Referring to the diagram, we have at station A an air-compressor, _c_, which draws its supply of air from the tank _e_, and delivers it, compressed to the necessary pressure, into the tank _d_. From the tank _d_ the air flows to the sending apparatus, _a_, and thence through the tube _f_ to the station B. Upon arrival at B it flows through the receiving apparatus _m_, and then by the pipe _l_ to the tank _j_. A second air-compressor, _o_, is located at station B, and it draws its supply of air from the tank _j_. The tank _j_ has an opening to the atmosphere, _i_, through which air can enter when the air-compressor draws more than is supplied from the pipe _l_. The opening _i_ in the tank _j_ serves as an escape for air when the air-compressor at station A is started before that at station B. Stations A and B are similar in their arrangements. At B the air-compressor _o_ delivers its compressed air to the tank _k_, from which it flows to the sending apparatus _n_, and thence through the tube _g_ back to station A. Upon its arrival at A it passes through the receiving apparatus and enters the tank _e_, which is open to the atmosphere at _h_. The tanks _d_ and _k_ serve as separators to remove from the air any dirt and oil coming from the compressors, and they form a cushion, deadening, to some extent, the pulsations of the compressors and making the current of air in the tubes more steady and uniform. The tanks _e_ and _j_ form traps to catch any moisture, oil, or dirt coming out of the tubes. Carriers are placed in the tubes and despatched by means of the sending apparatus _a_ and _n_. They are received from the tubes and delivered on to tables by means of the receiving apparatus _b_ and _m_. It will be seen that the arrangement is such that the air flows through one tube and returns through the other, the same air being used over and over again. Any air that escapes at the sending and receiving apparatus is replaced by an equal amount entering the tanks _e_ and _j_ from the atmosphere. By thus keeping the same air circulating in the tubes we prevent an accumulation of moisture in the tubes. The air is at its maximum pressure in the tanks _d_ and _k_. The pressure falls gradually as it flows along the tubes and is down to atmospheric when it enters the tanks _e_ and _j_. The pressure at the receivers, _b_ and _m_, is just sufficient to push the carriers out on to the tables. The construction of the sending and receiving apparatus will be described in another place. [Illustration: FIG. 25. DIAGRAM OF A TWO-STATION, ONE-COMPRESSOR LINE.]