THE MAIN POST-OFFICE, PHILADELPHIA.

The

Pneumatic Despatch Tube System

of the

Batcheller Pneumatic Tube Co.

ALSO

FACTS AND GENERAL INFORMATION
RELATING TO PNEUMATIC
DESPATCH TUBES

BY

B. C. BATCHELLER, B.Sc.

MECHANICAL ENGINEER

PHILADELPHIA
PRESS OF J. B. LIPPINCOTT COMPANY
1897

Copyright,

1896,

by

B. C. Batcheller.

I have been prompted to prepare this book by the frequent inquiries made regarding the details of our system of pneumatic tubes. These inquiries have come from people interested in our company, from others interested in companies that have purchased the right to use our apparatus, from people desirous of becoming interested in a pneumatic-tube business, from would-be purchasers of pneumatic tubes, and from people interested in pneumatic tubes from a scientific, engineering, or mechanical point of view. This book is not intended to be a treatise on pneumatic tubes. In the first chapter I have given a brief sketch of what has been done in the application of pneumatic tubes from the earliest records to the present time. The second chapter contains a description of the postal tubes in Philadelphia, and the third chapter describes our system in detail. Following this is a short chapter explaining the theory of pneumatic tubes, or the theory of the flow of air in long pipes, stating the more interesting facts and relations in as plain and simple a8 manner as possible. Mathematical formulæ have been purposely avoided.

Several plates showing the Philadelphia postal line have been kindly loaned to me by the Engineers’ Club of Philadelphia. They formerly appeared in a paper read by Mr. A. Falkenau before that club. I have also to thank the and Drill Co., the B. F. Sturtevant Co., the Wilbraham-Baker Blower Co., and J. B. Stewart for the use of electrotypes of their machines.

B. C. B. 

October 6, 1896.

The first one and one-half inch tubes laid under the direction of Mr. Clark were of iron with screwed joints. They gave much trouble from roughness upon the interior, which wore the carriers very rapidly, and from water that was drawn in through leaky joints. When the extensions were made in 1858 and afterwards, two and one-fourth inch lead tubes were used with plumber’s joints made over a heated mandrel, which made the joints very smooth upon the interior. The carriers were of gutta percha in the form of a cylinder closed at one end and fitted with a cap at the other. The outside was covered with felt or drugget.

When a carrier was to be despatched, a signal was sent to an attendant at the pump end of the tube, who closed that end and connected the tube to an exhausted chamber by opening a valve. As soon as the carrier arrived, he closed the valve and opened the tube, which allowed the carrier to drop out. Mr. Varley improved the method of operating the valves by making the air pressure do the work by means of cylinders and pistons when the attendant pressed a button. He also improved the carriers by doing14 away with the cap and using in its place an elastic band to hold the messages in place.

We have seen that Mr. Clark designed the first tube used in connection with the telegraph, and that it was a single tube, operated in one direction only by vacuum, being operated only when there were messages to send. This was extended and improved by Mr. Varley, who increased the diameter of the tubes from one and one-half inches to two and one-quarter inches, and operated them in both directions, using vacuum for sending in one direction and compressed air for sending in the other. The air current was maintained in the tubes only when messages were sent.

Great credit is due to Sir Rowland Hill, who, in 1855, had a proposed method of conveying mails in the city of London, through nine- and thirteen-inch tubes, thoroughly investigated. It was decided at this time that the saving in time over that consumed by mail carts would not warrant the expense of installing such a system.

In 1869 Messrs. Siemens Bros. received an order from the British government to install an experimental line of tubes between the central telegraph station and the general post-office. This was completed in 1870, and after a half-year’s test it was extended to Fleet Street, and finally to Charing Cross. The tubes were of iron, three inches in diameter, with flanged and bolted joints. It was found, after some experience, that there was no advantage in the circuit, so the up and down tubes were separated at Charing Cross Station and worked independently.

Fig. 2.
DIAGRAM ILLUSTRATING THE PNEUMATIC TUBE SYSTEM
LONDON RADIAL SYSTEM.

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From this beginning the English system developed into what has been termed a “radial system;” that is to say, one principal and several minor central pumping stations have been established, and from these radiate tubes to numerous sub-stations (see Fig. 2). Some of the stations are connected with double lines for sending in opposite directions. The out-going tube from the pumping station is worked17 by compressed air, and the incoming tube by exhaustion. Other stations are connected by single tubes, and they are operated alternately by compression and exhaustion. Intermediate stations are located on some of the lines. For the central station the Varley valves were found too expensive and troublesome to keep in order, so they were replaced by the Wilmot double sluice-valves, which are operated manually. In recent years the sluice-valves have been in turn replaced by what are termed D-boxes, a simpler form of apparatus. At the sub-stations the tube terminates in a box into which the carriers drop. As the system has been gradually extended, tubes two and three-sixteenths inches inside diameter have been used for short lines, and three-inch tubes for long lines. The tubes are of lead laid inside a cast-iron pipe which serves as a shield, protecting them from injury. They are laid in twenty-nine18 foot sections, the joints being made by soldering over a steel mandrel, which is afterwards drawn out by a chain. The joints in the cast-iron protecting pipe are made by caulking with yarn and lead. “Electric signals are used between the central and sub-stations, consisting of a single stroke bell and indicator, giving notice of the arrival and departure of carriers, and to answer the necessary questions required in working. Where there are intermediate stations the tubes are worked on the block system, as if it were a railway. Experience shows that, where great exactness of manipulation cannot be obtained, it is necessary to allow only one train in each section of a tube, whether worked by vacuum or pressure. But where there is no intermediate station, and where the tube can be carefully worked, carriers may be allowed to follow one another at short intervals in a tube worked by vacuum, although it is not perfectly safe to do so in one worked by pressure. In working by pressure it has been found that, notwithstanding a fair interval may be allowed, carriers are apt to overtake one another, for no two carriers travel in the same times, because of differences in fit, unless they are placed end to end. If signalling be neglected and a carrier happens to stick fast, being followed by several others, a block will ensue which it will be difficult to clear, while the single carrier could readily have been dislodged.” (Proceedings Institute of Civil Engineers, London, Vol. XLIII. p. 61.)

No changes have been made in the carriers from those used in the early experiments which have already been described.

19

The London system has grown until it now includes no less than forty-two stations and thirty-four miles of tubes. Similar systems have been established in connection with the telegraph in Liverpool, Manchester, Birmingham, Glasgow, Dublin, and New Castle. The tubes give a cheaper and more rapid means of despatching telegrams between sub-stations and central stations than transmission by telegraph, and local telegrams can be delivered in the sender’s handwriting.

Fig. 3.
DIAGRAM OF PART OF PARIS PNEUMATIC TUBE SYSTEM.
PARIS CIRCUIT SYSTEM.

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Several methods are used to compress and exhaust the air. The most novel method consists in having tanks in which a partial vacuum is produced by allowing water to flow out of them into the sewer, or in having the air compressed by allowing water from the city mains to flow into the tanks and displace the air. Water jets have also been used, operating similar to a steam-injector. At some of the stations water turbines drive the air-pumps, and at others steam-engines are used.

25

The tubes of the Paris system are of wrought iron, in lengths of from fifteen to twenty feet, the joints being made with flanges and bolts. The interior diameter is 2.559 inches with a maximum variation to 2.519 inches. The bends are made with a radius of from six to one hundred and fifty feet. Water frequently gives trouble by accumulating in the tubes. Traps are placed at low points to drain it off.

The speed of the trains of carriers in the Paris tubes is from fifteen to twenty-three miles per hour, and the average time that elapses from the receipt of a message until its delivery is from forty to forty-five minutes.

The most extensive use of small pneumatic tubes in this country has been in our large retail department stores for despatching cash to and from a centrally located cashier’s desk. Seamless brass tubes are usually used, and, since the tubes are short, the air is either compressed or exhausted by means of positive rotary blowers. At the outlying stations the tubes are usually open to the atmosphere, while at the central station simple forms of valves are used for sending and receiving. An outgoing and a return tube are always used, and the air is kept in constant circulation. The carriers are of metal with felt packing rings and open on the side. These cash-carrying systems have come into use during the past twenty-five years.

