HEADQUARTERS ARMY OF THE POTOMAC,
Camp near Rockville, Md., September 10, 1862-noon.

(Received 1.45 p. m.)

His Excellency the PRESIDENT:

In reply to your dispatch of this morning, I have the honor to state that General Pleasonton, at Barnesville, reports that a movement of the enemy last night is said to have been made across the Potomac from this side to the other side. We shall know the truth of this rumor soon. Pleasonton is watching all the fords as high as Conrad's Ferry, and has pickets out to the mouth of the Monocacy. He has sent out this morning to occupy Sugar Loaf Mountain, from which a large extent of country can be seen in all directions. General Burnside had his scouts out last night to Ridgeville, and within 3 miles of New Market. No enemy seen, with the exception of a few pickets. They were told that Stuart's cavalry, 5,000 in number, occupied New Market, and that the main rebel force, under Jackson, was still at Frederick. Burnside has sent a strong reconnaissance to-day to the mountain pass at Ridgeville. I propose, if the information I have received proves reliable regarding the natural strength of this position, to occupy it with a sufficient force to resist an advance of the enemy in that direction. I have scouts and spies pushed forward in every direction, and shall soon be in possession of reliable and definite information. The statements I get regarding the enemy's forces that have crossed to this side range from 80,000 to 150,000. I am perfectly certain that none of the enemy's troops have crossed the Potomac within the last twenty-four hours below the mouth of the Monocacy. I was informed last night by General Pleasonton that his information rendered it probable that Jackson's forces had advanced to New Market, with Stuart's cavalry at Urbana. In view of this, I ordered the army forward this morning to the line along the high ridge from Ridgeville through Damascus, Clarksburg, &c., but the information subsequently obtained from General Burnside's scouts, that the mass of the enemy was still at Frederick, induced me to suspend the movement of the right wing until I could verify the truth of the reports by means of Burnside's reconnaissance in force to-day. My extreme left advanced to Poolesville this morning. The work of reorganizing and refitting is progressing very satisfactorily under the new heads of staff departments.

Dispatch this instant received from General Pleasonton, dated Barnesville, 10.30 a. m., says:

My scouts occupy the ferry at the mouth of the Monocacy. They found no enemy, except a few pickets on the other side of the Monocacy, at Licksville. About 3 miles from that stream it was reported there was a force of 6,000 men.

GEO. B. McCLELLAN,

Major-General, Commanding.

WASHINGTON, D. C., September 10, 1862.

Major-General McCLELLAN, Rockville, Md.:

Scouts report that the remainder of the Confederate Army, under Smith [?] and Robertson, were to cross the Potomac last night at Edwards Ferry and Monocacy. No reliable account of numbers. They themselves say about 40,000.

H. W. HALLECK,

General-in-Chief.