HEADQUARTERS ARMY,
September 6, 1862.

Major-General McDEWELL, Arlington:

General Porter has been instructed to relieve the pickets of the Third Corps immediately, and to remove the heavy guns from Upton's Hill during the night, leaving an advance guard in the works there to hold it against an attack of pickets or inferior force.

S. WILLIAMS,

Assistant Adjutant-General.

HEADQUARTERS CORPS D'ARMEE,
Arlington, [September] 6, 1862 - 6.45 p. m.

Brigadier-General KING,

Or Commanding Officer of his Division, Upton's Hill:

Call in your pickets and put your division in march immediately for Leesborough or vicinity, via Aqueduct Bridge and Seventh street road.

By command of Major-General McDowell:

ED. SCHRIVER,

Colonel and Chief of Staff.

MUDDY BRANCH, MD., September 6, 1862.

Brigadier General R. B. MARCY, Chief of Staff:

My scouts on the tow-path, the mountain road, and the river road report everything quiet up to 3 a. m. No enemy visible. Should the enemy move in the direction of Washington from his present position, it will be by the Frederick road, his right resting on the Darnestown road; but some of my cavalry are so nervous I cannot make much out of their reports. My pickets on the Poolesville road are at Seneca Creek. They have just reported everything quiet on that road, and the enemy not in sight.

A. PLEASONTON,

Brigadier-General.

MUDDY BRANCH, MD.,

September 6, 1862 - 8.30 a.m.

General R. B. MARCY, Chief of Staff:

I have just learned that Lee's corps, said to number 30,000, crossed above yesterday, and moved down in the direction of Poolesville; and that Jackson is to move by the Frederick road, the design being an attack on Washington. This looks probable, for a flank movement by them to Baltimore would expose them fatally. Should this be the case, our point of defense must be Rockville and Brookville, and the line between them, connecting with Offutt's, Leesborough, and Tennallytown, would be the route for our troops and supplies. It this is the enemy's programme, his design will be to turn our right or pierce our center. He cannot go through on our troops and supplies. If this is the enemy's programme, his design will be to turn our right or pierce our center. He cannot go through on our left, by the Potomac River, on account of the broken country. This simplifies matters, and shows Brookville and Rockville should be well looked after. Please send me some cavalry, to scout in front of Brookville. I have one squadron there now, but want more that I can rely on. Brookville, Mechanicsville, and Rockville are in about the same line. Send Averell's brigade, with a battery, to Middlebrook, on the Rockville and Frederick road, and let them report