78 BULLETIN 949, U. S. DEPARTMENT OF AGRICULTURE. 
sions shown, however, and the diamonds set as indicated, the cores should come out 
very close to 25 mm. in diameter. The pressure should be applied always by hand 
and never automatically on account of the tendency of carbons to shatter if subjected 
to any appreciable impact. When drilling stone by hand, the pressure on the drill 
may be regulated in accordance with the character of the material being drilled. 
This is, of course, not the case if an automatic feed is used. A much greater pressure 
may be used when drilling fine-grained, homogeneous materials, such as trap, even 
if the rock is very hard, than in the case of coarse-grained, non-homogeneous materials 
or stone in which minerals of greater hardness than the mass of the rock are embedded. 
With a properly constructed drill, it should be possible to cut a core 4 inches in length 
from rock of medium hardness in about 10 minutes. Properly constructed drills 
will cut from 20 to 50 feet of rock, depending, of course, on the average hardness of the 
stone being used. 