The Western Union Telegraph Company uses small tubes to transmit its messages to a considerable extent in some of our large cities. In 1876 four lines were laid in New York City from the main office on Broadway: two to the branch office at No. 14 Broad Street, one to 134 Pearl Street, and one to the Cotton Exchange. Since then this company has laid a double line about two miles in length under Broadway to its up-town office. It also uses27 tubes to send messages from the receiving desks to the operating rooms within the buildings.

Many of our large hotels use pneumatic tubes to transmit messages to the different floors and offices of the buildings, taking the place of messenger, or bell boys, who formerly did this service.

We call especial attention to the fact that in all the systems that we have mentioned which are in use both in this country and in Europe, none of the tubes are larger than three inches internal diameter; also that in all the systems, except in Paris where the carriers are despatched in trains, the carriers are so light and move so slowly that they can be stopped by allowing them to come in contact with some solid object, such as a box into which the carriers drop. Very few of the tubes are more than two miles in length, and most of them are less than one mile. A speed of more than thirty miles per hour has seldom been attempted, and usually it is much less than this.

WM. J. KELLY,
President of the Pneumatic Transit Co.

Fig. 4.

Fig. 5.
SIX-INCH PNEUMATIC TUBES IN PROCESS OF BORING AT THE SHOP OF A. FALKENAU, PHILADELPHIA, PA.

For the transmission of mail it was planned to connect the main post-office with the sub-post-offices by tubes of a size large enough to carry all of the first-class and most of the other classes of mail matter. The sub-post-offices would be divided into groups, all of the offices in one group being connected to the same line, which would terminate at the main post-office. Most of the business would be between the main and individual sub-offices; in addition to this there would be some local mail sent between the sub-offices which, for offices in the same group, could be despatched directly without passing through the main office. The advantages to be gained by the use of these tubes over the present wagon service are very apparent. It places all the sub-post-offices in almost instant communication with the main office and with each other.

It was a part of the general plan to lay tubes from the main post-office to the railway stations, thereby hastening the despatch and receipt of mails to and from the trains.

It was expected that the bulk of the business would consist in the delivery of parcels from the retail stores to the private houses in the residence sections of the city. Of course it would not be practicable to lay a tube to each house, but with a station not more than four or five blocks away, the parcels would be sent through the tube to the nearest station, and then delivered by messengers to the houses with a minimum loss of time. Ladies could do their30 shopping and find their purchases at home when they returned.

The same tubes used for parcel delivery would also be used for a district messenger service. With numerous public stations in convenient locations, all the advantages of the European system would be realized in the quick despatch of letters and telegrams. Every one knows how much time is consumed by district messenger-boys in the delivery of messages, especially when they have to go long distances, and no argument is required to show that this time would be very much reduced by the use of pneumatic tubes, besides prompt delivery would be made much more certain.

The tubes of this system were to be six or eight inches in diameter, with a few small tubes in localities where the message service is very heavy.

Without going more into detail, such were in brief the plans of the promoters of this new company; but before launching such an enterprise, involving a large amount of capital, there were many engineering and mechanical problems to be solved. It was not simply a question of obtaining tubes and laying them in the streets, but ways and means for operating them must be devised. Up to this time only small tubes had been used for the transmission of telegrams, messages, cash, and other light objects. Now it was proposed to transmit heavy and bulky material. There was no experience for a guide.

In order to demonstrate the practicability of the system, the Electro-Pneumatic Transit Company had constructed32 in the basement of the Mills Building, on Broad Street, New York, a short line of small brass tubing, about two or three inches in diameter, with one branch, thus connecting three stations together. The tube was very short, probably not more than two hundred feet in length. The air-pressure required was very slight, probably not more than an ounce or two, being supplied by a small blower run by an electric motor.

At the junction of the branch and the main tube was located a switch that could be moved across the main tube and so deflect the approaching carrier into the branch. This switch was moved by an electro-magnet, or solenoid, that could be excited by pressing a button at the station from which the carrier was sent. When the carrier passed into the branch tube it set the switch back into its normal position, so that a second carrier, following the first, would pass along the main tube, unless the switch was again moved by pressing the button at the sending station.

This tube in the Mills Building worked well, but it was of a size only suited to the transmission of cash in a store or other similar service. It could not be said, because this tube worked well, that a larger and longer tube with numerous branches would work equally well. In fact, there are several reasons why such a tube would not operate satisfactorily. The method of operating the switches was impracticable. Suppose the branch tube had been located two miles away from the sending station and that it would take a carrier four minutes to travel from the sending station to the junction of the branch tube. Again, suppose that we have just despatched a carrier destined for a station33 on the main line beyond the junction, and that we wish to despatch the second carrier to be switched off into the branch tube, we must wait at least four minutes, until the first carrier has passed the junction, before we can press the button and set the switch for the second carrier which may be on its way. How are we to know when the first carrier has passed the junction, and when the second will arrive there, in order that we may throw the switch at the proper time? Must we hold our watch and time each carrier? It is plain that this is not practical. I take this as an illustration of the impracticability of the Clay-Lieb System as constructed in the Mills Building when extended to practical dimensions. I will not describe the mechanism and details of the system, which are ingenious, but will say in passing that the automatic sluice-gates, which work very well in a three-inch tube with carriers weighing an ounce or two and air-pressures of only a few ounces per square inch, would be useless and could not be made to operate in a six-inch tube with carriers weighing from eight to twenty-five pounds and an air-pressure of from five to twenty-five pounds per square inch. For further information the reader is referred to the patents of Clay and Lieb.

Fig. 6.
PIPE BORING APPARATUS.

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It was certainly a credit to the Pneumatic Transit Company and its managers that they were able to complete this first line of tubes so quickly and successfully under such trying circumstances. The tubes have been in successful operation from the opening until the present time, a period of nearly four years, and the repairs that38 have been made during that time have not required its stoppage for more than a few hours.

In the summer of 1895 the sub-post-office was removed from Chestnut Street to the basement of the Bourse (see Fig. 7). This required the abandonment of a few feet of tube on Chestnut Street and the laying of a slightly greater amount on Fourth Street, thus increasing the total length of the tubes a little. Wrought-iron tube, coated with some alloy, probably composed largely of tin or zinc, was used for this extension. The wrought-iron tube is not as good as the bored cast iron.

Fig. 7.
BOURSE BUILDING, PHILADELPHIA.

Fig. 8.
PNEUMATIC TUBES SUSPENDED IN THE BASEMENT OF THE MAIN POST-OFFICE.

The tubes were laid in the trench and supported by having the ground firmly tamped about them. Usually one tube was laid above the other, with an iron bracket between, but frequently this arrangement had to be departed from in order to avoid obstructions, such as gas- and water-pipes, sewers, man-holes, etc. The depth of the tubes below the pavement varied from two to six feet, and in one place, in order to pass under a sewer, the extreme depth of thirteen feet was reached. At the street crossings it was frequently difficult to find sufficient space to lay the tubes. At the intersection of Fifth and Chestnut Streets a six-inch water-main had to be cut and a bend put40 in. A seven-strand electric cable, used for telephoning and signalling, was laid upon the top of one of the tubes, protected by a strip of “vulcanized wood,” grooved to fit over the cable. The cable and protecting strip of wood were fastened to the tube by wrought-iron straps and bolts.

The tubes enter the main post-office on the Chestnut Street side, through one of the windows, and are suspended from the ceiling along the corridor in the basement for a distance of nearly two hundred feet. Fig. 8 shows the tubes thus suspended. They terminate upon the ground floor about the centre of the building, and near the cancelling machines.

Fig. 9.
DUPLEX AIR-COMPRESSOR IN THE BASEMENT OF THE MAIN POST-OFFICE.

Fig. 10.
TANKS AND TUBE IN THE BASEMENT OF THE MAIN POST-OFFICE.

Fig. 11.

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The writer was asked to design an automatic receiver43 to stop the carriers without shock upon their arrival at the stations, and to deliver them upon a table without appreciable escape of air,—something that would answer the requirements of the present plant.

Fig. 12.
TRANSMITTER.—PHILA.
SENDING APPARATUS.

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At the sub-post-office this sending apparatus is placed in a horizontal position, but its operation is the same.

Fig. 13.
RECEIVING AND SENDING APPARATUS IN THE SUB-POST-OFFICE.

Fig. 14.
APPARATUS AT SUB-STATION—PHILA.

