If the situation will admit of the battery being sunk, even as low as the soles of the embrazures, a great deal of labour may be saved. In batteries without embrazures, this method may almost always be adopted; and it becomes in some situations absolutely necessary in order to obtain earth for the epaulement; for when a battery is to be formed on the crest of the glacis, or on the edge of the counterscarp of the ditch, there can be no excavation but in the rear of the battery.
4. Batteries on a coast—generally consist of only an epaulement, without much attention being paid to the ditch; they are, however, sometimes made with embrazures, like a common gun battery; but the guns are more generally mounted on traversing platforms, and fire over the epaulement. When this is the case, the guns can seldom be placed nearer than 3¹⁄₂ fathoms from each other. The generality of military writers prefer low situations for coast batteries; but M. Gribauvale lays down some rules for the heights of coast batteries, which place them in such security, as to enable them to produce their greatest effect. He says the height of a battery of this kind, above the level of the sea, must depend upon the distance of the principal objects it has to protect or annoy. The shot from a battery to ricochet with effect, should strike the water at an angle of about 4 or 5 degrees at the distance of 200 yards. Therefore the distance of the object must be the radius, and the height of the battery the tangent to this angle of 4 or 5°; which will be, at the above distance of 200 yards, about 14 yards. At this height, he says, a battery may ricochet vessels in perfect security; for their ricochet being only from a height of 4 or 5 yards, can have no effect against the battery. The ground in front of a battery should be cut in steps, the more effectually to destroy the ricochet of the enemy. In case a ship can approach the battery so as to fire musquetry from her tops, a few light pieces placed higher up on the bank, will soon dislodge the men from that position, by a few discharges of case shot. It is also easy to keep vessels at a distance by carcasses, or other fire balls, which they are always in dread of.
Durtubie estimates, that a battery of 4 or 5 guns, well posted, will be a match for a first rate man of war.
To estimate the materials for a battery.
Fascines of 9 feet long are the most convenient for forming a battery, because they are easily carried, and they answer to most parts of the battery without cutting. The embrazures are however better lined with fascines of 18 feet. The following will be nearly the number required for a fascine battery of two guns or howitzers:
90 fascines of 9 feet long.
20 fascines of 18 feet—for the embrazures.
This number will face the outside as well as the inside of the epaulement, which if the earth be stiff, will not always be necessary; at least not higher than the soles of the embrazures on the outside. This will require five of 9 feet for each merlon less than the above.
A mortar battery will not require any long fascines for the lining of the embrazures. The simplest method of ascertaining the number of fascines for a mortar battery, or for any other plain breast work, is to divide the length of work to be fascined in feet, by the length of each fascine in feet, for the number required for one layer, which being multiplied by the number of layers required, will of course give the number of fascines for facing the whole surface. If a battery be so exposed as to require a shoulder to cover it in flank, about 50 fascines of 9 feet each will be required for each shoulder.
Each fascine of 18 feet will require 7 pickets.