All my cavalry is with Colonel Lowell, except one squadron, which went to the city by the Aqueduct road--by whose order I do not know. It should be moving up that road. Have sent several times, but cannot hear anything definite of that squadron.

M. D. HARDIN,

Brigadier-General, Commanding.

Major-General AUGUR.

FORT RENO, July 13, 1864.

(Received 5.10 p. m.)

The following just received:

ROCKVILLE--2.10 p. m.

The rear guard of the rebels, consisting of Jackson's brigade, in extreme rear, Bradley T. Johnson in the advance, a little stronger, passed through here half an hour ago. Our advance is half a mile ahead.

2.30 p. m.--My dispatch was here interrupted by report of a large number of rebels being met just through the town by my advance guard, who charged at once. My advance was then dismounted, and after a sharp skirmish there checked a good strong charge of the rebels, after being driven nearly through the town by them. Major Fry was formed at the junction of the two roads before entering Rockville. We fell back to the edge of the town and established a strong dismounted skirmish line, holding them. Learning they were endeavoring to flank us, I have retired to a situation, two miles from Rockville, slowly. My regiment in the town, I fear, was mostly enveloped by the enemy and are very severely whipped.

C. R. LOWELL, JR.,

Colonel Second Massachusetts Cavalry.

M. D. HARDIN,

Brigadier-General.

Major-General AUGUR,

Commanding Defenses.

DAWSONVILLE,

Seneca Creek [July 14, 1864]--11.30 a. m.

MAJOR: I have the honor to report that everything on the right flank is quiet. Scouts sent out beyond Brookeville, Middlebrook, and moving thence westward report no parties in that direction. There is no doubt that the rebel infantry has crossed at White's or Conrad's or the ford near mouth of the Monocacy. Bradley Johnson's command (which has rear guard) passed here between one and two hours ago. My advance is moving forward, and I shall not be surprised to find some of their cavalry in position this side of the river. A small scout, sent clear around to Frederick, has not yet reported. I inclose best memorandum I have seen of the force on this side of the Potomac. A good many stragglers are reported through the woods. Have picked up a few; their stories all agree. A citizen, just from Poolesville, reports Johnson just entering there as he left. He (the citizen) had lost horses, but being convinced that the trains and stolen property had been got across the river, he went no farther than Poolesville.

C. R. LOWELL, JR.,

Colonel, Commanding.

Major WHITTELSEY,

Assistant Adjutant-General.