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Fig. 15.
TERMINALS OF THE TUBE IN THE MAIN POST-OFFICE.

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Fig. 16.
RECEIVING APPARATUS AT THE MAIN POST-OFFICE.

Fig. 17.
APPARATUS AT THE MAIN OFFICE—PHILA.

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When a carrier arrives, after passing the slots that allow the air-current to flow into the branch pipe, it compresses50 the air in front of it against the gate. This compression checks its momentum, and it comes gradually to rest. The air compressed between the carrier and the sluice-gate operates to move the piston slide-valve, thereby admitting air to the gate cylinder under the piston, which rises, carrying with it the sluice-gate. The tube is now open to the atmosphere, and there is just sufficient pressure in the tube to push the carrier out on to a table arranged to receive it. As the carrier passes out of the tube it lifts a finger out of its path. This finger is located at E, Fig. 17, and when it is lifted by the passing carrier it moves the piston slide-valve, and the sluice-gate is closed. A valve is located in the branch-pipe that conducts the air to the return tank in the basement. If the pressure in the tube is not sufficient to push the carrier out on to the table, this valve is partially closed, thereby increasing the pressure to a desired amount.

Fig. 18.
CARRIER.

Fig. 19.
CARRIER.

Fig. 20.
MAIL CARRIER.—PHILA.

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The bearing-rings are made of fibrous woven material, especially prepared, and held in place by being clamped between two metal rings, one of which is riveted to the52 body of the carrier. Of course these rings wear out and have to be replaced occasionally, but their usual life is about one thousand miles. The rear end of the carrier is closed by a hinged lid and secured by a special lock. The lock consists of three radial bolts that pass through the body of the carrier and the rim of the lid. These bolts are thrown by three cams, attached to a short shaft that passes through the lid and has a handle or lever attached to it upon the outside of the lid. This cam-shaft is located out of the geometrical centre of the lid in such a position that when the lever or handle is swung around in the unlocked position, it projects beyond the periphery of the lid, and in this position the carrier will not enter the tube. When the lid is closed and locked, the lever lies across the lid in the position shown in Fig. 19, and when the carrier is in the tube it cannot become unlocked, for the lever cannot swing around without coming in contact with the wall of the tube. This insures against the possibility of the carriers opening during transit through the tube. The empty carriers weigh about nine pounds, and when filled with mail, from twelve to fifteen pounds. They have a capacity for two hundred ordinary letters, packed in the usual manner.

The time required for a carrier to travel from the main to the sub-post-office is sixty seconds, and from the sub- to54 the main post-office, fifty-five seconds. This difference of time in going and returning is due to the expansion of the air in the tube, as will be explained more fully in another place. The distance between the offices being two thousand nine hundred and seventy-four feet, gives an average speed of about fifty-two feet per second, or 35.27 miles per hour. Of course the speed can be increased by increasing the air-pressure, but this speed is found in practice to be ample for all requirements. In order to give some idea of the energy possessed by one of these carriers travelling at this speed, it may be said that if the end of the tube were left open and turned upward, an emerging carrier would rise about forty feet into the air. It is easy to imagine how apparatus, depending for its operation upon impact with a moving carrier, would be soon destroyed, as well as the carriers themselves. This is why receiving apparatus used with small tubes and light carriers cannot be applied to large tubes with heavy carriers.

No serious trouble has ever been experienced from carriers getting wedged in any part of these tubes.

Since the sub-office has been established in the Bourse, it has been made a distributing as well as receiving office. At least two more deliveries of mail are made each day in the Bourse building than in any other office building in the city.

All letters mailed in the sub-office with a special delivery stamp are despatched through the tube immediately.

It is now nearly four years since the system was put into operation. During that time more than thirty-five million letters have been transported, and all the repairs to the system have not required it to be stopped for more than a few hours. During the first year the Pneumatic Transit Company operated the tubes at their own expense, agreeing at the end of that time to take them out if the government so requested. Since the first year the government has paid the running expenses.

Such is the history of the first United States pneumatic postal system. Such is the history of the first pneumatic tubes of sufficient size to carry all the first class and most of the lower classes of mail, in this or any other country, so far as the writer knows.

While the Pneumatic Transit Company has ample field in the State of Pennsylvania to carry out the work which it has mapped out, a field broad enough to yield a good profit for the capital invested, there is no reason why the system should be limited to one State. So, in order to obtain a broader charter, covering all places where pneumatic service may be needed, a new corporation was formed and styled the Batcheller Pneumatic Tube Company.

It is impossible to lay down a rigid system equally well58 adapted to all places and purposes. We must accommodate ourselves somewhat to circumstances. For example, the post-office department may require one size of tube, arranged to operate in a particular way, while the requirements of a parcel delivery business would be utterly different. The geographical location of the stations will have much to do with the general arrangement; also the condition of the streets. Some of the streets of our large cities are so filled with water- and gas-pipes, electrical conduits, sewers, steam-pipes, etc., etc., that it is almost impossible to find space for pneumatic tubes, especially of large diameter. Railway or water facilities have much to do with the location of a central pumping station, on account of the coal supply. All of these and many other things have to be taken into consideration in planning a system for any locality.

We have an example of a peculiar location and conditions in a proposed line of tubes over the New York and Brooklyn bridge connecting the main post-offices of those cities. This would be in many respects a unique plant. Two air-compressors would be used, one at each office.

In order to give a general idea how a large number of stations can be connected into one system, the diagram Fig. 21 has been drawn.

We have already referred to the attempts of Clay and Lieb to devise means whereby several stations could be located along a main line and carriers be sent from any station to any other through the main line. Their method was to use branch tubes leading off from the main line with switches at the junctions. They deflected the air-current59 into the branch by placing an automatic closed valve in the main line just beyond the junction, returning the air from the branch to the main line just beyond the valve. The carriers were to open and close this valve automatically as they passed.

The branch and switch system has many attractions for the inventor, and upon first thought it would seem the most feasible solution of the problem. It has been the dream of more than one inventor, as the records of the patent-office show, but no one has succeeded in working it out. The current of air cannot be divided; carriers passing from the branch into the main line must not collide with other carriers running in the main line; a certain minimum distance must always be maintained between the carriers in the same tube; when a carrier is despatched it must go directly to the station for which it is intended without further attention from the sender and it must not interfere with other carriers; expense of manufacture prohibits the use of any but round smooth tubes up to eight inches in diameter, hence projections cannot be placed upon the carrier to give it an individuality and cause it to operate a switch at any particular point along the line; the carrier is free to rotate in a round tube about its longitudinal axis, therefore, its individuality must be indicated by some symmetrical marking about this axis, if it is to be automatic in its operation; the speed of the carrier is so high that electrical contacts placed in distinctive positions on the carriers cannot be used while it is in motion, for mechanism having inertia could not be moved during the short time that the electric circuit would be closed; only60 the simplest attachments can be made to the carrier, for constructional reasons and because of the rough usage that they receive. These and numerous other reasons make the problem most difficult. We have not attempted to solve it by the use of branch tubes and electrically operated switches, but have adopted the simpler and equally effective method of carrying the main line through each of the stations that it unites. In our system each carrier has an individuality determining the station at which it will be discharged from the tube. By a simple attachment to, the front end of the carrier, consisting of a circular metal disk, the sender so marks the carrier that it will pass all stations until it arrives at the station for which it was destined and will there pass out of the tube. In addition to this a method has been devised whereby carriers can be inserted into the tube without the possibility of collision with carriers already running in the tube.

Fig. 21.
A DIAGRAM SHOWING VARIOUS METHODS OF CONNECTING THE STATIONS OF A LARGE SYSTEM WITH PNEUMATIC TUBES.

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Referring now to the diagram, Fig. 21, we have here an imaginary system which we will suppose to be located in some large city. The two large squares I and II indicate central pumping stations, and the small squares A, B, C, D, etc., indicate receiving and sending stations. Some of the stations, such as A, B, C, D, E, F, and Y, which do a large amount of business and may be supposed to be large retail stores, are connected directly with the central station by double tubes, one for sending and the other for receiving carriers. Two smaller stores, such as G and H, may be located on the same line. At I, J, K, and L we have four stations, all connected by the same double line of tubes. These stations we will imagine to be located in the residence62 section of the city. Carriers containing parcels of merchandise or other matter destined for private residences would be sent from the stores A, B, C, etc., to the central station I, where they would be transferred to the line 2 and be adjusted to stop at the station nearest the residence to which the parcels were addressed. From this station the parcels would be delivered by messengers to the residences. If a carrier is to be sent from the central station I to station K, it will be so adjusted before it is put into the tube that it will pass stations I and J, but be discharged automatically from the tube when it arrives at station K. In a similar manner carriers can be despatched from station L to station I or from station J to station L. In passing through the central station the carriers are manually transferred from one line to another.

