THE HYDRAULIC EAM 



693 



designed by M. Sommellier and used in the work on the Mont Cenis 

 Tunnel. Here the inlet valve V\ and the waste valve F 2 are mechanically, 

 driven, and are coupled together. When the valve F 2 is opened the water 

 in the compressor finds its own level and air at atmospheric pressure is 

 drawn in through the air valve F 4 . F 2 is then closed and F x opened, 

 allowing pressure water to flow along the pipe A, and in virtue of its 

 pressure and momentum to compress the air in the chamber B, whence 

 it passes through the delivery valve F 3 into the air reservoir. V\ is then 

 closed, F 2 opened, and the cycle of operations repeated as before. Work- 

 ing under a head of 85 feet, this compressor delivers air at a pressure of 



. 337. Hydraulic Air Compressor. 



75 Ibs. per square inch. The machine is, however, somewhat cumbrous 

 for the amount of work which it is capable of performing. 



ART. 187. THE JET PUMP. 



The fact that the pressure energy of a water supply may be converted 

 into kinetic energy, with a consequent reduction of pressure, is taken 

 advantage of in the type of jet pump devised by Professor James Thomson 

 (about 1852). The pump, as usually constructed, is illustrated in Fig. 338. 

 Here water from the source of supply is led through the converging 

 passage P, its pressure diminishing as its velocity increases, and is 

 finally discharged into the delivery pipe through the diverging passage I) 

 It follows that at the section J, where its velocity is greatest, its pressure 



