Two-Station, One-Compressor Line.
—Fig. 25 is a diagram showing two stations, A and B, connected by a double line of tubes, both operated by one air-compressor located at station A. This is the arrangement used in the Philadelphia post-office line, and is the arrangement that will ordinarily be used for all two-station lines except where unusual conditions require something different. Station A is arranged precisely like station A in Fig. 24, so it need not be described again. The air flows from the sending apparatus a through the tube f to the receiving apparatus p at station B. From the receiver p it flows through the pipe l to the sending apparatus n and thence through the tube g back to station A. The receiver p at station B is what we will call a closed receiver,—i.e., it delivers the carrier from the tube on to the table without opening the tube to the atmosphere. The use of this form of receiver is made necessary by the fact that the air-pressure in the tube at this station is considerably above atmospheric. The air-pressure is at a maximum in the tank d. It falls gradually along the tube f, and when the air arrives at the receiver p, at station B, the pressure has fallen nearly to one-half its maximum amount in the tank d. On its return journey through the tube g the pressure continues falling until it reaches the atmospheric pressure when the air enters the tank e at station A.
The entire line of tube, going and returning, is operated by air at a pressure above the atmospheric. There is no exhausting in the return tube. It is distinctly a pressure system.
Fig. 26.
DIAGRAM OF A THREE- TO EIGHT-STATION LINE.
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Three- to Eight-Station Line.
—Thus far we have described only two-station lines. In Fig. 26 we have a diagram of three stations connected together by a double line of tubes, and the arrangement would be similar if it were extended to four, five, six, seven, or eight stations. The stations are called A, B, and H. Station A is arranged exactly the same as stations A in Figs. 24 and 25, therefore, needs no description. Station B, being an intermediate station, is quite differently arranged from any of the preceding. From station A the air flows through the tube f to station B, where it enters the automatic receiving and transferring apparatus, s. From this it flows through the tube f′ to the sending apparatus r, and thence through the tube f″ to the next station, which may be another intermediate station, C, or the terminal station H. Station H is arranged like station B, Fig. 25. The air from the tube f″ enters the receiver p, and is then returned, through the pipe l, to the sending apparatus n. From the sending apparatus n it continues on its return journey through the tube g to the intermediate station B, where it enters the receiver and transfer apparatus t, then passes to the sending apparatus q, and through the tube g″ back to the receiver b at station A. Thus we have followed the air-current out through one tube and back through the other. The current is kept circulating by the compressor located at station A. The pressure is at a maximum in the tank d, and falls gradually as the air flows along the tube until it returns to the tank e, when the pressure has fallen to atmospheric. A carrier is despatched from station A, and after passing through the tube f arrives at station B, where it stops momentarily in the automatic receiver and transfer apparatus s. If the carrier is intended for station B, and was properly adjusted when it was despatched at A, it will be discharged from the apparatus s on to the table u. But if it were intended for some other station and were so adjusted, after the delay of two or three seconds in the apparatus s, it will be automatically transferred to the tube f′, pass through the sending apparatus r, and go on its journey through tube f′ to the next station. If it is not discharged from the tube at any of the intermediate stations, it will finally arrive at the terminal station H and there stop. Just how the carriers are adjusted and the details of the receiving and transfer apparatus will be described hereafter. Carriers arrive at station B from H, or other stations on the line, through tube g, in the apparatus t, which either discharges them on to the table u or sends them on through the tube g′ and g″ to station A. Carriers are despatched from station B to station A by means of the sending apparatus q, and from station B to other stations along the line, C, D, E, F, G, and H, by means of the sending apparatus r. Thus, from B carriers can be sent and received in either direction. In order to prevent the possibility of a collision of carriers by attempting to despatch one at station B at the instant another is passing through the sending apparatus, an automatic lock is attached to each sending apparatus. Just outside the station B, say three hundred feet on each side, are located manholes, and in these manholes boxes are attached to the tube containing an electric circuit-closing apparatus, so arranged that when a carrier passes it will close an electric circuit leading to the sending apparatus in the station. These manholes and circuit-closers are shown and located on the diagram at v and w. Wires x and y lead from them to the sending apparatus r and q. When a carrier from station A passes the box v, it closes the electric circuit x, which sets a time-lock on the sending apparatus r, holding this apparatus locked, so that it is impossible to despatch a carrier for, say, twelve seconds, a sufficient time for the carrier coming from the station A to pass station B and get three hundred feet beyond it. After the twelve seconds have elapsed the sending apparatus is unlocked and a carrier can then be despatched. In a similar manner a carrier coming from station H, in passing the box w, closes the electric circuit y and locks the sending apparatus q for a sufficient length of time to let the carrier pass the station. This resembles, in some respects, the “block system” as used on railroads. A “block” about six hundred feet in length, depending upon the speed of the carriers, is made at each intermediate station with the station in the centre of the block. Whenever a carrier enters this “block” the sending apparatus at the station is locked, and a carrier cannot be inserted into the tube to collide with the one which is passing. It will be noted that a carrier in passing out of the “block” does not unlock the sending apparatus; this is done automatically at a definite time after the carrier entered the block. The unlocking is entirely independent of the carrier after it has entered the block, and the reason it is so arranged is this: suppose that a second carrier enters the “block” before the first one leaves it; if the first carrier unlocked the apparatus when it left the “block,” then it would be unlocked with the second carrier in the block and a collision might occur, but by arranging it as we have done, if a second carrier enters the “block” before the first has passed out, the sending apparatus remains locked for a period of time beginning with the arrival of the first carrier in the “block” and ending, say, twelve seconds after the arrival of the last carrier, which is sufficient time for the last carrier to pass out of the block. Of course, if a carrier becomes wedged in the tube a collision may occur, but this very seldom if ever happens. The details of the locking apparatus will be described in another place.
If stations A, B, ... and H were arranged on a loop, as shown in circuit 6, Fig. 21, then station H, Fig. 26, would be at the central, or station A. If it were a single loop, like circuit 4, Fig. 21, then there would be only one sending apparatus and one receiving and transferring apparatus at the intermediate stations.
A telephone circuit will include all stations, in order to give orders to the station attendants and to signal to the central station in case of an accident, when it might be necessary to stop the air-compressor. The telephone wires, in the form of a lead-covered cable, are laid in the same trench with the tubes and fastened to them.