348 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



ployed. The shots are fired by means of electricity, Xobel's 

 low tension fuses and exploder being used. 



Compressed air is used for the Ingersoll punches, the 

 Little (jriant air drill, and a Tangye-Snow pump. There 

 are air receivers underground as well as on the surface; in 

 fact, each district has its own receiver. The main air pipe is 

 six inches in diameter, from which three-inch branch pipes are 

 led into the slants, one and a half inch pipes being provided 

 for each bord. The air is compressed in a ]^orwalk tandem 

 compound compressor, with mechanical valves, and provided 

 with an overhead intercooler, supplemented by two Ingersoll- 



Fig. 218. Ingersoll-Serjeant Compressor. 



Sergeant single cylinder compressors. In the Norwalk com- 

 pressor the air and steam cylinders are arranged tandem. (Fig. 

 217.) The object of compounding the air cylinders is to aver- 

 age the resistance throughout the stroke, instead of having 

 an excessive maximum resistance at the end of the stroke, as 

 is the case with single compressors. By having the iinal pres- 

 sure in the intake cylinder comparatively light, the loss in 

 capacity due to clearance is reduced to the smallest amount; 

 moreover, the cylinders are w T ater jacketed, which helps to 



