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 | | |Dynamite (or of |Black Powder|Gun-cotton| Tonite) the Hole.| in 1 inch. |in 1 inch.| in 1 inch. ---------+------------+----------+------------ ins. | ozs. | ozs. | ozs. 1 | 0·419 | 0·419 | 0·670 1¼ | 0·654 | 0·654 | 1·046 1½ | 0·942 | 0·942 | 1·507 1¾ | 1·283 | 1·283 | 2·053 2 | 1·675 | 1·675 | 2·680 2¼ | 2·120 | 2·120 | 3·392 2½ | 2·618 | 2·618 | 4·189 2¾ | 3·166 | 3·166 | 5·066 3 | 3·769 | 3·769 | 6·030 ---------+------------+----------+------------
_Conditions of Disruption._--Having explained the law according to which the elastic gases evolved by an explosion act upon the surrounding rock, and shown how the force required to cause disruption may be calculated, it now remains to consider the conditions under which disruption may take place. Suppose a block of unfissured rock detached on all sides, as shown in plan, in Fig. 40, and a bore-hole placed in the centre of this block. If a charge be fired in this position, the lines of rupture will radiate from the centre towards any two, or towards all four of the unsupported faces of the block, because the forces developed will act equally in all directions, and the lines of rupture will be those of least resistance. Evidently this is the most favourable condition possible for the charge, since the rock offers an unsupported face on every side; and it is evident that the line of rupture must reach an unsupported face to allow of dislodgement taking place. Suppose, again, as shown in Fig. 41, the block to be unsupported on three sides only, and the charge placed at _h_. In this case, the lines of rupture may run to any two, or to all three, of the unsupported faces; and hence this will be the next most favourable condition for the action of the charge. The greatest useful effect, however, will be obtained in this case by placing the charge farther back at _h′_, when the lines of rupture must necessarily run to the opposite faces _b c_, and, consequently, the whole of the block will be dislodged. Assume another case, in which the rock is unsupported upon only two sides, as shown in Fig. 42, and the charge placed at _h_. In this case, the lines of rupture must run to each of the unsupported faces _a b_. Thus, it is evident that this condition, though still a favourable one for the good effect of the charge, is inferior to the preceding. As rock is never homogeneous in composition nor uniform in texture, the lines of rupture, which, as before remarked, will be those of least resistance, may reach the faces at any point, as at _m n_, _m′ n′_, or any point intermediate between these. But it will be seen that the useful effect will be greatest when these lines, radiating from the charge, make an angle of 180°, or, in other words, run in directly contrary directions, and that the useful effect diminishes with the angle made by these lines of rupture. Suppose, again, the rock to be unsupported upon one side only, as shown in Fig. 43, and the charge placed at _h_. In this case, the lines of rupture must run to the face _a_, and the condition must therefore be considered as less favourable than the preceding. As in those cases, the useful effect will depend upon the angle made by the lines of rupture _h m_ and _h n_, which angle may be very small, and which must necessarily be much less than 180°. A greater effect may be obtained, under this condition, by firing several charges simultaneously. If, for example, we have two charges placed, one at _h_, and the other at _h′_, and fired successively, the lines of rupture will run in or near the directions _h m_, _h n_, _h′ m′_, _h′ n′_, and the portion of rock dislodged will be _m h n h′ n′_. But if these two charges be fired simultaneously, the lines of rupture will be _h m_, _h o_, _h′ o_, _h′ n′_, and the mass of rock dislodged will be _m h h′ n′_. Simultaneous firing is in this way productive of a greatly increased useful effect in numerous cases, and the mining engineer, and the quarryman especially, will do well to direct their attention to this source of economy.