and the third where the shock is transmitted by elasticity. 

 When tlie latter sphere comes below the surface, the gases 

 ivuiain inside the rock; but when the surface intersects either 

 of the other two spheres, the gases blow up the rock, forming a 

 enne or cmter, whose apex is at the point of explosion, and 

 whieh is called the blasting-cone. The larger the blasting-cone 

 is. the greater is the amount of rock broken up; and the object 

 of the engineer should, therefore, always be so to regulate the 

 depth of the hole and the quantity of explosive as to secure the 

 largest possible blasting cone in each case. Experiments are 

 required to determine the most efficient depth of hole, and 

 quantity of explosive to be employed, since these differ in 

 different kinds of rock, with the position of the rock strata, 

 etc. ; but in ordinary practice, the depths of the holes are made 

 from l ft. to 2 ft. in the. heading and upper portion of the 

 tunnel, when drilled by hand ; and from 3 ft. to 5 ft. when 

 drilled by power drills. In the lower portion of the profile, the 

 holes are made deeper, from 3 ft. to 4 ft. when drilled by 

 hand, and exceeding 5 ft. when drilled by power. The dis- 

 tance of the holes apart should be about equal to the diameter 

 of the blasting-cone; as a general rule it is assumed that the 

 base of the blasting-cone has a diameter equal to twice the 

 depth of the hole. The following table gives the average 

 number of holes required in each part of the excavation for the 

 St. Gothard tunnel : 



HO. OF PART* NAME OF PABT XO. OF HOLES 



1. Heading 6 to 9 



2. Right wing of heading 3 to 5 



3. Left wing of heading 3 to 5 



4. Shallow trench with core 2 



6. Deepening of trench to floor 6 to 9 



6. Narrow mass of core to left 3 



7. Greater mass of core to left 6 to 9 



8. Culvert 1 



Total section 30 to 43 



The location of the parts numbered is shown by Fig. 15, p. 32. 



