312 - Pneumatic Transmission* [July, 



exhausts the air from one and compresses it into the other, 

 thus keeping up a continual circuit of current within the 

 tubes. 



The pneumatic system in use in Paris differs from the 

 foregoing principally in the use of water power instead of 

 steam-engine power for working the tubes, each station being 

 supplied with an arrangement for compressing air. Until 

 recently the transmission of carriers between stations was 

 effected by means of compression alone, produced by the 

 action of water upon a chamber full of atmospheric air. 

 This water is obtained from the River Ourcq, and is employed 

 in the following manner : — Three wrought-iron cylindrical 

 vessels are erected at each station, one of which is large and 

 the other two smaller, and of the same size as one another. 

 The larger vessel is connected by means of a pipe with the 

 water mains of the town, and an exhaust pipe leads from 

 the same vessel into the sewers ; each of these pipes is fitted 

 with a valve to enable the communication to be opened and 

 closed at pleasure. From the top of this vessel a pipe leads 

 into the first of the two smaller reservoirs, and these again 

 are connected together by a pipe fitted with a cock, whilst 

 the second smaller reservoir is in communication with the 

 pneumatic transmitter. In order to obtain a supply of com- 

 pressed air, the valve is closed in the tube communicating 

 with the sewers, and that leading to the water mains is 

 opened, allowing the supply water to rush into the larger 

 vessel, and to displace the air which previously filled it. 

 This displaced air is compressed into the two smaller reser- 

 voirs until the water has risen nearly up to the top of the 

 larger vessel. A cock, in the pipe communicating between 

 that and the smaller reservoirs, is then closed to prevent the 

 air returning. The water is then run out of the larger vessel 

 into the sewers, and it again becomes filled with air, which 

 is in turn compressed into the reservoirs as before, as soon 

 as it is required. 



The Pneumatic Dispatch Company in London employ a 

 pair of horizontal engines to drive a fan 22 feet in diameter, 

 by means of which the air can either be exhausted from, or 

 forced into, the tubes at pleasure. 



The latest form of pneumatic tube is that recently laid 

 down by Mr. Siemens, from the Post Office in St. Martins- 

 le-Grand towards Westminster, and which will probably be 

 extended, in course of time, in other directions. Several 

 entirely new features have been introduced in the construc- 

 tion of this tube, which we shall describe more fully at some 

 future time in our chronicles of engineering, when the system 



