84 TUXNKLINd 



in the efficiency of the cooling device to obtain low first cost, 

 economy in space, light weight, higher speed, greater durability, 

 and greater general availability. 



Air compressors are also distinguished as double acting and 

 simple acting. They are simple acting when the cylinder is 

 arranged to take in air at one stroke and force it out at the 

 next, and they are double acting when they take in and force 

 out air at each stroke. In form compressors may be simple or 

 duplex. They are simple when they have but one cylinder, 

 and duplex when they have two cylinders. A straight line or 

 direct acting compressor is one in which the steam and air 

 cylinders are set tandem. An indirect acting compressor is 

 one in which the power is applied indirectly to the piston rod 

 of the air cylinder through the medium of a crank. Mr. W. L. 

 Saunders writes in regard to direct and indirect compression 

 as follows : 



" The experience of American manufacturers, which has been more exten- 

 sive than that of others, has proved the value of direct compression as distin- 

 guished from indirect. By direct compression is meant the application of 

 power to resistance through a single straight rod. The steam and air cylinders 

 are placed tandein. Such machines naturally show a low friction loss because 

 of the direct application of power to resistance. This friction loss has been 

 recorded as low as 5/ ( , while the best practice is about 10% with the type which 

 conveys the power through the angle of a crank shaft to a cylinder connected 

 to the shaft through an additional rod." 



Receivers. Compressed air is stored in receivers which are 

 simply iron tanks capable of withstanding a high internal 

 pressure. The puipose of these tanks is to provide a reservoir 

 of compressed air, and also to allow the air to deposit its 

 moisture. From the receivers the air is conveyed to the work- 

 ings through iron pipes, which decrease gradually in diameter 

 from the receivers to the front. 



Rock Drills. The various forms of rock drills used in tun- 

 neling have been described in Chapter III., and need not be 

 considered in detail here except to say that American engi- 



