3. Because, although a corps thrown forward in front of the centre does not directly protect the wings as a real vanguard, it still contributes greatly to their security indirectly. For instance, the enemy cannot in ordinary cases pass by such a corps within a certain distance in order to effect any enterprise of importance against one of the wings, because he has to fear an attack in flank and rear. Even if this check which a corps thrown forward in the centre imposes on the enemy is not sufficient to constitute complete security for the wings, it is at all events sufficient to relieve the flanks from all apprehension in a great many cases. The van-guard of the centre, if much stronger than that of a wing, that is to say, if it consists of a special corps as advanced guard, has then not merely the mission of a van-guard intended to protect the troops in its rear from sudden surprise; it also operates in more general strategic relations as an army corps thrown forward in advance. The following are the purposes for which such a corps may be used, and therefore those which determine its duties in practice. 1. To insure a stouter resistance, and make the enemy advance with more caution; consequently to do the duties of a van-guard on a greater scale, whenever our arrangements are such as to require time before they can be carried into effect. 2. If the central mass of the army is very large, to be able to keep this unwieldy body at some distance from the enemy, while we still remain close to him with a more moveable body of troops. 3. That we may have a corps of observation close to the enemy, if there are any other reasons which require us to keep the principal mass of the army at a considerable distance. The idea that weaker look-out posts, mere partisan corps, might answer just as well for this observation is set aside at once if we reflect how easily a weak corps might be dispersed, and how very limited also are its means of observation as compared with those of a considerable corps. 4. In the pursuit of the enemy. A single corps as advanced guard, with the greater part of the cavalry attached to it, can move quicker, arriving later at its bivouac, and moving earlier in the morning than the whole mass.