At a later period, Vauban's method of constructing out works was considerably improved by Carmontagne. By enlarging the reduit, he diminished the terre-plein of the demi-lune, which in Vauban's system was too large. He made the ditch between the demi-lune and the reduit 55 feet, and constructed the latter so as to make its flanks about 55 feet, terminating in the same way as Vauban's. He also constructed larger works in the places of arms. The principal of these was a reduit D, of which the faces r s, 8 u, formed a very obtuse angle, and the flank -o, pre vented the part of the covered way behind from being en filaded. These flanks serve also to take the enemy in the rear when attempting to ascend the breach in the demi lune.
Ravelins might be still farther improved, by increasing the saliant angle so as to lengthen the faces ; but as that angle ought never to exceed 60 degrees, the most advan tageous form for a ravelin, is an equilateral triangle. The advantages of the ravelin, however, depend not a little on the angle of the polygon. The larger that angle is, the farther will the ravelins extend into the field, and of course the greater will be their command. This, indeed, is the principle on which all the modern improvements in forti fication are founded, and for the illustration of which we are chiefly indebted to St Paul, the inspector general of the French fortresses. We say illustration, for the disco very, we believe, is due to Montalembert and Belidor, par ticularly the latter.
The practice of enlarging the angle of the polygon has been adopted by succeeding engineers, not only from the advantageous position which it gives to the demi-lune, but also because it is found to form the strongest possible en ceinte. The proper application of this principle, and the late improvements in the construction of works in the places of arms, constitute the chief merit of the modern system of defence.
In Montalembert's method there are no bastions, but merely saliant and re-entering angles, the former 60 and the latter 90 degrees. His lines of defence, that is, the lines containing the angles, are from 300 to 550 feet.
In the preceding part of this article, we have consider ed the trace merely as a suite of lines, without regard to the thickness of the works which these lines represent. We shall now proceed to lay down the dimensions of the different parts, as they have been determined by the most skilful engineers.
The terre-plein of the rampart ought to be of such a breadth as to permit the soldiers to manoeuvre, and effec tually to resist all attempts to make a breach in it ; at the same time, if it be too wide, it is much exposed to the enemy's fire, occupies more space than can be spared from the other works, and becomes very expensive. By some
old engineers it was fixed at from 55 to 70 feet, a breadth often impracticable, and in all cases useless. Pagan re duced it to 25; but unless the quantity of artillery be very small, and the guns seldom require to be removed from one place to another, this space is too narrow. A piece of ordnance after being discharged, recoils from 18 to 23 feet, according to its calibre and the nature of its mount ing. The breadth of the terre-plein, therefore, cannot be less than from 33 to 40 feet, without embarrassing some part of the service. Perhaps 36 feet is the best me dium.
To prevent the rain lodging upon it, the surface of the terre-plein should incline towards the town at the rate of one inch of perpendicular descent in every foot of breadth. It ought also to be planted with rows of trees, particularly oak and elms, for the purpose of supplying wood for the artillery engines and carriages. The mass of the rampart is to be finished towards the town by a talud or slope, not, however, so steep but that the soldiers may ascend it with out being obliged to go round by the ramps or passages leading up to the terre-plein. Between the houses and the rampart, there ought, if possible, to be a street 14 to 20 feet wide ; and if there is not space sufficient for this other wise, the inner side of the rampart may be made more nearly perpendicular, and be supported by a revetment. It is always inconvenient to have houses close to the ram part, and ought therefore, if possible, to be avoided.
Engineers have differed in opinion with regard to the construction of bastions, some recommending the full, others the empty. The former are those in which the ground within is level with the rampart, the parapet only being higher ; the latter are such as have the ground con siderably lower. The empty bastions, it is said, afford fa cilities to the garrison for carrying on their mines, and the interior space is well adapted for magazines. The full bastions, however, are now generally preferred, except in cases where there is a scarcity of earth, as they are better calculated to resist an assault, while in the empty bastions the magazines are much exposed to the enemy's shells, and the entrenchments within are incapable of a great resistance.