74 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



A word now on the perforating machines. They were placed to the 

 number of ten on a carriage free to move, forwards or backwards, on 

 rails ; a second vehicle, a kind of tender behind this, carried the 

 reservoirs of water and compressed air (see Plate IV). The com- 

 pressed air, introduced through a box in a cylinder furnished with a 

 piston, communicated to the latter and to its rod the oscillating 

 movement which, transmitted to the cutters, caused the repeated strik- 

 ing of the tools on the rock. But besides this longitudinal or clashing 



FIG 48. Clearing the rubbish in the Alpine tunnel. 



movement, each ciitter possessed two other movements, indispensable 

 in the nature of the work to be accomplished by each of them. In 

 boring its hole, the cutter was obliged to turn gradually on itself like 

 a gimlet, and also to advance as the hole became deeper. These two 

 movements were produced by a small lateral machine moved like 

 the other, by compressed air, and serving at the same time to regu- 

 late the movement of the slide valves of the first, to act on a rachet 



