SEPTEMBER 16, 1862.-Reconnaissance toward Thoroughfare Gap and Aldie, Va.

Report of Brigadier General David B. Birney, U. S. Army.

HEADQUARTERS FIRST DIVISION, Seminary, Va., September 19, 1862.

SIR: I have the honor to report that the Second Pennsylvania Cavalry, Lieutenant-Colonel Brinton, proceeded, under my instructions, in pursuance of orders from Major-General Heintzelman, on a reconnaissance to within 2 miles of Thoroughfare Gap and 3 miles of Alide. They captured some 32 prisoners, 4 wagons, and a number of horses, which have been brought in. They came within a few miles of General Ewell, who was on his way to Richmond, and captured 3 of his body guard.

The enemy had withdrawn the force from Bristoe Station, and has placed it at Gainesville. A guide, reporting himself as being in the secret service of Generals Banks and Geary, reports 900 wagons at Ashby's Gap, guarded by two regiments of cavalry (800), and no force this side. I sent the guide to General Banks' headquarters with the prisoners. I should like him returned to me, if he is a trusty guide.

I sent to-day a cavalry force to Annandale, with instructions to take the road from that place to Accotink Village, thence to the Occoquon Ferry, with instructions to find out the haunts of a squadron of rebel cavalry, said to be foraging and scouting on the other side.

I sent two officers and 50 men, with a wagon, loaded with shovels and picks, to the Bull Run battle-field, to bury the dead. It seems that the One hundred and thirty-ninth Regiment Pennsylvania Volunteers, detailed for that duty, contented themselves with throwing a few clods of dirt over the dead, leaving uncovered hands and feet. The slight rains since have uncovered them.

The detail sent with flag of truce was unarmed.

Yours, respectfully,
D. B. BIRNEY,

Brigadier-General, Commanding.

Lieutenant-Colonel McKEEEVER,

Assistant Adjutant-General.

SEPTEMBER 16-19, 1862.-Reconnaissance from Upton's Hill to Leesburg, Va., and skirmish.

Report of Colonel J. Mansfield Davies, Second New York Cavalry.

HEADQUARTERS BYARD'S CAVALRY, Upton's Hill, Va., September 19, 1862.

I have the honor to report the return of the reconnaissance sent by me to the Bull Run Mountains.

It consisted of ten companies of cavalry (six of the Second New York, two of the Ninth Pennsylvania, two of the First New Jersey) and a section of Captain Buell's artillery. They left Upton's Hill, under command of Lieutenant-Colonel Kilpatrick, Second New York Cavalry, at 2 p.m., 16th instant; arrived at Dranesville same night. My instructions were for them to leave about midnight for Alide, or some gap