It was Philip Augustus, say the French writers, who first introduced engineers (_engigneurs_, or _engignours_, as they were called) into France, and restored the art of sieges. The engineers of that age were seldom charged with the construction of works of military defence, but, like Archimedes at Syracuse, and Longinus at Palmyra, they directed their attention principally to devising implements of war and the most effective manner of using them. Engines of war were at that time divided between the _engigneurs_ and the _artilliers_; the former being charged with the heavier machines, and the latter with the smaller weapons used for throwing projectiles. After the invention of gunpowder, the old battering-rams, cranes, helipoles, &c., disappeared, and with them the _engigneurs_, or masters of engines. The new inventions were united with the few old projectile machines that remained in the artillery, and the engineers were for a time left almost without employment. The revival of the art of fortification was very slow, and the modern system scarcely began to be developed till near the sixteenth century. We must omit for the present giving even an outline of the history of military engineering, and pass to the troops of this arm, as constituting an essential element of an army organization. The subject of fortification, and the history of its various changes, will be examined in the next chapter. The engineers, in modern army organization, constitute the fourth arm of service, as, compared with artillery, their relative numbers are about as two to three. They are divided in the same manner as the artillery, viz.:--1st, the staff; 2d, guards, or fort-keepers; 3d, artificers; and 4th, the troops. I. The officers constituting the staff of this corps are charged in time of peace with planning, constructing, and repairing all fortifications and other defensive works; the construction and preparation of all military materials, and stores connected with this arm; and (in our service) with the disbursements of money connected with these operations: in time of war they are charged with the attack and defence of military works, the laying out and construction of field defences, redoubts, intrenchments, roads, &c.; in the attack they form a part of the vanguard, to remove obstructions; and in retreat they form a part of the rear-guard, to erect obstacles, destroy roads, bridges, &c., so as to retard an enemy's pursuit.