CHAP, iv.] APPLICATIONS OF COMPRESSED AIR. 79 



carefully made, so that whilst being perfectly air-tight, the surface of 

 the tube is kept as smooth as any other part. 



The tubes are all worked from one centre, where the engines and 

 air-pumps are fixed : in London there are for the purpose of com- 

 pressing and exhausting air, 3 engines, each of 50-horse-power 

 (nominal). In the busy parts of the day, two engines are in use, 

 whilst the third is kept spare. The air-pumps are six in number, 

 and are of the diameter of 35 inches with a stroke of 3 feet. From 

 the pumps lead two large mains, one for " pressure," and the other 

 for "vacuum ;" these mains reach to the instrument gallery. The size 

 of the mains is so arranged that the intermittent action of the pumps 

 is obviated. 



The tubes are arranged in the gallery side by side, first those for 

 receiving only, then those for alternately sending and receiving, and 



FIG. 4t>. Section of carrier. 



lastly those for sending only. The tubes terminate in valves, which 

 are possessed of a double action, so that they can be used for sending 

 or receiving, or for both ; in connection with these valves are pipes 

 which communicate with the mains. 



Every carrier containing messages is signalled electrically, and its 

 arrival is also made known in a similar manner : this is particularly 

 necessary when an " up " and " down " traffic is carried through the 

 same tube. 



The carriers or pistons in which the messages are placed, are 

 made of a cylindrical box of gutta-percha. A section is shown 

 above. 



The portion shaded is the gutta-percha, which is covered with felt 

 or drugget projecting at the ends //. The front of the carrier is 

 provided with a buffer or piston b, just fitting the lead pipe. At the 



