When the direction of the trench is such that the men are exposed on both sides, it will be necessary to throw up an embankment both to the right and left. This operation is called the double sap, and is executed by two parties of sappers, working side by side. In this sap it will be necessary to frequently change the direction of the trench, or to throw up traverses, in order to cover the men at a distance from the sap-roller. Wing-traverses, on the side of the trench which is least exposed, some times serve the same purpose as a double sap.

Mines.—By mining, as a military term, we understand the operations resorted to for the demolition, with powder, of a military structure of any description. The term mine is applied both to the excavation charged with powder for the purpose of producing an explosion, and to the communications which lead to this excavation.

The place in which the charge of powder is lodged is called the chamber, the communication by which this place is reached the gallery, and the excavation made by the explosion is termed the crater.

The form of the crater caused by an explosion in ordinary soils is assumed to be a truncated cone, the diameter, c d, ([Fig. 53,]) of the lower circle being one-half the diameter, a b, of the upper circle. This form has never been ascertained to be exactly correct, but the theoretical results deduced from a mathematical discussion of this figure have been fully verified in practice. The radius, p b, of the upper circle is termed the crater radius; the line o p, drawn from the centre of the charge perpendicular to the surface where the explosion takes place, is termed the line of least resistance; the line o b, drawn from the centre of the powder to any point in the circumference of the upper circle, is termed the radius of explosion.

When the crater radius is equal to the line of least resistance, the mine is termed common; when this radius is greater than the line of least resistance, the mine is termed overcharged; and when the radius is less, undercharged. A mine of small dimensions, formed by sinking a shaft in the ground, is termed a fougasse. The term camouflet is applied to a mine used to suffocate the enemy's miner, without producing an explosion. Small mines made in rock or masonry, merely for the purpose of excavation, without any considerable external explosion, are called blasts.

From experiments made on common mines, whose line of least resistance did not exceed fifteen feet, it has been ascertained that the tenacity of the earth is completely destroyed around the crater to a distance equal to the crater radius, and that empty galleries would be broken in at once and a half that distance. It has also been proved by experiment, that the crater radius in overcharged mines may be increased to six times the line of least resistance, but not much beyond this; that within this limit the diameter of the crater increases nearly in the ratio of the square roots of the charge; and that empty galleries may be destroyed by overcharged mines at the distance of four times the line of least resistance.

By means of the deductions of physico-mathematical theory, and the results of experiments, rules have been determined by which the miner can calculate, with much accuracy, the charge necessary to produce a required result in any given soil.

In the earlier stages of the history of this art, mines were only used to open breaches and demolish masses of masonry; but in later times they have been employed as important elements in the attack and defence of places.

An isolated wall, only two or three feet thick, may readily be demolished by exploding one or two casks of powder placed in contact with its base. If the wall be five or six feet thick, the charges should be placed under the foundation. For walls of still greater thickness it will be best to open a gallery to the centre of the wall, a foot or two above its base, and place the powder in chambers thus excavated. Revetment walls may be overturned by placing the charges at the back of the wall, about one-third or one-quarter of the way up from the base. If placed too near the base, a breach will be made in the wall without overturning it.

To demolish a bridge of masonry the powder should be lodged in chambers excavated in the centre of the piers. When there is not time for excavating these chambers in the piers, a trench may be cut over the key of the arch, in which the powder is placed and exploded; or, the casks of powder may be suspended immediately under the arch, with the same results. Where a saving of powder is of consequence, small chambers may be excavated in the haunches of the arch, and the mine carefully tamped before firing it.