8 TUNNELING 



or as reaction turbines. Impulse turbines are those in which 

 the whole available energy of the water is converted into 

 kinetic energy before the water acts on the moving part of the 

 turbine. .Reaction turbines are those in which only a part of 

 the available energy of the water is converted into kinetic 

 energy before the water acts on the moving vanes. Impulse 

 turbines give efficient results with any head and quantity of 

 water, but they give better results when the quantity of water 

 varies and the head remains constant. Reaction turbines, on 

 the contrary, give better results when the quantity of water 

 remains constant and the head varies. These observations 

 indicate in a general way the form of turbine which will best 

 meet the particular conditions in each case. The number of 

 turbines required, arid their dimensions, will be determined in 

 each case by the number of horse-power required and the 

 quantity of water available. The power of the turbines is 

 transmitted to the air compressors or pumps by shafting and 

 gearing. 



Air Compressors. An air compressor is a machine usually 

 driven by steam, although any other power may be used by 

 which air is compressed into a receiver from which it may be 

 piped for use. For a detailed description of the various forms 

 of air compressors the reader should consult the catalogues of 

 the several makers and the various text-books relating to air 

 compression and compressed air. Air compressors, like other 

 machines, suffer a loss of power by friction. The greatest loss 

 of power, however, results from the heat of compression. 

 When air is compressed, it is heated, and its relative volume 

 is increased. Therefore, a cubic foot of hot air in the com- 

 pressor cylinder, at say, 60 Ibs. pressure, does not make a cubic 

 foot of air at 60 Ibs. pressure after cooling in the receiver. 

 In other words, assuming pressure to be constant, a loss of 

 volume results due to the extraction of the heat of compression 

 after the air leaves the compressor cylinder. To reduce the 

 amount of this loss, air compressors are designed with means 



