PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 
one-fourth-inch pipe, while the drill is held in its lower end by means 
of a headless set screw instead of the usual drift hole. In this way 
water may be carried in a very simple and convenient manner to the 
inside of the revolving drill. This method of supplying water to 
the drill has proved simpler and more satisfactory than the scheme 
described in Bulletin 44, in which the water is introduced at the side 
through a stationary brass ring. 
The Tobin bronze drill crown C, carrying the diamond points, is 
soldered to the lower end of the steel core tube A as shown in the cut. 
The crown should 
measure about one- 
eighth of an inch thick 
across its lower face, 
should be about 
three-sixteenths of an 
inch thick, and show 
a slight taper in its 
outer side, to allow 
clearance when cut- 
ting. Eight or ten 
holes are drilled in 
the lower face of the 
crown, of such size 
that the diamonds 
will wedge firmly into 
them, until nearly 
flush with the surf ace. 
Black diamonds or 
carbons. 
The diamonds used 
in the drills and saws 
should be dense, reg- 
ular in shape, and 
should range in size 
from one-sixteenth 
of an inch to three 
thirty-seconds of an 
inch in diameter. No flat, scaly, or porous diamonds should be used. 
Diamonds suitable for this work will usually weigh about one-tenth 
of a carat (0.02 gram) each. 
Diamond saw. 
A cut of the diamond saw is shown at A, figure 4. The saw 
proper is made of Russia iron and consists of a flat disk 8 inches in 
diameter and 0.03 inch thick. It is made as follows : 
The saw is firmly clamped between two circular wooden blocks 
7| inches in diameter, leaving one-eighth of an inch projecting all 
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Fig. 3.— Diamond core drill. 
