The command of a work is its height above the field, or above some other work, and is not to be confounded with relief. which is the height above the horizon. When a fortification has only' a partial command of the field, it is called a razing fortification ; and when it is morn elevated above the ground it gets the name of a high fire. If any part of a fortification cannot be seen from the parapet, it is called a dead angle.
The profile or a fortification, is a vertical section of the works from the extreme boundary of the glaci3 to the town. or the inside of the terre-plein. The object of a profile, is to exhibit the altitude and depression, as well as the inter nal structure of the different parts.
Various rules have been laid down by different writers, for determining the most advantageous proportions of the parts now defined. '['he following were given by Errard : 1st, That the saliant angle of the bastion should never be less than 60°, and always 90° when practicable.
2d, That a flanking and a flanked part should be within musket shot of one another, viz. from 700 to 900 feet.
3d, That every part or a fortification ought to be flanked, or seen from some other part.
4th, That all flanking parts ought to be so strong, as to resist the force of ordnance.
5th, That all the works of a fortification ought to be so proportioned in elevation, that the one nearer the place shall be higher than the one before it.
Notwithstanding the excellence of these rules, however, the trace of Errard was very defective. Ile left all the lines to be determined by the constructor ; the flank was given, but varied with the sides of the polygon, and, on irregular ground, was difficult to be traced. It was also too small, and rendered less easy to be defended, from being perpen dicular to the face. Succeeding engineers corrected these faults, by determining the various parts with more precision, and on more accurate principles. The improvements which followed, were adopted partly in one country, and partly in another, and thus gave rise to the different methods known by the French, Italian, Spanish, and Dutch. These me thods, however, may be considered as essentially the same, the difference among them being chiefly in the saliant an gle of the bastion, and the form of the flank.
Whatever method of fortification be adopted, it is of im portance to observe, that all the works ought to be traced out in the exterior polygon. If they are traced on the in terior, the different points will not be determined with the same accuracy. Having premised this, we proceed to lay down the rules for the most approved method of fortifying as it is now practised.
The gorge of the bastion should never be less than from 160 to 200 feet wide, so as to afford a free communication with the bastion, and room for mounting two or three guns.
The length of the flank depends on the length of the line of defence, and what it is required to defend : but it ought never to be less than from 180 to 200 feet, and should be perpendicular to the face which it is intended to flank.
The orillon should never be larger than to allow two or three guns to be placed on the covered plank. If it be more, it weakens the flank.
To increase the offensive power of the flanks, guns have sometimes been placed in vaults cut out of the rampart be neath. These vaults are called casenzates; but they have been objected to on account of their diminishing the strength of the flank, their annoying the gunners with smoke, and being very expensive. To remedy these de fects, and, at the same time, to reap the advantages of an auxiliary fire, lower flanks have been constructed before the other flanks, and behind the orillons. In these, the ordnance is covered by a parapet, and the ammunition is placed in a vault under the parapet of the higher flank. Compared with casemates, these lower flanks are undoubt edly an improvement, but, at the same time, they certainly diminish, in a considerable degree, the effect of the higher flank ; and it may therefore be justly questioned, how far they are, in many cases, advantageous.
The saliant angle of the bastion ought never to be less than 80 degrees, and if it could be made to exceed 90, it would give to the whole work a still more advantageous form. The flanked angle would be thus enlarged, the flanks lengthened, the faces more parallel to the field, and the bastions themselves might be at a greater distance from one another.