The square may be readily fortified, so as, by means of ravelins, counterguards, and tenailles, to be equally strong on every side. But the garrison which it is capable of con taining is comparatively small ; and it is scarcely possible to make it hold out more than from twelve to fifteen days. The same remarks are applicable to the pentagon, neither the one nor the other being fitted for an important position.
The hexagon is superior to either of the former. Its faces and flanks are sufficiently large ; it will admit of other outworks besides ravelins, counterguards, and tenailles, and will contain 2400 infantry and 100 cavalry. It can only be used, however, in situations where assistance can be quickly obtained, as it cannot be made to hold out longer than from twenty-two to thirty days.
The heptagon and octagon will contain a garrison of from 3000 to 3500 men; and, by the help of outworks and mines, may be rendered capable of a considerable resis tance. The defence of the heptagon may extend from twenty-two to thirty-five days, and that of the octagon from thirty to forty-five.
The enneagon, decagon, and endecagon, are capable of containing 4000 or 5000 men ; and consequently, from so large a garrison, can spare a greater force for the defence of outworks, than a polygon of fewer sides. If, with the outworks, they possess the advantage of inundation, they may be considered as places of the first order. Under these circumstances, they may hold out from forty to sixty days.
Dodecagons and polygons of more than twelve sides are considered as the strongest of all fortifications. They contain a garrison sufficient to defend not only the places themselves, but also the passages leading to them, as well as smaller forts in the neighbourhood. If all the parts are equally well constructed, the defence may be extended from forty to ninety days.
It is also to be observed, in comparing these various fi gures, that the polygons of a smaller number of sides are much more expensive than those of a greater number, if the same advantage in point of strength is to be derived from them ; at the same time that a much smaller army is required for besieging the former than the latter. A hex agon may be completely irivested with 25,000 men—an oc tagon by 35,000—but a decagon, or dodecagon, will re quire 50,000.
Places that require to be defended only from a coup-de main, such as passes, tops of mountains, harbours, &c. will be sufficiently protected by a fort, or small fortress. Of this kind are the forts near Antwerp, Dunkirk, Brest, Tou lon, Dover, and in almost all the colonies in the East and 'West Indies.
Intrenched camps are works raised for the protection of an army near a fortress, by taking advantage of moors, wa ter, kc. in the neighbourhood. Under this class of works may also be included the forts constructed about a large town to prevent a siege, as well as to cover the magazines in the place from the enemy's fire. If such works arc raised about a fortress, they should be sufficiently near, and strong enough to form an intrenchment. If the for tress be on the frontiers, such works are very useful.
Citadels are forts, or large reduits constructed within fortresses, for the purpose of.commanding the town .should the inhabitants prove refractory, and to serve as a retreat to the garrison when the place surrenders. Citadels con sist of an earthen rampart and a ditch, but ought always to be stronger than the lbrtress, that the enemy may not be tempted to attack them first. If possible, the ditch should be filled with water : if nut, it must have a revetement. The souterrains of the citadel should be large enough to contain the garrison ; and the communications with the town should be such, as that they may be destroyed by mines when necessary. The space between the town and the citadel is called the esIdanade, and is fiorn 300 to 400 feet wide. In citadels, as well as in all other forts, martel lo and other towers may be used with considerable advan tage.
The form of a fortress cdepends a good deal on the na ture of the ground on which it is constructed. If the ground be quite level, the fronts may be equal in every re spect, both in the trace and in the relief. If a river runs through a town, it should pass under the rampart, and be covered with a shell-proof arch. The entrance of these arches are to be shut up with iron doors on the outside ; but, if the river be navigable, on the inside. The water of the river is to be used, if possible, for filling the ditches at pleasure ; and for this purpose, a batterdeau is to be con structed across the river at its entrance, and along each side, so as to stop up the communication between the river and the ditch. By means of sluices in this batterdeau, the water may be thrown into the ditch, and let out by a simi lar batterdeau at the opposite side, when the river issues from the place. These batterdeaux ought to be six feet above the usual height of the surface of the water, but ne ver so high as to be seen by the enemy.