The depth of the ditch is usually not less than 6½, nor more than 10 feet, so that the workmen may dig it, and clear it out with the spade and pickaxe alone, without being obliged to have recourse to other means, which it is difficult to procure in the field.
The depth of the ditch being determined upon, its breadth is calculated so that the earth produced by its excavation may suffice as nearly as possible to the formation of the parapet. It is to be observed that earth newly excavated exceeds its former bulk by ⅙, or in other words, were it returned into the ditch, about a sixth part of the whole would not re-enter it.
The slopes of the scarp and counterscarp vary also with the nature of the soil. In loose earth, the scarp loaded with the weight of its parapet cannot support itself without a slope equal to its height: the slope of the counterscarp need not be more than ¾ of its height. In tenacious soils these slopes need not be more than half the height.
The ditch must not be less than 6½ feet deep, and 9 feet wide. When the ditches are seen and flanked, they may be made wide and shallow; but when the enemy cannot be seen in them, it is better to make them narrow and deep. When it is possible to fill the ditches with water, its depth should be 6½ feet.
The breadth of the ditch is calculated from the surface of the polygon n, b, a, m, d, i, of the profile of the parapet, whose dimensions are known. This surface should be to that of the polygon e, f, g, c, of the profile of the ditch, as 5 to 6, in order to compensate for the difference in volume before and after excavation. That my readers may not be driven to long calculations, I shall give in the sequel a table to serve for the draught of the principal profiles which may be needed. The profile No. 1, of this table, is employed when it can only be attacked by infantry, or pieces of small caliber, which must batter it from a distance.
The profiles Nos. 2 and 3 are constantly used for the construction of entrenchments intended to resist ordinary attacks. The profile No. 4 is only employed for more considerable works, such as têtes de pont, and great forts, which may be battered by 12-pounders. A rampart is sometimes added to it, which augments its relief and command. (fig. 5) The rampart should be from 16 to 24 feet in breadth, h, s, according to the nature of the gun carriages which are used; but ammunition must then be carried by hand, and 12 feet more ought to be added if it be intended to carry it in carriages behind the pieces. To diminish the breadth of the terra plain, and, consequently, the labour of its erection, stairs are made to mount to the banquette when cannon are not to be placed there, and when it is not necessary that carriages should move upon it.
When entrenchments must be made in the moment of combat, under the fire of an enemy, or to cover ones self quickly in a position about to be attacked, two ditches are made, an outer and an inner one. The inner ditch is only 1½ feet deep, and about 15 feet wide; the outer ditch is three feet deep, and its slope half the depth; the crest is raised within to a height of 6½ feet, and without, to that of 6 feet; the base of the outer slope is made equal to its height; the upper breadth of the outer ditch will be 7 feet, and the lower 4 feet, when the thickness of the parapet is 3 feet.
When it is wished to cover nothing more than a small post of observation, such as a great guard in front of a camp, etc. a simple parapet of the usual breast height is all that is requisite, taking the earth from within and omitting the outer ditch, (fig. 2).
The height, m h of the parapet is elevated 4 feet 3 inches, its base is three times the height, the depth of the inner ditch 2 feet, and its breadth at top 14 feet. (Savart, p. 86)
Ditches of Fortresses
Ditches full of water cannot be too wide; and when dry, they can neither be too narrow nor too deep. In the first case, the width increases the labour of crossing them; in the second, their bein made narrow augments the difficulty of opening the breach low enough for it to be accessible. They have generally a width of 30 to 40 yards before the body of the place, and from 20 to 25 in front of the half moon.
The advanced ditch f f, (fig. 56) is a ditch dug at the foot of the glacis of the covered way of the fortress. It is sometimes established in order to procure earth for the construction of the works of the fortress, when rocks, or other circumstances, do not allow giving the necessary depth to the ditch. When it can be filled with water at pleasure, it increases the defence. When it is possible that the enemy may drain it, without making himself master of it, or when it is made for the purpose of procuring earth, the side towards the fortress must be made of the same slope as the inner glacis, so that the enemy may not be sheltered from the fire of the garrison, when he has taken possession of it.
Source: Lallemand, Henri Dominique: A Treatise on Artillery (New York 1820)