In the defence, the strictest vigilance should be at all times exerted to guard against a surprise: sentinels are posted on all the most commanding points of the work; all the avenues of approach are most thoroughly guarded; and patroles are constantly scouring the ground in all directions. At night all these precautions are redoubled. Light and fire-balls are thrown out in front of the work to light up the ground, and discover the movements and approach of the enemy. Each man should have his particular post assigned to him, and be thoroughly instructed in the duties he will have to perform. All auxiliary arrangements, such as palisades, abattis, &c., should be defended with the utmost obstinacy; the longer the enemy is held in check by these obstacles, the longer will he be exposed to the grape and musketry of the main work. When he assaults the parapet, he will be opposed by the bayonet in front and a well-aimed fire in flank. While in the ditch, or as he mounts the scarp, hollow projectiles, incendiary preparations, stones, logs, &c., will be rolled down upon his head. But when the assaulting column has gained the top of the scarp, the bayonet forms the most effective means of resistance. The measures resorted to in the attack and defence of the larger class of field-works, will necessarily partake much of the nature of the operations employed in the attack and defence of permanent fortifications. II. The attack and defence of a fortress may be carried on either by a regular siege, or by irregular operations and an assault. The latter plan has sometimes been adopted when the works of the place were weak and improperly defended; where the time and means were wanting for conducting a regular siege; or where the assailants were ignorant of the means proper to be resorted to for the reduction of the fortress. Such operations, however, are usually attended by an immense sacrifice of human life, and the general who neglects to employ all the resources of the engineer's art in carrying on a siege, is justly chargeable with the lives of his men. In the siege of Cambrai, Louis XIV., on the solicitation of Du Metz, but contrary to the advice of Vauban, ordered the demi-lune to be taken by assault, instead of waiting for the result of a regular siege. The assault was made, but it was unsuccessful, and the French sustained great losses. The king now directed Vauban to take the demi-lune by regular approaches, which was done in a very short time, and with a loss of _only five men!_ Again, at the siege of Ypres, the generals advised an assault before the breaches were ready. "You will gain a day by the assault," said Vauban, "but you will lose a thousand men." The king directed the regular works to be continued, and the next day the place was taken with but little loss to the besiegers.