The first draw-bridge generally con nects with the body of the bridge passing over the ditch, and is drawn up by persons standing on the berm ; while that draw-bridge, which rises close up against the gate, is so contrived as to bury itself; for at least its whole thickness, into the masonry ; whereby its edges are secured from the grazing of shots ranging against the wall, and the possibility of wrench ing the draw-bridge out of its place is sufficiently obviated. The gates usually close in the ordinary way of all large ones, that is, in two leaves, meeting in the centre ; over them a portcullis is sometimes suspended horizontally ; its hinges being close behind the gates when shut. This immense machine resembles a very large harrow, and lets down, much like the ports of a ship, until it hangs vertically, close at the back of the gates, and being secured with long iron stays, beams of wood passing like window bars into the wall, and other de vices, it proves adequate to the repulsion of even a common sized petard. Some places have a succession of such gates and portcullices, one behind the other, which, added to the casematesbeing lined with defenders, renders it almost impos sible to force the passages. When mat ters are driven to extremity, owing to the raveline being possessed by the enemy, and by the defences on the curtain and flanks being rased, the gateways are then filled up with rubbish, either loose or in bags, &c. so that they are no longer passable.
The nakedness opposite the angle B, of bastion 3, is left purposely, to she w how easily a besieging army would effect a breach at that angle, provided no addi tional outworks were supplied ; for, as yet, we are to consider the fortress to consist only of the principal and the ra velines. A suitable train of battering cannon brought to act upon the point B, while other batteries were employed to silence the faces of the adjacent rave lines 0 and N, would, in a very few days, effect a breach, and give the besiegers a command of the ditch, by establishing themselves in a lodgment on the crest of the glacis g, at the saliant point q ; whence they would batter the flanks o S, and -zo v. Then, as nothing could oppose their passage over. the ditch, which if wet would be passed by sap, (i. e. by filling up with fascines, &c.) the angle B would be carried by storm for the matter would obviously rest on the numbers, and on the personal prow ess of the contending forces, The issue of such affairs have been so various, that it would be presumption to say the be siegers must succeed ; but if the breach be practicable, and the internal state of the bastion, as seen in No. 3, even though there should be an intrenchment of the gorge, i. e. from r to u, the chances would be in their favour, after the breach was gained. This mode of defence is perhaps the best in hollow bastions ; that is, in such as are not solid, but have deep areas within them, level with the streets of the town, &c. (called the
terre-pleine ;) but in a solid bastion, some defences should be internally constructed while the breach is making; of this some idea-may be formed by the flanked angle in bastion 4, where a rampart and ditch are made, to force those who may as cend the breach to quit the bastion. The gorge may also be fortified as in bastion 3, whereby much time may be gained, a matter often of the utmost im portance, either from the expectation of succours, or to favour the evacuation of the fortress altogether.
In bastions No. 3 and 4, the flanks t u, and w v, are not only curved, but they are double, presenting, of course, two tiers of cannon, of which the upper stand on the bastion, while the lower are just below the level of the berm, whereof they constitute a part, and cannot be discover ed beyond the crest of the glacis. These latter, therefore, cannot be battered from the approaches in the early stages of a siege : they lay, as it were, perdue, in reserve for the defence of the ditch. There are two little semi-circular projec tions at t and w, called orillons ; these serve not only to cover the flanks t u and w a from enfilade, but each mounts a gun which cannot he perceived until half way over the bridge, and which serve to defend the gate when assaulted, as well as to take the assailants in flank, and partly in reverse, (i. e. from behind,) as they advance to the attack. 'I hey are especially useful when a tenaille, as seen at P, is constructed in the dry ditch be fore the curtain S r; for when those who were placed in the tenailles, which com mand the interior of the raveline 0, and of the redoubt la, may be attacked in flank, and be obliged to retreat into the principal, along the caponnaire P these guns pour in grape along the inte rior of the tenaille, when it is in the hands of the enemy, and enfilade so as to cause its abandonment.
The caponnaire is a passage made be tween two parapets, each having a long talus, or slope, outwards ; as expressed by the small lines diverging from the path. It is commanded by (i. e. open to the fire) the flanks o s, and s r, and the centre of the curtain a r. I)emi.capon naires are common for the passage of troops Iron] one outwork to another, as seen at e e, in the raveline N, where they serve to shelter the narrow defiles lead ing into the intrenclimentsff, within the crown-work M. They have but one pa. rapet, which is open to the fire of the faces B t, and w c, also to the oblique fire of the curtain between 91 and v. By their exterior slope they serve to flank tile pas sage of the ditch of the raveline, in con junction with the faces B t, and C w, which fire over them, and command the whole interior of the horn-work M.