in the heading and upper portion of the tunnel, when drilled by hand; and from 6 to 8 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 6 ft. when drilled by power. The distance 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 in which the heading was excavated by machine drills while the other parts were excavated by hand drills: NO. OF PART.[5] NAME OF PART. NO. 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 5. 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 [5] The location of the parts numbered is shown by Fig. 14, p. 36. The quantity of explosives required for blasting depends upon the quality of the rock, since the force of the explosives must overcome the cohesion of the rock, which varies with its nature, and often differs greatly in rocks of the same kind and composition. The quantity of explosives required to secure the greatest efficiency in blasting any particular rock may be determined experimentally, but in practice it is usually deduced by the following rules: (1) The blasting force is directly proportional to the weight of the explosives used, and (2) the bulk of the blasted rock is proportional to the cube of the depth of the holes. It is usually assumed, also, that the explosive should fill at least one-fourth the depth of the hole. The following table gives the depth of holes and amount of dynamite used at each advance in the Fort George Tunnel illustrated on page 135.