PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 



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onc-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 surface . 



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 J inches in diameter, leaving one-eighth of an inch projecting all 



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Fig. 3.— Diamond core drill 



