Mechanics teach us the proportions of the machines in use, and how to increase or diminish their powers as occasion may require; and likewise to judge whether those which our own imagination suggests to us, will answer in practice. Hydraulics teach us how to conduct waters from one place to another, to keep them at a certain height, or to raise them higher. How fluently soever we may express ourselves in speaking or writing, we can never give so perfect an idea as by an exact drawing; and often in fortification both are wanted; for which reason the art of drawing is indispensibly necessary for engineers. To the qualities above mentioned, must be added activity and vigilance; both which are absolutely necessary in all operations of war, but especially in the attack of such places as are in expectation of succours. The besieged must have no time allowed them for consideration; one hour lost at such a juncture often proves irreparable. It is by their activity and vigilance, that engineers often bring the besieged to capitulate, much sooner than they would have done, if those engineers had not pushed on the attack with firmness and resolution. Want of vigilance and activity often proceed from irresolution, and that from weakness of capacity. As the office of an engineer requires great natural qualifications, much knowlege, study, and application, it is but reasonable that the pay should be proportioned to that merit which is to be the qualification of the person employed: he must be at an extraordinary expence in his education, and afterwards for books and instruments for his instruction and improvement, as well as for many other things; and that he may be at liberty to pursue his studies with application, he must not be put to shifts for necessaries. It should likewise be considered, that if an engineer do his duty, be his station what it will, his fatigue must be very great; and, to dedicate himself wholly to that duty, he should be divested of all other cares. The word engineer is of modern date in England, and was first used about the year 1650, when one captain Thomas Rudd had the title of chief engineer. In 1600, the title given to engineers, was trench-master; and in 1622, sir William Pelham, and after him sir Francis Vere, acted as trench-masters in Flanders. In the year 1634, an engineer was called camp-master general, and sometimes engine-master, being always subordinate to the master of the ordnance. At present the corps of _engineers_ in _England_, consists of 1 colonel in chief, 1 colonel en second, 1 chief engineer, 5 colonels, 6 lieutenant colonels, 18 captains, 15 captain lieutenants, and captains, 31 lieutenants, 16 second lieutenants. The establishment of the corps of _invalid engineers_, comprises a colonel, lieutenant colonel, captain, captain lieutenant and captain, first lieutenant, and second lieutenant.