If a river runs close by a place, no other work will be necessary on that side, than a single enceinte with a cover ed way ; but if there be a gate towards the river, it will re quire to be covered with a strong work called a tete-de pont, or head of a bridge. If a canal be carried from the river through the place, the sluices that form the commu nication roust also he strongly fortified, and the entrance of the canal or river into the place covered with demilunes, lunettes, &c. It will be found most advantageous to let the river or canal issue from the place at the curtain.
If a fortress be situated on a hill, the rampart should be constructed so as to enfilade the declivity of the hill as much as possible, independent of the outworks. Such for tresses are generally small, but very strong. If the situa tion of the place be in a moor, or ground any way impassa ble, it is still more easily fortified, particularly if there be a command of water. A single enceinte with demi-lunes before the gates, will in general be sufficient for such a place ; but if there is a more easy access on any side, it is to be covered by stronger works. These outworks ought to have the gorge enclosed by a wall with loop-holes.
When a fortress has a plain on one side, and a moor, height, or impassable marsh on the other, the side most open to attack should be nearly a straight line, while that towards the other may have a considerable degree of cur vature. With regard to the works necessary on each side, the observations we have already made will be found use ful; though, after all the directions that can be given, a great deal must in every case depend on the skill-and expe rience of the engineer. It may be observed in general, however, that fortifications on unequal eminences, or on eminences and plains, are always disadvantageously placed ; but when it is necessary to construct them on such places, the side most open to attack should have as open bastions as possible ; and if such sides have eminences in front pa rallel to them, or nearly so, it will be necessary, in many cases, to occupy these eminences by detached forts and other strong works.
When a place is situated on the sea coast, the side next the sea may be secured against a coup de main, by a single enceinte, or by strong detached works. When the place is at a distance from the sea, but conununicates with it by means of a long canal, forts are to be constructed along the canal at convenient distances : these are generally placed at the ends of dams or dikes, and so formed as readily to en filade the canal or its hank. If the forts can be approached
by vessels, they ought to be of masonry, and the guns mounted upon them should be 36 pounders. They ought also, in this case, to possess the means of making red hot balls.
Temporary or Field Fortifications.
Suet] is a general sketch of the modern system of per manent fortification, or the construction of fortresses that arc to be permanently used as places of strength. We shall now proceed to consider the most approved plan of field fortification, or the construction of temporary works for protecting an army, or a detachment in the field.
As field works are constructed of the same materials, and intended for a time to serve the same purposes as for tresses, many of the general principles laid down above in the construction of the latter, are equally applicable to the former. The strength of each consists of an earthen mound, with a ditch before it, and the only difference, therefore, between them, is in the form, size, and number of works, The first object in field fortification is, to give the work a form as will render it most capacious at a given expellee. The flanking defence should be as much as possible employed, and the flanked parts within the range of musket shot, that is, 400 or 500 feet. The saliant angles are always to he strongest, and dead angles, or points, that cannot be seen from any other part of the work, as much as possible avoided. To render the faces more nearly parallel to the field, the saliant angle should be very obtuse, and its defence will be considerably improved, if, instead of forming an angular point, it be truncated, round ed, or serrated. These different forms are represented in Plate CCLVIII. Fig. 5. A, B, C. Saliants like C, where the faces consist of a suite of small saliant angles, are use ful in removing all unflanked angles ; but the sides, forming each of the small saliants, ought never to exceed two feet ; when they arc more than this, they either weaken the pa rapet, or they render it necessary to increase its thickness, which both adds to the expense, and encroaches on the room within. Perhaps, upon the whole, the best form of a saliant is the circular represented by B. With these general remarks, we shall now proceed to enumerate the the principal works employed in field fortification.