HEADQUARTERS DEFENSES OF MARYLAND HEIGHTS, Fort Duncan, July 9, 1864.

Captain H. M. BURLEIGH,
Assistant Adjutant-General:

CAPTAIN: I have the honor to report that the captain of the canalboat which lies near here states that he went to his house in Sharpsburg this a. m., where he learned that about 1,000 or 1,500 rebels were guarding a large lot of stores they had collected in the vicinity at or near Shepherdstown, on the Maryland side of the Potomac ; that their pickets were up to Sharpsburg, and that they might soon move.

I am, captain, respectfully, yours, &c.,

G. F. MERRIAM,

Major and Chief of Artillery.

HANCOCK, July 9, 1864. (Received 12 m.)

Brigadier General B. F. KELLEY:

GENERAL: General Imboden left Williamsport at 9 a. m. yesterday with about 1,500 men, going to Sharpsburg. Reports say the enemy are concentrating at Frederick.

Respectfully,

H. PEASE,

Captain, &c.

HANCOCK, July 9, 1864.

General KELLEY:

A citizens just from Hagerstown reports but a small squad of rebels there this a. m. Reports place them at Frederick.

H. PEASE,

Captain, &c.

HEADQUARTERS, Monocacy, Md., July 9, 1864. (Received 9. 15 a. m.)

Major General H. W. HALLECK,
Chief of Staff:

My cavalry skirmished with that of the enemy all day yesterday between Frederick City and South Mountain. At 5 p. m. they marched down to attack me, but retired. Shortly after I received intelligence that Breckinridge, with a heavy force, was moving on the Washington pike, and was within six miles of Urbana. The report was founded upon a telegram from an operator at or near Point of Rocks. Upon this I withdrew from Frederick and returned to this point (Frederick City Junction), where my line covers the Washington road, while the Baltimore pike is two miles and a half on my right. The rebel force in front of me at Frederick City consisted of the brigades of McCausland and Johnson. My cavalry are reconnoitering up the Baltimore and Ohio Railroad and down the left bank of the Monocacy. Colonel Clendenin, my chief cavalry officer, reports that his men have found a skirmish line of the rebels about two miles and a half from the bridge at this Junction.

LEW. WALLACE,

Major-General.