At about 1 o'clock p.m. the vedettes at Annandale Station discovered some cavalry in front and gave due notice to the grand guard, but at the same instant a force of about 200 cavalry came in full gallop, and thus attacked the several pickets belonging to Company A, Forty-fifth Regiment New York Volunteers, on the left wing, between Annandale Station and the turnpike which leads from Alexandria to Fairfax Court-House. The vedettes were at first under the impression that it was our own cavalry, which made their daily round abbot this time, and it thus happened that they were overpowered and had to fall back into the woods, where, under the command of Captain Weller, they made a stand, firing on the enemy. The drummer, Feuerstein, gave the sign in time, and it was by his commendable zeal that the two companies at Cox's farm, under Captain Doebke, and a small squad of Mounted Rifles, under command of First Lieutenant William R. Parnell, were immediately dispatched on the battle-ground, and at once charged on the enemy and drove him back. First Lieutenant Parnell showed great bravery. The enemy fled towards Centreville. The detail of the Mounted Riffles and four companies of the Forty-fifth New York Volunteers pursued them for two miles; then they formed, as it was necessary, for immediate action.

Patrols were sent out towards Centreville and Fairfax Court-House, who on their return reported "All quiet." After having received the report at these headquarters, I ordered one squadron of cavalry, as well as a battery, of light artillery, as re-enforcement, who immediately, under my special command, departed to the scene of action. Two companies of the Mounted Rifles stand till now in readiness upon Mason's Hill, as likewise three companies of the Twenty-seventh Regiment Pennsylvania Volunteers stand as reserve at Rose Hill. The pickets are posted again as before.

The loss of the enemy is 2 prisoners, 2 killed lying outside of the lines, and 7 or 8 wounded. Our loss, as far as ascertained, is 11 killed-Carsten Huhnenberg, private of Company A, Forty-fifth Regiment New York Volunteers-and 12 men missing, also of Company A. There is 1 horse of the enemy killed in the woods and 1 horse captured.

I have further to report that Brevet Second Lieutenant Von Haythausen, of the Forty-fifth New York Volunteers, is missing since Sunday night, and is supposed to have gone over to the enemy. He took along with him a horse belonging to the orderly of the New York Mounted Rifles at Rose Hill. This attack of the enemy's cavalry was caused by a detachment that had been sent out foraging, by statement of the prisoners, although they seemed to be well informed of our position.

There have been arrested and sent to these headquarters the owner of Cox's farm, his son, and two laborers, as by report of officers and men they are suspected to be hot secessionists; but as it is contrary to general orders to arrest persons on suspicion, I ordered them to be set free, notwithstanding at several times in the night signal lights from their farm house have been seen. To avoid similar occurrences as just now reported, strict orders have been issued to the pickets to fire upon any cavalry nearing the same, unless they ride at a slow pace.

According to last night's report from the commander of the outposts there is nothing new to add.

Very respectfully, yours,

LOUIS BLENKER,

Brigadier-General.

General R. B. MARCY,

Chief of Staff of Major General McClellan, Commanding U. S. Army.