The efficacy of a fortress is plainly composed of two different elements, the passive and the active. By the first it shelters the place, and all that it contains; by the other it possesses a certain influence over the adjacent country, even beyond the range of its guns.
This active element consists in the attacks which the garrison may undertake upon every enemy who approaches within a certain distance. The larger the garrison, so much the stronger numerically will be the detachments that may be employed on such expeditions, and the stronger such detachments the wider as a rule will be the range of their operations; from which it follows that the sphere of the active influence of a great fortress is not only greater in intensity but also more extensive than that of a small one. But the active element itself is again, to a certain extent, of two kinds, consisting namely of enterprises of the garrison proper, and of enterprises which other bodies of troops, great and small, not belonging to the garrison but in co-operation with it, may be able to carry out. For instance, corps which independently would be too weak to face the