At the sub-post-office, after flowing through the receiving and sending apparatus, it enters the return tube and flows back to the main office, passing through the receiving apparatus there and then to a tank in the basement,—the left tank in Fig. 10. The air-compressor draws its supply from this tank, so that the air is used over and over again. This return tank has an opening to the atmosphere, which allows air to enter and make up for any leakage or escape at the sending and receiving apparatus, thereby maintaining the atmospheric pressure in the discharge end of the tube and in the suction of the compressor. The tank serves to catch any moisture and dirt that come out of the tube. Fig. 11 is a diagram showing the direction and course of the air-current. It will be noticed that both the out-going and return tube are operated by _pressure_, in distinction from _exhaust_. The air is forced around the circuit by the air-compressor. There is no exhausting from the return tube. The pressure of the air when it enters the tube at the main post-office is, say, seven pounds per square inch; when it arrives at the sub-post-office the pressure is about three and three-quarters pounds, and when it gets back to the main office and enters the return tank, the pressure is zero or atmospheric. Thus it will be seen that the pressure becomes less and less as the air flows along the tube. This is not the pressure that moves the carriers, but the pressure of the air in the tube, a pressure that exists when there are no carriers in the tube. It is the pressure that would be indicated if you should drill a hole into the tube and attach a gauge. [Illustration: FIG. 11.] =Terminal Apparatus.=—When the construction of this line was begun, it was the intention of the Pneumatic Transit Company to use the apparatus of the Electro-Pneumatic Transit Company, at both stations, for sending and receiving carriers, and so-called working-drawings were obtained for this purpose. The sending apparatus was constructed according to the designs furnished, but, upon examination of the drawings of the receiving apparatus, it was so apparent that it would not work as intended that it was never constructed. The writer was asked to design an automatic receiver 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. [Illustration: FIG. 12. _TRANSMITTER.—PHILA._ SENDING APPARATUS.]