QUARRYING. 93 



Railway Company, where the rock is chalk, the coefficient 

 adopted was 32. 



In this mine, 18,500 pounds of powder were used, and the 

 chalk rock thrown down amounted to about 700,000 tons, being 

 very nearly 38 tons per pound of powder. This was, however, 

 an altogether exceptional operation, and the nature and position 

 of the projecting mass of rock removed were such as to admit 

 of the powder exerting its power to great advantage, 



In ordinary quarrying, in hard rock, the yield per pound of 

 powder usually amounts to from 4 to 6 tons. 



After having ascertained, approximately, the weight of a 

 charge by the foregoing rule, it is well to check it by estimating, 

 as nearly as possible, the amount of rock which it is expected 

 the blast will throw out, and, in the case of hard compact rock, 

 allowing one pound of powder for every 4, 5, or 6 tons of rock, 

 as the arrangement of joints and other circumstances may 

 indicate. 



In this way, after a little experience has been gained, the 

 correct weight of charges may be decided with some degree of 

 accuracy. 



The system of measuring the charge by taking some propor- 

 tion, say J or \ of the depth of the hole, regardless, to a great 

 extent, of the size of hole or weight of rock to be moved, is 

 obviously a careless mode of apportioning a charge, which should 

 not be allowed. 



In some cases, it may be found convenient to charge the 

 holes or mines somewhat heavily, in order to break up the 

 stone, and thus obviate, to some extent, the necessity for doing 

 so afterwards by means of wedges, hand-drilling, or plaster- 

 firing. 1 



In estimating the space required by a charge of any given 

 weight, 30 cubic inches should be allowed for each pound of 

 powder. 



Some care is necessary in deciding which is the line of least 

 resistance, because it may be greatly affected by the arrange- 

 ment of joints, and the inclination of the beds of the rock. 



Although vertical holes involve a greater length of boring 

 than horizontal holes, they will usually be found to possess 



1 Laying small charges of guncotton or mtro-glycerine compound upon the 

 surface of stones, covering them with earth or other suitable substance, and then 

 firing them. 



