-Station, -Two-Compressor Line</span></p></td> -<td class="tdr vertb"><a href="#Two_Station_Two_Compressor_Line">69</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Two-Station, One-Compressor Line</span></p></td> -<td class="tdr vertb"><a href="#Two_Station_One_Compressor_Line">72</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Three- to Eight-Station Line</span></p></td> -<td class="tdr vertb"><a href="#Three_to_Eight_Station_Line">74</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Sending Apparatus</span></p></td> -<td class="tdr vertb"><a href="#The_Sending_Apparatus">79</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Sending Time-Lock</span></p></td> -<td class="tdr vertb"><a href="#Sending_Time_Lock">84</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Intermediate Station Time-Lock</span></p></td> -<td class="tdr vertb"><a href="#Intermediate_Station_Time_Lock">88</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Electro-Pneumatic Circuit-Closer</span></p></td> -<td class="tdr vertb"><a href="#The_Electro_Pneumatic_Circuit_Closer">91</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Open Receiver</span></p></td> -<td class="tdr vertb"><a href="#The_Open_Receiver">94</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Closed Receiver</span></p></td> -<td class="tdr vertb"><a href="#The_Closed_Receiver">99</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">The Intermediate Station Receiving and Transfer -Apparatus</span></p></td> -<td class="tdr vertb"><a href="#The_Intermediate_Station_Receiving_and_Transfer_Apparatus">106</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Carriers</span></p></td> -<td class="tdr vertb"><a href="#Carriers">115</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Air Supply</span></p></td> -<td class="tdr vertb"><a href="#Air_Supply">117</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Fans</span></p></td> -<td class="tdr vertb"><a href="#Fans">117</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Blowers</span></p></td> -<td class="tdr vertb"><a href="#Blowers">117</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Air Compressors</span></p></td> -<td class="tdr vertb"><a href="#Air_Compressors">118</a></td> -</tr><tr> -<td class="tdl padltwo"><span class="pagenum"><a name="Page_5" id="Page_5">5</a></span><p class="indent"><span class="smcap">The Tube, Line Construction, etc.</span></p></td> -<td class="tdr vertb"><a href="#The_Tube_Line_Construction_etc">122</a></td> -</tr> -<tr> -<td class="tdc padtone" colspan="2">CHAPTER IV.</td> -</tr> -<tr> -<td class="tdl" colspan="2"><p class="indent">FACTS AND GENERAL INFORMATION RELATING TO -PNEUMATIC TUBES.</p></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Definitions</span></p></td> -<td class="tdr vertb"><a href="#Definitions">124</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Intermittent and Constant Air-Current</span></p></td> -<td class="tdr vertb"><a href="#Intermittent_and_Constant_Air_Current">125</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Laws Governing the Flow of Air in Long Tubes</span></p></td> -<td class="tdr vertb"><a href="#Laws_Governing_the_Flow_of_Air_in_Long_Tubes">126</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Law of Pressure</span></p></td> -<td class="tdr vertb"><a href="#Law_of_Pressure">128</a></td> -</tr> -<tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Uses of Pressure Curves</span></p></td> -<td class="tdr vertb"><a href="#Uses_of_Pressure_Curves">130</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Law of Velocity</span></p></td> -<td class="tdr vertb"><a href="#Law_of_Velocity">130</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Characteristics of the Velocity Curve</span></p></td> -<td class="tdr vertb"><a href="#Characteristics_of_the_Velocity_Curve">132</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Uses of Velocity Curves</span></p></td> -<td class="tdr vertb"><a href="#Uses_of_Velocity_Curves">133</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Quantity of Air Used</span></p></td> -<td class="tdr vertb"><a href="#Quantity_of_Air_Used">134</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Temperature of the Air</span></p></td> -<td class="tdr vertb"><a href="#Temperature_of_the_Air">135</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Horse-Power</span></p></td> -<td class="tdr vertb"><a href="#Horse_Power">136</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Efficiency</span></p></td> -<td class="tdr vertb"><a href="#Efficiency">137</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Pressure and Exhaust Systems</span></p></td> -<td class="tdr vertb"><a href="#Pressure_and_Exhaust_Systems">138</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Laws expressed in Mathematical Formulæ</span></p></td> -<td class="tdr vertb"><a href="#Laws_Expressed_in_Mathematical">141</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Moisture in the Tubes</span></p></td> -<td class="tdr vertb"><a href="#Moisture_in_the_Tubes">142</a></td> -</tr><tr> -<td class="tdl padltwo"><p class="indent"><span class="smcap">Location of Obstructions in Tubes</span></p></td> -<td class="tdr vertb"><a href="#Location_of_Obstructions">143</a></td> -</tr></table></div> - -<hr class="chap" /> -<div class="chapter"> -<p><span class="pagenum"><a name="Page_7" id="Page_7">7</a></span></p> - -<h2 id="PREFACE">PREFACE.</h2></div> - -<p><span class="smcap">I have</span> been prompted to prepare this book by the frequent inquiries -made regarding the details of our system of pneumatic tubes. These -inquiries have come from people interested in our company, from -others interested in companies that have purchased the right to use -our apparatus, from people desirous of becoming interested in a -pneumatic-tube business, from would-be purchasers of pneumatic tubes, -and from people interested in pneumatic tubes from a scientific, -engineering, or mechanical point of view. This book is not intended to -be a treatise on pneumatic tubes. 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 time. The second chapter -contains a description of the postal tubes in Philadelphia, and the -third chapter describes our system in detail. Following this is a short -chapter explaining the theory of pneumatic tubes, or the theory of -the flow of air in long pipes, stating the more interesting facts and -relations in as plain and simple a<span class="pagenum"><a name="Page_8" id="Page_8">8</a></span> manner as possible. Mathematical -formulæ have been purposely avoided.</p> - -<p>Several plates showing the Philadelphia postal line have been kindly -loaned to me by the Engineers’ Club of Philadelphia. They formerly -appeared in a paper read by Mr. A. Falkenau before that club. I -have also to thank the and Drill Co., the B. F. Sturtevant Co., -the Wilbraham-Baker Blower Co., and J. B. Stewart for the use of -electrotypes of their machines.</p> - -<p class="right"> -B. C. B.<span class="add2em"> </span> -</p> - -<p class="smaller">October 6, 1896.</p> - -<hr class="chap" /> -<div class="chapter"> -<p><span class="pagenum"><a name="Page_9" id="Page_9">9</a></span></p> - -<h2 class="nobreak" id="CHAP_I">THE<br /> - -<br /><span class="smcap">Batcheller Pneumatic Tube System</span>.<br /> -<br />————————<br /> -<br /> - -CHAPTER I.<br /> -<br /> -<span class="smaller">A BRIEF HISTORICAL SKETCH.</span></h2></div> - -<div class="section-no-break"><h3 class="inline" id="Early_Records"><b>Early Records.</b></h3> -<p class="inline">—The earliest reference to pneumatic transmission -of which we find any record is a paper presented to the Royal Society -of London, by Denis Papin, in the year 1667, entitled “Double Pneumatic -Pump.” His plan was to exhaust the air from a long metal tube by two -large cylinders. The tube was to contain a piston, to which a carriage -was attached by means of a cord. The “American Cyclopædia” goes on to -say, “More than a century elapsed before any further effort in this -direction was made. Paucbrouke’s ‘Dictionnaire Encyclopédique des -Amusements des Sciences’ (1792) gives a description of a machine by -M. Van Estin, 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 -length, and having many curves. This plan seems, however, to have been -more an amusement than an attempt to introduce an industrial scheme. -With more regard to practical results, Medhurst, an engineer of London, -published a pamphlet on the subject in 1810. He proposed to move<span class="pagenum"><a name="Page_10" id="Page_10">10</a></span> small -carriages on rails in air-tight tubes or tunnels, by compressed air -behind, or by creating a partial vacuum in front. In 1812 he published -another pamphlet; but the plan was not put into successful operation, -principally from insufficient means of exhaustion. About 1832 he -proposed to connect the carriage inside such a tube with a passenger -carriage running on the top of the tube; and, although the latter -project has never been commercially successful, it was the first to -be practically attempted. More than a score of patents were taken out -on the Continent and in England and America, none of which met with -any practical success. Returning to the original idea of Denis Papin, -inventors attempted to accomplish pneumatic transmission by moving the -load inside the tube, and in course of time achieved success. In France -MM. Jarroux and Taisseau presented a project for atmospheric telegraphy -before the Academy of Sciences, and they were succeeded in the same -direction by MM. Brochet and Ardor.”</p></div> - -<div class="section-no-break"><h3 class="inline" id="Practical_Beginning"><b>Practical Beginning of the Art. The London Pneumatic -Telegraph.</b></h3><p class="inline">—London has the honor of being the first city to have -a practical system of pneumatic telegraphy. The first tubes were -installed by the Electric and International Telegraph Company, the work -being planned and carried out by their engineer, Mr. Josiah Latimer -Clark, in 1853 and 1854. The first tube to be laid was one and one-half -inches in diameter, and extended from the central station, Founder’s -Court, Lothbury, to the Stock Exchange, Throgmorton Street, a distance -of two hundred and twenty yards. The tube was operated intermittently -by connecting it to a vacuum chamber at the<span class="pagenum"><a name="Page_11" id="Page_11">11</a></span> central station. Carriers -were sent only in one direction. A steam-pump was used to maintain the -vacuum. Much experience was gained from the use of this first tube. In -1858 some improvements were made by Mr. C. F. Varley, and I can best -describe them by quoting from the discussion of Mr. Carl Siemens’s -paper on “Pneumatic Despatch Tubes: The Circuit System” before the -Institution of Civil Engineers, as recorded in the minutes of that -society. “Later, about the year 1858, when a pipe two and one-fourth -inches internal diameter was extended from Telegraph Street to Mincing -Lane, thirteen hundred and forty yards in length, the traffic was so -considerable that it was found desirable to have the power of sending -messages in both directions. To effect that a smaller pipe, one and -one-half inches in internal diameter, was laid between Telegraph -Street and Mincing Lane, with a view to carrying the vacuum to the -latter station, so as to take messages in the opposite direction. This -smaller pipe was found to so wiredraw the current that the pipe would -not work, the leakage past the carrier being too considerable; 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 -Lane. This chamber was constructed of timber, fourteen feet by twelve -feet broad and ten feet high, and was covered with lead. It was not -strong enough to withstand the pressure; for one day, a carrier having -stuck half-way, and when there was a higher vacuum than usual,—viz., -twenty-three inches of mercury,—it collapsed with a loud report. At -the time<span class="pagenum"><a name="Page_12" id="Page_12">12</a></span> the landlord of the house happened to be dining in the next -room, and he suddenly found himself, his table, dinner, and the door, -which was wrenched off its hinges, precipitated into the room amongst -the <i>débris</i> of the chamber. The windows were forced inwards, and those -on the opposite side of Mincing Lane and Leadenhall Street were drawn -outwards. The damage was considerable. This accident put an end, for a -time, to the attempt to send telegraph messages by means of a vacuum -conveyed through this smaller pipe. About that time he (Mr. Varley) -became the engineer-in-chief of the Electric Telegraph Company, and -got permission from the directors to introduce a new system,—viz., -compressed air,—though many persons contended that it would be -impossible to blow messages through a pipe, because all attempts to -blow air through long pipes had utterly failed; while others said -that, if messages were sent, they would go much slower than with the -vacuum.... In his (Mr. Varley’s) apparatus, for he was the first to -introduce compressed air, the reverse was found to be the case, and for -this reason: the tube did not consume power until a message was about -to be forwarded; and in a tube thirteen hundred yards in length, and -two and one-fourth inches in diameter, 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 tube, -consequently the carrier did not start until after fifteen seconds -had elapsed. When a message was sent by compressed air, it was sent -from the end at which the power was applied, and the carrier started -at once, thus gaining a start of fifteen<span class="pagenum"><a name="Page_13" id="Page_13">13</a></span> seconds; now, inasmuch as -the air in the tube had to be compressed, it started at a very high -velocity, and when it reached the other end the compressed air in -expanding gave it a higher velocity. The result was, in thirteen -hundred and forty yards, from Telegraph Street to Mincing Lane, -the carriers were drawn by vacuum, on an average, in from sixty to -seventy seconds, and were propelled by compressed air in about fifty -or fifty-five seconds, the difference of pressure in each case being -nearly equal.”</p></div> - -<p>The first one and one-half inch tubes laid under the direction of Mr. -Clark were of iron with screwed joints. They gave much trouble from -roughness upon the interior, which wore the carriers very rapidly, and -from water that was drawn in through leaky joints. When the extensions -were made in 1858 and afterwards, two and one-fourth inch lead tubes -were used with plumber’s joints made over a heated mandrel, which made -the joints very smooth upon the interior. The carriers were of gutta -percha in the form of a cylinder closed at one end and fitted with a -cap at the other. The outside was covered with felt or drugget.</p> - -<p>When a carrier was to be despatched, a signal was sent to an attendant -at the pump end of the tube, who closed that end and connected the tube -to an exhausted chamber by opening a valve. As soon as the carrier -arrived, he closed the valve and opened the tube, which allowed the -carrier to drop out. Mr. 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 button. He also improved the -carriers by doing<span class="pagenum"><a name="Page_14" id="Page_14">14</a></span> away with the cap and using in its place an elastic -band to hold the messages in place.</p> - -<p>We have seen that Mr. Clark designed the first tube used in connection -with the telegraph, and that it was a single tube, operated in one -direction only by vacuum, being operated only when there were messages -to send. This was extended and improved by Mr. Varley, who increased -the diameter of the tubes from one and one-half inches to two and -one-quarter inches, and operated them in both directions, using vacuum -for sending in one direction and compressed air for sending in the -other. The air current was maintained in the tubes only when messages -were sent.</p> - -<p>Great credit is due to Sir Rowland Hill, who, in 1855, had a proposed -method of conveying mails in the city of London, through nine- and -thirteen-inch tubes, thoroughly investigated. 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 system.</p> - -<div class="section-no-break"><h3 class="inline" id="The_Siemens_Circuit_System"><b>The Siemens Circuit System.</b></h3><p class="inline">—The next progressive step was -made by Siemens Brothers, of Berlin, who proposed a new system called -the “circuit system,” in which two tubes were used, the up tube -being connected to the down tube at the distant end. The air was -compressed into one end of the circuit and exhausted from the other, -and, furthermore, it was kept in constant circulation. Carriers were -despatched by inserting them into the air-current without stopping it, -in one direction in one tube or in the opposite direction in the other. -Another feature of the Siemens system was the placing of three or more<span class="pagenum"><a name="Page_15" id="Page_15">15</a></span> -stations on one double line of tubes. 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 carrier. This system -is described in detail in a paper read by Mr. Carl Siemens before the -Institution of Civil Engineers, London, November 14, 1871, Vol. XXXIII. -of the Proceedings. 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 -tube. One of the tube sections was open at both ends, and was used for -despatching carriers. A carrier was placed in it, then it was swung -into line with the main tube, when the air-current passing through -swept the carrier along. The other tube section contained a perforated -screen in one end and was used to receive carriers. When it was in line -with the main tube and a carrier arrived, the carrier was stopped by -striking the screen, then the tube section was swung to one side and -the carrier pushed out with a rod. A by-pass was provided for the air -around the apparatus so that its flow was not checked when the tube -section was swung. When a carrier was despatched to an intermediate -station, a signal was sent, and then the section of tube containing -the screen was interposed in the line of the tube to stop the carrier -upon its arrival. The carriers used by Mr. Siemens were made of -gutta-percha, papier maché, or tin, closed at one end and fitted -with a cover at the other. They were covered with felt, drugget, or -leather. The front ends of the carriers were provided with thick disks -of drugget or<span class="pagenum"><a name="Page_16" id="Page_16">16</a></span> leather fitting the tube loosely, and the opposite ends -were surrounded with pieces of the same material attached to them like -the leather of an ordinary lifting pump.</p></div> - -<p>In 1869 Messrs. Siemens Bros. received an order from the British -government to install an experimental line of tubes between the central -telegraph station and the general post-office. This was completed in -1870, and after a half-year’s test it was extended to Fleet Street, -and finally to Charing Cross. The tubes were of iron, three inches -in diameter, with flanged and bolted joints. It was found, after -some experience, that there was no advantage in the circuit, so the -up and down tubes were separated at Charing Cross Station and worked -independently.</p> - -<div class="section-no-break"><h3 class="inline" id="Recent_Improvements_in_the_London_System"><b>Recent Improvements in the London System.</b></h3><p class="inline">—In 1870 Mr. J. W. -Wilmot designed a double sluice-valve by means of which carriers could -be despatched continuously without stopping the flow of air in the -tubes. Mr. Wilmot further increased the working capacity of pneumatic -tubes when, in 1880, he invented an intermediate automatic signaller, -by means of which a carrier signals the passage of a given point on its -journey, showing that the section of the tube traversed is clear, thus -allowing a second carrier to be despatched before the first has reached -its destination.</p></div> - -<div class="figcenter chapter" id="fig_2"> -<img src="images/i_020.jpg" width="600" height="340" alt="" /> -<p class="caption"><span class="smcap">Fig. 2.</span><br /> -<span class="sans smallest">DIAGRAM ILLUSTRATING THE PNEUMATIC TUBE SYSTEM</span><br /> -LONDON RADIAL SYSTEM.</p> -<p class="largeimg"><a href="images/i_020_large.jpg" rel="nofollow">Larger image</a> (116 kB)</p> -</div> - -<p>From this beginning the English system developed into what has been -termed a “radial system;” that is to say, one principal and several -minor central pumping stations have been established, and from these -radiate tubes to numerous sub-stations (see Fig. 2). Some of the -stations are connected with double lines for sending in opposite -directions. The out-going tube from the pumping station is worked<span class="pagenum"><a name="Page_17" id="Page_17">17</a></span> by -compressed air, and the incoming tube by exhaustion. Other stations -are connected by single tubes, and they are operated alternately by -compression and exhaustion. Intermediate stations are located on some -of the lines. For the central station the Varley valves were found -too expensive and troublesome to keep in order, so they were replaced -by the Wilmot double sluice-valves, which are operated manually. In -recent years the sluice-valves have been in turn replaced by what are -termed D-boxes, a simpler form of apparatus. At the sub-stations the -tube terminates in a box into which the carriers drop. As the system -has been gradually extended, tubes two and three-sixteenths inches -inside diameter have been used for short lines, and three-inch tubes -for long lines. The tubes are of lead laid inside a cast-iron pipe -which serves as a shield, protecting them from injury. They are laid -in twenty-nine<span class="pagenum"><a name="Page_18" id="Page_18">18</a></span> foot sections, the joints being made by soldering over -a steel mandrel, which is afterwards drawn out by a chain. The joints -in the cast-iron protecting pipe are made by caulking with yarn and -lead. “Electric signals are used between the central and sub-stations, -consisting of a single stroke bell and indicator, giving notice of -the arrival and departure of carriers, and to answer the necessary -questions required in working. Where there are intermediate stations -the tubes are worked on the block system, as if it were a railway. -Experience shows that, where great exactness of manipulation cannot -be obtained, it is necessary to allow only one train in each section -of a tube, whether worked by vacuum or pressure. But where there is -no intermediate station, and where the tube can be carefully worked, -carriers may be allowed to follow one another at short intervals in -a tube worked by vacuum, although it is not perfectly safe to do so -in one worked by pressure. In working by pressure it has been found -that, notwithstanding a fair interval may be allowed, carriers are -apt to overtake one another, for no two carriers travel in the same -times, because of differences in fit, unless they are placed end to -end. If signalling be neglected and a carrier happens to stick fast, -being followed by several others, a block will ensue which it will be -difficult to clear, while the single carrier could readily have been -dislodged.” (<i>Proceedings Institute of Civil Engineers, London</i>, Vol. -XLIII. p. 61.)</p> - -<p>No changes have been made in the carriers from those used in the early -experiments which have already been described.</p> - -<p><span class="pagenum"><a name="Page_19" id="Page_19">19</a></span></p> - -<p>The London system has grown until it now includes no less than -forty-two stations and thirty-four miles of tubes. Similar systems -have been established in connection with the telegraph in Liverpool, -Manchester, Birmingham, Glasgow, Dublin, and New Castle. The tubes -give a cheaper and more rapid means of despatching telegrams between -sub-stations and central stations than transmission by telegraph, and -local telegrams can be delivered in the sender’s handwriting.</p> - -<div class="section-no-break"><h3 class="inline" id="An_Underground_Pneumatic_Railway"><b>An Underground Pneumatic Railway for Transportation of -Mail.</b></h3><p class="inline">—Before describing the systems used in the cities on the -Continent of Europe, we must notice a very large pneumatic tube, or -more properly called a pneumatic tunnel railway, constructed in London -for the transportation of mail from one of the railway stations. The -first railway of this type was constructed in 1863 by the Pneumatic -Despatch Company of London, and extended from Euston to the district -post-office in Eversholt Street, a distance of about eighteen -hundred feet. The tunnel was flat on the bottom, having a D-shaped -cross-section two feet eight inches by two feet eight inches. The -carriers or carriages were cradle-like boxes fitting the tunnel, and -they moved at a speed of seventeen miles per hour, carrying fifteen -mails daily. In 1872 two similar but larger tunnels were built from -Euston Station to the general post-office, a distance of fourteen -thousand two hundred and four feet, or two and three-quarters miles. -One was for the up traffic, and the other for the down. The tunnels -were four and a half feet wide by four feet high, the straight part -being built of cast iron and the<span class="pagenum"><a name="Page_20" id="Page_20">20</a></span> bends of brick. The line was operated -by a fan twenty-two feet in diameter, which forced the air into one -tunnel and exhausted it from the other, producing a vacuum of ten -inches of water, or six ounces per square inch. The carriages occupied -twelve minutes in traversing the tunnels, and there was one gradient -of one to fourteen. The carriages were ten feet four inches long and -weighed twenty-two hundredweight. “The system was able to transport -over the whole line, allowing for delays, an average of a ton per -minute.” The system was used to transport the mails in bulk, but it was -found to be slow and unsatisfactory, and was very soon abandoned.