CHAP, iv.] APPLICATIONS OF COMPRESSED AIR. 77 



wheel which impelled the piston and the cutter, and to force forward 

 the cylinder gradually as the boring of the hole of the rock advanced. 



Each perforating machine could give 200 strokes of the cutter in a 

 minute, consuming at each stroke a little less than a litre of compressed 

 air. The rate of advance of the work depended naturally partly on 

 the nature and hardness of the rock. 



The success of this application of compressed air as a motive 

 power, in an enterprise which could only employ steam with difficulty, 

 suggested the idea of extending the use of this power to other works ; 

 for instance, in countries where the water-courses produce falls, and 

 consequently natural power, they could be employed to compress 

 air, which could be distributed, through pipes, to the homes of a 

 labouring population, and thus solve the problem of the economic 

 distribution of power. In the meanwhile, while this use and trans- 

 formation of the force of water-falls is being realized and comes into 

 general use, it will be well to point out some of the special applica- 

 tions used in the present day. 



III. COMPRESSED Am POSTS COMPRESSED AIE EAILWAYS. 



A few years ago, the Administration of Telegraphic Lines established 

 in Paris a communication between the two stations of the Grand 

 Hotel and the Place de la Bourse. A tube 1,100 metres long and 

 O m '065 in diameter connected at each of its extremities two chambers 

 which served to introduce into it or to extract from it a piston carrying 

 despatches. This piston, cylindrical in form, is nothing more than a 

 box closed at one end and at the other furnished with a movable lid. 

 The despatches are placed under cover in the interior. A covering of 

 leather enables the piston to adapt itself exactly against the sides of 

 the tube, in a way to prevent the passage of the compressed air. 



Each chamber can by using two cocks be placed in communica- 

 tion at will either with the exterior free air, when the despatches- are 

 to be received, or with the reservoir of compressed air if the piston- 

 carriage is to be sent to the other station. 



As to the compression of the air, it is managed in a very simple 

 and economical way, with the assistance of the pressure from the water 

 from the town reservoirs, which at each of the two stations about 



