SOUTH BULL! AND UKLLAM.BI COLLIERIES. 103 



stroke, when the top set of ports do coincide with the piston 

 ports, no escape can take place because the connecting rod 

 head fits tightly inside the cylinder. 



This quadruples compressor is very simple in construction, 

 and is readily taken to pieces. The automatic unloading de- 

 vice shown at (K) is a bye-pass valve, which forms a connec- 

 tion between the delivery and suction side of the compressor. 



Fig. 120. Little Hardy Coal Cutter. 



It is automatically controlled, so that when the air pressure 

 reaches the desired limit the bye-pass valve opens, thus re- 

 lieving the motor of the load. As soon as the pressure falls to 

 a predetermined limit, this valve closes, and the compressor 

 commences to deliver air again. The air receiver between the 

 compressor and the air drills is 10ft. long by 3ft. diameter. 



