Sub-Post-Office Receiver.
—We have already explained that the air-pressure in the tube at the sub-post-office is about three and three-quarters pounds per square inch. With such a pressure we cannot open the tube to allow the carriers to come out. They must be received in a chamber that can be closed to the tube after the arrival of a carrier and then opened to the atmosphere. Furthermore, this chamber must act as an air-cushion to check the momentum of the carriers. Fig. 13 shows the sub-post-office apparatus when a carrier is being delivered from the receiving apparatus, or, as we will name it for convenience, the receiver. Fig. 14 is a drawing of the same apparatus, partly in section, that shows more clearly its method of operation. This drawing shows the sending apparatus in a different position from Fig. 13, but that is immaterial. The receiver consists of a movable section of tube, about twice the length of a carrier, closed at one end, supported upon trunnions, and normally in a position to form a continuation of the main tube from which the carriers are received. When a carrier arrives it runs directly into the receiver, which being closed at the end forms an air-cushion that stops the carrier without shock or injury. Just before reaching the receiving chamber the current of air passes out through slots in the walls of the tube into a jacket that conducts it to the sending apparatus, as shown in Fig. 14. At the closed end of the receiving chamber, or air-cushion, is a relief valve, normally held closed by a spring. As the carrier compresses the air in front of it, this valve opens and allows some of the air to escape, which prevents the carrier from rebounding into the tube. Under the outer end of the receiving chamber is a vertical cylinder, E, Fig. 14, supported upon the base-plate containing a piston. The piston of this cylinder is connected by a piston- and connecting-rod to the receiving chamber. When air is admitted to the cylinder under the piston, the latter rises and tilts the receiving chamber to an angle of about forty degrees, which allows the carrier to slide out. The receiving chamber carries a circular plate, C, that covers the end of the main tube when it is tilted. A small piston slide-valve, F, located near the trunnion of the receiving chamber, controls the admission and discharge of air to and from the cylinder E, upon the arrival of a carrier. When a carrier arrives and compresses the air in the air-cushion or receiving chamber, a small portion of this compressed air is forced through pipe G, to a small cylinder containing a piston and located just above the piston slide-valve F. The increased pressure acting on the piston moves it downward, and it in turn moves the slide-valve F. Thus it will be seen that the stopping of the carrier causes the receiving chamber to be tilted and the carrier slides out on to an inclined platform, K. This platform is hinged at one end, and supported at the angle seen in the figure by a counterweight. When a carrier rests upon it, the weight of the carrier is sufficient to bear it down into a horizontal position; in this position the carrier rolls off on to a table or shelf. The platform, K, is connected by rods, bell-cranks, etc., to the piston slide-valve, so that when it swings downward by the weight of a carrier, the slide-valve is moved upward into its normal position, and this causes the receiving chamber to tilt back into a horizontal position ready to receive the next carrier. The time that elapses from the arrival of a carrier until the receiving chamber has returned to its horizontal position is not more than three or four seconds. Nothing could operate in a more satisfactory manner.
Main Post-Office Receiver.
—At the main post-office we have a receiver of a different type. It will be remembered that the pressure in the return tube at the main post-office is nearly down to zero or atmospheric, so that we can open the tube to allow the carriers to pass out without noise or an annoying blast of air. Figs. 15 and 16 show the main-office apparatus, and Fig. 17 is a drawing of the same. Here the receiver consists of a section of tube closed by a sluice-gate, located at B, Fig. 17. The air-current passes out through slots in the tube into a branch pipe leading to the return tank in the basement. These slots are located about four feet back of the sluice-gate, so that the portion of the tube between the slots and the sluice-gate forms an air-cushion to check the momentum of the carriers. The sluice-gate is raised and lowered by a piston moving in a cylinder located just above the gate. The movement of this piston is controlled by a piston slide-valve in a manner similar to the apparatus at the sub-post-office. Air for operating the piston is conveyed through the pipe D, Fig. 17, from the pipe leading from the air-compressor to the sending apparatus. This air is at about seven pounds pressure per square inch.
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 compresses the air in front of it against the gate. This compression checks its momentum, and it comes gradually to rest. The air compressed between the carrier and the sluice-gate operates to move the piston slide-valve, thereby admitting air to the gate cylinder under the piston, which rises, carrying with it the sluice-gate. The tube is now open to the atmosphere, and there is just sufficient pressure in the tube to push the carrier out on to a table arranged to receive it. As the carrier passes out of the tube it lifts a finger out of its path. This finger is located at E, Fig. 17, and when it is lifted by the passing carrier it moves the piston slide-valve, and the sluice-gate is closed. A valve is located in the branch-pipe that conducts the air to the return tank in the basement. If the pressure in the tube is not sufficient to push the carrier out on to the table, this valve is partially closed, thereby increasing the pressure to a desired amount.