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.