In another part of the city we may have another central pumping station, II; and the two central stations may be connected by a double trunk line, 3. Again, we have lines radiating from this central station, as shown by station Y. There will be some localities where it will be an advantage to arrange the stations upon a loop, as shown in circuit 4, where stations S, T, U, V, W, and X are connected together in this way. Or we can combine the two arrangements of loop and direct line, as shown in circuit 5. Stations O and R are on the double line, but from O a loop is formed including stations N, M, P, and Q. Here it is supposed that the stations O and R do a much larger business with the central station II than the stations N, M, P, and Q, this being the principal reason for placing them on the double line. All carriers must be returned to the station from63 which they were sent, or others to replace them, otherwise there will be an accumulation of carriers at some of the stations. It is like a railway: there must be as many trains despatched in one direction as the other, each day. Station O can receive a carrier from the central station and return it directly, but when station N receives one it must be returned via M, P, Q, O, and R, a much longer route than that by which it was received. This disadvantage is compensated, when stations N, M, P, and Q do only a small amount of business, by the less cost of laying a single line. If a carrier is to be sent from M to N, it must go via P, Q, and O, being manually transferred at O from the “down” to the “up” line. P can send directly to Q, but Q must send to P via O, N, and M. R can send directly to O and O to R. Similarly in circuit 4 the carriers must all travel around the loop in the same direction, shown by the arrows. Station S can receive carriers directly from the central station, but they must return via U, W, X, V, and T.

Again, we may have a double-loop line, as indicated in the diagram by circuit 6. Here five stations, a, b, c, d, and e, are connected by a loop consisting of two lines of tube, in which the air circulates in one direction in one line and in the opposite direction in the other. Here b can send directly to c, c directly to b, and e to b via d and c, or via central and a. This is an arrangement that would be used where there is a large amount of business between the stations on the loop. As stated before, the best arrangement for any particular locality depends entirely upon circumstances.

64

If a larger than eight-inch tube is to be used for mail service, it should be not less than thirty-six inches. Carriers larger than eight inches cannot be handled: they are too heavy. They are also too heavy to slide through the tube, hence, must be mounted upon wheels. It is not practical to make a carrier on wheels less than eighteen or twenty-four inches, and the carrier must be at least twenty-four inches to contain a large mail-pouch. Now, if we are going to despatch mail-pouches through a pneumatic tube we must send more than one in a carrier, otherwise the service will be too slow. Such large carriers could not be despatched oftener than once or at most twice in a minute. Suppose we were to transport the mail from a railway station to a main post-office. A train arrives with, say, sixty pouches. If only one pouch could be put into a carrier and the carriers could be despatched at half-minute intervals, it would take thirty minutes to despatch all the pouches. Now, suppose we make the tube thirty-six inches. The carriers will be eight feet long and will contain from twelve to fifteen pouches. Five carriers would contain the entire train-load of mail, and they could be despatched in four or five minutes.

Fig. 22.
CROSS-SECTION OF A 36-INCH TUBE.

Fig. 23.
CARRIER FOR A 36-INCH TUBE.

At the stations considerable floor space or “yard room” would be required for side tracks, switches, etc. Usually the basement of a building would have to be utilized for the termination of such a tube. There are but few places in our large cities where the streets are so free from pipes, sewers, conduits, etc., that it would be practicable to build a thirty-six-inch pneumatic tube. When the service can be rendered by an eight-inch tube, the cost of installation favors its adoption. Steep grades cannot be ascended by these very large tubes, while the eight-inch tubes can be placed vertically. We do not say that there is no use for eighteen- and twenty-four-inch tubes, but the69 demand for them would be in special cases and we will not discuss them here. For ordinary mail and parcel service we recommend the use of six- and eight-inch tubes. An eight-inch carrier is twenty-four inches long, about seven inches inside diameter, and will contain five hundred ordinary letters. It weighs about thirteen pounds empty, and one can be despatched every six to ten seconds. We estimate that eighty per cent. of all the parcels delivered from a large retail department store could be wrapped up to go into these carriers. The minimum radius of curvature of an eight-inch tube is eight feet.

Fig. 24 represents a line of two stations with an air-compressor at each station. Such an arrangement is proposed for the line of postal tubes over the New York and Brooklyn bridge, or for any two stations located a very long distance apart, say six or eight miles.

Fig. 24.
DIAGRAM OF A TWO-STATION, TWO-COMPRESSOR LINE.

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Referring to the diagram, we have at station A an air-compressor, c, which draws its supply of air from the tank e, and delivers it, compressed to the necessary pressure, into the tank d. From the tank d the air flows to the sending apparatus, a, and thence through the tube f to the station B. Upon arrival at B it flows through the receiving71 apparatus m, and then by the pipe l to the tank j. A second air-compressor, o, is located at station B, and it draws its supply of air from the tank j. The tank j has an opening to the atmosphere, i, through which air can enter when the air-compressor draws more than is supplied from the pipe l. The opening i in the tank j serves as an escape for air when the air-compressor at station A is started before that at station B. Stations A and B are similar in their arrangements. At B the air-compressor o delivers its compressed air to the tank k, from which it flows to the sending apparatus n, and thence through the tube g back to station A. Upon its arrival at A it passes through the receiving apparatus and enters the tank e, which is open to the atmosphere at h. The tanks d and k serve as separators to remove from the air any dirt and oil coming from the compressors, and they form a cushion, deadening, to some extent, the pulsations of the compressors and making the current of air in the tubes more steady and uniform. The tanks e and j form traps to catch any moisture, oil, or dirt coming out of the tubes.

Carriers are placed in the tubes and despatched by means of the sending apparatus a and n. They are received from the tubes and delivered on to tables by means of the receiving apparatus b and m. It will be seen that the arrangement is such that the air flows through one tube and returns through the other, the same air being used over and over again. Any air that escapes at the sending and receiving apparatus is replaced by an equal amount entering the tanks e and j from the atmosphere. By thus keeping72 the same air circulating in the tubes we prevent an accumulation of moisture in the tubes.

The air is at its maximum pressure in the tanks d and k. The pressure falls gradually as it flows along the tubes and is down to atmospheric when it enters the tanks e and j. The pressure at the receivers, b and m, is just sufficient to push the carriers out on to the tables. The construction of the sending and receiving apparatus will be described in another place.

Fig. 25.
DIAGRAM OF A TWO-STATION, ONE-COMPRESSOR LINE.

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The entire line of tube, going and returning, is operated by air at a pressure above the atmospheric. There is no exhausting in the return tube. It is distinctly a pressure system.

Fig. 26.
DIAGRAM OF A THREE- TO EIGHT-STATION LINE.

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If stations A, B, ... and H were arranged on a loop, as shown in circuit 6, Fig. 21, then station H, Fig. 26, would be at the central, or station A. If it were a single loop, like circuit 4, Fig. 21, then there would be only one sending apparatus and one receiving and transferring apparatus at the intermediate stations.

A telephone circuit will include all stations, in order to give orders to the station attendants and to signal to the central station in case of an accident, when it might be necessary to stop the air-compressor. The telephone wires,79 in the form of a lead-covered cable, are laid in the same trench with the tubes and fastened to them.

Fig. 27.
SENDING APPARATUS.

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Fig. 28.
SENDING APPARATUS.—LONGITUDINAL SECTION.

