It would be impossible to afford any in structions regarding those works which may be conjoined to natural defences, so as to render the whole complete; such must depend entirely on local circum stances, of which the skilful engineer will not fail to take advantage.
Artificial fortification applies to every kind of defence, whether regular or ir regular, pure or mixed ; and has been di vided by the most celebrated engineers into two distinct kinds, vi:. offensive and defensive. The former relates princi pally to the various works used in attacks and sieges; the latter appertains to the more general purpose of securing towns, forming depots, commanding choice si tuations, defiles, &c. protecting harbours, and, in general, tending more to self-pre servation, and to control, than to the an noyance of others, or to the extension of dominion.
This important science is again sub divided into the permanent and the tem porary: the former being with the view to endure the test of ages, while the lat ter is confined principally to operations in the field ; and such works are, for the most part, abandoned, so soon as the oc casion for their construction may have subsided.
Defensive fortification consists of three systems, each of which has its particular uses : 1. The little which is usually adopted in the construction of works, having four or five sides, or citadels, various small or detached posts, horn works, crown works, &c. where the exterior of the de fences, that is, between the salient angles of the two bastions, does not exceed 350 yards.
2. The mean, which is of general use, and forms a very considerable portion of all regular fortifications, whose exterior sides of defence may be from 350 to about 400 yards.
3. The great, which is principally used where the exterior of the defences mea sures more than 380, and as far as 500 yards, or perhaps rather more; it is ob vious, that such very extensive fronts, even in a hexagon, or figure of six sides, would enclose an immense area ; conse quently would require a moderate army to man the defences. Hence we gene rally find this system composing only part of the works ; such as are on the borders of a lake, or of a marsh, or along the bank of a river; while the other sides are composed of the second or mean sys tem.
Such are the leading features and ap plications of the three systems, as settled by the celebrated Vauban, and adopted by the most distinguished professors of our own time. Occasional deviations have, however, been made in several in stances, with the approbation of pre-emi nent engineers ; but, for the most part, such have been with the view of conform ing to local necessity, and of effecting a saving, either of materials, where they were scarce, or to disbursement, where parsimony was an object. Anomalies of
this description are not to be considered as data whereby to be guided ; but it may be proper to study the practical ef fects of all innovations, however much they may be abstracted from received theories; for the most happy inventions generally experience much opposition, often, indeed, illiberal contempt, while in their infancy. This should not deter the man of genus from ushering his sugges tions to the notice of the world; for al though his inefficient measures may ap pear wild and eccentric, or eventually be ignominiously decried by those invinci bles, who, having learnt to work" by line and rule," neither will, nor can, correct their errors; yet there will never be wanting, in this enlightened age, persons who can both comprehend, and duly ap preciate, the effusions of a vigorous and sensible innovator.
We now come tothe description of the several defences, as regulated by and others of acknowledged skill, premis ing,, that many opinions have started, and other proportions been recommended by men of first rate talents, who have each their advocates. What is now offered may be deemed a concentration of their va rious hypotheses, so fitr as they could be assimilated into one general system : to give all would occupy volumes.
Fortifications may be considered as re gular, when the enclosed area is of such a form, as canbe inscribed in some regular figure ; such as a triangle, a rectangle, a circle, or an ellipsis : observing that such figure should, in a manner, fit the town, &c. it is intended to protect. It is usual to divide the perimeter, or whole outline of the figure, when it is either of the two latter forms, into as many faces, or por tions, as may admit of suitable defences in either of the three systems already de tailed. The number of faces must be re gulated as well by the form, as by the ex tent of the figure. Small circles may be di vided into five or six faces ; moderately extensive ovals may have six also; while the more extensive circles and ovals will require an additional face or more, in or der to reduce the exterior lines of defence within due bounds ; so that every part of each face may be within musket shot of those parts whereby it is flanked, or de fended. Whenever this funeamental prin ciple is disregarded, the plan will be pro portionally weaker, according to the un due prolongation of the faces, and the consequent deficiency of mutual support.