Cæsar at one time remarked that wise men anticipate possible difficulties, and decide beforehand what they will do, if certain possible occasions arise. Does not this process involve invention, in cases where the possible occasions are not of the ordinary and expectable kind? In such cases, does it not require imagination to foresee the possible occasions, and form a correct picture on the mind of the resulting situations? This being done, does it not require the exercise of the constructive faculty afterwards, to make a concrete and effective plan to meet them? If it be so, then we may reasonably declare that, of all the factors that contributed to the successes in Gaul of Cæsar, the most powerful single factor was his inventiveness. The final crisis came when Cæsar besieged Alesia, and Vercingetorix, who had taken refuge in it, sent out a call for succor, that was eagerly and promptly responded to; for it was plain to the barbarians that Cæsar, being held in position fronting a fortress that he could not successfully storm, would be in a precarious condition if attacked vigorously in his rear. Attacked vigorously he was; for the barbarians came in his rear with about 250,000 men; Cæsar having only 50,000, and the enemy in front having 80,000. But it required somewhat more than a month for the barbarians to unite and reach Alesia. With his wonted energy and resourcefulness, Cæsar had by this time cast up siege works all around the fortress, placed camps at strategic points, and constructed twenty-three block-houses. He dug a trench twenty feet deep around the place, and back of this began his other siege works. These included two parallel trenches fifteen feet broad and fifteen feet deep. Behind these he built a palisade twelve feet high, and to this he added a breastwork of pointed stakes; while at intervals of eighty feet he constructed turrets. In addition, he had branches cut from trees and sharpened on the ends; and these he fastened at the bottom of the trenches, so that the points projected just above the ground. In front of these he dug shallow pits, into which tapering stakes hardened in the fire were driven, projecting four inches above the ground. These pits were hidden with twigs and brushwood. Eight rows of these pits were dug, three feet apart; and in front of all stakes with iron hooks were buried in the ground at irregular intervals. When all this had been done on the side toward the fortress, Cæsar constructed parallel entrenchments of the same kind, to protect his rear; the two sets being so arranged with respect to each other that the same men could man both. Having constructed all these material appliances, he instituted a comprehensive system of drills, so that his men would know exactly how to utilize them under all probable contingencies.