| Nitro-glycerine | 52·50 |
| Nitrate of baryta | 16·40 |
| Charcoal | 2·85 |
| Sulphur | 25·75 |
| Kieselguhr | 22·50 |
| 100·00 |
Brain’s Powder.
—Brain’s powder is a nitro-glycerine compound, similar in character to lithofracteur. The exact composition of the base has never been published, so far as relates to the proportions of the ingredients. But it is composed of chlorate of potash, charcoal, and nitrated sawdust. The proportion of nitro-glycerine never exceeds 40 per cent. Horseley’s powder contains about the same proportion of nitro-glycerine in a base of chlorate of potash and nut-galls.
Cellulose Dynamite.
—In Germany, gun-cotton is used as an absorbent for nitro-glycerine, the compound being known as “Cellulose dynamite.” It is chiefly used for primers to explode frozen dynamite. It is more sensitive to blows than the kieselguhr dynamite.
CHAPTER III.
The Principles of Rock Blasting.
Line of Least Resistance.
—The pressure of a fluid is exerted equally in all directions; consequently the surrounding mass subjected to the force will yield, if it yield at all, in its weakest part, that is, the part which offers least resistance. The line along which the mass yields, or line of rupture, is called the “line of least resistance.” If the surrounding mass were perfectly homogeneous, it would always be a straight line, and it would be the shortest distance from the centre of the charge to the surface. Such, however, is never the case, and the line of rupture is, therefore, always a more or less irregular line, and often much longer than that from the centre direct to the surface. It will be obvious, on reflection, that the line of least resistance will be greatly dependent upon (1) the texture of the rock, which may vary from one point to another; (2) its structure, which renders it more easily cleavable in one direction than in another; (3) the position, direction, and number of the joints, which separate the rock into more or less detached portions; and (4) the number and relative position of the unsupported faces of the rock. All these circumstances must be ascertained, and the position and the direction of the bore-hole determined in accordance with them, in order to obtain the maximum effect from a given quantity of explosive. It must not be supposed, however, that this is a labour involving minute examination and long consideration. On the contrary, a glance is generally sufficient to enable the trained eye to estimate the value of those circumstances, and to determine accordingly the most effective position for the shot. In practice, the line of least resistance is taken as the shortest distance from the centre of the charge to the surface of the rock, unless the existence of joint planes, a difference of texture, or some other circumstance, shows it to lie in some other direction.