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We will, for convenience, call the system of swinging tubes B and C, with their supports, etc., the swing-frame or simply the frame. This frame is moved or swung from one position to the other by means of a cylinder and piston, H, placed in an inclined position under it. A lug, N, is cast on the tube B, to which the connecting-rod, O, is attached. The cross-head, P, slides upon an inclined guide, Q. On top of the cylinder is placed a controlling valve, made in the form of a piston slide-valve. The piston in the cylinder H is moved by the pressure of the air taken from the main tube through the pipe I. The apparatus is operated by a hand-lever, K. When this lever is pulled, it moves the sliding-head R, and this, through the spring S, moves the controlling valve, if the valve is not locked. If it is locked, pulling the lever simply compresses the spring S. When the controlling valve is moved to the right the air in the cylinder H escapes through the passage V and the port J to the atmosphere, and compressed air from the main tube flows through the pipe I, the passages T and U, to the cylinder H, under the piston, causing the piston to move up the inclined cylinder and swing the frame until the tube C is in line with the main tube. Carriers are despatched by placing them in the tube C, then pulling the lever K, and swinging the frame until the tube C is in line with the main tube. The carrier is then taken up and carried along by the current of air in the main tube.

Fig. 29.
SENDING APPARATUS.—CROSS-SECTION.

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84

Replacing the hand-lever K in its original position returns the frame to its normal position.

Fig. 30.
SENDING TIME-LOCK.

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Above the cylinder is a cross-head, H, that moves up and down between vertical guides. This cross-head is moved by the rod X, also shown in Fig. 27, that receives its motion from the swinging frame of the sending apparatus. A piston rod, I, attached to the piston in the cylinder, extends up through the travelling cross-head but is not attached to it. On the piston-rod are two enlargements, J and K, one made a solid part of it, the other formed by two nuts. The travelling cross-head H carries a pawl, L, that engages under the shoulder formed by the nuts K. This pawl is kept against the piston-rod by the spring M. The enlargement, J, on the piston-rod forms a shoulder that bears against the bell-crank, N, that connects with the bolt, O, which locks the controlling valve. In the present down position of the piston and piston-rod, the enlargement J, by pressing against the bell-crank N, holds the bolt O in an unlocked position. When a carrier is despatched the cross-head H is lifted by the rod X, and carries with it the piston and piston-rod, compressing the87 spring C. This upward movement of the piston-rod allows the bolt O to be thrown by a spring, not shown in the figure, and so lock the controlling valve of the sending apparatus. As the cross-head continues its upward movement, the pawl L comes in contact with the end of the screw P and disengages the piston-rod. This allows the piston to descend as rapidly as the oil can pass through the by-pass and cock G. When the piston has reached nearly to the bottom of the cylinder, the shoulder J, on the piston-rod, engages the bell-crank N and withdraws the bolt O, thereby unlocking the controlling valve. The time that the sending apparatus is locked depends upon the time required for the piston to descend. While the sending apparatus is locked against the sending of another carrier, it is not so locked that the swing-frame cannot be returned to its normal position and another carrier inserted ready to be sent as soon as the necessary time expires. This time is usually not more than ten seconds. Not only may the second carrier be placed in the tube C, Fig. 29, ready to be sent, but the handle K may be pulled and fastened in the notch a, thereby compressing the spring S, which, as soon as the controlling valve is unlocked, will move the valve and automatically despatch the carrier. The controlling valve is locked by the passage of a bolt through the hole b, in a block carried on the end of the valve stem, when it returns to the normal position shown in the figure. Usually little or no time will be lost in thus locking the sending apparatus, for the small amount of time that the apparatus is locked will be needed in handling the carriers.

88

When a carrier closes an electric circuit in passing one of the boxes located in a manhole about three hundred feet from an intermediate station, it indicates its approach to the station by exciting the electro-magnet A, Fig. 31. This magnet pulls down its armature B and raises the small piston valve C, which admits compressed air to a small chamber, D. The air is supplied to this chamber from the main tube through the pipe E. In one end of this chamber is fitted a piston, F, held to one end of its stroke by a spring, G. When compressed air is admitted to the chamber D, this piston is moved to the left, and by such movement throws the controlling valve of the sending apparatus into its normal position (shown in Fig. 29) and holds it there. This forms a positive lock, and, no matter in what position the sending apparatus may be, it puts the tube B, Fig. 29, into line with the main tube so that the approaching carrier can pass through the apparatus. The piston-rod H, Fig. 31, is connected to the finger d, Fig. 29, and by rocking this finger moves the controlling valve, or prevents it being moved by the handle K.

Fig. 31.
INTERMEDIATE STATION TIME-LOCK.

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Returning now to Fig. 31, we have on the top of the90 chamber D, in addition to the electro-magnet A and its armature B, a differential cylinder and piston, K, L, whose function is to close the valve C when the chamber D is filled with air. The piston K is smaller than the piston L, and sustains a constant air-pressure, supplied through the small pipe M M, from the pipe E, which leads to the main tube. When the chamber D becomes filled from the pipe E through the valve C, the pressure in the chamber moves the piston L upward against the pressure on the piston K, because of the greater area of the piston L. This movement of the differential piston raises the lever I, which passes through a slot in the stem of the differential piston, and thus closes the valve C. The air in the chamber now gradually escapes to the atmosphere through a small orifice Q; in fact it has been escaping here all the time while the chamber was being filled, but the opening through the valve C is so many times larger than the orifice Q that the escape of air was not sufficient to prevent the chamber from filling. Now, however, that all supply to the chamber is shut off, the air in the chamber is gradually being discharged through the orifice. When nearly all the air has escaped, the piston F will return to its normal position, shown in the figure, and unlock the controlling valve. The time required for the air to escape from the chamber, D, is the time that the sending apparatus will be locked, and this time can be regulated by varying the size of the orifice Q. The opening of the orifice, or the time that the sending apparatus is locked, is indicated by an index and dial, P.

This locking mechanism is secured to a bracket on the91 side of the large cylinder H, Fig. 27, in a position where it can be easily inspected. The moving parts of the electro-magnetic valve—for such is the valve C, with the magnet A, Fig. 31—are made very light, in order that they may respond easily and quickly to the closing of the electric circuit.

It is a disadvantage to have stations too numerous upon the same line, especially if they do a large amount of business, for each station will delay the sending of carriers from the others more or less, and the interference will be greatest during the busiest hours of the day. This condition is inherent in any system of large tubes where carriers have to be run a certain minimum distance apart, and cannot be overcome by any mechanism. But the disadvantage is greatly overshadowed by the advantage of being able to connect several stations by one line, instead of having to run independent lines from each station to the central, especially when the business of the individual stations is not sufficient to occupy a separate tube all the time. It makes it possible to have stations where otherwise the business would not warrant the cost of installation and expense of operation. We recommend the establishing of not more than eight stations on a line, and usually a smaller number than this, depending, of course, upon the amount of business to be done at each station.

Fig. 32.
ELECTRO-PNEUMATIC CIRCUIT-CLOSER.

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Fig. 33.
OPEN RECEIVER.

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Fig. 34.
OPEN RECEIVER.—LONGITUDINAL SECTION.

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Fig. 35. OPEN RECEIVER.—SLUICE-GATE MECHANISM.

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Referring to the longitudinal section, the apparatus is attached to the end of a pneumatic tube, A. The current of air from the tube A flows through the slots B into a pipe, C, that conducts it to a tank near the air-compressor. About the centre of the apparatus is a sluice-gate, E, that is raised and lowered by a piston in a vertical cylinder, F, located just above the sluice-gate. This piston is moved by air-pressure taken from some part of the system. When a carrier arrives from the tube A, it passes over the slots B and runs into the air-cushion D, where it comes gradually98 to rest. Checking the momentum of the carrier compresses the air in front of it considerably, and this excess of pressure is utilized to move a small slide-valve that controls the movement of the piston in the cylinder F, so that as soon as the carrier has come to rest the sluice-gate rises and allows the carrier to be pushed out with a low velocity on to a table. The small pipe G conducts a small portion of the air compressed in front of the retarded carrier to the controlling valve, H, seen in Figs. 33 and 35. Referring now to the section of the valve and cylinder, Fig. 35, the pipe G enters the top of a small valve-cylinder containing a hemispherical piston, I, that is held up by a spiral spring, J. This spring has just sufficient tension to hold the piston I up against the normal pressure of air in the tube. When a carrier arrives and compresses the air in the air-cushion, the excess of pressure forces the piston I down against the spring J, and moves the piston slide-valve K. This change of position of the slide-valve allows the air in the cylinder F to escape to the atmosphere through the passage L, passage P, and pipe M, while compressed air from some part of the main tube enters through the port N and passage O to the under side of the piston in the cylinder F. This moves the piston up, carrying with it the sluice-gate E.

