To return to fig. 3. It will be seen that the elevation of A would, on the cal. culation there assumed, be such as to carry the cordon of its terre-pleine so high, that its revetement could be bat tered from the glacis, D 0, without touch ing C or B. To remedy this, where such an exposure would be injurous, (for it is in some instances expedient, as above de scribed, to direct all the force exteriorly, especially where the outwork lays upon a navigable river, and that ships can be brought to bear upon the works,) the re doubt must be lowered to the same level of terre-pleine as the raveline : and, in deed, it may even, in some few instances, where A must be completely screened from the exterior, be proper to sink the terre-pleine of B so very low, that its can non may just graze the terre-pleine of the raveline C ; making the parapet of a due height, and forming a substantial shed of three inch boards, &c. nearly flat, over the banquette, at a foot above the crest of the parapet. This prevents the enemy from commanding the terre-pleine of the redoubt, while, at the same time, those defending it would do ample exe cution through the narrow slit, or open ing, between the crest and the shed. The latter would effectually keep off grenades, &c. and give the defenders great confidence. The dotted lines above the rampart A, spew the height to which its parapet would necessarily be raised, if B were elevated. even a foot and a half only above C. The dotted line from C towards B shews the level to which the genouliere, or crest of the lower slope of the embrazure, may be brought, so as to allow the fire from B to graze the terre pleine of C, and to drive the enemy from the raveline.
Relinquishing the explanation of what relates to the more scientific parts of the topic, we must now enter upon the de scription of many other matters, indis pensably necessary on this occasion. We have already spoken of bomb-proofs. These are vaulted chambers, either run ning under long arches, or groined, and standing on substantial walls and pillars. The term very properly implies, that the arches should be sufficiently strong to resist the fall of shells, or, at least, to prevent their penetrating into the cham bers. It generally demands, at least, a yard in thickness to produce such a firm resistance ; the masonry should be of the very best materials, and put together in a workmanlike manner. The casemates, thus arched over, should be further cover ed. with three or four feet of soil, where such an addition would not raise the terre pleine too high : for, by allowing a shell to bury itself completely, its splinters do not occasion half so much mischief, as when the explosion is more superficial ; although a quantity of soil may be thrown out. Perhaps the best contrivance for small casemates, defended by breast works, is the giving them such a slope as may cause shells to fall into pits, &c. so as to do no damage. Such pits answer well in the centres of round or polygon redoubts, where only a parapet is left around the top, and might, perhaps, be advantageously made in the centre of block-houses, &c.
In the formation of magazines, for ammunition, the arches are usually made double, that is, one greater arch covers two smaller ones ; the latter resting on a central pier, as seen in fig. 5. In this case every arch should be bomb proof, and the roof should be pitched; the end falling off by a gradual rounding, and the whole well supported by buttresses. The walls of magazines are generally double; the buttresses are sometimes pierced at their sides with small loop holes, for the admission of air and of light to the surrounding passage. All the fast enings should be of brass or of copper, and no wood or iron allowed in any part whatever.
The ramparts are ascended by means of long slopes, called ramps, laying in general parallel with and leaning against them : these ought always to be broad enough to admit a gun passing up and down, mounted on its carriage. The
ramps into solid bastions sometimes di verge into three branches, of which two lay along the insides of the adjoining curtains, while the third runs straight up in a line with the capital, i. e. with the centre of the gorge, pointing towards the saliant angle. Thus 11 C is the capi tal ofthe bastion 4. fig. 1. Horseshoe ca valiers have usually but one ramp, placed in the centre of the rear, as shewn by the two parallel lines proceeding from that in the bastion C, of the crown work M. fig. 1.
The proper arrangement of streets within fortilied towns is of the utmost importance, by contributing essentially to the ready resort of troops to their posts, and facilitating the supply of stores. Every avenue ought to have a barrier, both to keep the inhabitants under pro per controul, and to prevent the effects of various stratagems in behalf of a sur prise. Those houses which command the interior of the works should be al ways reserved for the habitations of the garrison, and should likewise be supplied with small quantities of anoitinition. The arsenals should be comuir cly cover ed from the fire of the en, triy's batter:es; and, together with every building ap propriated to the lodgment of troops, or of stores, &c. should be covered in with bomb proofs. The magazines should not be too large, but commodiously situated for the distribution of ammunition, and every precaution should be taken to keep all combustible stores as far as possible from the provision, &c. so that in case of the former taking fire, the garrison should not be necessitated to capitulate, owing to a want of subsistence.
Sally-ports are made under various parts of the works, to favour the sallies made occasionally, for the purpose of attacking the besiegers in their intrench meats, or for other essential purposes. These are generally galleries, which are shut up, except at the moments when in immediate use. Mines are frequently prepared in the first construction of a for tress ; but the galleries whereby they are entered are usually stopped up ; nor are they loaded, that is, filled with powder, until the period seems close at hand for their being serviceable. Such matters are carefully concealed from all but the engineers, and the superior officers. The supply of water, if from a river, or lake, should be very carefully secured ; for this purpose, it is often necessary to en large the outworks, so as to command sluices, &c. whereby the ditch or reser voirs are filled. If possible, large tanks should be kept in the garrison, or a pro per quantity of casks should he filled, especially inparts where vi ells cannot be dug : lest the besiegers should either drain off the lake, or get command of the sluices and block them up. If particular parts of the surrounding country can be inundated, it sometimes renders all at tempts to carry the place by breaching the walls utterly impracticable.'When this happens, and that the situation ad mits of its being completely invested, whereby it is meant that all access is cut ,) the place may fall in consequence f a blockade. The chances of war are, owever, greatly against the success of lockades ; for, if the garrison be strong nd well provided, it may make nunter s successful sallies against an army •hich must be greatly dispersed by sur unding the place ; while the diseases cident to fixed camps, had provisions, utrid water, constant watching, and pro ably the necessity of countervallation roughout its rear, to keep out partisans, r to repel such forces as may hover bout with the intention to relieve the lace, all combine to weaken, dishearten, nd cause relaxation among the be siegers. In this instance, the besieged,, who have but one object, namely the de fence of the works, have some advantage. But a good general will never sit down before a town he is not tolerably certain must fall in a given time.