By the time the main army arrives the reconnoissance will be sufficiently complete to enable the chief engineer to lay before the general the outline of his plan of attack, so as to establish the position of his depots and camp. These will be placed some two miles from the work, according to the nature of the ground. As they occupy a considerable extent of ground around the work, it will generally be necessary to form intrenchments strong enough to prevent succors of troops, provisions, &c., from being thrown into the place, and also to restrain the excursions of the garrison. The works thrown up between the camp and besieged place are termed the _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.