But let us enter a little more into the details of frontier defences,
and examine the character of the several systems which have been
successively proposed or adopted. Frontiers are divided into four
distinct classes, according as the state may be open on one or more
sides, or bounded by mountains, large rivers and lakes, or by the sea.

An open frontier is the most difficult of defence; and while there
exists a perfect uniformity among military men upon the vast importance
of fortifying such a frontier, there is an equal diversity of opinion
respecting the best manner of arranging these works. We shall here
mention three general systems of arranging forts for the defence of an
open country, each of which has been advocated at different times, and
afterwards received various modifications and additions. These three
systems comprise the main features of all others worthy of much
consideration. They are:--


1st. The system of continuous lines, proposed by Montalembert.

2d. A system of three lines of detached works, strongly recommended by
D'Arçon 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