There is just sufficient pressure in the tube in rear of the carrier to push the carrier past the gate and on to the table. As the carrier moves out it raises a finger, Q, Fig. 34, that projects into its path. Raising this finger extends the spring R, Fig. 33, and rotates the lever S, bringing the pawl T under the end of the controlling valve-stem.99 When the carrier has passed out and the finger Q is free to descend, the spring R rotates the lever S back to its original position, and thereby raises the controlling slide-valve, which causes the sluice-gate to close. By having the upward motion of the finger Q simply extend the spring R, and the downward motion, by the force of the spring, move the valve, we are enabled to have several carriers pass out of the tube together without having the sluice-gate close until the last carrier has passed out. If raising the finger Q moved the valve, then when the first carrier passed out, the gate would close down upon the second. Attached to the receiving apparatus and extending beyond it is a tube, U, cut away upon one side so that the carriers can roll out of it on to a table, and having in the end a buffer to stop the carriers if by any accident they come out of the tube with too much speed. This buffer consists of a piston covered with several layers of leather and having a stiff spring behind it. The whole apparatus is supported from the floor upon suitable standards, and, for an eight-inch tube, occupies a floor-space twelve feet long by two feet wide, not including the table.

This is the simplest form of receiving apparatus. Owing to conditions of pressure already explained, its use is confined principally to the pumping stations. The only care that it requires is an occasional cleaning and oiling.

Fig. 36.
CLOSED RECEIVER.

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Fig. 37.
CLOSED RECEIVER.—LONGITUDINAL SECTION.

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Fig. 38.
INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.

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Fig. 39.
INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.—VERTICAL SECTION.

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The carrier is discharged down a chute, L, which has a buffer at the bottom, and from the chute it rolls off on to a table. The buffer is made similar to the buffer in the open receiver already described. The cylinder and piston M, that operate to tilt the receiving tube D, are supported upon the base of the apparatus under the closed end of the receiving tube. The cross-head of the piston- and connecting-rods travels between guides that are made a part of the upper cylinder head. The movement of the piston in the cylinder M is controlled by a piston slide-valve exactly similar to the one shown in Fig. 35. The slide-valve is moved, in the same manner, by the air compressed ahead of the carrier when it is brought to rest in the air-cushion D. The air is conducted from the air-cushion to the controlling slide-valve through a small pipe, N, Fig. 36. This pipe leads to one of the trunnions, where it has a joint to allow for the tilting of the receiving tube. When the carrier is discharged from the receiving tube, it raises a finger, O, Fig. 37, located just outside the tube. Raising106 this finger pulls the rod P, Fig. 36, extends the spring Q, turns the lever R, and catches the pawl S, under the end of the controlling valve stem. When the carrier has passed down the chute and allowed the finger O to drop down, the spring Q turns the lever R back to its original position and moves the controlling valve. This causes the receiving tube to return to a horizontal position, where it is ready to receive the next carrier.

At first this apparatus may seem a little cumbersome, but nothing could work better. It is certain in its action and almost noiseless. Carriers are received, discharged, and the receiving tube returned to its normal position in four seconds, and it can be done in less time if necessary.

Fig. 40.
A DETAIL OF THE INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.

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The end of a carrier is represented at T. As the carrier settles down to the bottom of the receiving tube, it comes in contact with the ends of the needles and presses them down, they being supported by two springs U and V. As the needle R is moved down, it makes contact with the spring clip W, located just below it, and closes an electric circuit that includes the electro-magnet X, Figs. 38 and 39, on the valve of the rotating cylinder O. When this electro-magnet is excited it attracts its armature and moves the piston slide-valve Y, that admits air to the top of the piston in the cylinder O, and allows the air under the piston to escape to the atmosphere. The piston moves110 downward and revolves the wheel by means of a connecting rod.

Upon the end of the carrier T is placed a thin circular metal disk, f, which may be copper, brass, tin-plate or any metal that is not easily oxidized. The diameter of this disk of metal determines the station at which the carrier will be discharged from the tube. Disks of various diameters, that may be attached to the carrier, are represented by dashed lines, g, in Fig. 40. When the carrier comes in contact with the two needles R and S, if the circular metal disk on the front end of the carrier has a diameter sufficient to span the space between the two needles, in the position in which it is held, then an electric circuit, made by the wires a and b, will be closed through the needles and the metal disk on the carrier. The metal disk makes a short-circuit from one needle to the other. If the metal disk is not large enough to span the distance between the two needles, then the electric circuit remains broken.

Returning again to Fig. 39, we have the opening J, where the carriers are discharged, closed by a sluice-gate. This gate is opened and closed by a piston moving in a cylinder, h, shown in Fig. 38. A piston slide-valve, i, similar in all respects to the valve on the cylinder O, controls the movement of the piston in this cylinder and the sluice-gate to which it is attached. The slide-valve is moved in one direction, that opens the sluice-gate, by an electro-magnet in the circuit of the wires a and b, Fig. 40.

When the electric circuit made by these wires is closed by a disk on the front end of a carrier, short-circuiting the111 two needles, the valve is moved by the electro-magnet in the circuit, and the sluice-gate is opened. As the wheel, including the receiving tube and carrier, revolves, a lug, j, Fig. 38, on the outside of the wheel comes in contact with the open sluice-gate and the wheel can rotate no farther. A blast of air through the valve L, Fig. 39, assisted by gravity, pushes the carrier out of the receiving tube, through the opening J and down the chute Q, on to the receiving table.

Had the disk on the front end of the carrier been too small to span the distance between the two needles, the circuit would not have been closed, the sluice-gate would not have been opened, no obstruction would have been placed in the path of the lug j, on the wheel, and the wheel would have continued its rotation through ninety degrees until the receiving tube F came in line with the tube D. During the latter part of the rotation, a pin on the wheel engages a lever, k, Fig. 38, and turns a valve, l, Fig. 39, stopping the flow of air through the passage C, compelling it to take another route through the passage m, and the receiving tube F, taking with it the carrier into the tube D. When the carrier leaves the receiving tube and passes through either of the openings J or K, it engages one of the fingers, n or o, that lie in its path. These fingers are connected by rods and levers to the valves on the rotating and sluice-gate cylinders. The ejected carrier pushes these fingers to one side, and after it has passed the fingers return, by the force of a spring, to their former position and move the valves, causing the sluice-gate to close and the wheel to rotate backward into its normal112 position ready to receive the next carrier. The connection between the fingers and the valves is similar to the mechanism on the open and closed receivers, so need not be described in detail here.

The speed with which the carriers are ejected from the receiving tube through the opening J and down the chute Q is regulated by the valve L, which can be opened or closed by a hand-wheel, p. Before the wheel and receiving tube can be rotated, the needles must be withdrawn from the receiving tube, and this is accomplished by a small cylinder and piston, q, shown in Fig. 40. The needles and their encasement are attached to a cross-head, r, on the end of a hollow piston-rod, s. When air is admitted to the top of the piston in the rotating cylinder O, Fig. 39, it is also admitted through the pipe t, Fig. 38, to the cylinder and upper side of the piston q, Fig. 40. This moves the piston q down against the force of a spring, u, and withdraws the needles from the receiving tube. This takes place after the needles have served their purpose and before the wheel is rotated. The piston q has much less inertia than the wheel, therefore it moves much quicker. When the wheel begins to rotate it closes a valve, v, in the pipe t, Fig. 38, confining the air in the cylinder q, and preventing the needles from being raised by the spring u before the wheel returns to its normal position. If by any accident the needles should be raised, no serious harm would result, for their ends would simply bear against the face of the wheel. If this took place constantly, grooves might be worn in the face of the wheel; for this reason the valve v is provided.

In order to facilitate the inspection of the needles and113 electric contact springs W, they are contained in a cylindrical brass case, w, that is held in place beneath the receiving tube by two bolts. By removing the nuts from these bolts the entire mechanism can be removed, examined, and cleaned. It also gives easy access to the receiving tube. The receiving tube is long enough to receive two carriers, if it should ever happen that two arrive at the same time.

Fig. 41.
DIAGRAM OF CONTACT-DISKS AND NEEDLES.

