The Pneumatic Despatch Tube System of the Batcheller Pneumatic Tube Co

—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.”

—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 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 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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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 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.

—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.

—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.

—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.

—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 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.

—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 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.

—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.

—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.

—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.

—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 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.

—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.

—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.

—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.

—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. 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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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.

—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:

—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 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.