BORING 441 



The boring rods are allowed to have a percussive fall ; and are connected by iron 

 clamps to tho rope, which passing over a wheel fixed in the shear legs, is fastened to 

 tho lever D. An up-and-down motion is imparted to this by the wheel E, which has 

 two blocks or cams fixed in its periphery. 



The action of these cams gives two full strokes of the rods for every revolution of 

 tho wheel E. 



The stroke at commencing a borehole is about 1 foot 3 inches, but as the depth 



178 



increases, the stroke is shortened by attaching the ropo nearer to the centre of the 

 lever. 



At a depth of 800 feet this stroke would be not more than a few inches, the weight 

 of rods being sufficient to drive the chisel forward, with a very short stroke. 



When the rods have to be drawn for changing tho chisel they are attached to the 

 hook F, and are drawn up by means of a small drum on the main shaft to tho height of 

 the small wheel n ; tho first length is then unscrewed and put on one side, the hook F 