</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Berlin_Pneumatic_Telegraph"><b>The Berlin Pneumatic Telegraph.</b></h3><p class="inline">—In 1863 the Prussian -government applied to the firm of Siemens and Halske, of Berlin, -for a proposition to establish a system of pneumatic tubes in that -city for the transmission of telegraph messages. A proposition was -accordingly submitted, and the work was completed in 1865. This first -line consisted of two parallel wrought-iron tubes, two and one-half -inches in diameter, one tube being used exclusively to send in one -direction, and the other in the opposite direction. They extended from -the telegraph station to the Exchange, requiring a total length of -five thousand six hundred and seventy feet of tube. The two tubes were -looped together at the Exchange, and a continuous current of air was -made to circulate in them by a double-acting steam air-pump, located -at the telegraph station. Air was compressed into one end of the tube -and exhausted from the other. With nine inches of mercury pressure and -vacuum the passage was made in ninety-five seconds to the Exchange, -and<span class="pagenum"><a name="Page_21" id="Page_21">21</a></span> seventy-five seconds from the Exchange. It was similar to the -line established in London by the same firm some years later, which we -have already described, except that there was no intermediate station. -After the line had been in use for a year and a half, the Prussian -government had it extended, first, from the telegraph station to the -Potsdam gate, with an intermediate station at the Brandenburg gate. -After these preliminary experiments, further extensions were made until -a net-work of tubes extended over the city of Berlin, including no -less than thirty-eight stations and over twenty-eight miles of tubes; -but in laying down this net-work a departure was made from the Siemens -system. Air was no longer kept constantly circulating, but power was -stored up in large tanks, some being exhausted and others filled with -compressed air, which was used when required to send messages, usually -at intervals of five or fifteen minutes. The exhausted tanks were -permanently connected with the closed tubes, which were opened when -required for use. The tanks containing compressed air were connected -to the tubes when messages were sent. The internal diameter of the -tubes was 2.559 inches. They were laid in circuits, including several -stations in a circuit, and the carriers travelled only in one direction -around the circuit. Some outlying stations were connected by a single -tube with central pumping-stations, these single tubes being worked in -both directions. Years of experience have shown the disadvantages of -this circuit-system, and it has gradually been changed to the radial -system, such as is used in London, until now nearly all the stations -are grouped around the central pumping-stations, to which they are -connected<span class="pagenum"><a name="Page_22" id="Page_22">22</a></span> directly by radiating tubes. The Siemens apparatus has been -replaced by simpler and less expensive valves and receiving-boxes, the -latest form of which was designed and patented by Mr. Josef Wildemann.</p></div> - -<div class="figcenter" id="fig_3"> -<img src="images/i_026.jpg" width="570" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 3.</span><br /> -<span class="sans"><i>DIAGRAM OF PART OF PARIS PNEUMATIC TUBE SYSTEM.</i></span><br /> -PARIS CIRCUIT SYSTEM.</p> -<p class="largeimg"><a href="images/i_026_large.jpg" rel="nofollow">Larger image</a> (202 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Paris_Pneumatic_Telegraph"><b>The Paris Pneumatic Telegraph.</b></h3><p class="inline">—We will now glance at the system -used in Paris, which has some novel features. In 1865 it was decided -to establish a system, and the first experimental line, from Place de -la Bourse to the Grand Hôtel, on the Boulevard des Capuciens, was laid -in 1866. Instead of using a steam-engine to drive an air-compressor or -exhaust-pump, air was compressed in tanks by displacement with water -from the city mains. In 1867 this line was extended to Rue de Grennelle -St. Germain, with an intermediate station at the Rue Boissy d’Anglais, -and another line with stations at Rue Jean Jacques Rousseau, Hôtel du -Louvre in the Rue de Rivoli, the Rue des Saints Pères, and terminating -in the central station. In 1868 the system was changed to a polygonal -or circuit system by removing the station in the Rue de Rivoli to the -Place du Théâtre Français and connecting the latter directly with the -Bourse. Other changes and extensions were made in 1870 and 1871, until -three polygons or circuits were formed, with five or six stations -in each circuit, and several outlying stations were connected by -independent tubes. In the middle of the year 1875 seventeen stations -had been connected and plans were made for twenty-one more. Instead of -maintaining an air-current around each circuit by machinery located at -one of the stations on the circuit, at least three of the five or six -stations comprised in the circuit have means of supplying compressed -air or<span class="pagenum"><a name="Page_23" id="Page_23">23</a></span> of exhausting it, and each side of the polygon, or section of -the circuit between two stations, is operated independently of the -rest of the circuit (see Fig. 3). Carriers are sent on from station -to station around the circuit, either by compressed air from the last -station from which they were sent or by means of exhaustion at the -next station towards<span class="pagenum"><a name="Page_24" id="Page_24">24</a></span> which they are moving. The carriers are made up -in trains of from six to ten, with a piston behind them that fits the -tube closely and forces them ahead. Each carrier is addressed by means -of a label for its destined station. Trains are despatched around the -circuits at stated times, usually at fifteen-minute intervals. As they -arrive at the various stations, carriers are taken out and others put -in, and the trains sent on their way. The carriers consist of iron -cylinders, closed at one end, with a leather case that slides over -them and closes the open end. They weigh, when filled with thirty-five -messages, twelve and one-half ounces, and they will travel about -twelve hundred miles before the leather cover is so worn that it must -be thrown away. The pistons are made of a wooden cone, covered with -iron, and having a “cup-leather” upon the rear end that fits the tube -closely. The sending and receiving apparatus consists of sections -of tube closed at one end, having a door on the side, through which -carriers are inserted or despatched. A peculiar form of fork is used -for picking them out. The air is controlled by valves opened and closed -by hand.</p></div> - -<p>Several methods are used to compress and exhaust the air. 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 sewer, or in -having the air compressed by allowing water from the city mains to flow -into the tanks and displace the air. Water jets have also been used, -operating similar to a steam-injector. At some of the stations water -turbines drive the air-pumps, and at others steam-engines are used.</p> - -<p><span class="pagenum"><a name="Page_25" id="Page_25">25</a></span></p> - -<p>The tubes of the Paris system are of wrought iron, in lengths of from -fifteen to twenty feet, the joints being made with flanges and bolts. -The interior diameter is 2.559 inches with a maximum variation to 2.519 -inches. The bends are made with a radius of from six to one hundred -and fifty feet. Water frequently gives trouble by accumulating in the -tubes. Traps are placed at low points to drain it off.</p> - -<p>The speed of the trains of carriers in the Paris tubes is from fifteen -to twenty-three miles per hour, and the average time that elapses from -the receipt of a message until its delivery is from forty to forty-five -minutes.</p> - -<div class="section-no-break"><h3 class="inline" id="The_Pneumatic_Telegraph_of_other_Cities"><b>The Pneumatic Telegraph of other Cities.</b></h3><p class="inline">—A system similar to -the one just described is used in Vienna. It differs some in details of -apparatus, but the carriers are despatched around circuits in trains, -stopping at each station, where some carriers are removed and others -inserted. Brussels also is not without its system of pneumatic tubes -for the transmission of telegrams.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Pneumatic_Tubes_in_America"><b>Pneumatic Tubes in America.</b></h3><p class="inline">—Turning our attention now to our -own country, we cannot pass without mention some experiments of Alfred -E. Beach with pneumatic railways, made nearly thirty years ago. His -first experiment upon a large scale was made at the American Institute -Fair held in New York City in 1867. Here he had constructed a circular -wooden tube, one hundred and seven feet long and six feet in diameter. -A car that would seat ten people ran upon a track laid down inside the -tube, and was propelled by a helix fan ten feet in diameter, making -two hundred revolutions per minute.<span class="pagenum"><a name="Page_26" id="Page_26">26</a></span> He next tried his railway upon -a practical scale, constructing an eight-foot tunnel for two hundred -feet under Broadway, starting at the corner of Warren Street. A car -was propelled by a large rotary blower located in the basement of a -building near by. The blower was kept constantly running, and the car -was sent alternately in one direction and then the other by changing -valves at the blower. Few people know that this experimental line still -exists under Broadway as Mr. Beach left it.</p></div> - -<p>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 desk. Seamless brass tubes -are usually used, and, since the tubes are short, the air is either -compressed or exhausted by means of positive rotary blowers. At the -outlying stations the tubes are usually open to the atmosphere, while -at the central station simple forms of valves are used for sending and -receiving. An outgoing and a return tube are always used, and the air -is kept in constant circulation. The carriers are of metal with felt -packing rings and open on the side. These cash-carrying systems have -come into use during the past twenty-five years.</p> - -<p>The Western Union Telegraph Company uses small tubes to transmit its -messages to a considerable extent in some of our large cities. In -1876 four lines were laid in New York City from the main office on -Broadway: two to the branch office at No. 14 Broad Street, one to 134 -Pearl Street, and one to the Cotton Exchange. Since then this company -has laid a double line about two miles in length under Broadway to its -up-town office. It also uses<span class="pagenum"><a name="Page_27" id="Page_27">27</a></span> tubes to send messages from the receiving -desks to the operating rooms within the buildings.</p> - -<p>Many of our large hotels use pneumatic tubes to transmit messages to -the different floors and offices of the buildings, taking the place of -messenger, or bell boys, who formerly did this service.</p> - -<p>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 Europe, -none of the tubes are larger than three inches internal diameter; -also that in all the systems, except in Paris where the carriers are -despatched in trains, the carriers are so light and move so slowly that -they can be stopped by allowing them to come in contact with some solid -object, such as a box into which the carriers drop. Very few of the -tubes are more than two miles in length, and most of them are less than -one mile. A speed of more than thirty miles per hour has seldom been -attempted, and usually it is much less than this.</p> - -<hr class="chap" /> -<div class="chapter"> -<p><span class="pagenum"><a name="Page_28" id="Page_28">28</a></span></p> - -<h2 class="nobreak" id="CHAPTER_II">CHAPTER II.<br /> - -<br /><span class="smaller">THE PNEUMATIC TRANSIT COMPANY AND THE FIRST PNEUMATIC TUBES FOR THE -TRANSPORTATION OF UNITED STATES MAIL.</span></h2></div> - -<div class="section-no-break"><h3 class="inline" id="Organization"><b>Organization.</b></h3><p class="inline">—Early in the year 1892 several Philadelphia -gentlemen organized a corporation and obtained a charter in the State -of New Jersey to construct, lay, and operate pneumatic tubes for the -transmission of United States mail, packages, merchandise, messages, -etc., within the States of New Jersey and Pennsylvania. The corporation -was styled the Pneumatic Transit Company. Mr. William J. Kelly was -elected president, and the company is still under his management.</p></div> - -<div class="figcenter" id="i_034"> -<img src="images/i_034.jpg" width="375" height="500" alt="" /> -<p class="caption">WM. J. KELLY, - -<br />President of the Pneumatic Transit Co.</p></div> - -<div class="figcenter" id="fig_4"> -<img src="images/i_043.jpg" width="600" height="417" alt="" /> -<p class="caption"><span class="smcap">Fig. 4.</span></p></div> - -<div class="figcenter" id="fig_5"> -<img src="images/i_046.jpg" width="600" height="412" alt="" /> -<p class="caption"><span class="smcap">Fig. 5.</span> - -<br />SIX-INCH PNEUMATIC TUBES IN PROCESS OF BORING AT THE SHOP OF A. -FALKENAU, PHILADELPHIA, PA.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Aim_and_Object_of_the_Company"><b>Aim and Object of the Company.</b></h3><p class="inline">—When the Pneumatic Transit -Company was formed, it was the aim and object of its promoters to -construct an extensive system of underground tubes in the City of -Philadelphia which would serve, first, for the rapid transmission of -mail, second, for the quick delivery of merchandise from the large -retail stores, third, for the transmission of telegrams or messages -within the city limits, and, fourth, to conduct a general local express -business with greater speed than can be done in any other manner. To -accomplish this result sub-stations were to be located six or eight -blocks apart throughout a large portion of the city, and a central -station was to be established in the centre of the business<span class="pagenum"><a name="Page_29" id="Page_29">29</a></span> -section. Stations were also to be established in the more important -retail stores and large office buildings, and all of the stations were -to be connected by tubes forming one large system.</p></div> - -<p>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 matter. The sub-post-offices would be divided into groups, all -of the offices in one group being connected to the same line, which -would terminate at the main post-office. Most of the business would -be between the main and individual sub-offices; in addition to this -there would be some local mail sent between the sub-offices which, for -offices in the same group, could be despatched directly without passing -through the main office. The advantages to be gained by the use of -these tubes over the present wagon service are very apparent. It places -all the sub-post-offices in almost instant communication with the main -office and with each other.</p> - -<p>It was a part of the general plan to lay tubes from the main -post-office to the railway stations, thereby hastening the despatch and -receipt of mails to and from the trains.</p> - -<p>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 city. Of course it would not be practicable -to lay a tube to each house, but with a station not more than four or -five blocks away, the parcels would be sent through the tube to the -nearest station, and then delivered by messengers to the houses with a -minimum loss of time. Ladies could do their<span class="pagenum"><a name="Page_30" id="Page_30">30</a></span> shopping and find their -purchases at home when they returned.</p> - -<p>The same tubes used for parcel delivery would also be used for a -district messenger service. With numerous public stations in convenient -locations, all the advantages of the European system would be realized -in the quick despatch of letters and telegrams. Every one knows how -much time is consumed by district messenger-boys in the delivery of -messages, especially when they have to go long distances, and no -argument is required to show that this time would be very much reduced -by the use of pneumatic tubes, besides prompt delivery would be made -much more certain.</p> - -<p>The tubes of this system were to be six or eight inches in diameter, -with a few small tubes in localities where the message service is very -heavy.</p> - -<p>Without going more into detail, such were in brief the plans of the -promoters of this new company; but before launching such an enterprise, -involving a large amount of capital, there were many engineering and -mechanical problems to be solved. It was not simply a question of -obtaining tubes and laying them in the streets, but ways and means -for operating them must be devised. Up to this time only small tubes -had been used for the transmission of telegrams, messages, cash, and -other light objects. Now it was proposed to transmit heavy and bulky -material. There was no experience for a guide.</p> - -<div class="section-no-break"><h3 class="inline" id="The_Clay_Lieb_Patents"><b>The Clay-Lieb Patents.</b></h3><p class="inline">—The Pneumatic Transit Company at -this time turned to the Electro-Pneumatic Transit Company, of New -Jersey, a national company that<span class="pagenum"><a name="Page_31" id="Page_31">31</a></span> had been in existence since 1886, -and which claimed to own valuable patents, for the ways and means -to carry out its new enterprise. The patents were those of Henry -Clay and Charles A. Lieb, and the rights to use them in the State -of Pennsylvania were procured by the Pneumatic Transit Company, -under a contract entered into between the two companies. 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 tubes, -with electrically-operated switches at the junctions of the branches -with the main lines. 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 operate. Up to the present time only single- or double-line tubes -have been used, without branches. In the European systems, frequently -several stations are located along a line, but the carriers must stop -at each station, be examined, and if they are destined for another -station, they must be redespatched. 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 store. It is plain to be seen -that, if several of these stations could be connected by branches to a -main tube, a large amount of tubing would be saved—a most desirable -result. The advantages of such a system would be still greater for long -lines of tube laid under the pavements, extending to stations located -in different parts of a large city. It was such a result that the -patents of Clay and Lieb aimed to accomplish.</p></div> - -<p>In order to demonstrate the practicability of the system, the -Electro-Pneumatic Transit Company had constructed<span class="pagenum"><a name="Page_32" id="Page_32">32</a></span> in the basement of -the Mills Building, on Broad Street, New York, a short line of small -brass tubing, about two or three inches in diameter, with one branch, -thus connecting three stations together. The tube was very short, -probably not more than two hundred feet in length. The air-pressure -required was very slight, probably not more than an ounce or two, being -supplied by a small blower run by an electric motor.</p> - -<p>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 branch. This switch was moved by an electro-magnet, -or solenoid, that could be excited by pressing a button at the station -from which the carrier was sent. When the carrier passed into the -branch tube it set the switch back into its normal position, so that a -second carrier, following the first, would pass along the main tube, -unless the switch was again moved by pressing the button at the sending -station.</p> - -<p>This tube in the Mills Building worked well, but it was of a size -only suited to the transmission of cash in a store or other similar -service. It could not be said, because this tube worked well, that -a larger and longer tube with numerous branches would work equally -well. In fact, there are several reasons why such a tube would not -operate satisfactorily. The method of operating the switches was -impracticable. 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 tube. -Again, suppose that we have just despatched a carrier destined for a -station<span class="pagenum"><a name="Page_33" id="Page_33">33</a></span> on the main line beyond the junction, and that we wish to -despatch the second carrier to be switched off into the branch tube, -we must wait at least four minutes, until the first carrier has passed -the junction, before we can press the button and set the switch for -the second carrier which may be on its way. How are we to know when -the first carrier has passed the junction, and when the second will -arrive there, in order that we may throw the switch at the proper time? -Must we hold our watch and time each carrier? It is plain that this is -not practical. I take this as an illustration of the impracticability -of the Clay-Lieb System as constructed in the Mills Building when -extended to practical dimensions. I will not describe the mechanism and -details of the system, which are ingenious, but will say in passing -that the automatic sluice-gates, 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 inch, 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 inch. For further information the reader is referred -to the patents of Clay and Lieb.</p> - -<div class="section-no-break"><h3 class="inline" id="Franchises_and_First_Government_Contract"><b>Franchises and First Government Contract.</b></h3><p class="inline">—In the spring of -1892 an ordinance was passed by Common and Select Councils, and signed -by the Mayor of the City of Philadelphia, permitting the Pneumatic -Transit Company to lay pneumatic tubes in the streets of that city. -At the time this franchise was granted negotiations were in progress -with the post-office department, in Washington, for the construction -of a six-inch pneumatic<span class="pagenum"><a name="Page_34" id="Page_34">34</a></span> tube, connecting the East Chestnut Street -sub-post-office, at Third and Chestnut Streets, with the main -post-office, at Ninth and Chestnut Streets, for the transmission of -mail. This sub-post-office was selected because more mail passes -through it daily than any other sub-office in the city, it being -located near the centre of the banking district. Negotiations were -delayed by various causes, so that the contract with the government was -not signed until October, 1892.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Search_for_Tubes"><b>Search for Tubes.</b></h3><p class="inline">—It was at this time that the writer was -first employed by the Pneumatic Transit Company, as engineer, to -superintend the construction of this line. The company commenced at -once to carry out its contract with the United States government, both -the post-office department and the company being very desirous of -having the work completed before winter. The time was very short for -such an undertaking, but wrought-iron tubes had already been ordered -of a well-known firm who manufacture pipe and tubing of all kinds. -After waiting four or five weeks the first lot of tubes were finished, -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 intended. The next thing that suggested itself was -seamless drawn brass tubes. While they would be very expensive, the -process of manufacture makes them eminently suited for the purpose, -but they could not be obtained in time. A city ordinance prohibits -the opening of the streets of Philadelphia during the winter months -except in extreme cases. Accurate tubes must be had, and had quickly. -It then occurred to the writer that it might be possible to bore a -sufficient quantity<span class="pagenum"><a name="Page_35" id="Page_35">35</a></span> of ordinary cast-iron water-pipe and fit -the ends accurately together to answer our purpose. Inquiry was made -at nearly all the machine-shops in the city, to ascertain how many -boring-machines could be put upon this work of boring nearly six -thousand feet of six-inch pipe. It was found impossible to get the work -done in time, if it was to be done in the usual manner of boring with -a rigid bar. At last a man was found in Mr. A. Falkenau, engineer and -machinist, who was prepared to contract for the construction of twelve -special boring-machines and to bore all the tubing required. Suffice it -to say, that the machines were built, and about six thousand feet of -tubes were bored, between November 8 and December 31.</p></div> - -<div class="figcenter" id="fig_6"> -<img src="images/i_049.jpg" width="600" height="228" alt="" /> -<p class="caption"><span class="smcap">Fig. 6.</span> - -<br /><span class="sans smallest"><i>PIPE BORING APPARATUS.</i></span></p> - -<p class="largeimg"><a href="images/i_049_large.jpg" rel="nofollow">Larger image</a> (92 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="Method_of_Manufacturing_Tubes"><b>Method of Manufacturing Tubes.</b></h3><p class="inline">—The process of boring was novel -in some respects, and might be termed reaming rather than boring. Figs. -4 and 5 show the interior of the shop and the twelve machines. Fig. 6 -is a drawing of one of the machines. A long flexible bar rotated the -cutter-head, which was pulled through the tube, in distinction from -being pushed. In order to give the feeding motion, a screw was attached -to the cutter-head and extended through the tube in advance of it. 