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.