A work may have only one re-entering or salient angle, or it may have several such ; the plane of defilement, how ever, remains always the same, as described above ; but care is, in this case, to be taken not to injure the flanking defence too much, and also not to make the dead angles too large. Several planes of defilement will, in this case, frequently be necessary. Inclosed works commonly re quire an enormous height to cover the men in them ; and as this can seldom be allowed in field fortifications, tra verses preferred for this purpose.
In this case, the position of the traverses is directed in such a manner as to take up the smallest room, and at the same time to cover the greatest space. The nearer the traverse is towards the commanding height, the more space it covers, and the smaller the relief may be, but the traverses are the higher. Their defilement is made out in a similar manner to that above described ; but care is to be taken in placing them so as not to block up the com munication. The only case where the traverses will not be wanting, is when the ground on which the work is con structed slopes, and when by this its interior is covered. The traverses should always be as nearly parallel to the heights as possible ; and therefore, if a work is command ed from several heights, the traverses will cross each other.
The most difficult case is, when a work is situated in a tunnel, or every where surrounded with commanding heights. In this case a block-house, or a corps de garde, is to be constructed in the middle of the work, which is secured from balls, and which, at the same time, serves as a traverse for the men who defend the work, or to secure its interior parts.
Lines or works connected with each other, and forming intrenchments, should always be strong enough to resist guns ; and therefore, if they are not above 900 paces dis tant from the height, their defilement is to be arranged accordingly. If such heights, therefore, occur, they are either to be occupied, or the position is to be somewhat altered, which is in general easily done, as the engineer will not be so much restricted on any given place as in permanent fortifications. However, in all cases where heights occur, and where the lines always remain com manded, advanced works on these heights are of the great est use.
Should a valley between two heights require to be for tified, strong advanced works will be advantageous. If, however, the valley is very narrow, the line may have very salient parts on the heights, and be frequently broken ; but if these lines do not cover the places behind them sufficiently, then traverses must be chosen, and construct ed at such places as may be deemed necessary.
The next important point to be considered, is the ap plication of the works to the field. Small works and single posts may be established by officers of infantry, but large and composed works only are to be directed by engineers.
An army may be in want of fortifications in different cases, viz. if it has to cover a large part of a country ; iF it has to act at another place, while a part of the frontier is to be well covered ; if an army goes to a distance from its frontiers in au open country ; it it fears an attack from a superior enemy ; if it is inclined to make its retreat in sight of a superior enemy ; and if an army besieges a fortress.
To cover a frontier, every obstacle that can be procured, as well as every thing that tends to strengthen a line, ought to be employed. The use of fortified lines has by some been recommended, by others it has been consider ed as unnecessary. The former, however, appear to be right, in as far as the lines serve to protect and to inclose a country.
Such lines as serve to protect a frontier, extend from two parts, where the enemy cannot pass, or where the line can be apprised, and for their better support, they gene rally have fortresses along them at certain intervals. Towns, rivers, rivulets, habitations, and all other hnpedi ments to obstruct the enemy's advancing, should in such cases be made use of.
To protect whole parts of a frontier, fortified places, or rather fortresses, are the most effectual ; hut where there are none, large towns, and other places commanding the roads, the rivers, and the vallies, by which an enemy may approach, should be fortified in such a manner as may enable them to sustain a slight attack, and to obstruct the enemy's advancing for a few days. In this respect, strong places, consisting of fortified and frontier posts, are of the greatest advantage, if properly chosen and well construct ed. They are generally surrounded with strong works, which defend each other, and also all the avenues or pass ages and roads, as well as the ground leading towards them. In this respect, inundations, abattis, crenelled walls, houses fitted up like block-houses, and all other possible means of defence, should be used. Sometimes houses• must be pulled down, and outworks constructed, if by these means a. town can be rendered stronger and less open to attack.