. Space does not permit of our entering further on this subject : the reader is therefore referred to the article Military,' in the Royal Engineer Aide Memoire? It may be stated, however, that the main gallery which, passing along the counterscarp, serves as the base of the system of mines, is termed the magistral gallery ; the advanced galleries are termed listening galleries. If there be an inter mediate gallery, parallel to the magistral, it is called the envelope; and those giving access from one to the other, galleries of communication.
In offensive mining, the earth is supported by wood-work. There are two different systems of mining, one with what are termed mining frames and sheeting, the other with cases. Mining frames for shafts consist of four pieces, two 44 feet long, and two 3 feet long, about 9i inches by 3 inches thick. The long pieces are made with tenons, which fit into mortices cut in the short pieces, which are further notched to one-third of their thickness; these being fitted together form tho frames, which are placed horizontally in the shaft at intervals of 4 feet, and retain the sheeting planks, are outside them and prevent the earth falling in,) in their places.
Gallery frames differ from shaft frames in being composed of only three pieces, two uprights called stanchions, and a top piece called the eapsili The system of mining with cases is more expeditious than with frames and sheeting ; they have long been known as Dutch cases, and were introduced into the British service by General Sir Charles I'aslcy. They consist of four pieces about 1 foot wide, namely, two stanchions, a eapaill, and a groundsill; and for ordinary work are cut out of 2 inch deal. The stanchions have tenons, 2' long by 3" wide, which fit into mortices of corresponding dimensions cut in the end of the capsill and groundsill. These form rectangular cues, which are placed touching one another if the soil is light, no sheeting being men/ 1. Used for the descent of ditches and passage of cannon.
2. Used for the passage of troops, two deep.
3. Ordinarily employed for general purposes of attack, giving the miner room to work easily either on both or one knee, and is the most rapidly executed.
4 & 5. Employed for short distances, about 10 or 12 feet only, for placing charges, &c., as being more easily tamped.
When cases are used, mines can be driven at nearly twice the rate that they can with frames and sheeting : namely, great galleries and shafts about 1 foot an hour, common galleries about 1i feet an hour.
Ventilation requires to be well looked after in mines, for not only do the gases generated by the explosions of powder collect in the descending and ascending portions of galleries and often stifle the miner, but even under ordinary circumstances the air is so vitiated by respi ration that a branch cannot safely be driven more than 60 feet. Hence openings should be made to the surface of the ground, and, if possible, concealed from the enemy, and communications made from adjacent galleries to create a draught. It also often becomes necessary to use artificial means of forcing air down into a mine, by means of a blower or bellows and metal pipes carried to the end of the mine.
The chamber in which the powder is placed is a cubical excavation formed on one side of the gallery, very little larger than is necessary to enable it to receive the box which contains the powder. When this is deposited, the vertical face of the chamber is covered with boards, which are kept in their places by short timbers fixed in horizontal positions between them and the opposite side of the gallery. The latter is then filled up with earth, well rammed, to an extent in the length of the gallery greater than that of what is called the line of least resistance; that is, a line imagined to be drawn from the chamber perpendicularly to the nearest surface where the crater would be formed. The mass of earth thus filling the gallery is called the tamping of the mine. A train of powder in a canvas hose, forming a tube about three-quarters of an inch in diameter, and for security contained in a wooden trough called an auget, or a casing-tube, is laid from the box in the chamber through the tamping to the place where the fire is to be applied; to its extremity is attached a piece of port-fire, which, being lighted, the fire communicates by means of the hese with the powder in the chamber, and an explosion takes place. Mines are also fired by means of what is called Bickford's fuse, which con sists of powder encased in a kind of rope made of spun yarn, tarred. This fuse is waterproof, and not so liable to be accidentally ignited, and is especially adapted for damp ground. Electricity is also now constantly employed in firing mines. It was used by the Russians in their mines at Sebastopol, and by the English in the demolition of the docks, &c.