Part 304

A glacis, properly countermined, and every advantage taken of it to retard the besiegers, may, with proper management, prolong a siege at least 2 months; and if the rest of the works are also countermined, and properly defended, they may add another month to the siege. Every system of countermines must depend upon the system of fortification to which they are to be adapted; the general principle for their regulation is, that the galleries should occupy situations, from which branches can be most readily run out under the most probable points of the besieger’s batteries and approaches. The general system of countermines commonly used in a place prepared before hand, is as follows: the principal or magistral gallery runs all round the work, under the banquette of the covert way, and across the places of arms, having the entrances at the re-entering places of arms. Nearly parallel to this at 20, 25 or 30 fathoms distance is another gallery, called the envellope. These two galleries are connected by galleries of communication, under the gutters of the re-entering parts of the glacis, and under the ridges of the salient parts. From the envellope are run out about 15 or 16 fathoms, galleries in directions parallel to the capitals of the works, and at 23 fathoms distance from each other. These are called listeners.

Sometimes, shafts are sunk from the end of these listeners, and by connecting these shafts, a second envellope formed. Behind the escarps of the different works, galleries are likewise made, about the level of the bottom of the ditch; from whence branches may be run out into or under the foundations of the walls; and if the ditch be dry, galleries of communication may be made from these to the magistral gallery; and from which communications branches may be run out for chambers to annoy the besiegers in their passage of the ditch. The entrances to the escarp galleries are by means of posterns, which descend from behind the interior slope of the rampart.

If a place be not countermined before hand, a great deal may be done even after the investment of the place, to prolong the siege by countermines. In this case, the first thing to be done immediately that the place is invested, is to sink a shaft in each of the places of arms of the covert way; one in each branch of the covert way opposite that part of the bastion where the breach will most probably be made; and one in the flanked angle of each bastion. Those on the covert way will be on the banquette, and sunk to about 18 inches below the bottom of the ditch. Those in the bastions to about 12 feet below the bottom of the ditch. Thus prepared, the moment the side on which the attack is to be made can be ascertained, galleries must be carried on from these shafts on the side attacked along the capitals, in the form of trefles, or double T; and advanced as far into the country as the time will admit. Communication galleries may likewise be driven between these different works on the covert way, and from them to the work in the bastion; which will prevent the enemy gaining possession of their entrances. All these works may be carried on after the investment of the place; and be in sufficient forwardness by the time the enemy gains the third parallel.

The following rules are given by Vauban for fougasses, or small mines, having the diameter of the excavation equal to double the line of least resistance. The side of the chamber must be exactly a sixth part of the depth of the shaft. The side of the box to hold the powder exactly a ninth part of the depth of the shaft.

These remarks respecting mines are principally extracted from the General Essay on Fortification before mentioned, written in French and published at Berlin, 1799.

Counter-MINES, are those made by the besieged, whereas mines are generally made by the besiegers. Both mines and counter-mines are made in the same manner, and for the like purposes, viz. to blow up their enemies and their works; only the principal galleries and mines of the besieged, are usually made before the town is besieged, and frequently at the same time the fortification is built, to save expence.

Eventer la MINE, Fr. to spring a mine. When used figuratively, this expression signifies to discover a plot, or make it known. It is likewise used to express the failure of any expedition or undertaking.

Definitions of MINES. A mine is a subterraneous cavity made according to the rules of art, in which a certain quantity of powder is lodged, which by its explosion blows up the earth above it.

It has been found by experiments, that the figure produced by the explosion is a paraboloid, and that the centre of the powder, or charge, occupies the focus.

The place where the powder is lodged is called the chamber of the mine, or fourneau.

The passage leading to the powder is called the gallery.

The line drawn from the centre of the chamber, perpendicular to the nearest surface of the ground, is called the line of least resistance.

The pit or hole, made by springing the mine, is called the excavation.

The fire is communicated to the mines by a pipe or hose, made of coarse cloth, whose diameter is about one and a half inch, called a saucisson, (for the filling of which near half a pound of powder is allowed to every foot) extending from the chamber to the entrance of the gallery, to the end of which is fixed a match, that the miner who sets fire to it may have time to retire, before it reaches the chamber.

To prevent the powder from contracting any dampness, the saucisson is laid in a small trough, called an auget made of boards, three and a half inch broad, joined together, lengthwise, with straw in it, and round the saucisson, with a wooden cover nailed upon it.

Foyer, Fr. Focus or centre of the chamber, some authors call the end of the saucisson that comes within the work, and which is to be set fire to, the foyer, or focus: but by most people, this is generally understood to be the centre of the chamber.

Galleries and chambers of MINES. Galleries made within the fortification, before the place is attacked, and from which several branches are carried to different places, are generally 4 or 4¹⁄₂ feet wide, and 5 or 5¹⁄₂ feet high. The earth is supported from falling in by arches and walls, as they are to remain for a considerable time; but when mines are made to be used in a short time, then the galleries are but 3 or 3¹⁄₂ feet wide, and 5 feet high, and the earth is supported by wooden frames or props.

The gallery being carried on to the place where the powder is to be lodged, the miners make the chamber. This is generally of a cubical form, large enough to hold the wooden box, which contains the powder necessary for the charge: the box is lined with straw and sand-bags, to prevent the powder from contracting dampness.

The chamber is sunk something lower than the gallery, if the soil permits; but where water is to be apprehended, it must be made higher than the gallery; otherwise the besieged will let in the water, and spoil the mine.

Quantities of powder to charge, MINES. Before any calculation can be made of the proper charge for a mine, the density and tenacity of the soil in which it is to be made, must be ascertained, either by experiment, or otherwise; for, in soils of the same density, that which has the greatest tenacity, will require the greatest force to separate its parts. The density is determined by weighing a cubic foot (or any certain quantity) of the soil; but the tenacity can only be determined by making a mine. The following table contains experiments in 6 different soils, which may be of some assistance to form a judgment of the nature of the soil, when an actual experiment cannot be had.

+-------------------------------+--------+-------------+ | |Density.| Tenacity. | | +--------+-------------+ | | Weight |Quantity of | | | of |power to | | | 1 cubic|raise 1 cubic| | Nature of the soil. | foot. |fathom. | +-------------------------------+--------+-------------+ |1. Loose earth or sand | 95 pds.| 8 pds. | +-------------------------------+--------+-------------+ |2. Common light soil |124 | 10 | +-------------------------------+--------+-------------+ |3. Loam, or strong soil |127 | 12¹⁄₄ | +-------------------------------+--------+-------------+ |4. Potter’s clay, or stiff soil|135 | 13¹⁄₂ | +-------------------------------+--------+-------------+ |5. Clay, mixed with stones |160 | 16 | +-------------------------------+--------+-------------+ |6. Masonry |205 | 21¹⁄₂ | +-------------------------------+--------+-------------+

All the requisites in mining may be determined by the following problems, which admit of 4 cases; for any 3 of the articles below being given, the 4th may thence be found.

1. The nature of the soil,

2. The diameter of the excavation,

3. The line of least resistance,

4. The charge.

PROBLEM. I.

Given the nature of the soil, the diameter of the excavation, and the line of least resistance, to find the charge.