=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. [Illustration: FIG. 26. DIAGRAM OF A THREE- TO EIGHT-STATION LINE.]