The quantity of powder, required to charge mines, depends upon the nature of the soil. That which is more tenacious, will require the greatest force to separate its parts. The density may be learned, comparatively speaking, by determining the specific gravity of each kind of soil. The requisites in mining may be ascertained by four simple problems, which relate to the nature of the soil, the diameter of the excavation, the line of least resistance, and the charge.
Table of the quantity of Gunpowder, to raise a cubic fathom of different kinds of Soil.
| DENSITY. | TENACITY. | |
| NATURE OF THE SOIL. | Weight of 1 cubic foot. | Quantity of powder to raise 1 cubic fathom. |
| 1. Loose earth or sand. | 95 lbs. | 8 lbs. |
| 2. Common light soil. | 124 | 10 |
| 3. Loam or strong soil. | 127 | 12¼ |
| 4. Potters' clay, or stiff soil. | 135 | 13½ |
| 5. Clay, mixed with stones. | 160 | 16 |
| 6. Masonry. | 205 | 21½ |
The gallery and chamber being ready to be loaded, a strong box of wood is made of the size and figure of the chamber, being about one-third or one-fourth larger than is required for containing the necessary quantity of powder. Against the sides and bottom of the box is put some straw, and this straw is covered over with empty sand bags, to prevent the powder from contracting any dampness. A hole is made in the side, next the gallery, near the bottom, for the saucisson to pass through, which is fixed to the middle of the bottom, by means of a wooden peg, to prevent its loosening from the powder, or to hinder the enemy (if he should reach the entrance) from being able to tear it out. This done, the powder is brought in sand bags, and thrown loosely in the box, and covered also with straw and sand bags. Upon this is put the cover of the box, pressed down very tight with strong props; and, to render them more secure, planks are also put above them, against the earth, and wedged in as fast as possible.
This done, the vacant spaces between the props are filled up with stones and dung, and rammed in the strongest manner. The least neglect in this work will considerably alter the effect of the mine. Then the auget, or small trough, is laid from the chamber to the entrance of the gallery, with some straw at the bottom; and the saucisson laid in it, with straw over it. Lastly, it must be shut with a wooden cover, nailed upon it. Great care must be taken in stopping up the gallery, not to press too hard upon the auget, for fear of spoiling the saucisson, which may hinder the powder from taking fire, and prevent the mine from springing. The gallery is stopped up with stones, earth and dung, well rammed, six or seven feet further from the chamber than the length of the line of the least resistance.[40]
Before closing this article, short as it is, compared with a full view of the subject, which belongs exclusively to engineering, we shall notice, from Belidor, the globe of compression in mines. If we imagine a large globe of earth, homogeneous in all its parts, and a certain quantity of powder lodged in its centre, so as to produce a proper effect without bursting the globe; by setting fire to the powder, it is evident that the explosion will act all round, to overcome the obstacles which oppose its motion; and as the particles of the earth are porous, they will compress each other in proportion as the flame increases, and the capacity of the chamber increases likewise: but the particles of the earth next to the chamber will communicate a part of their motion to those next to them, and those to their neighbours; and this communication will thus continue in a decreasing proportion, till the whole force of explosion is entirely spent; and the particles of earth beyond this term will remain in the same state as they were at first. The particles of earth, that have been acted upon by the force of explosion, will compose a globe, which Mr. Belidor calls the globe of compression. He observed, that, when a mine exploded, and threw up the ground over it, its action was, at the same time, felt in a circular direction, throughout the surrounding ground, to a distance at least equal to the oblique line drawn from the centre of inflammation to the edge of the funnel.
Mines and counter-mines are now called offensive and defensive mines. The hole made by the explosion is called the entonnoir, crater, or funnel.
In the system of counter-mines, we have the magistral gallery, or gallery of the counterscarp, which is that extended below the covered way, from which branches are pushed to overthrow the works and batteries of the beseiger, that crown it; the enveloping gallery that communicates with the other passages, called the galleries of communication, and is nearly parallel with the first at the distance of from forty to sixty yards. Other galleries are pushed forward, leaving the enveloping gallery, projecting at least thirty yards, and having spaces between them of about fifty yards in width; so that the enemy's miner, whose work may be heard under ground about thirty yards, may not pass between any two of them without being discovered. These are called listening galleries. It may be observed, that, from these galleries, branches are carried forward to establish chambers under the works of the enemy. Those who wish to acquire information on this, and other subjects, connected with attack and defence, and on some branches of engineering, would do well to consult the French work of Bousmard.
There are likewise small mines called Fougasses, used in the defence of field works. They are seldom more than ten feet beneath the surface, and are placed at the expected points of attack, usually nine feet from the salient angles, and without the counterscarp. The chest of powder and the saucisson are placed as usual. Barrels or casks and even grenades are used.
Sec. XXXII. Of the Means of Increasing the Strength of Gunpowder for Mining.