William Swinton, Campaigns of the Army of the Potomac

The task of forming an artillery establishment was facilitated by the fact that the country possessed in the regular service a body of accomplished and energetic artillery officers.¹ As basis of organization it was decided to form field-batteries of six guns (never less than four guns, and the guns of each battery to be of uniform calibre);² and these were assigned to divisions, not to brigades, in the proportion of four batteries to each division; one of which was to be a battery of Regulars, and the captain of the Regular battery was in each case appointed commandant of the artillery of the division. In addition, it was determined to create an artillery reserve of a hundred guns and a siege-train of fifty pieces. This work was pushed forward with so much energy, that whereas, when General McClellan took command of the army, the entire artillery establishment consisted of nine imperfectly equipped batteries of thirty guns, before it took the field this service had reached the colossal proportions of ninety-two batteries of five hundred and twenty guns, served by twelve thousand five hundred men, and in full readiness for active field-duty.³

With equal energy the formation of the engineer establishment was entered upon; and this included not only the training of engineer companies and the Corps of Topographical Engineers, but the organization of engineer and bridge-trains and equipage adequate for an army of first-class proportions. At the same time, the entire system of the defences of Washington, both for the northern and southern side of the Potomac,