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 began. Since it -was not necessary that the tubes should be perfectly straight, a method -of this kind was permissible, in which the cutters could be allowed to -follow the cored axis of the tube. Air from a Sturtevant blower was -forced through the tubes during the process of boring, for the double -purpose of clearing the chips from<span class="pagenum"><a name="Page_37" id="Page_37">37</a></span> the cutters and keeping them cool. -After the tubes were reamed, each piece had to be placed in a lathe, -have a counter-bore turned in the bottom of the bell, and have the -other end squared off and turned for a short distance on the outside to -fit the counter-bore of the next section.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Laying_and_Opening_the_Tubes_for_Traffic"><b>Laying and Opening the Tubes for Traffic.</b></h3><p class="inline">—The first tubes were -laid about the middle of November, but December 1 came before the work -was completed and special permission had to be obtained from the city -to carry on the work after that date. All work was suspended during -the holidays in order not to interfere with the holiday trade of the -stores on Chestnut Street. Severe frosts prevailed at that season, so -that when the work was begun again, after the holidays, 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 tubes. Several times the -trench was filled with snow by unusually heavy storms. Notwithstanding -all these delays and annoyances, the work was pushed forward, when -a less determined company would have given it up, and was finally -completed. A formal opening took place on February 17, 1893, when Hon. -John Wanamaker, then Postmaster-General, sent through the tube the -first carrier, containing a Bible wrapped in the American flag.</p></div> - -<p>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 circumstances. The tubes -have been in successful operation from the opening until the present -time, a period of nearly four years, and the repairs that<span class="pagenum"><a name="Page_38" id="Page_38">38</a></span> have been -made during that time have not required its stoppage for more than a -few hours.</p> - -<p>In the summer of 1895 the sub-post-office was removed from Chestnut -Street to the basement of the Bourse (see Fig. 7). This required the -abandonment of a few feet of tube on Chestnut Street and the laying of -a slightly greater amount on Fourth Street, thus increasing the total -length of the tubes a little. Wrought-iron tube, coated with some -alloy, probably composed largely of tin or zinc, was used for this -extension. The wrought-iron tube is not as good as the bored cast iron.</p> - -<div class="figcenter" id="fig_7"> -<img src="images/i_054.jpg" width="456" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 7.</span> - -<br />BOURSE BUILDING, PHILADELPHIA.</p></div> - -<div class="figcenter" id="fig_8"> -<img src="images/i_057.jpg" width="600" height="473" alt="" /> -<p class="caption"><span class="smcap">Fig. 8.</span> - -<br />PNEUMATIC TUBES SUSPENDED IN THE BASEMENT OF THE MAIN POST-OFFICE.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Description_of_the_Tubes_Method_of_Laying_etc"><b>Description of the Tubes, Method of Laying, etc.</b></h3><p class="inline">—This Chestnut -Street line consists of two tubes, one for despatching carriers from -and the other to the main post-office. The distance between the two -stations is two thousand nine hundred and seventy-four feet, requiring -five thousand nine hundred and forty-eight feet of tube. The inside -diameter of the tube is six and one-eighth inches, and it was made in -sections each about eleven feet long, with “bells” cast upon one end, -in order to join the sections with lead and oakum, calked in the usual -manner of making joints in water- and gas-pipes, with this exception, -that at the bottom of the bell a counter-bore was turned to receive -the finished end of the next section. By thus machining the ends of -each section of tube and having them fit accurately together, male and -female, a practically continuous tube was formed with no shoulders -upon the interior to obstruct the smooth passage of the carriers. -Joints made in this way possess another great advantage over flanged -and bolted joints, in that they are slightly yielding without<span class="pagenum"><a name="Page_39" id="Page_39">39</a></span> -leaking, and so allow for expansion and contraction due to changes of -temperature. Each joint takes care of the expansion and contraction of -its section, which is very slight, but if all were added together would -amount to a very large movement. Another advantage of the “bell” joint -is that it permits slight bends to be made in the line of tube without -requiring special bent sections. Where short bends had to be made, at -street corners, in entering buildings, and other similar places, brass -tubes were used, bent to a radius of not less than six feet, or about -twelve times the diameter of the tube. (One of the brass bends may be -seen in Fig. 10.) The bends were made of seamless tubing, bent to the -desired form and radius in a hydraulic machine. To prevent them from -being flattened in the process of bending, they were filled with resin, -which was afterwards melted out. Flanges were screwed and soldered -to the ends of the bent brass tubes, and they were bolted to special -flanged sections of the iron tube.</p></div> - -<p>The tubes were laid in the trench and supported by having the ground -firmly tamped about them. Usually one tube was laid above the other, -with an iron bracket between, but frequently this arrangement had to -be departed from in order to avoid obstructions, such as gas- and -water-pipes, sewers, man-holes, etc. The depth of the tubes below the -pavement varied from two to six feet, and in one place, in order to -pass under a sewer, the extreme depth of thirteen feet was reached. At -the street crossings it was frequently difficult to find sufficient -space to lay the tubes. At the intersection of Fifth and Chestnut -Streets a six-inch water-main had to be cut and a bend put<span class="pagenum"><a name="Page_40" id="Page_40">40</a></span> in. A -seven-strand electric cable, used for telephoning and signalling, -was laid upon the top of one of the tubes, protected by a strip of -“vulcanized wood,” grooved to fit over the cable. The cable and -protecting strip of wood were fastened to the tube by wrought-iron -straps and bolts.</p> - -<p>The tubes enter the main post-office on the Chestnut Street side, -through one of the windows, and are suspended from the ceiling along -the corridor in the basement for a distance of nearly two hundred -feet. Fig. 8 shows the tubes thus suspended. They terminate upon the -ground floor about the centre of the building, and near the cancelling -machines.</p> - -<div class="figcenter" id="fig_9"> -<img src="images/i_064.jpg" width="600" height="412" alt="" /> -<p class="caption"><span class="smcap">Fig. 9.</span> - -<br />DUPLEX AIR-COMPRESSOR IN THE BASEMENT OF THE MAIN POST-OFFICE.</p></div> - -<div class="figcenter" id="fig_10"> -<img src="images/i_067.jpg" width="479" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 10.</span> - -<br />TANKS AND TUBE IN THE BASEMENT OF THE MAIN POST-OFFICE.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Air_Compressor"><b>Air-Compressor—Method of Circulating the Air.</b></h3><p class="inline">—The current of -air that operates the tubes is supplied by a duplex air-compressor -located in the basement of the main post-office. This machine is -shown in Fig. 9, and requires no detailed description, as it does not -differ materially from air-compressors used for other purposes. The -stroke is twenty-four inches, the diameter of the steam-cylinders -ten inches, and the air-cylinders eighteen inches. The air-cylinders -are double acting, with poppet-valves, and have a closed suction. -The speed of the machine varies slightly, being controlled by a -pressure-regulator that maintains a practically constant pressure -in the tank that feeds the tube. The engines develop a little over -thirty horse-power under normal conditions. The pressure of the air -as it leaves the compressor is usually six or seven pounds per square -inch. Compressing the air heats it to about 156° F., but this is not -sufficient to require water-jackets about the air-cylinders. From the -compressor the air<span class="pagenum"><a name="Page_41" id="Page_41">41</a></span> flows to a tank, shown on the right in -Fig. 10, where any oil or dirt contained in the air is deposited. The -principal purpose of the tank is, however, to form a cushion to reduce -the pulsations in the air caused by the periodic discharge from the -cylinders of the compressors, and make the current in the tube more -steady. 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-office. At the sub-post-office, after flowing through the -receiving and sending apparatus, it enters the return tube and flows -back to the main office, passing through the receiving apparatus there -and then to a tank in the basement,—the left tank in Fig. 10. The -air-compressor draws its supply from this tank, so that the air is used -over and over again. This return tank has an opening to the atmosphere, -which allows air to enter and make up for any leakage or escape at the -sending and receiving apparatus, thereby maintaining the atmospheric -pressure in the discharge end of the tube and in the suction of the -compressor. The tank serves to catch any moisture and dirt that come -out of the tube. Fig. 11 is a diagram showing the direction and course -of the air-current. It will be noticed that both the out-going and -return tube are operated by <i>pressure</i>, in distinction from <i>exhaust</i>. -The air is forced around the circuit by the air-compressor. There is no -exhausting from the return tube. The pressure of the air when it enters -the tube at the main post-office is, say, seven pounds per square inch; -when it arrives at the sub-post-office the pressure is about three and -three-quarters pounds, and when it gets back to the<span class="pagenum"><a name="Page_42" id="Page_42">42</a></span> main office and -enters the return tank, the pressure is zero or atmospheric. Thus it -will be seen that the pressure becomes less and less as the air flows -along the tube. This is not the pressure that moves the carriers, but -the pressure of the air in the tube, a pressure that exists when there -are no carriers in the tube. It is the pressure that would be indicated -if you should drill a hole into the tube and attach a gauge.</p></div> - -<div class="figcenter" id="fig_11"> -<img src="images/i_071.jpg" width="600" height="265" alt="" /> -<p class="caption"><span class="smcap">Fig. 11.</span></p> -<p class="largeimg"><a href="images/i_071_large.jpg" rel="nofollow">Larger image</a> (94 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="Terminal_Apparatus"><b>Terminal Apparatus.</b></h3><p class="inline">—When the construction of this line was -begun, it was the intention of the Pneumatic Transit Company to use the -apparatus of the Electro-Pneumatic Transit Company, at both stations, -for sending and receiving carriers, and so-called working-drawings -were obtained for this purpose. The sending apparatus was constructed -according to the designs furnished, but, upon examination of the -drawings of the receiving apparatus, it was so apparent that it would -not work as intended that it was never constructed.</p></div> - -<p>The writer was asked to design an automatic receiver<span class="pagenum"><a name="Page_43" id="Page_43">43</a></span> to stop the -carriers without shock upon their arrival at the stations, and to -deliver them upon a table without appreciable escape of air,—something -that would answer the requirements of the present plant.</p> - -<div class="figcenter" id="fig_12"> -<img src="images/i_072.jpg" width="600" height="405" alt="" /> -<p class="caption"><span class="smcap">Fig. 12.</span> - -<br /><span class="sans smallest"><i>TRANSMITTER.—PHILA.</i></span> - -<br />SENDING APPARATUS.</p> -<p class="largeimg"><a href="images/i_072_large.jpg" rel="nofollow">Larger image</a> (143 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Sender"><b>The Sender.</b></h3><p class="inline">—The sending apparatus is for the purpose of -enabling the operator to place a carrier in the tube without allowing -the air to escape. In other words, it is a means of despatching -carriers. The apparatus for this purpose, already referred to, is -simply a valve. A side view and section of it are shown in Fig. 12. -Fig. 15 is a view of the apparatus in the main post-office. The sending -apparatus is seen on the left. Fig. 13 is a view of the sub-post-office -apparatus, and here a man may be seen in the act of despatching a -carrier. Referring to<span class="pagenum"><a name="Page_44" id="Page_44">44</a></span> the section, Fig. 12, it will be seen that the -sending apparatus consists of a short section of tube supported on -trunnions and enclosed in a circular box. Normally this short section -of tube stands in line with the main tube, and the air-current passes -directly through it. It is shown in this position in the figure. When a -carrier is to be despatched, this short section of tube is rotated by a -handle until one end comes into coincidence with an opening in the side -of the box. In this position the air flows through the box around the -movable tube. A carrier can then be placed in the short section of tube -and be rotated by the handle into line with the main tube. The carrier -will then be carried along with the current of air. A circular plate -covers the opening in the box where the carrier is inserted when the -sending apparatus is closed.</p></div> - -<p>At the sub-post-office this sending apparatus is placed in a horizontal -position, but its operation is the same.</p> - -<div class="figcenter" id="fig_13"> -<img src="images/i_076.jpg" width="600" height="437" alt="" /> -<p class="caption"><span class="smcap">Fig. 13.</span> - -<br />RECEIVING AND SENDING APPARATUS IN THE SUB-POST-OFFICE.</p></div> - -<div class="figcenter" id="fig_14"> -<img src="images/i_078.jpg" width="600" height="315" alt="" /> -<p class="caption"><span class="smcap">Fig. 14.</span> - -<br /><span class="sans smallest"><i>APPARATUS AT SUB-STATION—PHILA.</i></span></p> - -<p class="largeimg"><a href="images/i_078_large.jpg" rel="nofollow">Larger image</a> (151 kB)</p> -</div> - -<div class="figcenter" id="fig_15"> -<img src="images/i_081.jpg" width="600" height="409" alt="" /> -<p class="caption"><span class="smcap">Fig. 15.</span> - -<br />TERMINALS OF THE TUBE IN THE MAIN POST-OFFICE.</p> -<p class="largeimg"><a href="images/i_081_large.jpg" rel="nofollow">Larger image</a> (397 kB)</p> -</div> - -<div class="section-no-break"><h3 class="inline" id="Sub_Post_Office_Receiver"><b>Sub-Post-Office Receiver.</b></h3><p class="inline">—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 inch. With such a pressure we -cannot open the tube to allow the carriers to come out. 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 atmosphere. Furthermore, -this chamber must act as an air-cushion to check the momentum of the -carriers. Fig. 13 shows the sub-post-office apparatus when a carrier -is being delivered from the receiving apparatus, or, as we will name -it for convenience, the receiver. Fig. 14 is a drawing of the same -apparatus, partly in section, that shows more clearly its method -of<span class="pagenum"><a name="Page_46" id="Page_46">46</a></span> operation. This drawing shows the sending apparatus in a -different position from Fig. 13, but that is immaterial. The receiver -consists of a movable section of tube, about twice the length of a -carrier, closed at one end, supported upon trunnions, and normally -in a position to form a continuation of the main tube from which the -carriers are received. When a carrier arrives it runs directly into -the receiver, which being closed at the end forms an air-cushion -that stops the carrier without shock or injury. 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 -apparatus, as shown in Fig. 14. At the closed end of the receiving -chamber, or air-cushion, is a relief valve, normally held closed by -a spring. As the carrier compresses the air in front of it, this -valve opens and allows some of the air to escape, which prevents the -carrier from rebounding into the tube. Under the outer end of the -receiving chamber is a vertical cylinder, E, Fig. 14, supported upon -the base-plate containing a piston. The piston of this cylinder is -connected by a piston- and connecting-rod to the receiving chamber. -When air is admitted to the cylinder under the piston, the latter rises -and tilts the receiving chamber to an angle of about forty degrees, -which allows the carrier to slide out. The receiving chamber carries -a circular plate, C, that covers the end of the main tube when it is -tilted. A small piston slide-valve, F, located near the trunnion of -the receiving chamber, controls the admission and discharge of air to -and from the cylinder E, upon the arrival of a carrier. When a carrier -arrives and<span class="pagenum"><a name="Page_47" id="Page_47">47</a></span> compresses the air in the air-cushion or receiving -chamber, a small portion of this compressed air is forced through pipe -G, to a small cylinder containing a piston and located just above the -piston slide-valve F. The increased pressure acting on the piston moves -it downward, and it in turn moves the slide-valve F. 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 platform, K. -This platform is hinged at one end, and supported at the angle seen -in the figure by a counterweight. When a carrier rests upon it, the -weight of the carrier is sufficient to bear it down into a horizontal -position; in this position the carrier rolls off on to a table or -shelf. The platform, K, is connected by rods, bell-cranks, etc., to the -piston slide-valve, so that when it swings downward by the weight of -a carrier, the slide-valve is moved upward into its normal position, -and this causes the receiving chamber to tilt back into a horizontal -position ready to receive the next carrier. 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 seconds. Nothing -could operate in a more satisfactory manner.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Main_Post_Office_Receiver"><b>Main Post-Office Receiver.</b></h3><p class="inline">—At the main post-office we have a -receiver of a different type. It will be remembered that the pressure -in the return tube at the main post-office is nearly down to zero or -atmospheric, so that we can open the tube to allow the carriers to pass -out without noise or an annoying blast of air. Figs. 15 and 16 show -the main-office apparatus, and Fig. 17 is a drawing of the same. Here -the receiver consists of a section of<span class="pagenum"><a name="Page_48" id="Page_48">48</a></span> tube closed by a sluice-gate, -located at B, Fig. 17. The air-current passes out through slots in the -tube into a branch pipe leading to the return tank in the basement. -These slots are located about four feet back of the sluice-gate, so -that the portion of the tube between the slots and the sluice-gate -forms an air-cushion to check the momentum of the carriers. The -sluice-gate is raised and lowered by a piston moving in a cylinder -located just above the gate. The movement of this piston is controlled -by a piston slide-valve in a manner similar to the apparatus at the -sub-post-office. Air for operating the piston is conveyed through the -pipe D, Fig. 17, from the pipe leading from the air-compressor to the -sending apparatus. This air is at about seven pounds pressure per -square inch.</p></div> - -<div class="figcenter" id="fig_16"> -<img src="images/i_085.jpg" width="600" height="360" alt="" /> -<p class="caption"><span class="smcap">Fig. 16.</span> - -<br />RECEIVING APPARATUS AT THE MAIN POST-OFFICE.</p></div> - -<div class="figcenter" id="fig_17"> -<img src="images/i_086.jpg" width="600" height="267" alt="" /> -<p class="caption"><span class="smcap">Fig. 17.</span> - -<br /><span class="sans smallest"><i>APPARATUS AT THE MAIN OFFICE—PHILA.</i></span></p> -<p class="largeimg"><a href="images/i_086_large.jpg" rel="nofollow">Larger image</a> (83 kB)</p> -</div> - -<p>When a carrier arrives, after passing the slots that allow the -air-current to flow into the branch pipe, it compresses<span class="pagenum"><a name="Page_50" id="Page_50">50</a></span> the air in -front of it against the gate. This compression checks its momentum, -and it comes gradually to rest. The air compressed between the -carrier and the sluice-gate operates to move the piston slide-valve, -thereby admitting air to the gate cylinder under the piston, which -rises, carrying with it the sluice-gate. The tube is now open to the -atmosphere, and there is just sufficient pressure in the tube to push -the carrier out on to a table arranged to receive it. As the carrier -passes out of the tube it lifts a finger out of its path. This finger -is located at E, Fig. 17, and when it is lifted by the passing carrier -it moves the piston slide-valve, and the sluice-gate is closed. A valve -is located in the branch-pipe that conducts the air to the return tank -in the basement. If the pressure in the tube is not sufficient to -push the carrier out on to the table, this valve is partially closed, -thereby increasing the pressure to a desired amount.</p> - -<div class="figcenter" id="fig_18"> -<img src="images/i_089.jpg" width="172" height="500" alt="" /> -<p class="caption"><span class="smcap">Fig. 18.</span> - -<br />CARRIER.</p></div> - -<div class="figcenter" id="fig_19"> -<img src="images/i_092.jpg" width="600" height="294" alt="" /> -<p class="caption"><span class="smcap">Fig. 19.</span> - -<br />CARRIER.</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Carrier"><b>The Carrier.</b></h3><p class="inline">—We have frequently spoken of the carrier, which -contains the mail and other parcels that are transported from one -office to the other. In Fig. 13, showing the sub-post-office apparatus, -we see one of these carriers being despatched by the attendant and -another being delivered from the tube. In Fig. 15 several carriers -may be seen standing on the floor. Fig. 18 shows a carrier with the -lid open, ready to receive a charge of mail, and Fig. 19 shows the -same closed, ready for despatching. The construction of the carrier -is shown by the drawing, Fig. 20. The body of the carrier is steel, -about one-thirty-second of an inch in thickness. It is made from a -flat sheet, bent into a cylinder, riveted, and soldered. The<span class="pagenum"><a name="Page_51" id="Page_51">51</a></span> -length outside is eighteen inches, and the inside diameter is five and -one-quarter inches. The front end is made of a convex disk of steel, -stamped in the desired form, and secured to the body of the carrier -by rivets, with the convex side inward. It is necessary to have a -buffer upon the front end of the carrier to protect it from blows -that it might receive, and this buffer is made by filling the concave -side of the front head with felt, held in place by a disk of leather -and a central bolt. The leather disk is made of two pieces, riveted -together, with a steel washer between. The steel washer is attached -to the head of the bolt. The carrier is supported in the tube on two -bearing-rings, located on the body of the carrier a short distance from -each end. 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 wedged. This is a very important feature in the -construction of carriers, but does not appear to have been utilized in -other systems.</p></div> - -<div class="figcenter" id="fig_20"> -<img src="images/i_094.jpg" width="600" height="157" alt="" /> -<p class="caption"><span class="smcap">Fig. 20.</span><br /> - -<span class="sans smallest"><i>MAIL CARRIER.—PHILA.</i></span></p> -<p class="largeimg"><a href="images/i_094_large.jpg" rel="nofollow">Larger image</a> (57 kB)</p></div> - -<p>The bearing-rings are made of fibrous woven material, especially -prepared, and held in place by being clamped between two metal rings, -one of which is riveted to the<span class="pagenum"><a name="Page_52" id="Page_52">52</a></span> body of the carrier. Of course these -rings wear out and have to be replaced occasionally, but their usual -life is about one thousand miles. The rear end of the carrier is closed -by a hinged lid and secured by a special lock. The lock consists of -three radial bolts that pass through the body of the carrier and the -rim of the lid. These bolts are thrown by three cams, attached to a -short shaft that passes through the lid and has a handle or lever -attached to it upon the outside of the lid. 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 position, it -projects beyond the periphery of the lid, and in this position the -carrier will not enter the tube. When the lid is closed and locked, -the lever lies across the lid in the position shown in Fig. 19, and -when the carrier is in the tube it cannot become unlocked, for the -lever cannot swing around without coming in contact with the wall of -the tube. This insures against the possibility of the carriers opening -during transit through the tube. The empty carriers weigh about nine -pounds, and when filled with mail, from twelve to fifteen pounds. They -have a capacity for two hundred ordinary letters, packed in the usual -manner.</p> - -<div class="section-no-break"><h3 class="inline" id="Operation_of_the_Tubes"><b>Operation of the Tubes.