Until just previous to the disastrous Bull Run campaign, no higher organization than that of brigades was adopted; but a day or two before the march commenced, General McDowell organized the brigades into divisions. These were reorganized by General McClellan as the two and three years volunteers joined the army. The organization of corps was made in the spring of 1862, just before the commencement of the Peninsula campaign, and is now the organization of the army. The complete organization is now as follows: _Regiments_, generally of ten companies. _Brigades_, of four or more regiments. _Divisions_, generally of three brigades. _Corps_, generally of three divisions. The various staffs have gradually been organized, until they now stand (in the Army of the Potomac) as follows: At the headquarters of the army: _A Chief of Staff._ _An Assistant Adjutant-General._ _A Chief Quartermaster._ _A Chief Commissary._ _A Chief of Artillery._ _An Assistant Inspector-General._ _A Medical Director._ _A Judge Advocate-General._ _An Ordnance Officer._ _A Provost Marshal-General._ _A Chief Engineer._ _A Signal Officer._ _Aides-de-Camp._ The rank of these officers, as the staff is now composed, is as follows: The chief of staff, a major-general; the assistant adjutant-general, chief of artillery, and provost marshal, brigadier-generals; assistant inspector-general, a colonel; medical director, chief engineer, judge advocate-general, majors; the signal officer, chief commissary, and ordnance officer, captains; the aides, of various ranks, lieutenants, captains, and majors. Most of these officers do not derive their rank from their position on the staff, but it has been given them in the volunteer organization, or pertains to them in the line of the regular or volunteer army. All the department officers (meaning all except aides) have a number of assistants, and the general officers have staffs and aides of their own, to which they are entitled by law. The total number of officers on duty at the headquarters may amount to fifty or more, and there is plenty of work for all of them during a campaign. Besides the regular staff, constituted as above related, there are the officers of an infantry regiment which furnishes guards and escorts, and officers of cavalry squadrons detailed to furnish orderlies. The headquarters of the army is therefore a town of considerable population. At the headquarters of the different corps the staffs are as follows: _An Assistant Adjutant-General_--Lieutenant-colonel. _A Chief Quartermaster_--Lieutenant-colonel. _A Chief Commissary_--Lieutenant-colonel. _An Assistant Inspector-General_--Lieutenant-colonel. [These officers derive their rank from their position, under a law of Congress.] _A Medical Director_--being detailed from the senior surgeons of the regular or Volunteer army, and ranking as a major.