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.
From the important character of these duties as connected with the means essential to a national defence, and the vast amount of money expended in these operations, it is evident that a high order of acquirements should be deemed necessary to qualify one to perform the duties of a military engineer.

This officer requires a knowledge of chemistry, to guide his choice of materials for mortars, cements, and mastics; of mineralogy and geology, for selecting stone; of botany, for timber and the means of preventing its decay; of mathematics, in laying out his work and calculating the thickness and stability of his walls, embankments, &c.; of mechanical philosophy, in constructing his machinery; of military engineering, in his plans of fortifications; and of all the higher branches of military science, in selecting positions for these works, such that they shall have the proper relations to the means of national defence, and to the grand operations of armies in the field.

The avenues to appointment to this corps are guarded, in most European armies, with special care, to prevent the influence of money, politics, or family connections; and in our own army it is now specified by law of Congress, that the vacancies shall be filled only from the most distinguished graduates of the military academy.