_line of countervallation_, and
those on the exterior side of the camp form the _line of
circumvallation_. These lines are generally about six hundred yards
apart. It is not unusual in modern warfare to dispense with lines of
circumvallation, (except a few detached works for covering the parks of
the engineers and artillery,) and to hold the succoring army in check by
means of an opposing force, called the _army of observation_.

The measures of defence resorted to by the garrison will, of course, be
subordinate, in some degree, to those of attack. As soon as any danger
of an investment is apprehended, the commanding general should collect
into the place all the necessary provisions, forage, military munitions,
&c., to be found in the surrounding country; all useless persons should
be expelled from the garrison; a supply of timber for the works of the
engineers and artillery, fascines, gabions, palisades, &c., prepared;
all ground within cannon range around the work levelled; hedges and
trees cut down; holes filled up; temporary buildings demolished or
burnt; and all obstacles capable of covering an enemy and interrupting
the fire of the work, removed.

During this period the engineer troops and working parties detached from
the other arms will be most actively employed. As soon as the investing
corps makes its appearance, bodies of light troops are thrown out to cut
off reconnoitring parties, and, if possible, to draw the enemy into
ambush. To facilitate these exterior operations, and to prevent a
surprise, several guns of long range are placed on the salients of the
bastions and demi-lunes, and others, loaded with grape, in the
embrasures of the flanks, so as to sweep the ditches. About one-third of
the garrison may be employed in exterior operations, and the other
two-thirds in arranging the means of defence in the interior.

_Second period._--As soon as the engineers have completed their
reconnaissances and determined on the front of attack, and all the other
preparations are made, the general will direct the opening of the
trenches. The ground being previously marked out, battalions of light
troops, termed _guards of the trenches_, as soon as it is dark, are
placed about thirty yards in front of the first parallel, (A. Fig. 54,)
with smaller sections, and sentinels about the same distance further in
advance. These guards lie down, or otherwise conceal themselves from the
fire of the work. The engineer troops and