</b></h3><p class="inline">—The tubes are kept in constant -operation during the day, and six days of the week. The air-compressor -is started at nine o’clock in the morning and runs until seven in the -evening, except during the noon hour, the air flowing in a constant -steady current through the tubes. When a carrier is placed in the -tube it is carried along in the current without appreciably affecting -the load on the compressor. Carriers may be despatched<span class="pagenum"><a name="Page_53" id="Page_53">53</a></span> at six-second -intervals, and when they are despatched thus frequently at each office, -there will be eighteen carriers in the tube at the same time. If ten -carriers per minute are despatched from each office, and each carrier -contains two hundred letters, the tube has a carrying capacity of two -hundred and forty thousand letters per hour, which is far beyond the -requirements of this office. About five hundred carriers a day are -despatched from each office. This varies considerably on different -days and at different seasons of the year. Experience has taught that -a certain period of time should elapse between the despatching of -carriers, in order that they may not come in contact with each other, -and that the receivers may have time to act. With the present plant -this period is made about six seconds. In order to make it impossible -for carriers to be despatched more frequently than this, time-locks -are attached to the sending apparatus. One of these locks may be seen -in Fig. 13, connected to the handle of the sending apparatus. It is -so arranged that when a carrier is despatched a weight is raised and -allowed to fall, carrying with it a piston in a cylinder filled with -oil. While the weight is rising and falling the sending apparatus is -locked, but becomes unlocked when the weight is all the way down. A -by-pass in the cylinder permits the oil to flow from one side of the -piston to the other, and the size of this by-pass can be regulated, -thus determining the time that the weight shall take in descending. -This makes a simple and effective time-lock that does not get out of -order.</p></div> - -<p>The time required for a carrier to travel from the main to the -sub-post-office is sixty seconds, and from the sub- to<span class="pagenum"><a name="Page_54" id="Page_54">54</a></span> the main -post-office, fifty-five seconds. This difference of time in going and -returning is due to the expansion of the air in the tube, as will -be explained more fully in another place. The distance between the -offices being two thousand nine hundred and seventy-four feet, gives -an average speed of about fifty-two feet per second, or 35.27 miles -per hour. Of course the speed can be increased by increasing the -air-pressure, but this speed is found in practice to be ample for all -requirements. In order to give some idea of the energy possessed by -one of these carriers travelling at this speed, it may be said that -if the end of the tube were left open and turned upward, an emerging -carrier would rise about forty feet into the air. It is easy to imagine -how apparatus, depending for its operation upon impact with a moving -carrier, would be soon destroyed, as well as the carriers themselves. -This is why receiving apparatus used with small tubes and light -carriers cannot be applied to large tubes with heavy carriers.</p> - -<p>No serious trouble has ever been experienced from carriers getting -wedged in any part of these tubes.</p> - -<div class="section-no-break"><h3 class="inline" id="Benefits_of_the_System"><b>Benefits of the System.</b></h3><p class="inline">—The advantages to the post-office -department by the adoption of this system have been numerous, and the -post-office officials who are familiar with the operation of the tubes -frequently speak in high terms of their usefulness. Formerly the mail -was transported from one office to the other by a wagon making a trip -every half-hour. Considerable time has been saved by the greater speed -of transit, but even more time is gained by keeping the mail moving -instead of allowing it to accumulate and then despatching it in bulk. -With the<span class="pagenum"><a name="Page_55" id="Page_55">55</a></span> pneumatic system a letter posted in the sub-post-office will -reach its destination just as quickly as if posted at the main office, -and sometimes more quickly. Let us take an example, first, with the old -wagon service. Suppose that you drop a letter in the sub-post-office; -it lies there, say, fifteen minutes waiting for the departure of the -next wagon; it is put into a pouch with hundreds of other letters, and -ten minutes are consumed in transporting it to the main office. When -it arrives there the pouch is thrown on the floor at the entrance of -the building; in a few minutes, more or less, a clerk takes the pouch, -throws it on a truck and wheels it around to the cancelling machines, -where it may lie for five or ten minutes more before being opened, -and then perhaps five minutes will elapse before your letter reaches -the cancelling machine. 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 cancelled. Now with the pneumatic tube service -forty minutes of this time will be saved; 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 machines. 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 use. People -frequently post letters requesting that they be sent through the tube; -of course they would be sent in that way if the request was not made, -but it shows that the public recognize the better<span class="pagenum"><a name="Page_56" id="Page_56">56</a></span> service. Formerly -mail was collected from the street boxes in the banking section of the -city and the collectors carried it to the main office. After the tubes -were installed this mail was carried to the sub-post-office to be sent -through the tube, 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 machines,—a double saving in time besides making -their labor much lighter and enabling them to do more useful work.</p></div> - -<p>Since the sub-office has been established in the Bourse, it has been -made a distributing as well as receiving office. At least two more -deliveries of mail are made each day in the Bourse building than in any -other office building in the city.</p> - -<p>All letters mailed in the sub-office with a special delivery stamp are -despatched through the tube immediately.</p> - -<p>It is now nearly four years since the system was put into operation. -During that time more than thirty-five million letters have been -transported, and all the repairs to the system have not required it -to be stopped for more than a few hours. During the first year the -Pneumatic Transit Company operated the tubes at their own expense, -agreeing at the end of that time to take them out if the government so -requested. Since the first year the government has paid the running -expenses.</p> - -<p>Such is the history of the first United States pneumatic postal system. -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 mail, in -this or any other country, so far as the writer knows.</p> - -<hr class="chap" /> -<div class="chapter"> -<p><span class="pagenum"><a name="Page_57" id="Page_57">57</a></span></p> - -<h2 class="nobreak" id="CHAPTER_III">CHAPTER III.<br /> - -<br /><span class="smaller">THE SYSTEM AND APPARATUS OF THE BATCHELLER PNEUMATIC TUBE COMPANY.</span></h2></div> - -<div class="section-no-break"><h3 class="inline" id="General_Arrangement"><b>General Arrangement and Adaptability of the System.</b></h3><p class="inline">—The -experience gained in the construction and operation of the Philadelphia -post-office tubes has naturally suggested improvements that can be -made in future construction, and, furthermore, it has taught us what -the requirements will be of an extensive system of tubes laid in the -streets of our cities, both for the transmission of mail and for -a general commercial business. Since the Philadelphia post-office -tube was completed, 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 other. It is the purpose of the present chapter to -describe this system.</p></div> - -<p>While the Pneumatic Transit Company has ample field in the State of -Pennsylvania to carry out the work which it has mapped out, a field -broad enough to yield a good profit for the capital invested, there -is no reason why the system should be limited to one State. So, in -order to obtain a broader charter, covering all places where pneumatic -service may be needed, a new corporation was formed and styled the -<span class="smcap">Batcheller Pneumatic Tube Company</span>.</p> - -<p>It is impossible to lay down a rigid system equally well<span class="pagenum"><a name="Page_58" id="Page_58">58</a></span> adapted to -all places and purposes. We must accommodate ourselves somewhat to -circumstances. For example, the post-office department may require -one size of tube, arranged to operate in a particular way, while the -requirements of a parcel delivery business would be utterly different. -The geographical location of the stations will have much to do with the -general arrangement; also the condition of the streets. Some of the -streets of our large cities are so filled with water- and gas-pipes, -electrical conduits, sewers, steam-pipes, etc., etc., that it is almost -impossible to find space for pneumatic tubes, especially of large -diameter. Railway or water facilities have much to do with the location -of a central pumping station, on account of the coal supply. All of -these and many other things have to be taken into consideration in -planning a system for any locality.</p> - -<p>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 cities. This would be in many respects a unique -plant. Two air-compressors would be used, one at each office.</p> - -<p>In order to give a general idea how a large number of stations can be -connected into one system, the diagram Fig. 21 has been drawn.</p> - -<p>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 line. -Their method was to use branch tubes leading off from the main line -with switches at the junctions. They deflected the air-current<span class="pagenum"><a name="Page_59" id="Page_59">59</a></span> into -the branch by placing an automatic closed valve in the main line just -beyond the junction, returning the air from the branch to the main line -just beyond the valve. The carriers were to open and close this valve -automatically as they passed.</p> - -<p>The branch and switch system has many attractions for the inventor, -and upon first thought it would seem the most feasible solution of -the problem. It has been the dream of more than one inventor, as the -records of the patent-office show, but no one has succeeded in working -it out. The current of air cannot be divided; carriers passing from the -branch into the main line must not collide with other carriers running -in the main line; a certain minimum distance must always be maintained -between the carriers in the same tube; 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 -carriers; expense of manufacture prohibits the use of any but round -smooth tubes up to eight inches in diameter, 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 line; the carrier is -free to rotate in a round tube about its longitudinal axis, therefore, -its individuality must be indicated by some symmetrical marking about -this axis, if it is to be automatic in its operation; 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 motion, for -mechanism having inertia could not be moved during the short time that -the electric circuit would be closed; only<span class="pagenum"><a name="Page_60" id="Page_60">60</a></span> the simplest attachments -can be made to the carrier, for constructional reasons and because of -the rough usage that they receive. These and numerous other reasons -make the problem most difficult. We have not attempted to solve it by -the use of branch tubes and electrically operated switches, but have -adopted the simpler and equally effective method of carrying the main -line through each of the stations that it unites. In our system each -carrier has an individuality determining the station at which it will -be discharged from the tube. By a simple attachment to, the front end -of the carrier, consisting of a circular metal disk, 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 -tube. 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 tube.</p> - -<div class="figcenter" id="fig_21"> -<img src="images/i_104.jpg" width="600" height="413" alt="" /> -<p class="caption"><span class="smcap">Fig. 21.</span><br /> - -A DIAGRAM SHOWING VARIOUS METHODS OF CONNECTING THE STATIONS OF A LARGE -SYSTEM WITH PNEUMATIC TUBES.</p> -<p class="largeimg"><a href="images/i_104_large.jpg" rel="nofollow">Larger image</a> (94 kB)</p></div> - -<p>Referring now to the diagram, Fig. 21, we have here an imaginary system -which we will suppose to be located in some large city. The two large -squares I and II indicate central pumping stations, and the small -squares A, B, C, D, etc., indicate receiving and sending stations. Some -of the stations, such as A, B, C, D, E, F, and Y, which do a large -amount of business and may be supposed to be large retail stores, are -connected directly with the central station by double tubes, one for -sending and the other for receiving carriers. Two smaller stores, such -as G and H, may be located on the same line. At I, J, K, and L we have -four stations, all connected by the same double line of tubes. These -stations we will imagine to be located in the residence<span class="pagenum"><a name="Page_62" id="Page_62">62</a></span> section of -the city. Carriers containing parcels of merchandise or other matter -destined for private residences would be sent from the stores A, B, C, -etc., to the central station I, where they would be transferred to the -line 2 and be adjusted to stop at the station nearest the residence to -which the parcels were addressed. From this station the parcels would -be delivered by messengers to the residences. If a carrier is to be -sent from the central station I to station K, it will be so adjusted -before it is put into the tube that it will pass stations I and J, but -be discharged automatically from the tube when it arrives at station -K. In a similar manner carriers can be despatched from station L to -station I or from station J to station L. In passing through the -central station the carriers are manually transferred from one line to -another.</p> - -<p>In another part of the city we may have another central pumping -station, II; and the two central stations may be connected by a double -trunk line, 3. Again, we have lines radiating from this central -station, as shown by station Y. There will be some localities where it -will be an advantage to arrange the stations upon a loop, as shown in -circuit 4, where stations S, T, U, V, W, and X are connected together -in this way. Or we can combine the two arrangements of loop and direct -line, as shown in circuit 5. Stations O and R are on the double line, -but from O a loop is formed including stations N, M, P, and Q. Here it -is supposed that the stations O and R do a much larger business with -the central station II than the stations N, M, P, and Q, this being -the principal reason for placing them on the double line. All carriers -must be returned to the station from<span class="pagenum"><a name="Page_63" id="Page_63">63</a></span> which they were sent, or others -to replace them, otherwise there will be an accumulation of carriers -at some of the stations. It is like a railway: there must be as many -trains despatched in one direction as the other, each day. Station O -can receive a carrier from the central station and return it directly, -but when station N receives one it must be returned via M, P, Q, O, -and R, a much longer route than that by which it was received. This -disadvantage is compensated, when stations N, M, P, and Q do only a -small amount of business, by the less cost of laying a single line. -If a carrier is to be sent from M to N, it must go via P, Q, and O, -being manually transferred at O from the “down” to the “up” line. P -can send directly to Q, but Q must send to P via O, N, and M. R can -send directly to O and O to R. Similarly in circuit 4 the carriers must -all travel around the loop in the same direction, shown by the arrows. -Station S can receive carriers directly from the central station, but -they must return via U, W, X, V, and T.</p> - -<p>Again, we may have a double-loop line, as indicated in the diagram -by circuit 6. Here five stations, <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, and <i>e</i>, are -connected by a loop consisting of two lines of tube, in which the air -circulates in one direction in one line and in the opposite direction -in the other. Here <i>b</i> can send directly to <i>c</i>, <i>c</i> directly to -<i>b</i>, and <i>e</i> to <i>b</i> via <i>d</i> and <i>c</i>, or via central and <i>a</i>. This -is an arrangement that would be used where there is a large amount -of business between the stations on the loop. As stated before, the -best arrangement for any particular locality depends entirely upon -circumstances.</p> - -<p><span class="pagenum"><a name="Page_64" id="Page_64">64</a></span></p> - -<div class="section-no-break"><h3 class="inline" id="Size_of_Tubes"><b>Size of Tubes.</b></h3><p class="inline">—The pneumatic-tube system that we are describing -is not limited to any particular size of tube. The size is usually -determined by the number and size of packages to be transported. -A small tube, two or three inches in diameter, is best suited for -telegrams and messages; mail, parcels, etc., require a six- or -eight-inch tube, while mail pouches and bulky material, a thirty-six -inch or possibly larger tube. We divide tubes into three classes, -according to their size, naming them small, large, and very large -tubes. By small tubes we mean those not larger than three or possibly -four inches in diameter. Large tubes are those having a diameter more -than four inches and not more than eight inches. Very large tubes -include all that are more than eight inches. This classification is -for convenience, but it has a deeper significance. For example, in -the transportation of mail, it must either be handled in bulk, that -is, in pouches, or in broken-bulk, that is, loose or tied up in small -packages. There are many advantages in transporting it in broken-bulk, -in fact, there are very few places where it could be handled in any -other way. For this service six- or eight-inch tubes—not larger—are -best suited. The carriers are light enough to be easily handled; they -are not so large in capacity as to make it necessary to wait for an -accumulation of mail to fill them; they can be delivered from the tube -on to tables at any point in the building where the mail is wanted, -for cancelling, distribution, or pouching, thus rendering a very rapid -service; the mail is kept moving in an almost constant stream, keeping -the postal employees more uniformly employed; special carriers can be -despatched with “special<span class="pagenum"><a name="Page_65" id="Page_65">65</a></span> delivery” letters. In other words, the most -rapid service can be rendered by this size of tube.</p></div> - -<p>If a larger than eight-inch tube is to be used for mail service, -it should be not less than thirty-six inches. Carriers larger than -eight inches cannot be handled: they are too heavy. They are also too -heavy to slide through the tube, hence, must be mounted upon wheels. -It is not practical to make a carrier on wheels less than eighteen -or twenty-four inches, and the carrier must be at least twenty-four -inches to contain a large mail-pouch. Now, if we are going to despatch -mail-pouches through a pneumatic tube we must send more than one in a -carrier, otherwise the service will be too slow. Such large carriers -could not be despatched oftener than once or at most twice in a minute. -Suppose we were to transport the mail from a railway station to a main -post-office. A train arrives with, say, sixty pouches. If only one -pouch could be put into a carrier and the carriers could be despatched -at half-minute intervals, it would take thirty minutes to despatch -all the pouches. Now, suppose we make the tube thirty-six inches. -The carriers will be eight feet long and will contain from twelve to -fifteen pouches. Five carriers would contain the entire train-load of -mail, and they could be despatched in four or five minutes.</p> - -<div class="figcenter" id="fig_22"> -<img src="images/i_109.jpg" width="600" height="410" alt="" /> -<p class="caption"><span class="smcap">Fig. 22.</span><br /> - -CROSS-SECTION OF A 36-INCH TUBE.</p></div> - -<div class="figcenter" id="fig_23"> -<img src="images/i_110.jpg" width="600" height="222" alt="" /> -<p class="caption"><span class="smcap">Fig. 23.</span><br /> -CARRIER FOR A 36-INCH TUBE.</p></div> - -<div class="section-no-break"><h3 class="inline" id="System_of_Very_Large_Tubes"><b>System of Very Large Tubes.</b></h3><p class="inline">—The cross-section of a -thirty-six-inch tube is shown in Fig. 22. It is built flat on the -bottom and sides, with an arched top. The floor is of concrete -containing creosoted ties; the side walls and top are of brick, -plastered with cement upon the interior. The two tubes may be built -one above the other<span class="pagenum"><a name="Page_68" id="Page_68">68</a></span> or side by side, depending upon the condition -of the streets, but one common separating wall will serve for both. -The carriers, one of which is shown in Fig. 23, run on two rails laid -close to the sides of the tube. At curves a guard-rail is placed upon -the side wall, making it impossible for a carrier to leave the track. -The carriers are made of hard wood with an iron frame, and are as light -as consistent with the service required of them. They are open on top. -Their outside dimensions are thirty-four inches by thirty-four inches -by eight feet. The sending and receiving apparatus for these very large -tubes have to be specially designed for each particular station, so no -attempt will be made here to describe them. The air-pressure required -depends upon the length of the line. If it were not more than six or -eight ounces a fan would be used to maintain the air-current, but for -pressures above this, up to a pound or two per square inch, some form -of positive blower would be used.</p></div> - -<p>At the stations considerable floor space or “yard room” would be -required for side tracks, switches, etc. Usually the basement of a -building would have to be utilized for the termination of such a tube. -There are but few places in our large cities where the streets are so -free from pipes, sewers, conduits, etc., that it would be practicable -to build a thirty-six-inch pneumatic tube. When the service can be -rendered by an eight-inch tube, the cost of installation favors its -adoption. Steep grades cannot be ascended by these very large tubes, -while the eight-inch tubes can be placed vertically. We do not say that -there is no use for eighteen- and twenty-four-inch tubes, but the<span class="pagenum"><a name="Page_69" id="Page_69">69</a></span> -demand for them would be in special cases and we will not discuss them -here. For ordinary mail and parcel service we recommend the use of -six- and eight-inch tubes. An eight-inch carrier is twenty-four inches -long, about seven inches inside diameter, and will contain five hundred -ordinary letters. It weighs about thirteen pounds empty, and one can -be despatched every six to ten seconds. We estimate that eighty per -cent. of all the parcels delivered from a large retail department store -could be wrapped up to go into these carriers. The minimum radius of -curvature of an eight-inch tube is eight feet.</p> - -<div class="section-no-break"><h3 class="inline" id="Two_Station_Two_Compressor_Line"><b>General Arrangement of Apparatus in the Stations. Two-Station, -Two-Compressor Line.</b></h3><p class="inline">—We will now proceed to a description of -our system in detail. Figs. 24, 25, and 26 are diagrams showing how -the tubes, air-compressor, tanks, sending and receiving apparatus -are connected together at the stations. These diagrams are drawn to -represent an eight-inch tube, but essentially the same arrangement -would be used for smaller tubes.</p></div> - -<p>Fig. 24 represents a line of two stations with an air-compressor at -each station. Such an arrangement is proposed for the line of postal -tubes over the New York and Brooklyn bridge, or for any two stations -located a very long distance apart, say six or eight miles.</p> - -<div class="figcenter" id="fig_24"> -<img src="images/i_113.jpg" width="600" height="102" alt="" /> -<p class="caption"><span class="smcap">Fig. 24.</span><br /> - -DIAGRAM OF A TWO-STATION, TWO-COMPRESSOR LINE.</p> -<p class="largeimg"><a href="images/i_113_large.jpg" rel="nofollow">Larger image</a> (83 kB)</p></div> - -<p>Referring to the diagram, we have at station A an air-compressor, <i>c</i>, -which draws its supply of air from the tank <i>e</i>, and delivers it, -compressed to the necessary pressure, into the tank <i>d</i>. From the tank -<i>d</i> the air flows to the sending apparatus, <i>a</i>, and thence through -the tube <i>f</i> to the station B. Upon arrival at B it flows through the -receiving<span class="pagenum"><a name="Page_71" id="Page_71">71</a></span> apparatus <i>m</i>, and then by the pipe <i>l</i> to the tank <i>j</i>. A -second air-compressor, <i>o</i>, is located at station B, and it draws its -supply of air from the tank <i>j</i>. The tank <i>j</i> has an opening to the -atmosphere, <i>i</i>, through which air can enter when the air-compressor -draws more than is supplied from the pipe <i>l</i>. The opening <i>i</i> in the -tank <i>j</i> serves as an escape for air when the air-compressor at station -A is started before that at station B. Stations A and B are similar in -their arrangements. At B the air-compressor <i>o</i> delivers its compressed -air to the tank <i>k</i>, from which it flows to the sending apparatus <i>n</i>, -and thence through the tube <i>g</i> back to station A. Upon its arrival at -A it passes through the receiving apparatus and enters the tank <i>e</i>, -which is open to the atmosphere at <i>h</i>. The tanks <i>d</i> and <i>k</i> serve -as separators to remove from the air any dirt and oil coming from the -compressors, and they form a cushion, deadening, to some extent, the -pulsations of the compressors and making the current of air in the -tubes more steady and uniform. The tanks <i>e</i> and <i>j</i> form traps to -catch any moisture, oil, or dirt coming out of the tubes.