HEADQUARTERS ARMY OF THE POTOMAC,
Near Rockville, Md., September 10, 1862.

General McCLELLAN, Brookville, Md.:

The following dispatches have been received since you left:

BARNESVILLE-4.45 p. m.

General R. B. MARCY:

Your dispatches at 12 noon and 1.40 p. m. received. General Franklin and his corps arrived, and I submitted to him your directions. I made three attempts to-day to dislodge the rebels from the mountain, but they have a very strong position, and are in force. As soon as Franklin's corps was in position, I withdrew my attacking force. We lost 1 man, mortally wounded. I shall place my command behind Franklin tonight, and let them rest, which they are very much in need of, having been two or three days in the saddle, incessantly. I think the rebels will make a strong stand at Sugar Loaf Mountain. General Franklin inclines to an opposite view, and I trust he is correct.

A. PLEASONTON,

Brigadier-General, Commanding.

BARNESVILLE-4.45 p. m.

General R. B. MARCY:

Just heard from Monocacy. The enemy last night attempted to blow up the aqueduct. The force at Sykesville came down from Frederick yesterday to effect this, and it is thought they will again attempt it to-night. The signal officer, who was a long time on Sugar Loaf Mountain and knows it well, says it is the best position the rebels can get to make a stand, and he believes the rebels will defend it strongly. This is the best information I can get relative to the mountain position. Unless otherwise ordered, General Franklin will attack to-morrow.

A. PLEASONTON,

Brigadier-General, Commanding.

BARNESVILLE-6 p. m.

General R. B. MARCY:

Captain Reno, First Cavalry, reports from Hyattstown that the rebels fired on him with one piece of artillery, and that cavalry is now entering the town. He reports the enemy with cavalry, infantry, and artillery. I have directed him, if pressed, to fall back here. This looks like supporting their position on Sugar Loaf Mountain.

A. PLEASONTON.

Brigadier-General, Commanding.

R. B. MARCY,

Chief of Staff.

BROOKVILLE, [MD.] - 11.15 p. m.

(Received Washington, D. C., September 10, 1862.)

General R. B. MARCY:

Direct General Franklin to remain at Barnesville to-morrow, and put himself in communication with General Sumner, at Clarksburg. Direct him to carry Sugar Loaf to-morrow, if possible. If he finds the enemy there in very strong force, let him await the result of Sumner's advance on Hyattstown, and communicate with me at Clarksburg, when I will arrange to cut off the garrison of Sugar Loaf. But the earlier we gain the Sugar Loaf the better. Direct Couch to remain where he is, and watch all the fords in his vicinity. Let Peck's division follow the same route as fast as it arrives.

GEO. B. McCLELLAN,

Major-General.