The operations of defence during this period are so directed as to harass the workmen in the trenches and retard the advance of the works of attack. Garrison pieces of long range and large howitzers are brought forward on the salients of the bastions and demi-lunes of attack, so as to fire in ricochet along the capitals on which the boyaux must be pushed: light and fire-balls are thrown out as soon as it becomes dark, to light up the ground occupied by the besiegers, thus exposing them to the fire of the work and to the attacks of the sortie parties. These parties are composed of light troops who charge the guards and compel the workmen to abandon their sapping tools and stand upon the defence. They are most effective when the besiegers commence the second parallel, as the guards in the first parallel are not so immediately at hand to protect the workmen. When the sortie detachment has driven these workmen from the trenches, instead of pursuing them into the first parallel, it will display itself in battle order to cover the engineer troops, (who should always accompany the detachment in this enterprise,) while they fill up the trenches and destroy the implements of the besiegers. When the guards of the trenches appear in force, the detachment will retire in such a way, if possible, as to draw the enemy within range of the grape and musketry of the collateral works. These sorties, if successful, may be frequently repeated, for they tend very much to prolong the siege. The best time for making them is an hour or two before day, when the workmen and guards are fatigued with the labors of the night. While the besiegers are establishing their enfilading batteries, a strong fire of solid shot and shells will be concentrated on the points selected for their construction. The garrison will also labor during this period to put the work into a complete state of defence: constructing all necessary palisadings, traverses, blindages, barriers; and strengthening, if necessary, the covering of the magazines.
Third period.—After the completion of the third parallel, the crowning of the covered way may be effected by storm, by regular approaches, or (if the work is secured by defensive mines) by a subterranean warfare.
In the first case stone mortar-batteries are established in front of the third parallel, which, on a given signal, will open their fire in concert with all the enfilading and mortar batteries. When this fire has produced its effect in clearing the outworks, picked troops will sally forth and carry the covered way with the bayonet, sheltering themselves behind the traverses until the sappers throw up a trench some four or five yards from the crest of the glacis, high enough to protect the troops from the fire of the besieged. It may afterwards be connected with the third parallel by boyaux.
When the covered way is to be crowned by regular approaches, a double sap is pushed forward from the third parallel to within thirty yards of the salient of the covered way; the trench is then extended some fifteen or twenty yards to the right or left, and the earth thrown up high enough to enable the besiegers to obtain a plunging fire into the covered way, and thus prevent the enemy from occupying it. This mound of earth is termed a trench cavalier, (O). Boyaux are now pushed forward to the crowning of the covered way and the establishing of breach batteries, (J). Descents are then constructed into the ditches, and as soon as these batteries have made a breach into the walls of the bastions and outworks, the boyaux are pushed across the ditches and lodgments effected in the breaches. The demi-lune is first carried; next the demi-lune redoubt and bastion; and lastly, the interior retrenchments and citadel. In some cases the breaches are carried by assault, but the same objection is applicable here as in the storming of the covered way; time is gained, but at an immense expense of human life.
If the place is defended by mines it will be necessary for the besiegers to counteract the effects of these works by resorting to the slow and tedious operations of a subterranean warfare. In this case a fourth trench is formed in front of the third parallel; shafts are sunk in this, about six yards apart, for establishing overcharged mines; as soon as the galleries of the besieged are destroyed by the explosion of these mines, the covered way is attacked by storm; other mines are established on the terre-plain of the covered way to destroy the entrance to the galleries, and thus deprive the besieged of the use of their entire system of mines.
The measures of defence during this period must embrace every thing calculated to retard the works of the besiegers. This may be most effectually accomplished by maintaining a constant fire of grape and musketry on the heads of the sap, and throwing grenades, shells, &c., into the trenches, to harass and destroy the workmen. As the musketry fire of the besiegers now becomes very destructive to the artillerists at the guns, strong musket-proof blinds are arranged to mask the mouths of the embrasures when the guns are not in battery, and also sloping blindages to cover the men when serving at the pieces. The possession of the outworks should be disputed inch by inch, and when the besiegers have reached the ditch of the body of the place, sorties, and every species of projectile, should be employed to drive off the sappers, and to retard the construction of their works. In fine, all the resources of the engineer's art should be put in requisition for the defence of the breach, and the final assault should be vigorously resisted by the bayonet, and by a well-sustained fire from all the collateral works.
With respect to the relative strength of the opposing forces it may be well to remark, that if the fortress is properly constructed the garrison will be able to resist a besieging army six times as numerous as itself. Such is the estimate of the best engineers.[[48]]
A good knowledge of the several subjects discussed in this chapter may be derived from the writings of Vauban, Cormontaigne, and Noizet de St. Paul, on the attack and defence of places and field fortification ; the several manuels used in the French service on sapping, mining, and pontoniering; Col. Pasley's experiments on the operations of a siege, sapping, mining, &c.; Douglas's work on military bridges; Macauley's work on field fortification; and Professor Mahan's Treatise on Field Fortification. This last is undoubtedly the very best work that has ever been written on field fortification, and every officer going into the field should supply himself with a copy.