346 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



The borer then comes along with a No. 2 Little Giant air 

 drill. The cutting portion is an ordinary auger, such as is used 

 for boring holes in coal, but in this case it is revolved by com- 

 pressed air instead of by hand, and does it work very much 

 quicker. The operator holds the machine by two handles, in 

 one of which is the valve for turning the air on or off. The 

 handles serve to guide the machine, the forward motion is given 

 by pressing against it with the body. Boring is done by wages 

 men, who bore a hole 6ft. deep in 1J rnimites. Fig-. 216 gives 



Fig. 216. "Little Giant" Piston Air Drills. 



a section of the working portion of the machine. These drills 

 are of the balanced piston type, and consist of four single 

 acting cylinders arranged in pairs, each pair of pistons being 

 connected to opposite wrists of a double crank shaft. Each 

 piston of each pair travels in opposite directions at all parts of 

 the stroke, thereby insuring a smooth running machine. The 

 crank shaft revolves in an enclosed chamber designed to be 

 kept partly filled with the lubricant. This machine weighs 

 151bs., makes 600 revolutions per minute with OOlbs. pres- 

 sure, and uses 20 cubic feet free air per minute. 



