The first of these systems was proposed in 1790, and for a time attracted considerable notice in France, but has long since been discarded, as being utterly incompatible with the principles of the military art. A writer, however, of some pretensions in this country, recommends its adoption for the defence of Baltimore and the shores of the Chesapeake. The same author would dispense entirely with our present system of fortifications on the sea-coast, and substitute in their place wooden Martello towers! This would be very much like building 120 gun ships at Pittsburg and Memphis, for the defence of the Ohio and the Mississippi rivers, and sending out duck-boats to meet the enemy on the Atlantic! In the second system, the works on the extreme frontier are to be placed about thirty or forty miles apart, and those of the second and third lines respectively thirty or forty miles in rear of the first and second lines, and opposite the intervals. In the third system, first recommended by Vauban and more recently by Rogniat, the works are to be arranged in the same manner as in that of D'Arcon, but the distance between them is to be from seventy to one hundred miles, and each fort arranged for covering a large intrenched camp. Either of these last two systems is well suited to the defence of an open frontier. The former is applied to the side of France towards Belgium, and the latter, with certain modifications, to the defence of Western Germany. The first line of fortifications on the northern frontier of France consists of Dunkirk, Lille, Valenciennes, Conde, Quesnoy, Rocroi, Charlemont, Mezieres, and Sedan; the second line, of Calais, Andres, St. Omer, Bethune, Arras, Douai, Chambrai, Landrecies, and Avesnes; the third line, of Boulogne, Montreuil, Hesdin, Abbeville, Amiens, Bapaume, Peronne, Ham, and Laon. For mountainous frontiers it is deemed necessary to secure all the important passes with small redoubts or military works, and to defend with strong forts the grand interior strategic points on which these communications are directed. For a frontier of moderate extent there may be some six or eight gorges in the mountains by which an army might penetrate; but it will always be found that these roads concentrate on two or three points in the great valleys below. Take, for example, the frontier of France towards Switzerland and Italy. The passes of the mountains are secured by the little works of Fort L'Ecluse, Fort Pierre-chatel, Fort Barraux, Briancon, Mont Dauphin, Colmars, Entrevaux, and Antibes; while Besancon, Grenoble, and Toulon, form a second line; and Lyons a grand central depot.