UPPER MARLBOROUGH, MD., September 10, 1861.

Brigadier General JOSEPH HOOKER:

SIR: I have to inform you that the cavalry, under command of Captain Hamblin, arrived here at 1 this p. m. Lieutenant-Colonel Potter, having received definite instructions, leaves immediately for Butler and Queen Anne, and shall proceed to-morrow morning towards Lower marlborough, covering the ground that has not already been explored by Lieutenant-Colonel Potter's command, and, if found expedient, shall go still farther down the country.

During the day I have been trying to gain such information as may aid me in future operations. I find by conversation with leading men of the town that Federal troops have been expected here for some time past, and they therefore were not disappointed in seeing us come.

ROBERT COWDIN,

Colonel, Commanding First Regiment Massachusetts Volunteers.

HEADQUARTERS CORPS OF OBSERVATION,
Poolesville, September 10, 1861.

Major S. WILLIAMS, Headquarters Army of the Potomac:

MAJOR: I have the honor to report for the information of the major-general commanding that while there seems to be no increase of force on the opposite side of the river, there was considerable activity to be noticed in improving the defensive works on the road from Edwards Ferry to Leesburg. The same works cover the approaches to Leesburg from the Chain Bridge and Alexandria turnpike. The pickets have many of them been withdrawn from opposite Conrad's Ferry and patrols are less frequent.

The work on the entrenchment above noticed is carried on so ostentatiously, that it may be stratagem to deceive us as to their real intentions, but I am inclined to think not.

Two colored teamers deserted from Waterford on the 7th instant and reached our lines yesterday. One of them is quite intelligent. They report that the Eighth Virginia Volunteers is at Waterford and a corps of cavalry 300 strong have headquarters at Lovettsville; that two Mississippi and one South Carolina regiment occupy the vicinity of Leesburg. They also state that the rebel forces have sufficient beef, corn meal, coffee, and sugar, but are short of salt; that until within a few days they had no coffee or sugar, but received a supply from Manassas Gap; that the people in Loudoun Country are destitute of coffee, sugar, and salt, none being offered for sale in any of the village groceries.

They report that the Eighth Regiment Virginia Volunteers had plenty of ammunition, one of the teamsters stating that he drove an ammunition wagon loaded with forty boxes, while the men carried their cartridge-boxes full. One of these men brought off the diary of a cavalry officer, in which I find that the force which recently passed from Leesbourg to Lovettsville, &c., consisted of two regiments infantry, 200 cavalry, and two pieces of cannon. One of these regiments has since returned to Leesburg.

The health of this command is good, and its discipline constantly improving.

Very respectfully, I am, your obedient servant,

CHAS. P. STONE,

Brigadier-General, Commanding.