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To show how the apparatus at the various stations is arranged to correspond with the disks of various sizes attached to the front of the carriers, a diagram, Fig. 41, has been made, in which the needles at the bottom of the receiving tubes of the apparatus at six intermediate stations are represented at A, B, C, D, E, and F. Six disks of different sizes are represented at a, b, c, d, e, and f. The needles are placed farthest apart at station A and nearer together at each succeeding station until we arrive at station F, where they are nearest together. If we wish to send a carrier to station A from the central, we place the largest disk, a, upon the front end of it. When it arrives at station A, it closes the electric circuit between the needles and is discharged from the tube. Should we wish to send a carrier to station D, then we place the disk d upon the front end of it. When the carrier arrives at the station A, the disk is not large enough to span the needles; therefore the sluice-gate is not opened and the carrier is sent on in the tube. When it arrives at stations B and C, the same thing occurs again, but when it reaches station D, the needles are sufficiently close together so that the disk makes an electric circuit between them, and the carrier is discharged from115 the tube, as was intended when despatched. Since the carriers always travel in the same direction in a tube, the first station at which they arrive where the needles are near enough together to have both touch the disk, will be the station at which the carrier was intended to stop. Carriers can be despatched from any station, but if we wish to send from say D to A, they must either travel around a loop or be sent through a return tube in which the needles are arranged in the reverse order. If no disk is placed on the carrier, it will go to the last station on the line.

There are other attachments that might be made to the front end of the carriers in order to have them stop at any desired station along a line. We have worked out two other systems which are entirely mechanical in their operation, not using electric circuits and electro-magnets to move the valves. While such a mechanical system has some advantages over the present combined mechanical and electrical system, yet there is one great advantage in the latter, and that is the simplicity of the attachment made to the carrier. A round flat disk of tin-plate is attached to the front end; it is something that is not in the way; it does not prevent standing the carriers on end in racks to fill them; it is not easily injured, and only those who have had experience can realize the rough usage that the carriers receive; it is quickly and easily attached to the carrier, and it is so cheap that when bent it can be thrown away.

Many experiments have been made to find the best material for bearing-rings, but thus far nothing better than a specially-prepared woven fabric has been found. These rings will run about a thousand miles, when they become so reduced in diameter that they have to be replaced by new ones.

The most essential elements of a carrier are strength, lightness, and security of the contents. Aluminum has frequently been proposed as a suitable material for the bodies of carriers, but for the same weight steel is much stronger, especially in thin rolled sheets, and for this reason it has been used.

One of the most perplexing problems that presented itself in working out the details of the system was to design a secure and reliable lock for the lids of the carriers. We believe that the one which has been adopted fulfils all requirements in a satisfactory manner.

Some experiments have been made with carriers that open on the side, but structurally they are weak and unsuited to stand the blows that carriers frequently receive. They are not so easily and quickly filled and emptied as those that open on the end. These remarks apply to carriers for large tubes. In small tubes for the transportation of cash in retail stores, carriers with side openings are found convenient.

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When United States mail is sent through tubes not used exclusively for postal service, carriers with special locks can be used, so that they can be opened only by post-office employees.

Three types of blowing machines are used,—viz., centrifugal fans, positive blowers, and air-compressors.

Fig. 42.
THE STURTEVANT STEEL PRESSURE BLOWER.

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Fig. 43.
ROOT’S POSITIVE PRESSURE BLOWER.

Fig. 44.
SECTION OF ROOT’S TRUE CIRCLE BLOWER.

Fig. 45.
THE GREEN BLOWER.

Fig. 46.
SECTION OF THE GREEN BLOWER.

Fig. 47.
RAND COMPOUND COMPRESSOR OF MODERATE SIZE.

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We are always hoping that wrought-iron or steel tubes will be so much improved in uniformity of dimensions and smoothness of interior that we can use them, but our experiments thus far have been discouraging. It may be123 that some of the new processes of making tubes will give us what we want, but we have not yet found it.

Small tubes and the short bends of large tubes are made of brass, it being the most suitable material. It would be very difficult to bend iron tubes without involving great expense. The thickness of the bent portion of an eight-inch tube is usually three-sixteenths of an inch and never less than one-eighth of an inch.

Where the ground is firm, no other support is needed for the tubes than to tamp the earth solidly about them. In order to economize space in the streets, it is customary to lay the tubes one above the other; and it is very convenient, although not necessary, to separate them by cast-iron saddle brackets. Such an arrangement has to be frequently departed from in order to overcome obstructions in the streets and to get through narrow passages. At all low points in a tube line, traps are provided to catch any moisture that may accumulate. These traps are made accessible for frequent inspection by means of man-holes or otherwise. The tube is usually laid about three feet below the pavement. This distance has frequently to be varied, but it never becomes so small as to render the tubes liable to injury from heavy trucks passing over the pavement.

We will now discuss, in an elementary way, the theory of pneumatic tubes, in order to understand more clearly their modus operandi and the principles upon which they should be constructed. Let us begin with the definition of a pneumatic tube.

These instruments are not usually pictured in our minds when we hear or see the term pneumatic tube used. Instead of these, we think of the brass tubes that we have seen in the large retail stores in some of our cities for conveying cash from the various counters to the centrally located cashier’s desk. Again narrowing our definition to125 conform more nearly with the mental picture presented, we will define a pneumatic tube as a long tube for the purpose of transporting material in carriers by means of a current of air in the tube. This, like all definitions, is not entirely satisfactory, if we examine it critically, but it will answer our present purpose.

The second and more usual method of operation consists in maintaining a constant current of air in the tube and in having the carriers inserted and ejected at the ends of the tube without stopping the current of air for any appreciable length of time. It is analogous to launching boats in a rapidly flowing stream, allowing them to float down stream and then withdrawing them. When the boats are in the stream they present little obstruction to the flow of water and check its speed but very little. In order to126 compute the speed with which the boat will pass from one point to another, we only have to know the speed of the stream between those points when no boat is in it. The presence of the boat does not change the speed appreciably. So it is with carriers in a pneumatic tube: they are carried along with the current of air. The air flows nearly as rapidly when a carrier is in the tube as when there is none. The friction of the carrier against the inner surface of the tube creates a slight drag, but it checks the speed of the air only a little. Therefore, in order to know the speed with which a carrier will be transported from one station to another through a pneumatic tube, we need only to know the velocity with which the air flows through the tube when no carrier is present. Of course there are special cases of heavy carriers, or carriers having a large amount of friction from their packing, or of tubes not laid horizontally, where the resistance of the carrier must be taken into consideration, but for our present purpose we will neglect all of these conditions.

Fig. 48.
PRESSURE AND VELOCITY CURVES.

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In order to make our ideas and thoughts as clear as possible let us represent them by a diagram, Fig. 48. We will suppose that a tank, A, is kept constantly filled with compressed air at a pressure of ten pounds per square128 inch, from some source of supply. We will suppose that the pressure of the air in this tank never changes, air being supplied as fast as it flows away. Next, let us assume that a tube eight inches in diameter inside and one mile long (five thousand two hundred and eighty feet) is connected to the tank at one end and left open to the atmosphere at the other. The air will flow in a constant stream from the tank into the atmosphere, for the reason that air is being supplied to the tank as fast as it flows away.

Now we will draw a smooth curve through the tops of all our vertical lines, and we have a curve, E, G, I, K, L, representing the pressure in the tube at every point. It falls gradually from ten pounds to zero, but it does not fall in exact proportion to the distance from the tank. Such a fall of pressure would be represented by the straight dash-line, E, L. The reason why the true pressure-curve is not a straight line, and lies above a straight line, is because air is an elastic fluid and expands, becoming larger in volume as the pressure diminishes. The straight dash-line represents the fall of pressure of an inelastic fluid, like water, when flowing in the tube.

The fall of pressure along the tube is analogous to the fall of level along a flowing stream. In fact, we frequently speak of the descent of a stream as the “head of water” when it is used for power purposes, and we mean by this the pressure the water would exert if it were confined in a pipe. The descent, or change of level, in the bed of a stream is necessary to keep the water flowing against the friction of the banks. The descent of the water imparts130 energy to overcome the friction. In a similar manner, we must have a fall of pressure along the pneumatic tube to overcome the friction of the air against the interior surface of the tube. We find another analogue in the flow of the electric current along a wire; here there is a fall of potential necessary to overcome the resistance of the wire. Since power has to be expended to compress the air and impart to it its pressure, when this pressure disappears we know that the air must be losing its energy or doing work, and we look to see what becomes of it. In the present case, we find that most of this work is expended in overcoming the friction between the air and the surface of the tube.

