Casemates have frequently been resorted to as a certain means of defence ; but besides the general objection that they are expensive, it does not appear that they are so effec tual as might at first be supposed. The most useful works of this kind, are the double caponniers, for the defence of the ditches. These are covered with a shell proof arch, and sur rounded with a ditch 12 feet wide, and 6 feet below the bottom of the ditch in which they arc placed. The chief object of this ditch is to prevent the enemy's approaching so near as to throw inflammable matter into the loop-holes of the capon niers.
To cover the gorge of a work, a single wall is some titiles constructed with loop-holes, and stairs behind it to communicate with the terre-plein. Perhaps the best de fence for any gorge, is a tower like Montalembert's, or the English Martello tower, as it serves not only to cover the work, but also to protect the men in retreating to the gar rison.
When casemates are constructed, they ought to be at least eight feet in height, and as much in width, with loop holes at the distance of everyl•three feet, both to give the soldiers the command of the ground immediately under the work, and to allow the smoke to disperse. If the capon niers are of a considerable length, they ought to have tra verses at short distances, or walls with loop-holes, that they may be more obstinately defended should the enemy get possession of one extremity.
It was formerly observed that, at a very early period, mines were employed by the beseiged for overturning the towers and other works of the enemy constructed near the place. After the invention of gunpowder, this mode of attack became more effectual, and of course more common. According to the modern system, a mine consists of a small subtei raneous chamber or cavity, filled with gunpow der, which by its explosion destroys the works raised above or near the spot. The part containing the powder is called the fourneau, and the passage leading to it the gallery. When a work of this kind is made by the be siegers, it is properly called a mine, and a countermine when employed by the garrison. The construction, how ever, is the same in both cases.
When mines are to be used in the defence of a place, the galleries ought to be constructed before the siege com mences, having branches running from them in various directions. The former ought to be of masonry, the lat ter may be constructed of timber. The works before which mines are to be cut, ought to be of sufficient strength to resist the enemy till such time as the mine be finished, if it has not been previously constructed. A mine, indeed, ought never to be used but before a strong and important work, where it is expected that the enemy will make a fomidablc attack. When mines are employed under an outwork for the purpose of dislodging the enemy, should they get possession of it, they ought not to be too near the escarps lest they should be discovered by the enemy's mi ner. In general, they ought to be at least 12 feet distant
from the revetement of the work, and if possible lower than the ground of the fosse. The principal galleries should be from four to five feet wide, and from six to six and a half feet high. Three feet square will be sufficient for the branches. Several rows of mines before one ano ther may sometimes he useful ; but they ought not to be too near, lest the explosion of one should loosen the earth about the others.
Having pointed out the general disposition of the differ ent works with regard to the trace, the next important point is to determine the relief. This is done by means of ver tical sections or profiles; and hence a place is said to be well profiled when the relief is so regulated as that the different works cover each other without preventing their command of the field.
When a fortification is situated in a plain, all the fronts are fortified in the same manner ; but if the ground be un even, regard must be had to the different heights around as well as within the place. When the parts of a work arc well arranged in this respect, the work is said to be sell defiled.
With regard to the relief of a fortification, in general it may be observed, that the parapet of the Most distant out work ought to have the command of the field, and every succeeding one the command of that before it, All revete meats should he completely eovered,—the covered way be fore the demi-lune should have a command of at least six feet,—the glacis before the places of arms 7 feet six inch es,—and the faces of the bastion so much as to fire over the saliant angle of the demi-lune. Some engineers have proposed to make the rampart in every case of a determin ed height. Vauban fixed this at 22 feet. Some have re commended 14 and others 30. From the very nature of the thing, however, it is obvious that no such rule can be applicable to all places. The great object in every forti fication is to bring as many fires as possible to bear on one point, and therefore the relief must depend on the number and distance of the outworks,—the dimensions of the trace, —the distance of the enemy's parallels or trenches,—and various other circumstances. But as the commands or re lative heights of the different works must always bear a certain relation to one another, these heights may be deter mined by the following proportions, To find the height of the saliant angle of the bastion. As the distance between the saliant angle of time tion and the saliant angle of the place of arms : To the distance between the angle of the shoulder and the enemy's third parallel before the demi-lune :: So is the height of the glacis before the place of arms +3 feet: To the height of the crest of the parapet at the sail= ant angle.