</p> - -<p>Carriers are placed in the tubes and despatched by means of the sending -apparatus <i>a</i> and <i>n</i>. They are received from the tubes and delivered -on to tables by means of the receiving apparatus <i>b</i> and <i>m</i>. It will -be seen that the arrangement is such that the air flows through one -tube and returns through the other, the same air being used over and -over again. Any air that escapes at the sending and receiving apparatus -is replaced by an equal amount entering the tanks <i>e</i> and <i>j</i> from the -atmosphere. By thus keeping<span class="pagenum"><a name="Page_72" id="Page_72">72</a></span> the same air circulating in the tubes we -prevent an accumulation of moisture in the tubes.</p> - -<p>The air is at its maximum pressure in the tanks <i>d</i> and <i>k</i>. The -pressure falls gradually as it flows along the tubes and is down to -atmospheric when it enters the tanks <i>e</i> and <i>j</i>. The pressure at the -receivers, <i>b</i> and <i>m</i>, is just sufficient to push the carriers out on -to the tables. The construction of the sending and receiving apparatus -will be described in another place.</p> - -<div class="figcenter" id="fig_25"> -<img src="images/i_116.jpg" width="600" height="102" alt="" /> -<p class="caption"><span class="smcap">Fig. 25.</span><br /> -DIAGRAM OF A TWO-STATION, ONE-COMPRESSOR LINE.</p> -<p class="largeimg"><a href="images/i_116_large.jpg" rel="nofollow">Larger image</a> (34 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="Two_Station_One_Compressor_Line"><b>Two-Station, One-Compressor Line.</b></h3><p class="inline">—Fig. 25 is a diagram showing -two stations, A and B, connected by a double line of tubes, both -operated by one air-compressor located at station A. This is the -arrangement used in the Philadelphia post-office line, and is the -arrangement that will ordinarily be used for all two-station lines -except where unusual conditions require something different. Station -A is arranged precisely like station A in Fig. 24, so it need not be -described again. The air flows from the sending apparatus <i>a</i> through -the tube <i>f</i> to the receiving apparatus <i>p</i> at station B. From the -receiver <i>p</i> it flows through the pipe <i>l</i> to the sending apparatus -<i>n</i> and thence through the tube <i>g</i> back to station A. The receiver -<i>p</i> at station B is what we will call a closed receiver,—<i>i.e.</i>, it -delivers the carrier from the tube on to the table without opening -the tube to the atmosphere. 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 atmospheric. The air-pressure is at a maximum in -the tank <i>d</i>. It falls gradually along the tube <i>f</i>, and when the air -arrives at the receiver <i>p</i>, at station B, the pressure has<span class="pagenum"><a name="Page_74" id="Page_74">74</a></span> fallen -nearly to one-half its maximum amount in the tank <i>d</i>. On its return -journey through the tube <i>g</i> the pressure continues falling until it -reaches the atmospheric pressure when the air enters the tank <i>e</i> at -station A.</p></div> - -<p>The entire line of tube, going and returning, is operated by air at a -pressure above the atmospheric. There is no exhausting in the return -tube. It is distinctly a <i>pressure</i> system.</p> - -<div class="figcenter" id="fig_26"> -<img src="images/i_118.jpg" width="600" height="144" alt="" /> -<p class="caption"><span class="smcap">Fig. 26.</span><br /> -DIAGRAM OF A THREE- TO EIGHT-STATION LINE.</p> -<p class="largeimg"><a href="images/i_118_large.jpg" rel="nofollow">Larger image</a> (98 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="Three_to_Eight_Station_Line"><b>Three- to Eight-Station Line.</b></h3><p class="inline">—Thus far we have described only -two-station lines. In Fig. 26 we have a diagram of three stations -connected together by a double line of tubes, and the arrangement -would be similar if it were extended to four, five, six, seven, or -eight stations. The stations are called A, B, and H. Station A is -arranged exactly the same as stations A in Figs. 24 and 25, therefore, -needs no description. Station B, being an intermediate station, is -quite differently arranged from any of the preceding. From station -A the air flows through the tube <i>f</i> to station B, where it enters -the automatic receiving and transferring apparatus, <i>s</i>. From this -it flows through the tube <i>f</i>′ to the sending apparatus <i>r</i>, and -thence through the tube <i>f</i>″ to the next station, which may be -another intermediate station, C, or the terminal station H. Station H -is arranged like station B, Fig. 25. The air from the tube <i>f</i>″ -enters the receiver <i>p</i>, and is then returned, through the pipe <i>l</i>, to -the sending apparatus <i>n</i>. From the sending apparatus <i>n</i> it continues -on its return journey through the tube <i>g</i> to the intermediate station -B, where it enters the receiver and transfer apparatus <i>t</i>, then passes -<span class="pagenum"><a name="Page_76" id="Page_76">76</a></span>to the sending apparatus <i>q</i>, and through the tube <i>g</i>″ back to -the receiver <i>b</i> at station A. Thus we have followed the air-current -out through one tube and back through the other. The current is kept -circulating by the compressor located at station A. The pressure is -at a maximum in the tank <i>d</i>, and falls gradually as the air flows -along the tube until it returns to the tank <i>e</i>, when the pressure has -fallen to atmospheric. A carrier is despatched from station A, and -after passing through the tube <i>f</i> arrives at station B, where it stops -momentarily in the automatic receiver and transfer apparatus <i>s</i>. If -the carrier is intended for station B, and was properly adjusted when -it was despatched at A, it will be discharged from the apparatus <i>s</i> -on to the table <i>u</i>. But if it were intended for some other station -and were so adjusted, after the delay of two or three seconds in -the apparatus <i>s</i>, it will be automatically transferred to the tube -<i>f</i>′, pass through the sending apparatus <i>r</i>, and go on its journey -through tube <i>f</i>′ to the next station. If it is not discharged from -the tube at any of the intermediate stations, it will finally arrive -at the terminal station H and there stop. Just how the carriers are -adjusted and the details of the receiving and transfer apparatus will -be described hereafter. Carriers arrive at station B from H, or other -stations on the line, through tube <i>g</i>, in the apparatus <i>t</i>, which -either discharges them on to the table <i>u</i> or sends them on through -the tube <i>g</i>′ and <i>g</i>″ to station A. Carriers are despatched -from station B to station A by means of the sending apparatus <i>q</i>, and -from station B to other stations along the line, C, D, E, F, G, and -H, by means of the sending apparatus <i>r</i>. Thus, from B carriers can -be sent and received in either direction. In order to<span class="pagenum"><a name="Page_77" id="Page_77">77</a></span> 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 -apparatus, an automatic lock is attached to each sending apparatus. -Just outside the station B, say three hundred feet on each side, are -located manholes, and in these manholes boxes are attached to the tube -containing an electric circuit-closing apparatus, so arranged that -when a carrier passes it will close an electric circuit leading to the -sending apparatus in the station. These manholes and circuit-closers -are shown and located on the diagram at <i>v</i> and <i>w</i>. Wires <i>x</i> and <i>y</i> -lead from them to the sending apparatus <i>r</i> and <i>q</i>. When a carrier -from station A passes the box <i>v</i>, it closes the electric circuit <i>x</i>, -which sets a time-lock on the sending apparatus <i>r</i>, holding this -apparatus locked, so that it is impossible to despatch a carrier for, -say, twelve seconds, a sufficient time for the carrier coming from the -station A to pass station B and get three hundred feet beyond it. After -the twelve seconds have elapsed the sending apparatus is unlocked and -a carrier can then be despatched. In a similar manner a carrier coming -from station H, in passing the box <i>w</i>, closes the electric circuit <i>y</i> -and locks the sending apparatus <i>q</i> for a sufficient length of time to -let the carrier pass the station. This resembles, in some respects, -the “block system” as used on railroads. A “block” about six hundred -feet in length, depending upon the speed of the carriers, is made at -each intermediate station with the station in the centre of the block. -Whenever a carrier enters this “block” the sending apparatus at the -station is locked, and a carrier cannot be inserted into the tube to -collide<span class="pagenum"><a name="Page_78" id="Page_78">78</a></span> with the one which is passing. It will be noted that a carrier -in passing out of the “block” does not unlock the sending apparatus; -this is done automatically at a definite time after the carrier entered -the block. The unlocking is entirely independent of the carrier after -it has entered the block, and the reason it is so arranged is this: -suppose that a second carrier enters the “block” before the first one -leaves it; if the first carrier unlocked the apparatus when it left the -“block,” then it would be unlocked with the second carrier in the block -and a collision might occur, but by arranging it as we have done, if a -second carrier enters the “block” before the first has passed out, the -sending apparatus remains locked for a period of time beginning with -the arrival of the first carrier in the “block” and ending, say, twelve -seconds after the arrival of the last carrier, which is sufficient time -for the last carrier to pass out of the block. Of course, if a carrier -becomes wedged in the tube a collision may occur, but this very seldom -if ever happens. The details of the locking apparatus will be described -in another place.</p></div> - -<p>If stations A, B, ... and H were arranged on a loop, as shown in -circuit 6, Fig. 21, then station H, Fig. 26, would be at the central, -or station A. If it were a single loop, like circuit 4, Fig. 21, -then there would be only one sending apparatus and one receiving and -transferring apparatus at the intermediate stations.</p> - -<p>A telephone circuit will include all stations, in order to give orders -to the station attendants and to signal to the central station in case -of an accident, when it might be necessary to stop the air-compressor. -The telephone wires,<span class="pagenum"><a name="Page_79" id="Page_79">79</a></span> in the form of a lead-covered cable, are laid in -the same trench with the tubes and fastened to them.</p> - -<div class="figcenter" id="fig_27"> -<img src="images/i_123.jpg" width="600" height="492" alt="" /> -<p class="caption"><span class="smcap">Fig. 27.</span><br /> -SENDING APPARATUS.</p> -<p class="largeimg"><a href="images/i_123_large.jpg" rel="nofollow">Larger image</a> (470 kB)</p></div> - -<div class="figcenter" id="fig_28"> -<img src="images/i_124.jpg" width="600" height="588" alt="" /> -<p class="caption"><span class="smcap">Fig. 28.</span><br /> -SENDING APPARATUS.—LONGITUDINAL SECTION.</p> -<p class="largeimg"><a href="images/i_124_large.jpg" rel="nofollow">Larger image</a> (283 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Sending_Apparatus"><b>The Sending Apparatus.</b></h3><p class="inline">—We have, in the preceding pages, -frequently spoken of the sending apparatus, and have described it -as mechanism by which carriers are inserted into the tube. In the -Philadelphia postal line this apparatus consisted of a large valve, -operated by hand. For an eight-inch tube such a valve would be too -large and heavy to be manually operated. Furthermore, that type of -apparatus is not suited to an intermediate station, where carriers -have to pass through it. To meet all of these requirements we have -designed an apparatus, of which Fig. 27 is a side elevation, Fig. 28 -a longitudinal section, and Fig. 29 a cross-section. Referring to the -longitudinal section, Fig. 28, the sending apparatus is shown inserted -into the line of a pneumatic tube, A, A. We have a movable section of -tube, B, that can be swung about the large bolt, G, at the top, into -and out of line with the main tube, A, A. When the section of tube B -is being swung to one side, the air-current has a by-pass through the -slots E and F and the U-shaped pipe D. The joints at the ends of the -movable section B are packed with specially-formed leathers. Referring -to the cross-section, Fig. 29, when the movable section of tube B is -swung out of line with the main tube, another and similar tube, C, -takes its place. The two movable tubes, B and C, are made in one piece, -so that they must always move together. They are connected together at -each end by plates, M, that serve not only as connecting-plates, but -covers for the ends of the main-line tube while the tubes B and C are -being moved. The tubes<span class="pagenum"><a name="Page_82" id="Page_82">82</a></span> B and C swing between four plates or wings, -L, that extend out on each side of the apparatus. They serve as guards, -and, at certain positions of the swinging tubes, prevent the air from -escaping.</p></div> - -<p>We will, for convenience, call the system of swinging tubes B and C, -with their supports, etc., the swing-frame or simply the frame. This -frame is moved or swung from one position to the other by means of a -cylinder and piston, H, placed in an inclined position under it. A lug, -N, is cast on the tube B, to which the connecting-rod, O, is attached. -The cross-head, P, slides upon an inclined guide, Q. On top of the -cylinder is placed a controlling valve, made in the form of a piston -slide-valve. The piston in the cylinder H is moved by the pressure of -the air taken from the main tube through the pipe I. The apparatus is -operated by a hand-lever, K. When this lever is pulled, it moves the -sliding-head R, and this, through the spring S, moves the controlling -valve, if the valve is not locked. If it is locked, pulling the lever -simply compresses the spring S. 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 atmosphere, and compressed air from the main tube -flows through the pipe I, the passages T and U, to the cylinder H, -under the piston, causing the piston to move up the inclined cylinder -and swing the frame until the tube C is in line with the main tube. -Carriers are despatched by placing them in the tube C, then pulling -the lever K, and swinging the frame until the tube C is in line with -the main tube. The carrier is then taken up and carried along by the -current of air in the main tube.</p> - -<div class="figcenter" id="fig_29"> -<img src="images/i_126.jpg" width="600" height="604" alt="" /> -<p class="caption"><span class="smcap">Fig. 29.</span><br /> -SENDING APPARATUS.—CROSS-SECTION.</p> -<p class="largeimg"><a href="images/i_126_large.jpg" rel="nofollow">Larger image</a> (448 kB)</p></div> - -<p><span class="pagenum"><a name="Page_84" id="Page_84">84</a></span></p> - -<p>Replacing the hand-lever K in its original position returns the frame -to its normal position.</p> - -<div class="figcenter" id="fig_30"> -<img src="images/i_128.jpg" width="277" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 30.</span><br /> -SENDING TIME-LOCK.</p> -<p class="largeimg"><a href="images/i_128_large.jpg" rel="nofollow">Larger image</a> (174 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="Sending_Time_Lock"><b>Sending Time-Lock.</b></h3><p class="inline">—In any system of large pneumatic tubes -a short time should elapse between the despatching of carriers, in -order that they may not collide in the tube, and to give the receiving -apparatus at the stations time to act. To insure the impossibility -of having carriers despatched too rapidly, we place on the sending -apparatus a time-lock that will automatically lock it for a determined -length of time after each carrier is despatched, the time-lock being -adjustable for any desired time. The time-lock, W, is shown attached -to the sending apparatus in Fig. 27. When the swing-frame is swung -to despatch a carrier, it pulls up the rod X by means of a link and -bell-crank, Y, thereby locking the controlling valve of the cylinder H -and starting the time-lock W, which will unlock the controlling valve -after the required time has elapsed. The details of this time-lock -are shown in Fig. 30. It consists of a long vertical cylinder, A, -containing a piston, B, and a spiral spring, C, that tends to force the -piston to the bottom of the cylinder. The cylinder is filled with oil, -and holes, D, in the piston allow the oil to pass freely through it -when it is moved upward in the cylinder. When the piston moves downward -an annular collar, E, forming a valve, closes the holes in the piston -and prevents the oil from passing through. Extending from one side of -the piston to the other is a by-pass, F, in the wall of the cylinder. -When the piston moves downward the displaced oil is forced to flow -through this by-pass. A small cock, G, is arranged in the by-pass to -throttle the stream of oil<span class="pagenum"><a name="Page_86" id="Page_86">86</a></span> flowing through it. The opening in this -cock, or the amount of throttling, is indicated on the outside by -an index and dial, Z (see Fig. 27). When the piston B is raised and -allowed to descend by the force of the spring C, it forces the oil -through the by-pass F and the cock G. If the latter is wide open the -piston will descend quickly, but if it is nearly closed the piston -will descend very slowly. In other words, the time of descent can be -regulated by opening and closing the cock G. The reading on the dial Z -can be made seconds of time that elapse while the piston is descending.</p></div> - -<p>Above the cylinder is a cross-head, H, that moves up and down between -vertical guides. This cross-head is moved by the rod X, also shown -in Fig. 27, that receives its motion from the swinging frame of the -sending apparatus. A piston rod, I, attached to the piston in the -cylinder, extends up through the travelling cross-head but is not -attached to it. On the piston-rod are two enlargements, J and K, one -made a solid part of it, the other formed by two nuts. The travelling -cross-head H carries a pawl, L, that engages under the shoulder formed -by the nuts K. This pawl is kept against the piston-rod by the spring -M. The enlargement, J, on the piston-rod forms a shoulder that bears -against the bell-crank, N, that connects with the bolt, O, which locks -the controlling valve. In the present down position of the piston and -piston-rod, the enlargement J, by pressing against the bell-crank N, -holds the bolt O in an unlocked position. When a carrier is despatched -the cross-head H is lifted by the rod X, and carries with it the piston -and piston-rod, compressing the<span class="pagenum"><a name="Page_87" id="Page_87">87</a></span> spring C. This upward movement of the -piston-rod allows the bolt O to be thrown by a spring, not shown in the -figure, and so lock the controlling valve of the sending apparatus. -As the cross-head continues its upward movement, the pawl L comes in -contact with the end of the screw P and disengages the piston-rod. This -allows the piston to descend as rapidly as the oil can pass through the -by-pass and cock G. When the piston has reached nearly to the bottom of -the cylinder, the shoulder J, on the piston-rod, engages the bell-crank -N and withdraws the bolt O, thereby unlocking the controlling valve. -The time that the sending apparatus is locked depends upon the time -required for the piston to descend. While the sending apparatus is -locked against the sending of another carrier, 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 -expires. This time is usually not more than ten seconds. Not only may -the second carrier be placed in the tube C, Fig. 29, ready to be sent, -but the handle K may be pulled and fastened in the notch <i>a</i>, thereby -compressing the spring S, which, as soon as the controlling valve is -unlocked, will move the valve and automatically despatch the carrier. -The controlling valve is locked by the passage of a bolt through the -hole <i>b</i>, in a block carried on the end of the valve stem, when it -returns to the normal position shown in the figure. Usually little or -no time will be lost in thus locking the sending apparatus, for the -small amount of time that the apparatus is locked will be needed in -handling the carriers.</p> - -<p><span class="pagenum"><a name="Page_88" id="Page_88">88</a></span></p> - -<div class="section-no-break"><h3 class="inline" id="Intermediate_Station_Time_Lock"><b>Intermediate Station Time-Lock.</b></h3><p class="inline">—We have another time-lock -attached to the sending apparatus that has been already referred -to in describing the “block system” used at intermediate stations; -a time-lock to prevent carriers being inserted into the tube at -intermediate stations while another carrier is passing that station. -This time-lock is shown in Fig. 27 at W´, and is shown in detail by a -sectional drawing, Fig. 31.</p></div> - -<p>When a carrier closes an electric circuit in passing one of the boxes -located in a manhole about three hundred feet from an intermediate -station, it indicates its approach to the station by exciting the -electro-magnet A, Fig. 31. This magnet pulls down its armature B and -raises the small piston valve C, which admits compressed air to a small -chamber, D. The air is supplied to this chamber from the main tube -through the pipe E. In one end of this chamber is fitted a piston, F, -held to one end of its stroke by a spring, G. When compressed air is -admitted to the chamber D, this piston is moved to the left, and by -such movement throws the controlling valve of the sending apparatus -into its normal position (shown in Fig. 29) and holds it there. This -forms a positive lock, and, no matter in what position the sending -apparatus may be, it puts the tube B, Fig. 29, into line with the main -tube so that the approaching carrier can pass through the apparatus. -The piston-rod H, Fig. 31, is connected to the finger <i>d</i>, Fig. 29, -and by rocking this finger moves the controlling valve, or prevents it -being moved by the handle K.</p> - -<div class="figcenter" id="fig_31"> -<img src="images/i_132.jpg" width="600" height="438" alt="" /> -<p class="caption"><span class="smcap">Fig. 31.</span><br /> - -INTERMEDIATE STATION TIME-LOCK.</p> -<p class="largeimg"><a href="images/i_132_large.jpg" rel="nofollow">Larger image</a> (199 kB)</p></div> - -<p>Returning now to Fig. 31, we have on the top of the<span class="pagenum"><a name="Page_90" id="Page_90">90</a></span> chamber D, in -addition to the electro-magnet A and its armature B, a differential -cylinder and piston, K, L, whose function is to close the valve C -when the chamber D is filled with air. The piston K is smaller than -the piston L, and sustains a constant air-pressure, supplied through -the small pipe M M, from the pipe E, which leads to the main tube. -When the chamber D becomes filled from the pipe E through the valve -C, the pressure in the chamber moves the piston L upward against the -pressure on the piston K, because of the greater area of the piston -L. This movement of the differential piston raises the lever I, which -passes through a slot in the stem of the differential piston, and thus -closes the valve C. The air in the chamber now gradually escapes to -the atmosphere through a small orifice Q; in fact it has been escaping -here all the time while the chamber was being filled, 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 filling. -Now, however, that all supply to the chamber is shut off, the air in -the chamber is gradually being discharged through the orifice. When -nearly all the air has escaped, the piston F will return to its normal -position, shown in the figure, and unlock the controlling valve. -The time required for the air to escape from the chamber, D, is the -time that the sending apparatus will be locked, and this time can be -regulated by varying the size of the orifice Q. The opening of the -orifice, or the time that the sending apparatus is locked, is indicated -by an index and dial, P.</p> - -<p>This locking mechanism is secured to a bracket on the<span class="pagenum"><a name="Page_91" id="Page_91">91</a></span> side of the -large cylinder H, Fig. 27, in a position where it can be easily -inspected. The moving parts of the electro-magnetic valve—for such is -the valve C, with the magnet A, Fig. 31—are made very light, in order -that they may respond easily and quickly to the closing of the electric -circuit.</p> - -<p>It is a disadvantage to have stations too numerous upon the same line, -especially if they do a large amount of business, for each station -will delay the sending of carriers from the others more or less, and -the interference will be greatest during the busiest hours of the -day. This condition is inherent in any system of large tubes where -carriers have to be run a certain minimum distance apart, and cannot be -overcome by any mechanism. But the disadvantage is greatly overshadowed -by the advantage of being able to connect several stations by one -line, instead of having to run independent lines from each station to -the central, especially when the business of the individual stations -is not sufficient to occupy a separate tube all the time. It makes -it possible to have stations where otherwise the business would not -warrant the cost of installation and expense of operation. We recommend -the establishing of not more than eight stations on a line, and usually -a smaller number than this, depending, of course, upon the amount of -business to be done at each station.</p> - -<div class="figcenter" id="fig_32"> -<img src="images/i_136.jpg" width="600" height="381" alt="" /> -<p class="caption"><span class="smcap">Fig. 32.</span><br /> -ELECTRO-PNEUMATIC CIRCUIT-CLOSER.