CHAP, iv.] APPLICATIONS OF COMPRESSED AIR. 73 



from the fall came through the pipe A, the valve a of which was 

 alternately opened and closed, whilst the valve b of the pipe 13 was 

 itself closed and opened ; a special little machine produced the 

 working of these valves. Finding a opened and c shut, the water, 

 with its acquired velocity, passed through the tube C, and, by rising, 

 compressed the air brought from the outside by the valve e. This one 

 was closed, whilst the air, more and more compressed, forced the 

 valve d and introduced itself into the receiver E. Then the valve b, 

 on being opened, whilst a was closed, the water escaped by the pipe 

 B, c being opened and admitting a fresh quantity of exterior air, 

 another arrangement compressed it and introduced it again into the 

 receiver E. 



Since then, engineers have substituted double-action compression- 

 pumps for the hydraulic rams. They are of much more simple con- 

 struction and require less power. The following are a few details of 

 the way in which these machines were worked at Modane. 



Twelve compression-pumps received their motion from six hydrau- 

 lic wheels put in motion by the fall of the Arc. Each of them con- 

 sists of a piston which receives backward and forward movement in 

 a horizontal cylindrical body. At the two extremities of the cylinder 

 are fixed two vertical tubes, each furnished with two valves : an 

 admission valve, which is at the lower part of the conically formed 

 tube, which admits the air from the exterior, and a valve introducing 

 the compressed air by the ascending of the water and allowing it to 

 penetrate into the corresponding reservoir. 



The movement of the piston, by forcing the water into one of 

 the cylinders, lowers its level in -the other. The air is then com- 

 pressed in the first and rarefied in the second. 



Taking into account the losses occasioned by escapes, the twelve 

 compression-pumps compressed in the mean, in twenty-four hours, 

 116,000 cubic metres of air at the ordinary pressure, and the pressure 

 at which this air was supplied to the perforating machines attained 7 

 atmospheres. 



Such a considerable quantity of air would not have been necessary, 

 if the boring machines alone had had to be put in motion. In fact, the 

 tube which conducted the compressed air from the compression reser- 

 voirs at the bottom of the gallery both supplied power for the perforating 

 machines, and air for the workings of the whole tunnel. 



