ALEXANDRIA, VA., SEPTEMBER 1, 1862.

Major General GEORGE B. McCLELLAN:

Have received your instructions in regard to ascertaining the strength of troops here, and will collect information immediately. Colonel Allabach's brigade consists of the One hundred and twenty-third, One hundred and thirty-first, One hundred and thirty-third, and One hundred and thirty-fourth Pennsylvania-aggregate present, 3,923, all of which, except 165, are for duty. We know the composition of nearly all the other commands, but not their strength. Do you wish me to send to Upton's Hill for the strength of Cox's division? We expect to have telegraphic communication with that point by 6 p.m.

S. WILLIAMS,

Assistant Adjutant-General.

HEADQUARTERS ARMY OF THE POTOMAC,
September 1, 1862-6 p.m.

Major General GEORGE B. McCLELLAN:

Your dispatch in regard to the disposition to be made of the troops in this vicinity has been received and your instructions have been executed. I have also taken measures to bring about the return of Major Haller's command to these headquarters. Have also directed McInryre's squadron to report to you in Washington on arrival at Alexandria. There is now telegraphic communication with Upton's Hill. Your horses are about leaving for Washington.

S. WILLIAMS,

Assistant Adjutant-General.

HEADQUARTERS,
Alexandria, Va., September 1, 1862-10 p.m.

General GEORGE B. McCLELLAN:

Brigadier General E. B. Tyler's command consists of the Ninety-first, One hundred and twenty-sixth, and One hundred and twenty-ninth Pennsylvania; for duty, 2,158. He also has a section of Barnes' battery (C), First New York Artillery. Brigadier-General Slough, military governor of Alexandria, has the Thirty-third Massachusetts-for duty, 1,150 men; Thirty-fourth Massachusetts, 850, and the Sixty-eighth Illinois, 400 strong-the latter a three-months' regiment, whose term expires on the 28th instant. I have sent the composition and strength of Alabach's brigade in a previous dispatch. Woodbury's Engineer Brigade, Fifteenth and Fiftieth New York, with one squadron Twelfth Pennsylvania Cavalry, is near fort Lyon, 946 strong. There are also detached commands, as follows: Second New York Artillery, 429 strong; two regiments of Devens' brigade, 1,200 strong; Twenty-fourth Michigan, just going into Fort Lyon, a full and new regiment; part of Duane's battalion; three companies of Thirty-second Massachusetts, of Porter's corps; about 2,500 recruits and stragglers, under Colonel Warner, now being armed and sent to Fort Whipple; about 5,000 recruits and stragglers, under Colonel Belknap, of whom nearly 1,000 mostly from Peck, would be efficient, if armed; five companies of the Forty-sixth New York, wagon guard of Stevens' division, 270; 60 pioneers, of Banks' corps, with tools only; two companies, a wagon