the surface of the earth.  Sometimes a quantity of powder is placed under the shells, so as to project them into the air previous to their explosion.  The stone fougasse is formed by making a funnel-shaped excavation, some five or six feet deep, and placing at the bottom a charge of powder enclosed in a box, and covered with a strong wooden shield; several cubic yards of pebbles, broken stone, or brickbats, are placed against the shield, and earth well rammed round, to prevent the explosion from taking place in the wrong direction.  These mines are fired by means of powder hose, or by wires connected with a galvanic battery.

The defensive mines employed to blow up the besiegers’ works, are generally common mines with the lines of least resistance seldom greater than fifteen feet.  All the main galleries and principal branches of mines for a permanent fortification are constructed at the same time with the other portions of the work, leaving only the secondary branches, chambers, &c., to be made during the siege.  For the general arrangement of these galleries, and the precautions necessary for their protection from the operations of the besiegers, reference must be made to treatises specially devoted to the discussion of this subject.

Mines can seldom be employed with advantage in works of slight relief, and liable to an assault.  But if judiciously arranged in the plan of their construction, and well managed during the operations of the siege, they contribute very materially to the length of the defence.

Attack and defence.—­This subject admits of two natural divisions:  1st, of intrenchments, and 2d, of permanent works.

I. Intrenchments maybe attacked either by surprise, or by open force.  In either case the operations should be based on exact information of the strength of the works and the number and character of the garrison—­information that can be obtained from spies, deserters, and prisoners, and confirmed by examinations or reconnaissances made by officers of engineers.  By these means a pretty accurate knowledge may be obtained of the natural features of the ground exterior to the works; their weak and strong points; and their interior arrangements for defence.

In an attack by surprise, the troops should consist of a storming party and a reserve of picked men.  The attacking column is preceded by a company of sappers armed with axes, shovels, picks, crowbars, &c.; bags of powder are also used for blowing down gates, palisades, &c.  All the operations must be carried on with the utmost dispatch.  The time most favorable for a surprise is an hour or two before day, as at this moment the sentinels are generally less vigilant, and the garrison in a profound sleep; moreover, the subsequent operations, after the first surprise, will be facilitated by the approach of day.  Under certain circumstances, it may be advisable to make false attacks at the same time with the true one, in order to distract the attention of the garrison from the true point of danger.  But false attacks have, in general, the objection of dividing the forces of the assailants as well as of the assailed.  In all attacks by surprise, secrecy is the soul of the enterprise.