</p> -<p class="largeimg"><a href="images/i_136_large.jpg" rel="nofollow">Larger image</a> (153 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Electro_Pneumatic_Circuit_Closer"><b>The Electro-Pneumatic Circuit-Closer.</b></h3><p class="inline">—There is one piece -of mechanism used in connection with the sending apparatus that we -have yet to describe, and that is the circuit-closing device located -in the manholes in the street. Since the carriers travel at a high -rate of speed, they<span class="pagenum"><a name="Page_92" id="Page_92">92</a></span> should not be made to operate any mechanism by -impact with fingers or levers protruding into the tube when it can be -avoided, even though the work to be done is so slight as the closing -of an electric circuit, for the repeated impacts cannot fail to work -injury to the carriers and the mechanism to be operated, no matter how -carefully they are designed. To avoid such impacts, we have designed -the electro-pneumatic circuit-closer, shown by the drawing in Fig. 32. -It is operated by a passing carrier, but pneumatically rather than -mechanically. In the figure we have a pneumatic tube, A, A, in which a -carrier, B, is moving in the direction indicated by the arrow. At two -points, about twenty or thirty feet apart, two small holes are tapped -into the tube and pipes, C and D, are screwed in. These pipes lead to -two chambers in a cast-iron box, F, separated by a diaphragm, E. This -diaphragm is insulated electrically from the box supporting it, and -is connected with the wire G. Just out of contact with the diaphragm -is an insulated screw, H, connected with the wire I. These wires lead -to the time-lock, already described, on the sending apparatus at the -station. When no carrier is passing, the air-pressure is the same on -both sides of the diaphragm, but when a carrier enters that part of the -tube between the two points where the pipes C and D are connected, the -equality of pressure on opposite sides of the diaphragm is destroyed. -There is always a slightly greater pressure in rear of the carrier than -in front of it, equal to the frictional resistance of the carrier in -the tube. It is this difference of pressure in front and in rear of -the carrier that moves it through the tube. When the carrier is in<span class="pagenum"><a name="Page_94" id="Page_94">94</a></span> -the position shown in the figure, the same difference of pressure will -exist on opposite sides of the diaphragm, and it will be deflected into -contact with the screw H, thereby closing the electric circuit. When -the carrier has passed, equality of pressure on opposite sides of the -diaphragm is established and the diaphragm takes its normal position, -out of contact with the screw H. This apparatus is easily attached to -the tube, and it contains no mechanism to get out of order.</p></div> - -<div class="figcenter" id="fig_33"> -<img src="images/i_138.jpg" width="600" height="299" alt="" /> -<p class="caption"><span class="smcap">Fig. 33.</span><br /> -OPEN RECEIVER.</p> -<p class="largeimg"><a href="images/i_138_large.jpg" rel="nofollow">Larger image</a> (107 kB)</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Open_Receiver"><b>The Open Receiver.</b></h3><p class="inline">—Wherever the pressure in the tube is down -nearly to atmospheric, we can use an open receiver to discharge the -carriers from the tube. This is a receiver that opens the tube to -the atmosphere and allows the carrier to come out. Such a receiver -is used at the main post-office in the Philadelphia postal-line, and -was described in the last chapter. The present receiver is similar in -operation, but contains some improvements in details. Fig. 33 is a side -elevation of the apparatus, Fig. 34 is a longitudinal section, and -Fig. 35 is a cross-section through the cylinder and valve, showing the -sluice-gate.</p></div> - -<div class="figcenter" id="fig_34"> -<img src="images/i_139.jpg" width="600" height="268" alt="" /> -<p class="caption"><span class="smcap">Fig. 34.</span><br /> - -OPEN RECEIVER.—LONGITUDINAL SECTION.</p> -<p class="largeimg"><a href="images/i_139_large.jpg" rel="nofollow">Larger image</a> (87 kB)</p> -</div> - -<div class="figcenter" id="fig_35"> -<img src="images/i_140.jpg" width="330" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 35.</span> - -OPEN RECEIVER.—SLUICE-GATE MECHANISM.</p> -<p class="largeimg"><a href="images/i_140_large.jpg" rel="nofollow">Larger image</a> (193 kB)</p> -</div> - -<p>Referring to the longitudinal section, the apparatus is attached to -the end of a pneumatic tube, A. The current of air from the tube -A flows through the slots B into a pipe, C, that conducts it to a -tank near the air-compressor. About the centre of the apparatus is a -sluice-gate, E, that is raised and lowered by a piston in a vertical -cylinder, F, located just above the sluice-gate. This piston is moved -by air-pressure taken from some part of the system. When a carrier -arrives from the tube A, it passes over the slots B and runs into the -air-cushion D, where it comes gradually<span class="pagenum"><a name="Page_98" id="Page_98">98</a></span> to rest. Checking the -momentum of the carrier compresses the air in front of it considerably, -and this excess of pressure is utilized to move a small slide-valve -that controls the movement of the piston in the cylinder F, 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 table. The -small pipe G conducts a small portion of the air compressed in front -of the retarded carrier to the controlling valve, H, seen in Figs. 33 -and 35. Referring now to the section of the valve and cylinder, Fig. -35, the pipe G enters the top of a small valve-cylinder containing a -hemispherical piston, I, that is held up by a spiral spring, J. This -spring has just sufficient tension to hold the piston I up against -the normal pressure of air in the tube. When a carrier arrives -and compresses the air in the air-cushion, the excess of pressure -forces the piston I down against the spring J, and moves the piston -slide-valve K. This change of position of the slide-valve allows the -air in the cylinder F to escape to the atmosphere through the passage -L, passage P, and pipe M, 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 F. This moves the piston up, carrying with -it the sluice-gate E.</p> - -<p>There is just sufficient pressure in the tube in rear of the carrier -to push the carrier past the gate and on to the table. As the carrier -moves out it raises a finger, Q, Fig. 34, that projects into its -path. Raising this finger extends the spring R, Fig. 33, and rotates -the lever S, bringing the pawl T under the end of the controlling -valve-stem.<span class="pagenum"><a name="Page_99" id="Page_99">99</a></span> When the carrier has passed out and the finger Q is free -to descend, the spring R rotates the lever S back to its original -position, and thereby raises the controlling slide-valve, which causes -the sluice-gate to close. By having the upward motion of the finger Q -simply extend the spring R, and the downward motion, by the force of -the spring, move the valve, 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 out. If raising the finger Q moved -the valve, then when the first carrier passed out, the gate would close -down upon the second. Attached to the receiving apparatus and extending -beyond it is a tube, U, cut away upon one side so that the carriers can -roll out of it on to a table, and having in the end a buffer to stop -the carriers if by any accident they come out of the tube with too much -speed. This buffer consists of a piston covered with several layers -of leather and having a stiff spring behind it. The whole apparatus -is supported from the floor upon suitable standards, and, for an -eight-inch tube, occupies a floor-space twelve feet long by two feet -wide, not including the table.</p> - -<p>This is the simplest form of receiving apparatus. Owing to conditions -of pressure already explained, its use is confined principally to the -pumping stations. The only care that it requires is an occasional -cleaning and oiling.</p> - -<div class="figcenter" id="fig_36"> -<img src="images/i_143.jpg" width="600" height="375" alt="" /> -<p class="caption"><span class="smcap">Fig. 36.</span><br /> - -CLOSED RECEIVER.</p> -<p class="largeimg"><a href="images/i_143_large.jpg" rel="nofollow">Larger image</a> (222 kB)</p></div> - -<div class="figcenter" id="fig_37"> -<img src="images/i_144.jpg" width="600" height="436" alt="" /> -<p class="caption"><span class="smcap">Fig. 37.</span><br /> - -CLOSED RECEIVER.—LONGITUDINAL SECTION.</p> -<p class="largeimg"><a href="images/i_144_large.jpg" rel="nofollow">Larger image</a> (196 kB)</p> -</div> - -<div class="section-no-break"><h3 class="inline" id="The_Closed_Receiver"><b>The Closed Receiver.</b></h3><p class="inline">—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 -atmosphere, so much so that the tube cannot be opened to allow the -carrier<span class="pagenum"><a name="Page_102" id="Page_102">102</a></span> to pass out without an annoying blast of air and a high -velocity of the carrier. This apparatus is similar to the receiver used -in the sub-post-office of the Philadelphia postal line, but contains -several modifications and improvements tending towards simplification. -Fig. 36 shows it in elevation, and Fig. 37 in longitudinal section. -As in the open receiver just described, the air from the tube A is -deflected through slots B into a branch pipe, C, that conducts it from -the receiving apparatus to the sending apparatus and return tube. The -carriers arrive from the tube A, pass over the slots B, where the air -makes its exit, and run into an air-cushion, D. This air-cushion is -a tube about twice the length of the carrier, closed at one end, and -supported upon trunnions. When the carrier has been brought to rest, -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 out; the tube -then returns to its original position. If the end of the air-cushion -was closed perfectly tight the carrier, after coming to rest, would -rebound and might be caught in the joint between the stationary and -movable parts of the apparatus, when the air-cushion tube tilted. To -prevent the rebounding of the carrier a relief-valve, E, has been -placed in the head of the air-cushion tube. It is held closed against -the normal pressure in the tube by a spiral spring, 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 G, down the pedestal H, to the atmosphere. When the air-cushion -or receiving tube D is tilted to discharge a carrier,<span class="pagenum"><a name="Page_103" id="Page_103">103</a></span> the circular -plate I covers the end of the main tube. In order to prevent carriers -sticking in the receiving tube when it is tilted, and to insure their -prompt discharge, the pipe J is provided. In the tilted position of -the receiving tube, the end of this pipe coincides with the end of the -main tube, from which it receives air to hasten the discharge of the -carrier. A check-valve, K, prevents the air from flowing backward in -this pipe when a carrier is being received in the air-cushion chamber. -The opening of this check-valve can be adjusted by a screw, thereby -regulating the speed of ejection of the carrier.</p></div> - -<div class="figcenter" id="fig_38"> -<img src="images/i_147.jpg" width="432" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 38.</span><br /> - -INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.</p> -<p class="largeimg"><a href="images/i_147_large.jpg" rel="nofollow">Larger image</a> (292 kB)</p> -</div> - -<div class="figcenter" id="fig_39"> -<img src="images/i_148.jpg" width="495" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 39.</span><br /> - -INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.—VERTICAL -SECTION.</p><p class="largeimg"><a href="images/i_148_large.jpg" rel="nofollow">Larger image</a> (326 kB)</p> -</div> - -<p>The carrier is discharged down a chute, L, which has a buffer at the -bottom, and from the chute it rolls off on to a table. The buffer is -made similar to the buffer in the open receiver already described. -The cylinder and piston M, that operate to tilt the receiving tube D, -are supported upon the base of the apparatus under the closed end of -the receiving tube. The cross-head of the piston- and connecting-rods -travels between guides that are made a part of the upper cylinder -head. The movement of the piston in the cylinder M is controlled by -a piston slide-valve exactly similar to the one shown in Fig. 35. -The slide-valve is moved, in the same manner, by the air compressed -ahead of the carrier when it is brought to rest in the air-cushion -D. The air is conducted from the air-cushion to the controlling -slide-valve through a small pipe, N, Fig. 36. This pipe leads to one -of the trunnions, where it has a joint to allow for the tilting of -the receiving tube. When the carrier is discharged from the receiving -tube, it raises a finger, O, Fig. 37, located just outside the tube. -Raising<span class="pagenum"><a name="Page_106" id="Page_106">106</a></span> this finger pulls the rod P, Fig. 36, extends the spring -Q, turns the lever R, and catches the pawl S, under the end of the -controlling valve stem. When the carrier has passed down the chute and -allowed the finger O to drop down, the spring Q turns the lever R back -to its original position and moves the controlling valve. This causes -the receiving tube to return to a horizontal position, where it is -ready to receive the next carrier.</p> - -<p>At first this apparatus may seem a little cumbersome, but nothing could -work better. It is certain in its action and almost noiseless. Carriers -are received, discharged, and the receiving tube returned to its normal -position in four seconds, and it can be done in less time if necessary.</p> - -<div class="section-no-break"><h3 class="inline" id="The_Intermediate_Station_Receiving_and_Transfer_Apparatus"><b>The Intermediate Station Receiving and Transfer Apparatus.</b></h3><p class="inline">—One -other form of receiving apparatus remains to be described, 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 station. A side elevation of the apparatus is shown in -Fig. 38 and a vertical section in Fig. 39. The tubes are led into an -intermediate station, carried upward, and then, with a bend of one -hundred and eighty degrees, are connected to the top of the receiving -and transfer apparatus, as shown in the diagram, Fig. 26. The object of -this arrangement will be seen as we describe the apparatus. Referring -to the sectional drawing, Fig. 39, the connection of the tube A is seen -at the top. As in the other receivers, the current of air arriving -from the tube A is deflected through slots, B, into a passage, C, made -in the frame of the apparatus. From this passage it enters the tube D -through<span class="pagenum"><a name="Page_107" id="Page_107">107</a></span> the slots E. The tube D leads to the sending apparatus and -on to the next station, as seen in Fig. 26. The carriers are received -in a closed section of tube F, which forms an air-cushion, similar to -the closed receiver last described. This receiving tube F is made a -part of what we might term a wheel. This wheel fits accurately into a -circular casing and is supported by two trunnions or axles, upon which -it revolves. The wheel has a broad flat rim, G, that covers the end of -the tube at H when the wheel is revolved, and, in the normal position -in which it is shown in the figure, covers the interior openings I, J, -K, and L, in the casing. 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 casing. From the bottom of the receiving -tube F a passage, M, leads past a check-valve, N, to the tube D. When -a carrier arrives from the tube A, it descends into the receiving tube -F, compressing the air in front of it. This compressed air begins to -escape through the passage M, but the high velocity of it closes the -check-valve N as much as possible. A stop on the stem of the valve -prevents it being closed entirely. The small opening past the valve -allows some of the air to pass, thereby preventing the carrier from -rebounding on the air-cushion. As soon as the carrier has come to rest, -the check-valve N, by its own weight, opens wide, and the carrier, by -its weight, settles gradually down to the bottom of the receiving tube. -The wheel containing the receiving tube and the carrier will then be -revolved by the cylinder and piston O, which is operated by compressed -air taken from the tube through the pipe P. If the carrier<span class="pagenum"><a name="Page_108" id="Page_108">108</a></span> is for this -station, the wheel will rotate through an angle of forty-five degrees -and discharge the carrier through the opening J, down the chute Q, from -which it will roll on to a table arranged to receive it. If, however, -the carrier is intended for some other station, the wheel will rotate -through an angle of ninety degrees and discharge the carrier through -the opening K into the tube D, and it will go on its way to the next -station. This selection of carriers is brought about in a comparatively -simple manner. At the bottom of the receiving tube F there are two -vertical needles, R and S, shown upon a larger scale in Fig. 40. 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 -apparatus. Wires <i>a</i> and <i>b</i> make connection with the needles through -metal plugs that form a guide for the needles, and through the springs -U and V. Directly below the needle R is an insulated spring clip, W, -held by two bolts and connected to the wire <i>e</i>.</p></div> - -<div class="figcenter" id="fig_40"> -<img src="images/i_152.jpg" width="409" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 40.</span><br /> -A DETAIL OF THE INTERMEDIATE STATION RECEIVING AND TRANSFER -APPARATUS.</p> -<p class="largeimg"><a href="images/i_152_large.jpg" rel="nofollow">Larger image</a> (360 kB)</p></div> - -<p>The end of a carrier is represented at T. As the carrier settles down -to the bottom of the receiving tube, it comes in contact with the -ends of the needles and presses them down, they being supported by -two springs U and V. As the needle R is moved down, it makes contact -with the spring clip W, located just below it, and closes an electric -circuit that includes the electro-magnet X, Figs. 38 and 39, on the -valve of the rotating cylinder O. When this electro-magnet is excited -it attracts its armature and moves the piston slide-valve Y, that -admits air to the top of the piston in the cylinder O, and allows the -air under the piston to escape to the atmosphere. The piston moves<span class="pagenum"><a name="Page_110" id="Page_110">110</a></span> -downward and revolves the wheel by means of a connecting rod.</p> - -<p>Upon the end of the carrier T is placed a thin circular metal disk, -<i>f</i>, which may be copper, brass, tin-plate or any metal that is not -easily oxidized. The diameter of this disk of metal determines the -station at which the carrier will be discharged from the tube. Disks of -various diameters, that may be attached to the carrier, are represented -by dashed lines, <i>g</i>, in Fig. 40. When the carrier comes in contact -with the two needles R and S, if the circular metal disk on the front -end of the carrier has a diameter sufficient to span the space between -the two needles, in the position in which it is held, then an electric -circuit, made by the wires <i>a</i> and <i>b</i>, will be closed through the -needles and the metal disk on the carrier. The metal disk makes a -short-circuit from one needle to the other. If the metal disk is not -large enough to span the distance between the two needles, then the -electric circuit remains broken.</p> - -<p>Returning again to Fig. 39, we have the opening J, where the carriers -are discharged, closed by a sluice-gate. This gate is opened and -closed by a piston moving in a cylinder, <i>h</i>, shown in Fig. 38. A -piston slide-valve, <i>i</i>, similar in all respects to the valve on the -cylinder O, controls the movement of the piston in this cylinder and -the sluice-gate to which it is attached. The slide-valve is moved in -one direction, that opens the sluice-gate, by an electro-magnet in the -circuit of the wires <i>a</i> and <i>b</i>, Fig. 40.</p> - -<p>When the electric circuit made by these wires is closed by a disk -on the front end of a carrier, short-circuiting the<span class="pagenum"><a name="Page_111" id="Page_111">111</a></span> two needles, -the valve is moved by the electro-magnet in the circuit, and the -sluice-gate is opened. As the wheel, including the receiving tube and -carrier, revolves, a lug, <i>j</i>, Fig. 38, on the outside of the wheel -comes in contact with the open sluice-gate and the wheel can rotate -no farther. A blast of air through the valve L, Fig. 39, assisted by -gravity, pushes the carrier out of the receiving tube, through the -opening J and down the chute Q, on to the receiving table.</p> - -<p>Had the disk on the front end of the carrier been too small to span -the distance between the two needles, the circuit would not have been -closed, the sluice-gate would not have been opened, no obstruction -would have been placed in the path of the lug <i>j</i>, on the wheel, and -the wheel would have continued its rotation through ninety degrees -until the receiving tube F came in line with the tube D. During the -latter part of the rotation, a pin on the wheel engages a lever, <i>k</i>, -Fig. 38, and turns a valve, <i>l</i>, Fig. 39, stopping the flow of air -through the passage C, compelling it to take another route through -the passage <i>m</i>, and the receiving tube F, taking with it the carrier -into the tube D. When the carrier leaves the receiving tube and passes -through either of the openings J or K, it engages one of the fingers, -<i>n</i> or <i>o</i>, that lie in its path. These fingers are connected by rods -and levers to the valves on the rotating and sluice-gate cylinders. -The ejected carrier pushes these fingers to one side, and after it has -passed the fingers return, by the force of a spring, to their former -position and move the valves, causing the sluice-gate to close and the -wheel to rotate backward into its normal<span class="pagenum"><a name="Page_112" id="Page_112">112</a></span> position ready to receive -the next carrier. The connection between the fingers and the valves is -similar to the mechanism on the open and closed receivers, so need not -be described in detail here.</p> - -<p>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 -L, which can be opened or closed by a hand-wheel, <i>p</i>. Before the -wheel and receiving tube can be rotated, the needles must be withdrawn -from the receiving tube, and this is accomplished by a small cylinder -and piston, <i>q</i>, shown in Fig. 40. The needles and their encasement -are attached to a cross-head, <i>r</i>, on the end of a hollow piston-rod, -<i>s</i>. When air is admitted to the top of the piston in the rotating -cylinder O, Fig. 39, it is also admitted through the pipe <i>t</i>, Fig. -38, to the cylinder and upper side of the piston <i>q</i>, Fig. 40. This -moves the piston <i>q</i> down against the force of a spring, <i>u</i>, and -withdraws the needles from the receiving tube. This takes place after -the needles have served their purpose and before the wheel is rotated. -The piston <i>q</i> has much less inertia than the wheel, therefore it -moves much quicker. When the wheel begins to rotate it closes a valve, -<i>v</i>, in the pipe <i>t</i>, Fig. 38, confining the air in the cylinder <i>q</i>, -and preventing the needles from being raised by the spring <i>u</i> before -the wheel returns to its normal position. If by any accident the -needles should be raised, no serious harm would result, for their ends -would simply bear against the face of the wheel. If this took place -constantly, grooves might be worn in the face of the wheel; for this -reason the valve <i>v</i> is provided.</p> - -<p>In order to facilitate the inspection of the needles and<span class="pagenum"><a name="Page_113" id="Page_113">113</a></span> electric -contact springs W, they are contained in a cylindrical brass case, -<i>w</i>, that is held in place beneath the receiving tube by two bolts. By -removing the nuts from these bolts the entire mechanism can be removed, -examined, and cleaned. It also gives easy access to the receiving tube. -The receiving tube is long enough to receive two carriers, if it should -ever happen that two arrive at the same time.</p> - -<div class="figcenter" id="fig_41"> -<img src="images/i_157.jpg" width="600" height="405" alt="" /> -<p class="caption"><span class="smcap">Fig. 41.</span><br /> -DIAGRAM OF CONTACT-DISKS AND NEEDLES.</p> -<p class="largeimg"><a href="images/i_157_large.jpg" rel="nofollow">Larger image</a> (101 kB)</p></div> - -<p>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 -carriers, a diagram, Fig. 41, has been made, in which the needles at -the bottom of the receiving tubes of the apparatus at six intermediate -stations are represented at A, B, C, D, E, and F. Six disks of -different sizes are represented at <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, <i>e</i>, and <i>f</i>. -The needles are placed farthest apart at station A and nearer together -at each succeeding station until we arrive at station F, where they -are nearest together. If we wish to send a carrier to station A from -the central, we place the largest disk, <i>a</i>, upon the front end of it. -When it arrives at station A, it closes the electric circuit between -the needles and is discharged from the tube. Should we wish to send a -carrier to station D, then we place the disk <i>d</i> upon the front end of -it. When the carrier arrives at the station A, the disk is not large -enough to span the needles; therefore the sluice-gate is not opened -and the carrier is sent on in the tube. When it arrives at stations -B and C, the same thing occurs again, but when it reaches station D, -the needles are sufficiently close together so that the disk makes an -electric circuit between them, and the carrier is discharged from<span class="pagenum"><a name="Page_115" id="Page_115">115</a></span> the -tube, as was intended when despatched. Since the carriers always travel -in the same direction in a tube, the first station at which they arrive -where the needles are near enough together to have both touch the disk, -will be the station at which the carrier was intended to stop. Carriers -can be despatched from any station, but if we wish to send from say D -to A, they must either travel around a loop or be sent through a return -tube in which the needles are arranged in the reverse order. If no disk -is placed on the carrier, it will go to the last station on the line.