Wet ditches, which have stagnant waters, are liable to great inconveniences. They are said to be well calculated to prevent sudden surprises and assaults; but we are convinced of the contrary, especially during a hard frost. Some again assert, that they stop all communication between ill-disposed persons in the garrison and the besiegers. Every man with the least experience, must be of a different opinion.
Wet ditches might certainly be so constructed, as to let the surface of the water remain 12 or 15 feet above the level of the adjacent country. In which case they would serve as large reservoirs, and not only contribute to the defence of a fortified place, but enrich the grounds by being occasionally let out. The additional value which the neighboring meadows would bear from these seasonable overflowings, might in some degree compensate for the expence of the fortification. During a siege, these waters, with proper management, must give considerable uneasiness to the enemy that invests the place.
To answer this double purpose, the ditch must be separated into several large basons, which might be filled or emptied at discretion, as often as circumstances would require.
Dry ditches. There are some ditches which may be filled at will; and others which cannot, except by extraordinary means. If they should be intended to answer the purpose of agriculture, aqueducts might be constructed, or the waters poured in through artificial channels. In which case the ditches would not require much depth. The glacis might be raised in such a manner as to serve to dam in the body of water, and to afford a second glacis from whence the besieger might be considerably embarrassed.
Ditches that are lined, ditches whose counterscarp is supported, and kept up by a stone or brick wall.
Ditches that are not lined, whose counterscarp is supported by earth covered with sods. These ditches are not so secure as the former, on account of the breadth which must be given to the talus, and by which an enemy might easily surprise a place.
So that ditches in fortification may be briefly distinguished under three separate heads, viz:
Dry ditches, which from the facility with which they may be repaired, and their capability of containing other works proper for their security, are in most instances preferable to any others.
Wet ditches that are always full of water, and consequently must have bridges of communication which are liable to be destroyed very frequently during a siege.
Wet ditches are subject to many inconveniences, are ill calculated to favor sallies, and have only the solitary advantage of preventing a surprise.