=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