</p> - -<p>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 line. We have worked out two other systems which are entirely -mechanical in their operation, not using electric circuits and -electro-magnets to move the valves. While such a mechanical system has -some advantages over the present combined mechanical and electrical -system, yet there is one great advantage in the latter, and that is the -simplicity of the attachment made to the carrier. A round flat disk of -tin-plate is attached to the front end; it is something that is not -in the way; it does not prevent standing the carriers on end in racks -to fill them; it is not easily injured, and only those who have had -experience can realize the rough usage that the carriers receive; it -is quickly and easily attached to the carrier, and it is so cheap that -when bent it can be thrown away.</p> - -<div class="section-no-break"><h3 class="inline" id="Carriers"><b>Carriers.</b></h3><p class="inline">—The carriers are similar in all respects to those -used in the Philadelphia postal-line, that have been described in the -preceding chapter and illustrated in Figs. 18, 19, and 20. When there -are intermediate stations upon<span class="pagenum"><a name="Page_116" id="Page_116">116</a></span> the lines, means are provided for -attaching disks to the front end of the carriers. The disks have a -central stem that secures them to the bolt in the centre of the head, -and are so arranged that they can be quickly attached or removed.</p></div> - -<p>Many experiments have been made to find the best material for -bearing-rings, but thus far nothing better than a specially-prepared -woven fabric has been found. These rings will run about a thousand -miles, when they become so reduced in diameter that they have to be -replaced by new ones.</p> - -<p>The most essential elements of a carrier are strength, lightness, and -security of the contents. Aluminum has frequently been proposed as a -suitable material for the bodies of carriers, but for the same weight -steel is much stronger, especially in thin rolled sheets, and for this -reason it has been used.</p> - -<p>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 carriers. We believe that the one which has been -adopted fulfils all requirements in a satisfactory manner.</p> - -<p>Some experiments have been made with carriers that open on the side, -but structurally they are weak and unsuited to stand the blows that -carriers frequently receive. They are not so easily and quickly filled -and emptied as those that open on the end. These remarks apply to -carriers for large tubes. In small tubes for the transportation of cash -in retail stores, carriers with side openings are found convenient.</p> - -<p><span class="pagenum"><a name="Page_117" id="Page_117">117</a></span></p> - -<p>When United States mail is sent through tubes not used exclusively for -postal service, carriers with special locks can be used, so that they -can be opened only by post-office employees.</p> - -<div class="section-no-break"><h3 class="inline" id="Air_Supply"><b>Air Supply.</b></h3><p class="inline">—This completes the description of the special -apparatus used in this system, but we have yet to say something -regarding the machines that supply the air. In Paris the water from the -city mains has been used to compress or exhaust the air used in small -tubes, but to operate large tubes in most of our cities steam is the -only available power. Except in isolated cases, an independent steam -plant will be erected to supply the air for a system of tubes. This -plant should be designed with a view to obtaining the maximum economy -in coal consumption, labor, water, cartage, and incidental expenses. -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 plants.</p></div> - -<p>Three types of blowing machines are used,—viz., centrifugal fans, -positive blowers, and air-compressors.</p> - -<div class="section-no-break"><h3 class="inline" id="Fans"><b>Fans.</b></h3><p class="inline">—Very large tubes of moderate length can be operated by -ordinary centrifugal fans. These fans are capable of supplying air -under a pressure not exceeding ten or twelve ounces per square inch -with very good efficiency. They are the simplest and most inexpensive -of all blowing-machines.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Blowers"><b>Blowers.</b></h3><p class="inline">—When tubes have a length and diameter that require a -pressure from one to four pounds per square inch, some form of positive -blower of the Root type can<span class="pagenum"><a name="Page_118" id="Page_118">118</a></span> be used with economy. Their construction -is familiar to nearly every one at all interested in machinery, so we -need give no space to their description here.</p></div> - -<div class="figcenter" id="fig_42"> -<img src="images/i_162.jpg" width="600" height="357" alt="" /> -<p class="caption"><span class="smcap">Fig. 42.</span><br /> - -THE STURTEVANT STEEL PRESSURE BLOWER.</p> -<p class="largeimg"><a href="images/i_162_large.jpg" rel="nofollow">Larger image</a> (324 kB)</p></div> - -<div class="figcenter" id="fig_43"> -<img src="images/i_163_1.jpg" width="600" height="313" alt="" /> -<p class="caption"><span class="smcap">Fig. 43.</span><br /> - -ROOT’S POSITIVE PRESSURE BLOWER.</p></div> - -<div class="figcenter" id="fig_44"> -<img src="images/i_163_2.jpg" width="600" height="532" alt="" /> -<p class="caption"><span class="smcap">Fig. 44.</span><br /> - -SECTION OF ROOT’S TRUE CIRCLE BLOWER.</p></div> - -<div class="figcenter" id="fig_45"> -<img src="images/i_164_1.jpg" width="600" height="346" alt="" /> -<p class="caption"><span class="smcap">Fig. 45.</span><br /> - -THE GREEN BLOWER.</p></div> - -<div class="figcenter" id="fig_46"> -<img src="images/i_164_2.jpg" width="600" height="452" alt="" /> -<p class="caption"><span class="smcap">Fig. 46.</span><br /> - -SECTION OF THE GREEN BLOWER.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Air_Compressors"><b>Air-Compressors.</b></h3><p class="inline">—By far the greater number of our tubes require -an air-pressure of more than five pounds per square inch. For such -air supply we recommend some form of air-compressor, and usually this -is driven by a steam-engine, which forms a part of the compressor. -In making our selection we should bear in mind the conditions under -which the compressor will run. Usually it must be kept in constant -operation at least ten hours per day, and frequently for a much longer -period. This makes it important that the compressor be substantially -built and supported upon a solid and firm foundation. The bearings -should be broad, of good wearing material that has a low coefficient of -friction, and provided at all times with ample lubrication. If poppet -valves are used in the air-cylinders, and they are most common, the -speed in revolutions per minute should not be high. Duplex are better -than single cylinder compressors, because they deliver the air in a -more steady stream,—the pulsations are less. For constant running, -economy of steam is an important item; therefore some good type of -cut-off valve should be provided. The air-cylinders should not be -water-jacketed unless the pressure is above twenty-five pounds per -square inch. It is better to use the air as warm as possible, for it -will soon be cooled after entering the tube. A speed-governor should -be provided with compressors which are to run at constant speed, but -usually they will be run to maintain a constant pressure in the tank,<span class="pagenum"><a name="Page_119" id="Page_119">119</a></span> -and to this end a good and reliable form of pressure-governor should -be provided, together with some reliable safety device to stop the -engine when the speed exceeds a safe limit. But most important of all -is to have the valves of the air-cylinders large in area; otherwise -the efficiency of the machine will be very low. With machines working -under eighty pounds pressure, a difference in pressure of one pound -on opposite sides of the valves has but little effect, but when -the machine is only compressing to five or ten pounds, one pound -is a very large proportion of the total pressure and reduces the -efficiency. Besides these few suggestions, only the requirements<span class="pagenum"><a name="Page_122" id="Page_122">122</a></span> of -good engineering need be demanded. In Figs. 42, 43, 44, 45, 46, and -47 we show a fan, two blowers, and an air-compressor suited to the -requirements of pneumatic-tube service that can be found in the market, -and that are built by responsible concerns. We believe they are all -good of their kind, but do not recommend any particular make.</p></div> - -<div class="section-no-break"><h3 class="inline" id="The_Tube_Line_Construction_etc"><b>The Tube, Line Construction, etc.</b></h3><p class="inline">—Up to the present time -we have found no material better suited for the straight parts of -pneumatic tubes than cast iron, machined upon the interior. It gives a -smooth and accurate tube. It can be made in most convenient lengths. It -is strong and not easily deformed. The bell-joint, calked with lead and -oakum, having the tubes fitted together male and female at the bottom -of the bell, is the best joint yet devised for pneumatic tubes. It is -slightly yielding, accommodating itself to slight changes of length -of tube due to changes of temperature, and it allows slight bends -to be made at each joint. The joints are very accurate, presenting -no shoulders to obstruct the passage of carriers. The joints can be -made by men accustomed to laying water- and gas-pipe. The cast iron -is so stiff that it is not distorted in calking, as may be done with -wrought-iron tube. The principal objections to its use are the expense -of boring and the readiness with which it corrodes upon the interior.</p></div> - -<div class="figcenter" id="fig_47"> -<img src="images/i_168.jpg" width="600" height="379" alt="" /> -<p class="caption"><span class="smcap">Fig. 47.</span><br /> -RAND COMPOUND COMPRESSOR OF MODERATE SIZE.</p> -<p class="largeimg"><a href="images/i_168_large.jpg" rel="nofollow">Larger image</a> (570 kB)</p></div> - -<p>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 them, but our experiments thus far have been discouraging. It -may be<span class="pagenum"><a name="Page_123" id="Page_123">123</a></span> that some of the new processes of making tubes will give us -what we want, but we have not yet found it.</p> - -<p>Small tubes and the short bends of large tubes are made of brass, it -being the most suitable material. It would be very difficult to bend -iron tubes without involving great expense. 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 inch.</p> - -<p>Where the ground is firm, no other support is needed for the tubes -than to tamp the earth solidly about them. In order to economize space -in the streets, it is customary to lay the tubes one above the other; -and it is very convenient, although not necessary, to separate them by -cast-iron saddle brackets. Such an arrangement has to be frequently -departed from in order to overcome obstructions in the streets and to -get through narrow passages. At all low points in a tube line, traps -are provided to catch any moisture that may accumulate. These traps -are made accessible for frequent inspection by means of man-holes -or otherwise. The tube is usually laid about three feet below the -pavement. This distance has frequently to be varied, but it never -becomes so small as to render the tubes liable to injury from heavy -trucks passing over the pavement.</p> - -<hr class="chap" /> -<div class="section"> -<p><span class="pagenum"><a name="Page_124" id="Page_124">124</a></span></p> - -<h2 class="nobreak" id="CHAPTER_IV">CHAPTER IV.<br /> - -<br /><span class="smaller">FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC TUBES.</span></h2></div> - -<p><span class="smcap">We</span> will now discuss, in an elementary way, the theory of pneumatic -tubes, in order to understand more clearly their <i>modus operandi</i> and -the principles upon which they should be constructed. Let us begin with -the definition of a pneumatic tube.</p> - -<div class="section-no-break"><h3 class="inline" id="Definitions"><b>Definitions.</b></h3><p class="inline">—A pneumatic tube is a tube containing air. This -is perhaps the broadest and most comprehensive definition that can -be given, but we usually associate with the idea of a pneumatic tube -the use to which it is put. 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 air. This is still a very -broad definition, including all kinds of material for transportation, -for every conceivable purpose. It places no limit upon the dimensions -of the tube nor the manner of its operation. This definition would -include the toy commonly known as a putty-blower, and the pneumatic gun.</p></div> - -<p>These instruments are not usually pictured in our minds when we hear -or see the term pneumatic tube used. Instead of these, 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 desk. Again narrowing our definition to<span class="pagenum"><a name="Page_125" id="Page_125">125</a></span> -conform more nearly with the mental picture presented, 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 tube. This, like all -definitions, is not entirely satisfactory, if we examine it critically, -but it will answer our present purpose.</p> - -<div class="section-no-break"><h3 class="inline" id="Intermittent_and_Constant_Air_Current"><b>Intermittent and Constant Air-Current.</b></h3><p class="inline">—Having thus defined -a pneumatic tube, there are two ways in which we may operate it to -transport our carriers containing mail, packages, or other matter. The -first method consists in storing our compressed air in a suitable tank, -or by exhausting the air from the tank; then, when we wish to despatch -a carrier we place it in the tube and connect the tube with the tank by -opening a valve. As soon as the carrier arrives at the distant end of -the tube the valve is closed and the air soon ceases to flow. When a -long interval of time elapses between the despatching of carriers, this -is the most economical method of operation, 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 carrier.</p></div> - -<p>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 time. It is analogous to launching boats -in a rapidly flowing stream, allowing them to float down stream and -then withdrawing them. When the boats are in the stream they present -little obstruction to the flow of water and check its speed but very -little. In order to<span class="pagenum"><a name="Page_126" id="Page_126">126</a></span> compute the speed with which the boat will pass -from one point to another, we only have to know the speed of the stream -between those points when no boat is in it. The presence of the boat -does not change the speed appreciably. So it is with carriers in a -pneumatic tube: they are carried along with the current of air. The air -flows nearly as rapidly when a carrier is in the tube as when there -is none. The friction of the carrier against the inner surface of the -tube creates a slight drag, but it checks the speed of the air only a -little. Therefore, in order to know the speed with which a carrier will -be transported from one station to another through a pneumatic tube, -we need only to know the velocity with which the air flows through the -tube when no carrier is present. Of course there are special cases of -heavy carriers, or carriers having a large amount of friction from -their packing, or of tubes not laid horizontally, where the resistance -of the carrier must be taken into consideration, but for our present -purpose we will neglect all of these conditions.</p> - -<div class="section-no-break"><h3 class="inline" id="Laws_Governing_the_Flow_of_Air_in_Long_Tubes"><b>Laws Governing the Flow of Air in Long Tubes.</b></h3><p class="inline">—This leads us -to study the laws governing the flow of air in long tubes, omitting -for the present the presence of a carrier. Since tubes operated -intermittently have become obsolete, we will only consider the case -of a constant current of air, this being what we have to deal with in -practice.</p></div> - -<div class="figcenter" id="fig_48"> -<img src="images/i_174.jpg" width="600" height="408" alt="" /> -<p class="caption"><span class="smcap">Fig. 48.</span><br /> - -PRESSURE AND VELOCITY CURVES.</p> -<p class="largeimg"><a href="images/i_174_large.jpg" rel="nofollow">Larger image</a> (152 kB)</p> -</div> - -<p>In order to make our ideas and thoughts as clear as possible let us -represent them by a diagram, Fig. 48. We will suppose that a tank, A, -is kept constantly filled with compressed air at a pressure of ten -pounds per square<span class="pagenum"><a name="Page_128" id="Page_128">128</a></span> inch, from some source of supply. We will suppose -that the pressure of the air in this tank never changes, air being -supplied as fast as it flows away. Next, let us assume that a tube -eight inches in diameter inside and one mile long (five thousand two -hundred and eighty feet) is connected to the tank at one end and left -open to the atmosphere at the other. The air will flow in a constant -stream from the tank into the atmosphere, for the reason that air is -being supplied to the tank as fast as it flows away.</p> - -<div class="section-no-break"><h3 class="inline" id="Law_of_Pressure"><b>Law of Pressure.</b></h3><p class="inline">—First, let us consider the pressure of the -air at various points in the tube. We will, for convenience, represent -the pressure in the tank by a vertical line, D E, ten units in length, -since the pressure is ten pounds per square inch. Now let us go to a -point on the tube one quarter of a mile (one thousand three hundred -and twenty feet) from the tank, drill a hole in the tube, attach a -pressure-gauge and measure the pressure of the air at this point. -We shall find it to be about 7.91 pounds per square inch; or, 2.09 -pounds below the pressure in the tank. We will represent this on our -diagram by another vertical line, F G, having a length of 7.91 units. -Again let us measure the pressure in the tube at a point one-half a -mile (two thousand six hundred and forty feet) from the tank. Here we -find it to be about 5.61 pounds per square inch, and we represent it -by the vertical line, H I, having 5.61 units of length. We note that -the pressure is 4.39 pounds below the pressure in the tank. We are at -the middle point of the tube and the pressure has fallen to nearly, -but not quite, one-half the pressure in the tank. We will now go to -a point three-quarters of a mile (three thousand nine<span class="pagenum"><a name="Page_129" id="Page_129">129</a></span> hundred and -sixty feet) from the tank, and here the pressure is about 3.01 pounds -per square inch. We represent it by the vertical line, J K. Lastly, -we measure the pressure very near the end of the tube, one mile from -the tank, and find it to be about zero, or the same as the pressure of -the atmosphere. All of our measurements have been in pounds above the -atmospheric pressure; to express them in absolute pressure, we should -add to each the pressure of the atmosphere, which is 14.69 pounds, -nearly.</p></div> - -<p>Now we will draw a smooth curve through the tops of all our vertical -lines, and we have a curve, E, G, I, K, L, representing the pressure in -the tube at every point. It falls gradually from ten pounds to zero, -but it does not fall in exact proportion to the distance from the tank. -Such a fall of pressure would be represented by the straight dash-line, -E, L. The reason why the true pressure-curve is not a straight line, -and lies above a straight line, is because air is an elastic fluid and -expands, becoming larger in volume as the pressure diminishes. The -straight dash-line represents the fall of pressure of an inelastic -fluid, like water, when flowing in the tube.</p> - -<p>The fall of pressure along the tube is analogous to the fall of level -along a flowing stream. In fact, we frequently speak of the descent of -a stream as the “head of water” when it is used for power purposes, -and we mean by this the pressure the water would exert if it were -confined in a pipe. The descent, or change of level, in the bed of a -stream is necessary to keep the water flowing against the friction of -the banks. The descent of the water imparts<span class="pagenum"><a name="Page_130" id="Page_130">130</a></span> energy to overcome the -friction. In a similar manner, we must have a fall of pressure along -the pneumatic tube to overcome the friction of the air against the -interior surface of the tube. We find another analogue in the flow of -the electric current along a wire; here there is a fall of potential -necessary to overcome the resistance of the wire. Since power has to be -expended to compress the air and impart to it its pressure, when this -pressure disappears we know that the air must be losing its energy or -doing work, and we look to see what becomes of it. In the present case, -we find that most of this work is expended in overcoming the friction -between the air and the surface of the tube.</p> - -<div class="section-no-break"><h3 class="inline" id="Uses_of_Pressure_Curves"><b>Uses of Pressure Curves.</b></h3><p class="inline">—The pressure curve teaches us many -things. Suppose we were to establish stations on this tube at the -quarter, half, three-quarter, and mile points; we see at once that -intermediate-station or closed receivers, described in the last -chapter, must be used at all of the stations except the mile point at -the end of the tube, 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 out, but at the end of the tube we could use the open -receiver. In designing our sending and receiving apparatus for each -station, we look to this pressure curve to tell us the pressure which -we shall have on the pistons in our cylinders, and are thereby enabled -to make them with proper proportions for the work that they have to do.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Law_of_Velocity"><b>Law of Velocity.</b></h3><p class="inline">—Next let us see what the velocity of the air -is in the tube. Suppose that we have some convenient means of measuring -the velocity of the air at any<span class="pagenum"><a name="Page_131" id="Page_131">131</a></span> point, in feet per second or miles -per hour, with some form of anemometer. We will have our measurements -taken at the five points where we measured the pressure,—viz., at -the tank, one-quarter, one-half, three-quarters and one mile from the -tank. We will represent the velocities by a diagram similar to the -one used for pressures. At the tank we find the air entering the tube -with a velocity of 59.5 feet per second (40.6 miles per hour). We draw -the vertical line M N, to represent this. At the quarter mile point -the velocity is sixty-five feet per second (44.4 miles per hour) an -increase in the first quarter of a mile of 5.5 feet per second. We -construct the vertical line O P. At the half-mile point the velocity is -72.4 feet per second (49.4 miles per hour); at the three-quarter mile -point it is eighty-three feet per second (56.8 miles per hour); and at -the end of the tube, one mile from the tank, the air comes out of the -tube with a velocity of 100.4 feet per second (68.5 miles per hour), -about 1.7 times faster than it entered the tube at the tank. Drawing -all the vertical lines to represent these velocities, and drawing a -smooth curve line through the tops of our vertical lines, we have the -curve of velocities, N, P, R, T, V, for all points along the tube. It -is an increasing velocity and increases more rapidly as we approach the -end of the tube. This is shown more clearly by drawing the straight -dashed line N V.</p></div> - -<p>If the fluid flowing in the tube were inelastic, like water, then the -curve of velocities would be a straight horizontal line, for the water -would not come out of the tube any faster than it went in. But we are -dealing with air, which is an elastic fluid, and, as we stated before, -it expands as<span class="pagenum"><a name="Page_132" id="Page_132">132</a></span> the pressure is reduced and becomes larger in volume. -It is this expansion that increases its velocity as it flows along the -tube. It must go faster and faster to make room to expand. 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 time, to prevent -an accumulation of air in the tube, and since it increases in volume as -it flows through the tube, it follows that its velocity must increase.</p> - -<div class="section-no-break"><h3 class="inline" id="Characteristics_of_the_Velocity_Curve"><b>Characteristics of the Velocity Curve.