HARBOUR CONSTRUCTION, 



The form of drills used was that of a cross ; but they were 

 costly to make and sharpen. Flat drills of the ordinary form 

 did not answer, on account of their corners wearing away too 

 fast, especially when boring holes that were horizontal, or nearly 

 so. This was obviated, to some extent, by increasing the wearing 

 surface, but they were not found to work so satisfactorily as 

 drills of the cross form. 



The cost per lineal foot of drilling by hand was only about 

 one-half of the cost of drilling by steam ; but the hand-drills 

 were only 2 inches wide, diminishing slightly to give clearance 

 as the boring advanced ; whereas the steam-drills were 3J inches 

 wide, diminishing to about 2 inches at the bottom of a hole, 

 say, 20 feet deep. 



The larger holes, drilled by steam, enabled the charges to be 

 placed well down at the bottom, which, especially in holes that 

 were horizontal, was found to be of great advantage. Indeed, 

 the quantity of rock thrown out by charges placed in the large 

 machine-bored holes was about double, per lineal foot of hole, 

 that thrown out by charges placed in the smaller hand-bored 

 holes. The cost of drilling by steam and hand, per ton of stone, 

 was therefore about equal. 



Some very good results were obtained by simultaneously 

 firing, by means of electricity, groups of 5 or 6 charges, placed 

 in machine-bored holes. 



The speed with which drilling can be done is necessarily 

 governed by the texture of the rock and the size of the drill used. 



The following statement will give the reader a fair general 

 idea of the speed of drilling by hand in hard rock : 



Usual average 



depth drilled 



per hour. 



feet. 

 1-17 

 1-04 

 2-17 

 1-43 

 1-20 



0-80 

 060 

 0-50 

 0-40 

 6-00 

 4-50 



" Blasting and Quarrying of Stone," Burgoyne. 



