L3 : w :: L′3 : w′;
whence
w′ = wL′3
L3.
129. The following charges corresponding to lines of least resistance are from the works of Sir John Burgoyne.
| Line of least resistance. | Charge of powder. | |
|---|---|---|
| 2 feet, | 0 lbs. | 4 oz. |
| 4 feet, | 2 lbs. | 0 oz. |
| 6 feet, | 6 lbs. | 12 oz. |
| 8 feet, | 16 lbs. | 0 oz. |
130. After the powdered stone is removed, the powder is placed in the lower part of the hole; after which a wad of turf, or some other light material, follows; next the tamping of powdered brick, dried clay, or something similar, and finally a stopper of wet clay, or some other firm substance. A hole is left through all, communicating with the powder by ramming the tamping around a wire; through this hole a fuse is inserted by which to light the charge. The most perfect tamping would offer a resistance as great as that by the natural rock. A great improvement upon the above method is the sand blast; the powder is put in, and the hole filled with loose, dry sand, simply poured in and settled by a gentle stirring, but not at all rammed; the explosion of the powder spreads the sand as a wedge, and causes the power of the blast to be exerted sideways. In some cases a small cone of wood has been placed (base down) in the hole with the sand, which aids very much in stopping the exit of the blast through the drill.
131. Of late years an admirable method of lighting large charges simultaneously has been employed, namely, voltaic electricity.
132. A gigantic example of the application of this method has been furnished by the English engineers in overthrowing a portion of Round Down Cliff, about two miles from Dover, (England). Two chambers, 13 × 5½ × 4½, and one 10 × 5½ × 4 feet were cut in the rock. Within these were placed fifty bags of powder, amounting in all to eight and one half tons. The charges were lighted by the voltaic system, by which operation a mass of rock (chalk) 380 × 360 × 80 feet, amounting to 400,000 cubic yards, was thrown into the sea, and by which there was estimated to have been saved nearly $40,000.
133. The following table from Colonel Pasley’s memoranda on mining, shows the capacity of different drills for powder, by weight, and also the depth of holes of different diameters, to contain one pound of powder.
| Diameter of hole in inches. | Ounces of powder in one inch depth. | Powder in one foot deep. | Depth of hole in inches to contain one pound. | |
|---|---|---|---|---|
| lbs. | oz. | |||
| 1 | 0.4 | 0 | 5.0 | 38.2 |
| 1½ | 0.9 | 0 | 11.3 | 16.9 |
| 2 | 1.7 | 1 | 4.1 | 9.5 |
| 2½ | 2.6 | 1 | 15.4 | 6.1 |
| 3 | 3.7 | 2 | 13.2 | 4.2 |
| 3½ | 5.1 | 3 | 13.5 | 3.1 |
| 4 | 6.7 | 5 | 0.4 | 2.4 |
| 4½ | 8.4 | 6 | 5.7 | 1.9 |
| 5 | 10.5 | 7 | 13.6 | 1.5 |
| 5½ | 12.7 | 9 | 8.0 | 1.3 |
| 6 | 15.1 | 11 | 4.9 | 1.0 |