First period. The object of the investment of the place is to cut off all communication between the work and the exterior, thus preventing it from receiving succors, provisions, and military munitions, and also to facilitate a close reconnoissance of the place by the engineers, who should always accompany the investing corps, and pursue their labors under its protection.  This corps should be composed chiefly of light troops—­cavalry, light infantry, horse artillery, “brigades of engineers and mounted sappers,”—­who march in advance of the besieging army, and, by a sudden movement, surround the work, seize upon all the avenues of approach, and carry off every thing without the work that can be of service either to the garrison or to the besiegers.  To effect this object, the enterprise must be conducted with secrecy and dispatch.

The investing corps is now distributed around the work in the most favorable positions for cutting off all access to it, and also to prevent any communication with the exterior by detachments from the garrison, and even single individuals are sent out to give intelligence to a succoring army or to reconnoitre the operations of the besieging corps.  These posts and sentinels, called the daily cordon, are placed some mile or mile and a half from the work, and beyond the reach of the guns.  But in the night-time these posts are insufficient to accomplish their object, and consequently as soon as it is dark the troops move up as close to the work as possible without being exposed to the fire of musketry.  This arrangement constitutes the nightly cordon.

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.