The rules laid down above for constructing the profile of a fortification, are applicable only to level ground. When the country is uneven, it becomes a problem of considera ble intricacy•to determine the heights of the different works. The method now commonly practised is as follows : A horizontal plane is supposed to pass through the highest point of the ground within the distance of 2400 feet of the intended enceinte. From this plane, whicn is called the plane of comparison, vertical lines are drawn to every remarkable point of the place to be fortified, form ing as it were a chart of soundings, in which the plane of comparison represents the surface of the sea. These ver tical lines, as in the case of soundings, are called cottes, their different lengths being marked on the plane. If these lengths differ no more than from two to Nee feet, the ground may be considered as level, and fortified accord ingly. If the inequalities are greater than this, regard must be had to them in determining the relief.
From what has been already said, it may easily be per ceived, that the crests of all the parapets, on the same front of a fortification, terminate in a plane more or less inclined to the horizon, as the different works are more or less ele vated above one another. In the same manner, the surfa ces of all the terre-pleins, and indeed of any other corres ponding parts of the works, lie in a plain parallel to the frmer, and as far distant from it as these parts are lower than the parapets. Such planes, in general, are called planes of defilement ; and that which passes through the covered way, the plane of view or of site, because it deter mines all the others. The plane of view ought to have such a position, that it will pass from four to six feet above the highest point within 2400 feet of the front. As three points are necessary in determining any plane, the three employed in this case are, two in the line intended for the covered way, and one on the summit, or rather five to six feet above the summit of the highest gi ound about the place. If the line joining the two first points be horizontal, the cottes of the plane of view, or the distances of the differ ent points in the plane of view from the plane of observa tion, may be found by the rules already given for determin ing the relief of a fortification or level ground. If that line is not horizontal, the angle of its inclination is first to be ascertained, and from that the cottes may be easily computed.
If an eminence running along a front be nearly of an equal height, the front ought to be parallel to it ; but if the eminence is higher at one extremity, the front ought to be at a greater distance from the higher part than from the lower.
If a front runs across an eminence, the parts of the front at the bottom on each side are to be covered by saliant works on the top of the eminence, and the whole front may then be constructed on one plane of view. If it crosses two eminences with a valley between, it will require two planes of view, always taking care that there be salient works on the eminences, by which the enemy may he obli ged to open his trenches at a greater distance. If the place be surrounded by eminences on all sides, each front will re quire a plane of view for itself.
In our introductory remarks to this article, we took oc casion to enumerate some of the leading advantages which a country derives from fortifications in general, and parti cularly from those on the frontiers. With regard to the position of the latter, it may be observed, that if the coun try be mountainous or woody, a few fortifications on the most commanding positions will form a sufficient cordon for opposing an invading army. This cordon ought to be either a straight line, or convex towards the enemy, as in this direction it affords greater facility to the different posts in assisting one another. If the country be level and open, it will be necessary to have two or three lines or cot-dons of fortified places, that the enemy may be forced to encoun ter at least ten or twelve sieges before he can enter the country in great strength. If a river forms the frontier line, the fortifications are to be placed at the junction of the river with other rivers—on islands, eminences, or any other advantageous position. Care should also be taken to cover the sluices that may be constructed for inundating the country, when that measure is rendered necessary, Fortifications should always be equally distributed accord ing to their strength, so that the weaker places may not be all on one part of the frontier. But the side which chiefly demands the attenttion of the engineer, is the sea coast, which is always more easily penetrated, and conse quently requires the assistance of every advantageous posi% tion that can possibly be obtained.
The enceinte of a fortification may be a figure of any number of sides, from a square upwards, the length of the side in each being 1100 feet. All figures, however, are by no means equally advantageous ; and therefore it is of con siderable importance to determine the comparative merits of each.