From the completion of the third parallel to the reduction of the place.
_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.