—When the line of least resistance is known, it remains to determine the quantity of the explosive compound required to overcome the resistance along that line. This matter is one of great importance, for not only is all excess waste, but this waste will be expended in doing mischief. In mining operations, the dislodged rock is violently projected, and the air is vitiated in an unnecessary degree; and in quarrying, stones are shattered which it is desirable to extract in a sound state. The evil effects of overcharging, in occasioning the formation of noxious gases, was pointed out in the last chapter. Of course it is not possible so to proportion a charge to the resistance that the rock shall be just lifted out, and no more; because neither the force developed by the charge, nor the value of the resistance can be known with precision. But a sufficient approximation may be easily arrived at to enable us to avoid the loud report that is indicative of wasted force.
Charges of an explosive compound of uniform strength produce effects that vary as the weight of those charges, that is, a double charge will move a double mass. And, as homogeneous masses vary as the cube of any similar line within them, the general rule is established that charges of powder capable of producing the same effects are to each other as the cubes of the lines of least resistance. Generally, the quantity of black blasting powder requisite to overcome the resistance will vary from 1⁄20 to 1⁄30 of the cube of the line of least resistance, the latter being measured in feet and the former in pounds. Thus, if the rock to be blasted be moderately strong limestone, for example, and the shortest distance from the centre of the charge to the surface of the rock be 3 feet, we shall have 3 × 3 × 3 = 27, the cube of the line, and 27⁄25 lb. = 12⁄25 lb., or about 1 lb. 1 oz., as the weight of the powder required. If dynamite be used, and we assume it to be four times as strong as common black powder, of course, only one-fourth of this quantity will be required. Also if gun-cotton, or cotton-powder, be used, and we assume its strength to be three times that of black powder, one-third only will be needed. Again, if Curtis’s and Harvey’s new extra-strong mining powder fired by a detonator be employed, we may assume it to be twice as strong as common black powder fired by the ordinary means, and consequently we shall need only one-half the quantity indicated by the formula.
It is neither practicable nor desirable that such calculations and measurements as these should be made for every blast; their practical value lies in this, namely, that if the principles involved in them be clearly understood, the blaster is enabled to proportion his charges by sight to the resistance to be overcome, with a sufficient degree of precision. A few experiments in various kinds of rock, followed by some practice, will enable a man to acquire this power.
As it is a common and a convenient practice to make use of the bore-hole as a measure of the quantity of explosive to be employed, we have calculated the following table:—
| Diameter of the Hole. | Black Powder in 1 inch. | Gun- cotton in 1 inch. | Dynamite (or Tonite) in 1 inch. | |
|---|---|---|---|---|
| ins. | ozs. | ozs. | ozs. | |
| 1 | 0·419 | 0·419 | 0·670 | |
| 1 | 1⁄4 | 0·654 | 0·654 | 1·046 |
| 1 | 1⁄2 | 0·942 | 0·942 | 1·507 |
| 1 | 3⁄4 | 1·283 | 1·283 | 2·053 |
| 2 | 1·675 | 1·675 | 2·680 | |
| 2 | 1⁄4 | 2·120 | 2·120 | 3·392 |
| 2 | 1⁄2 | 2·618 | 2·618 | 4·189 |
| 2 | 3⁄4 | 3·166 | 3·166 | 5·066 |
| 3 | 3·769 | 3·769 | 6·030 | |
Fig. 40.
Fig. 41.
Fig. 42.