But a work may be of such a character as to render it unnecessary to resort to all the works of attack which would be required for the reduction of a regular bastioned fort, on a horizontal site. For example: the nature of the ground may be such as to enable the troops to approach to the foot of the glacis, without erecting any works whatever; of course, all the works up to the third parallel may in this case be dispensed with without any violation of the rules of a siege. Again, the point of attack may be such that the other parts of the place will not flank the works of approach; here a single line of _boyaux_ and short parallels may be all-sufficient. But for the purpose of discussion, we will here suppose the place besieged to be a regular bastioned work on a horizontal site, (Fig. 54.) The operations of the siege may be divided into three distinct periods. 1st. The preliminary operations of the attack and defence previous to the opening of the trenches. 2d. The operations of the two parties from the opening of the trenches to the establishment of the third parallel. 3d. 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. This arrangement constitutes the _nightly cordon_.