The system of continuous lines, proposed by Montalembert.
2d.

A system of three lines of detached works, strongly recommended by D'Arcon and others.
3d.

A system proposed by Vauban, and advocated by Rogniat, consisting of lines of very strong works, placed at considerable distances from each other and covering large _intrenched camps_.
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.