</b></h3><p class="inline">—This velocity curve is -both interesting and surprising, if we have not given the subject any -previous thought. It might occur to us that the air expands in volume -in the tube, and we might reason from this fact that the velocity of -the air would increase as it flowed through the tube, but very few of -us would be able to see that the rate of increase of velocity also -increases. That is to say, it gains in velocity more rapidly as it -approaches the open end of the tube. If the velocity were represented -on the diagram by a straight horizontal line, we should know that it -was constant in all parts of the tube, which would be the case if water -were flowing instead of air. If it were represented by a straight -inclined line, like the dashed line N V, then we should know that -the velocity increased as the air flowed along the tube, but that it -increased at a uniform rate. The slope of the line would indicate the -rate of increase. Neither of these suppositions represent correctly -the velocity of the air at all points in the tube; this can only be -done by a curved line such as we have shown. The slope of the curve at -any point represents<span class="pagenum"><a name="Page_133" id="Page_133">133</a></span> the rate of increase of velocity of the air at -that point. If the curve is nearly horizontal, then we know that the -velocity does not increase much; but if the curve is steep, then we -know that it is increasing rapidly, the actual velocity being indicated -by the vertical height of the curve above the horizontal line M U.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Uses_of_Velocity_Curves"><b>Use of Velocity Curves.</b></h3><p class="inline">—Besides being interesting, a knowledge -of the velocity of the air at all points in a tube is of much practical -value. It gives us the time a carrier will take in going from one -station to another. Usually the first questions asked, when it is -proposed to lay a pneumatic tube from station A to station B, are, -How quickly can you send a carrier between these points? How much -time can be saved? These questions are answered by constructing a -velocity curve. Since the velocity changes at every point along a -tube, to get the time of transit between two points we must know the -average velocity of the air between those points. 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 points, and then -taking the average of all these heights; but there is a more exact -and easier method by means of a mathematical formula. As such formula -would be out of place here, we will not give it; suffice it to say, -that the average velocity of the air between the tank and the end of -the tube, in the case we have assumed, is about seventy-three feet per -second (49.7 miles per hour), a little less than one-half the sum of -the velocities at the two ends, and a little more than the velocity -at the half-mile point. Knowing the average velocity, we can tell -how<span class="pagenum"><a name="Page_134" id="Page_134">134</a></span> long it takes for a particle of air, and it will be nearly the -same for a carrier, to travel from the tank to the end of the tube, -by dividing the distance in feet by the average velocity in feet per -second. This we find to be one minute 12.3 seconds. Since the air moves -more rapidly as it approaches the open end of the tube, 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 -end. The last quarter of a mile will be covered in a little more than -fourteen seconds, while the first quarter will require a little more -than twenty-one seconds. This difference is surprising, and it becomes -even more marked in very long tubes with high initial pressures. 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 line.</p></div> - -<p>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 station. It gives us the length of the “blocks” in our -“block system.” When we know the velocity and weight of our carriers, -we can compute the energy stored up in them, and from this the length -we need to make our air-cushions so as not to have the air too highly -compressed. It would be impossible to design our apparatus properly if -we did not know the laws that govern the flow of air in the tubes.</p> - -<div class="section-no-break"><h3 class="inline" id="Quantity_of_Air_Used"><b>Quantity of Air Used.</b></h3><p class="inline">—The next important fact that we learn -from the velocity curve is the quantity of air that flows through -the tube each second or minute. If we<span class="pagenum"><a name="Page_135" id="Page_135">135</a></span> 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 feet, we have the number of cubic feet -of air at atmospheric pressure discharged from the tube per unit of -time. The same quantity of air must be supplied to the tank in order -to maintain a constant flow in the tube. In the present case that we -have assumed, the tube is eight inches in diameter; therefore the -cross-sectional area is 0.349 square foot. The velocity of the air as -it comes out of the end of the tube is 100.4 feet per second; therefore -about thirty-five cubic feet of air are discharged from the tube each -second, or two thousand one hundred cubic feet per minute. This same -amount must be supplied to the tank A in order to maintain the pressure -constant, but when it is compressed so that it exerts a pressure of -ten pounds per square inch, the two thousand one hundred cubic feet -will only occupy a space of one thousand two hundred and fifty cubic -feet, if its temperature does not change. This leads us to consider the -effect of temperature changes.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Temperature_of_the_Air"><b>Temperature of the Air.</b></h3><p class="inline">—If the air is allowed to become heated -by compression, as is the case in practice, we have a new set of -conditions. If the air in the tank A is hot,—that is, warmer than the -surrounding atmosphere,—it will by radiation cool somewhat before it -enters the tube, and it will be still further cooled when it expands -in the tube. Again, if its temperature falls below the temperature of -the ground in which the tube is laid, it will absorb heat from the -ground, and this will tend to keep up its temperature; so in practice -we have very complicated relations between the temperature, pressure, -and volume<span class="pagenum"><a name="Page_136" id="Page_136">136</a></span> of the air. These relations cannot be exactly expressed by -mathematical formulæ, and we will make no attempt so to express them, -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 feet, so that we can without appreciable error compute the -pressures and velocities as if it were constant. Now, if the air in the -tank A is hot, we must raise the pressure a little above ten pounds -per square inch to obtain the velocities given on our diagram. When -the air cools it contracts in volume, or, if the volume cannot change, -being fixed by the limits of the containing vessel, then the pressure -is reduced, so by raising the pressure in the tank A a little above ten -pounds, we compensate the loss of pressure.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Horse_Power"><b>Horse-Power.</b></h3><p class="inline">—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 tube. I say estimate, -because we have to take into consideration the efficiency of the -air-compressor, and that is not an absolutely fixed quantity: it varies -with different types of machines and with their construction. When -working with pressures of less than ten pounds, the friction of the -machine is an important factor. The area and construction of the valves -in the air-cylinders is another very important factor. If the valves -are not large enough or do not open promptly, our cylinders will not -be filled with air at each stroke; this will reduce the efficiency of -the machine. In practice we go to a manufacturer of air-compressors and -tell him how much air must be compressed per minute, and the pressure -to which it must be compressed,<span class="pagenum"><a name="Page_137" id="Page_137">137</a></span> with other conditions, and then he -tells us what size of machine we shall require and the horse-power of -the machine approximately. He is supposed to know the efficiency of his -own machines. We may endeavor to prove his estimate by computations of -our own. To give some idea of the horse-power required to supply the -air needed to operate our eight-inch tube one mile long, 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-power. From -the horse-power of the steam-engine we can easily compute the coal that -will be consumed under boilers of the usual type.</p></div> - -<div class="section-no-break"><h3 class="inline" id="Efficiency"><b>Efficiency.</b></h3><p class="inline">—It will be noted that most of the power is not used -primarily in moving the carriers, but to move the air through the tube. -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 despatched. 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 air, we should find this so-called efficiency to -be very low. It is analogous to pulling the carrier with a long rope -and dragging the rope on the ground. Much more power would be consumed -by the rope than by the carrier. But a business man would define the -efficiency of a pneumatic tube as the ratio of the cost of transporting -his letters, parcels, etc., to the cost of transporting them with equal -speed in any other way. Defined in this practical manner the efficiency -of a pneumatic tube is high. We do not care what becomes of the power -so long as it accomplishes our purpose.</p></div> - -<p><span class="pagenum"><a name="Page_138" id="Page_138">138</a></span></p> - -<div class="section-no-break"><h3 class="inline" id="Pressure_and_Exhaust_Systems"><b>Pressure and Exhaust Systems.</b></h3><p class="inline">—We have noticed that pneumatic -tubes have not always been operated by compression of the air, but -that some of the small tubes used in the telegraph service of European -cities have been operated by exhausting the air. The two systems are -sometimes distinguished by calling one a pressure system and the other -an exhaust system. These terms are very misleading, for an exhaust -system is a pressure system. The current of air is kept flowing in a -tube by maintaining a difference of pressure at the two ends, 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 atmospheric. In either -case it is pressure that causes the air to flow. It happens that we -are living in an atmosphere of about fifteen pounds pressure per -square inch, and it is very convenient oftentimes in our computations -to take the pressure of the atmosphere as our zero, and reckon all -other pressures above and below this. If all our pressure scales read -from absolute zero, the pressure of a perfect vacuum, then all this -confusion would be avoided. We have not used the absolute zero in -our diagram, because all our gauges are graduated with their zero at -atmospheric pressure, and it is customary to speak of pressures above -and below the atmospheric.</p></div> - -<p>It is very natural to ask the question, why are tubes sometimes -operated by compressing the air and at other times by exhausting it? -We answer by saying that it is usually a question of simplicity and -convenience that determines which system shall be used. Some of the -cash systems in the stores use compressed air in the out-going<span class="pagenum"><a name="Page_139" id="Page_139">139</a></span> tubes -from the cashier’s desk and exhaust the air from the return tubes. Both -ends of the tubes are then left open at the counters, no sending or -receiving apparatus being required there. The carriers are so light, -their velocity so low, 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 counters, and they can be despatched by -simply placing them at the open end of the tube into which the air is -flowing. The currents of air entering and leaving the tubes are not -so strong as to cause any special annoyance. At the cashier’s desk -some simple receiving and sending apparatus has to be used, but it is -better to concentrate all of the apparatus at one point rather than -have it distributed about the store, as would be the case if the double -system were not used. In the London pneumatic telegraph both methods -of operation have been in use. Double lines were laid, and the engines -exhausted the air from one tube and forced it into the other. The -exhausted tube was therefore used to despatch in one direction, while -the other tube, operated by the compressed air, served for despatching -in the opposite direction. So far as I know, both work equally well.</p> - -<p>In operating large tubes, that is to say, tubes six or eight inches -in diameter, there is an advantage in using compressed, rather than -exhausted air, in the construction of the sending and receiving -apparatus, especially when the tubes are very long. With an ample -supply of compressed air always at hand, the air-cushions can be made -shorter and more effective in bringing the carriers quickly to rest.<span class="pagenum"><a name="Page_140" id="Page_140">140</a></span> -With exhausted air the cushions are ineffective, and consequently -must be made very long in order to stop the carrier before it strikes -the closed end of the tube. 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 buffers. Again, when compressed air is -used, we have a larger difference in pressure between the pressure in -the tube and the atmosphere to operate our mechanism by cylinders and -pistons. With an exhaust system carriers are not so easily ejected -from the tubes of the receiving apparatus; we could not use the simple -form of open receiver. Again, if the tubes are laid in wet ground, and -a leak occurs in any of the joints, water will be drawn in if air is -being exhausted from the tube, while it will be kept out if compressed -air is used.</p> - -<p>In regard to the question of relative economy of the two systems, we -will say that when long tubes are used, requiring high pressures, or, -more strictly speaking, a large difference of pressure, to maintain the -desired velocity of air-current, there seems to be some advantage in -using an exhaust system. The reason is this: the friction of the air in -the tube, which absorbs most of the power, increases as the air becomes -heavier and more dense. When the air is exhausted from the tube, we -are using a current of rarefied air, and this moves through the tube -with less friction and, consequently, a higher velocity, for the same -difference of pressure, than the more dense compressed air. But for -short tubes that require only a small difference of pressure, this -advantage becomes very small, and is overbalanced by other advantages -of a compressed air system. So, taking<span class="pagenum"><a name="Page_141" id="Page_141">141</a></span> everything into consideration, -there is not so much to be said in favor of an exhaust system.</p> - -<div class="section-no-break"><h3 class="inline" id="Laws_Expressed_in_Mathematical"><b>Laws Expressed in Mathematical Formulæ.</b></h3><p class="inline">—While we have -heretofore purposely avoided all complicated mathematical formulæ, it -may not be out of place here to give a few of the more simple relations -that exist between the pressure, velocity, length and diameter of -the tubes, etc. In two tubes having the same diameter, with the same -pressures maintained at each end, but of different lengths, the mean -velocities of the air in the tubes will bear the inverse ratio to the -square roots of the lengths of the tubes. This is expressed by the -following proportion:</p></div> - -<p class="center"><i>u</i> : U :: √L : √<i>l</i></p> - -<p class="noindent"><i>u</i> and U represent the mean velocities of the air in the -two tubes and <i>l</i> and L the respective lengths of tubes.</p> - -<p>A similar but direct ratio exists between the mean velocities and the -diameters of the tubes, thus:</p> - -<p class="center"> -<i>u</i> : U :: √<i>d</i> : √D<br /> -</p> - -<p class="noindent">This relation, however, is only approximately true for -tubes differing greatly in diameter.</p> - -<p>The relation of the pressure to other factors is not so simply -expressed. For example, in two tubes of the same length and diameter, -the relation between the pressures at the ends and the mean velocity of -the air may be expressed as follows:</p> - -<p class="center"> -<span class="add4em">(</span><i>p</i><sub><span class="smaller">0</span></sub><sup><span class="smaller">2</span></sup> - <i>p</i><sub><span class="smaller">1</span></sub><sup><span class="smaller">2</span></sup>)<sup><span class="smaller">3/2</span></sup><span class="add1em">(</span>P<sub><span class="smaller">0</span></sub><sup><span class="smaller">2</span></sup> - P<sub><span class="smaller">1</span></sub><sup><span class="smaller">2</span></sup>)<sup><span class="smaller">3/2</span></sup><br /> -<i>u</i> : U :: ———— : ————<br /> -<span class="add4em">(<i>p</i></span><sub><span class="smaller">0</span></sub><sup><span class="smaller">3</span></sup> - <i>p</i><sub><span class="smaller">1</span></sub><sup><span class="smaller">3</span></sup>)<span class="add2em">(P<sub><span class="smaller">0</span></sub><sup><span class="smaller">3</span></sup> - P<sub><span class="smaller">1</span></sub><sup><span class="smaller">3</span></sup>)</span> -</p> - -<p class="center"> -<span class="add4em">(</span><i>p</i><sub><span class="smaller">0</span></sub><sup><span class="smaller">2</span></sup> - <i>p</i><sub><span class="smaller">1</span></sub><sup><span class="smaller">2</span></sup>)<sup><span class="smaller">3/2</span></sup><span class="add1em">(</span>P<sub><span class="smaller">0</span></sub><sup><span class="smaller">2</span></sup> - P<sub><span class="smaller">1</span></sub><sup><span class="smaller">2</span></sup>)<sup><span class="smaller">3/2</span></sup><br /> -<i>u</i> : U :: ———— : ————<br /> -<span class="add4em">(<i>p</i></span><sub><span class="smaller">0</span></sub><sup>3</sup> - <i>p</i><sub><span class="smaller">1</span></sub><sup><span class="smaller">3</span></sup>)<span class="add2em">(P<sub><span class="smaller">0</span></sub><sup><span class="smaller">3</span></sup> - P<sub><span class="smaller">1</span></sub><sup><span class="smaller">3</span></sup>)</span> -</p> - -<p><span class="pagenum"><a name="Page_142" id="Page_142">142</a></span></p> - -<p class="noindent">where <i>u</i> and U are the respective -mean velocities, <i>p</i><sub>0</sub> and P<sub>0</sub> the respective pressures at the -initial ends of the tubes, and <i>p</i><sub>1</sub> and P<sub>1</sub> the respective -pressures at the final ends of the tubes,—the pressures being measured -above absolute zero.</p> - -<p>There are other relations that can be similarly expressed, but for them -we must refer the reader to a mathematical treatise on the subject.</p> - -<div class="section-no-break"><h3 class="inline" id="Moisture_in_the_Tubes"><b>Moisture in the Tubes.</b></h3><p class="inline">—When pressures of more than five pounds -per square inch are used, 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 tube. It is seldom more than a slight dampness, or at -most a degree of wetness equal to that seen on the outside of a pitcher -of ice-water on a warm day. A slight amount of moisture in the tube is -not objectionable, for it serves as a lubricant to the carriers; but -when it is present in considerable quantity it becomes objectionable -and even annoying. This moisture is brought into the tube with the air, -and is deposited upon the walls of the tube when their temperature -is sufficiently below that of the atmosphere. The atmosphere always -contains more or less moisture in the state of vapor. The capacity of -air for water-vapor depends upon its temperature, being greater the -higher the temperature, but it is a fixed and definite quantity at any -given temperature. When the air contains all the water-vapor it can -hold at a certain temperature, it is said to be saturated. If it is -not saturated, we express the amount that it contains in per cent. of -the amount it would contain if it were saturated, and this is termed -the “relative humidity.”<span class="pagenum"><a name="Page_143" id="Page_143">143</a></span> For example, if the air is three-fourths -saturated, we say the “relative humidity” is seventy-five; but if the -temperature changes, the “relative humidity” changes also. Suppose -the temperature to-day is seventy-five degrees Fahrenheit, and that -the “relative humidity” is eighty, a cubic foot of air then contains -0.00107 pound of water-vapor. Now suppose this air enters a pneumatic -tube and is cooled by expansion and contact with the colder walls of -the tube to sixty degrees. At this temperature a cubic foot of air can -contain only 0.00082 pound of water-vapor when it is saturated. Now, -each cubic foot of air brought into the tube brings with it 0.00107 -pound of vapor, and after it is cooled down to sixty degrees it cannot -hold it all, consequently the difference, or 0.00025 pound, must be -deposited in the tube. Under these conditions one hundred thousand -cubic feet of air will deposit twenty-five pounds of water in the tube.</p></div> - -<p>In the system of pneumatic tubes built and operated by the Batcheller -Pneumatic Tube Company, the presence of a large quantity of moisture -in the tube is prevented by using the same air over and over again. A -little moisture may be deposited when the tube starts into operation, -but the amount does not increase appreciably, as very little fresh air -is admitted after starting.</p> - -<div class="section-no-break"><h3 class="inline" id="Location_of_Obstructions"><b>Location of Obstructions in Tubes.</b></h3><p class="inline">—In regard to the removal -of obstructions in the tubes, I have had little or no experience; -therefore under this heading I am satisfied to quote from the “Minutes -of the Proceedings of the Institution of Civil Engineers,” London, -Volume XLIII.</p></div> - -<p>“Intimately connected with the working of the tubes is<span class="pagenum"><a name="Page_144" id="Page_144">144</a></span> the removal of -obstructions which occur from time to time, causing not unfrequently -serious inconvenience and delay. The most general cause of obstruction -is a stoppage of the train arising from accident to the tube, to the -carriers or piston, or to the transmitting apparatus. In such cases the -delay is generally very brief, it being for the most part sufficient to -reverse the pressure on the train from the next station, and to drive -it back to the point it started from. If one or more of the carriers -break in the tube, reverse pressure is also generally sufficient to -remove the obstacle; but where this fails, the point of obstruction -must be ascertained. This is done by carefully observing the variations -of air pressure in the reservoir when placed in connection, first -with a line of known length, and then with the obstructed tube. By -this means the position of the obstruction can be ascertained within -one hundred feet. Or the tube may be probed with a long rod up to a -length of two hundred feet. A very ingenious apparatus, by M. Ch. -Bontemps, is shown in Figs. 49 and 50, and is employed to ascertain -the exact position of the obstruction. It acts by the reflection of -sound-waves on a rubber diaphragm. A small metal disk is cemented to -the rubber, and above this is a pointed screw, D. An electric circuit -is closed where the points C and D are brought in contact. To locate an -obstruction a pistol is fired into the tube as shown, and the resulting -wave, traversing the tube at the rate of three hundred and thirty -metres a second, strikes the obstruction and is then reflected against -the diaphragm, which in its turn reflects it to the obstacle, whence it -returns to the diaphragm. By this means indications are marked on the<span class="pagenum"><a name="Page_146" id="Page_146">146</a></span> -recording cylinder, and if the interval of time between the first and -second indications be recorded, the distance of the obstacle from the -membrane is easily ascertained. The chronograph employed is provided -with three points; the first of these is placed in a circuit, which -is closed by the successive vibrations of the diaphragm; the second -corresponds to an electric regulator, marking seconds on the cylinder; -and the third subdivides the seconds there marked. Fig. 50 indicates a -record thus made. In this case the obstacle is situated at a distance -of sixty-two metres, and the vibration marks thirty-three oscillations -per<span class="pagenum"><a name="Page_147" id="Page_147">147</a></span> second. The interval occupied by two successive marks from the -diaphragm on the paper corresponds to twelve oscillations, and the -distance of the obstruction is then calculated by the following formula:</p> - -<p class="center"> -D = 0.5 × 330 × <sup>12</sup>⁄<sub>33</sub> = 60 metres;</p> - -<p class="noindent">so that the distance of the obstacle is recorded within two -metres.</p> - -<div class="figcenter" id="fig_49"> -<img src="images/i_192.jpg" width="600" height="410" alt="" /> -<p class="caption"><span class="smcap">Fig. 49.</span><br /> - -OBSTRUCTION-RECORDING APPARATUS.</p> -<p class="largeimg"><a href="images/i_192_large.jpg" rel="nofollow">Larger image</a> (359 kB)</p></div> - -<div class="figcenter" id="fig_50"> -<img src="images/i_193.jpg" width="557" height="600" alt="" /> -<p class="caption"><span class="smcap">Fig. 50.</span><br /> - -OBSTRUCTION-RECORDING APPARATUS.</p></div> - -<p>“Amongst the special causes of accident may be mentioned the accidental -absence of a piston to the train, breaking of the piston, the freezing -up of a piston in the tube, and even forgetting the presence of a -train, which has caused the entire service to be one train late -throughout the day. Finally, the tubes themselves are sometimes broken -or disturbed during street repairs, resulting of course in a complete -cessation of traffic in the system till the damage is made good.”</p> - -<hr class="chap" /> -<h2>Transcriber’s Notes:</h2> - -<p>The spelling, punctuation and hyphenation are as the original, with the exception of apparent typographical errors which have been corrected.</p> - -<pre style='margin-top:6em'> -*** END OF THE PROJECT GUTENBERG EBOOK THE PNEUMATIC DESPATCH TUBE SYSTEM -OF THE BATCHELLER PNEUMATIC TUBE CO. *** - -This file should be named 63952-h.htm or 63952-h.zip - -This and all associated files of various formats will be found in: -http://www.gutenberg.org/6/3/9/5/63952/ - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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