If the fluid flowing in the tube were inelastic, like water, then the curve of velocities would be a straight horizontal line, for the water would not come out of the tube any faster than it went in. But we are dealing with air, which is an elastic fluid, and, as we stated before, it expands as132 the pressure is reduced and becomes larger in volume. It is this expansion that increases its velocity as it flows along the tube. It must go faster and faster to make room to expand. Since the same actual quantity of air in pounds must come out of the tube each minute as enters the tube at the other end in the same time, to prevent an accumulation of air in the tube, and since it increases in volume as it flows through the tube, it follows that its velocity must increase.

This velocity curve shows us the velocity of the carriers at each station along the line and enables us to regulate our time-locks and to locate the man-holes and circuit-closers connected with each intermediate station. It gives us the length of the “blocks” in our “block system.” When we know the velocity and weight of our carriers, we can compute the energy stored up in them, and from this the length we need to make our air-cushions so as not to have the air too highly compressed. It would be impossible to design our apparatus properly if we did not know the laws that govern the flow of air in the tubes.

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It is very natural to ask the question, why are tubes sometimes operated by compressing the air and at other times by exhausting it? We answer by saying that it is usually a question of simplicity and convenience that determines which system shall be used. Some of the cash systems in the stores use compressed air in the out-going139 tubes from the cashier’s desk and exhaust the air from the return tubes. Both ends of the tubes are then left open at the counters, no sending or receiving apparatus being required there. The carriers are so light, their velocity so low, and the air-pressure varies so little from the pressure of the atmosphere that the carriers can be allowed to drop out of the tubes on to the counters, and they can be despatched by simply placing them at the open end of the tube into which the air is flowing. The currents of air entering and leaving the tubes are not so strong as to cause any special annoyance. At the cashier’s desk some simple receiving and sending apparatus has to be used, but it is better to concentrate all of the apparatus at one point rather than have it distributed about the store, as would be the case if the double system were not used. In the London pneumatic telegraph both methods of operation have been in use. Double lines were laid, and the engines exhausted the air from one tube and forced it into the other. The exhausted tube was therefore used to despatch in one direction, while the other tube, operated by the compressed air, served for despatching in the opposite direction. So far as I know, both work equally well.

In operating large tubes, that is to say, tubes six or eight inches in diameter, there is an advantage in using compressed, rather than exhausted air, in the construction of the sending and receiving apparatus, especially when the tubes are very long. With an ample supply of compressed air always at hand, the air-cushions can be made shorter and more effective in bringing the carriers quickly to rest.140 With exhausted air the cushions are ineffective, and consequently must be made very long in order to stop the carrier before it strikes the closed end of the tube. This does not apply to small tubes where the carriers are so light that they can be stopped without injury by allowing them to strike solid buffers. Again, when compressed air is used, we have a larger difference in pressure between the pressure in the tube and the atmosphere to operate our mechanism by cylinders and pistons. With an exhaust system carriers are not so easily ejected from the tubes of the receiving apparatus; we could not use the simple form of open receiver. Again, if the tubes are laid in wet ground, and a leak occurs in any of the joints, water will be drawn in if air is being exhausted from the tube, while it will be kept out if compressed air is used.

In regard to the question of relative economy of the two systems, we will say that when long tubes are used, requiring high pressures, or, more strictly speaking, a large difference of pressure, to maintain the desired velocity of air-current, there seems to be some advantage in using an exhaust system. The reason is this: the friction of the air in the tube, which absorbs most of the power, increases as the air becomes heavier and more dense. When the air is exhausted from the tube, we are using a current of rarefied air, and this moves through the tube with less friction and, consequently, a higher velocity, for the same difference of pressure, than the more dense compressed air. But for short tubes that require only a small difference of pressure, this advantage becomes very small, and is overbalanced by other advantages of a compressed air system. So, taking141 everything into consideration, there is not so much to be said in favor of an exhaust system.

u : U :: √L : √l

u and U represent the mean velocities of the air in the two tubes and l and L the respective lengths of tubes.

A similar but direct ratio exists between the mean velocities and the diameters of the tubes, thus:

u : U :: √d : √D

This relation, however, is only approximately true for tubes differing greatly in diameter.

The relation of the pressure to other factors is not so simply expressed. For example, in two tubes of the same length and diameter, the relation between the pressures at the ends and the mean velocity of the air may be expressed as follows:

(p₀² - p₁²)^3/2(P₀² - P₁²)^3/2
u  :  U  ::  ————    :    ————
(p₀³ - p₁³)(P₀³ - P₁³)

(p₀² - p₁²)^3/2(P₀² - P₁²)^3/2
u  :  U  ::  ————    :    ————
(p₀³ - p₁³)(P₀³ - P₁³)

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where u and U are the respective mean velocities, p₀ and P₀ the respective pressures at the initial ends of the tubes, and p₁ and P₁ the respective pressures at the final ends of the tubes,—the pressures being measured above absolute zero.

There are other relations that can be similarly expressed, but for them we must refer the reader to a mathematical treatise on the subject.

In the system of pneumatic tubes built and operated by the Batcheller Pneumatic Tube Company, the presence of a large quantity of moisture in the tube is prevented by using the same air over and over again. A little moisture may be deposited when the tube starts into operation, but the amount does not increase appreciably, as very little fresh air is admitted after starting.

“Intimately connected with the working of the tubes is144 the removal of obstructions which occur from time to time, causing not unfrequently serious inconvenience and delay. The most general cause of obstruction is a stoppage of the train arising from accident to the tube, to the carriers or piston, or to the transmitting apparatus. In such cases the delay is generally very brief, it being for the most part sufficient to reverse the pressure on the train from the next station, and to drive it back to the point it started from. If one or more of the carriers break in the tube, reverse pressure is also generally sufficient to remove the obstacle; but where this fails, the point of obstruction must be ascertained. This is done by carefully observing the variations of air pressure in the reservoir when placed in connection, first with a line of known length, and then with the obstructed tube. By this means the position of the obstruction can be ascertained within one hundred feet. Or the tube may be probed with a long rod up to a length of two hundred feet. A very ingenious apparatus, by M. Ch. Bontemps, is shown in Figs. 49 and 50, and is employed to ascertain the exact position of the obstruction. It acts by the reflection of sound-waves on a rubber diaphragm. A small metal disk is cemented to the rubber, and above this is a pointed screw, D. An electric circuit is closed where the points C and D are brought in contact. To locate an obstruction a pistol is fired into the tube as shown, and the resulting wave, traversing the tube at the rate of three hundred and thirty metres a second, strikes the obstruction and is then reflected against the diaphragm, which in its turn reflects it to the obstacle, whence it returns to the diaphragm. By this means indications are marked on the146 recording cylinder, and if the interval of time between the first and second indications be recorded, the distance of the obstacle from the membrane is easily ascertained. The chronograph employed is provided with three points; the first of these is placed in a circuit, which is closed by the successive vibrations of the diaphragm; the second corresponds to an electric regulator, marking seconds on the cylinder; and the third subdivides the seconds there marked. Fig. 50 indicates a record thus made. In this case the obstacle is situated at a distance of sixty-two metres, and the vibration marks thirty-three oscillations per147 second. The interval occupied by two successive marks from the diaphragm on the paper corresponds to twelve oscillations, and the distance of the obstruction is then calculated by the following formula:

D = 0.5 × 330 × ¹²⁄₃₃ = 60 metres;

so that the distance of the obstacle is recorded within two metres.

Fig. 49.
OBSTRUCTION-RECORDING APPARATUS.

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Fig. 50.
OBSTRUCTION-RECORDING APPARATUS.

“Amongst the special causes of accident may be mentioned the accidental absence of a piston to the train, breaking of the piston, the freezing up of a piston in the tube, and even forgetting the presence of a train, which has caused the entire service to be one train late throughout the day. Finally, the tubes themselves are sometimes broken or disturbed during street repairs, resulting of course in a complete cessation of traffic in the system till the damage is made good.”

Transcriber’s Notes:

The spelling, punctuation and hyphenation are as the original, with the exception of apparent typographical errors which have been corrected.