100 



HARBOUR CONSTRUCTION. 



Radius Q* 



rill. 



Radius f2" 



Drills should be arranged in sets, and their widths should 

 diminish at the rate of about ^ 6 inch for every 18 inches drilled 

 A in hard rock. A some- 



what smaller allowance 

 will suffice in softer rock, 

 as the drills do not wear 

 away at the sides so fast. 

 Thus in a hole 18 feet 

 deep, assuming it to be 

 started with a 2J-inch 

 drill, the last drill would 

 measure only 1^-g inch. 1 



Drills as shown at A 

 and B, Fig. 18, are used for 

 deep holes. The cutting 

 edge of the first or enter- 

 ing drill is rather more 

 rounded than that of the 

 following drills, it being 

 found easier to bore the 

 first length of a hole true 

 with a drill thus formed 

 than with the squarer 

 tools. Drills of the forms 

 shown at A and B cut 

 triangular holes. Dia- 

 mond-pointed drills (C) 

 cut faster than square or 

 slightly rounded tools. 

 They cut circular holes, 

 and are found more con- 

 venient for " pitching " 

 holes than those of other 

 forms. They will not, 

 however, stand heavy 

 hammering, and are chiefly 

 used for drilling the short 

 plug-holes say from 3 to 

 4 inches deep by means of which blocks of stone are cut or 



1 The steam-drills mentioned on p. 97 were for the most part used for boring 

 horizontal holes, the wear on the sides of the drills being thus much greater than 

 in the boring of vertical holes. 



/# Drill, 



Drill. 



SCALE HALF SIZE. 

 FIG. 18. Drills used in quarrying. 



