HDQRS. CAVALRY CORPS, ARMY OF THE POTOMAC, Aldie,
June 20, 1863-1 p. m.

Brigadier General D. McM. GREGG, Commanding Second Division:

GENERAL: Since the last dispatch to you, information has been received that the Second Army Corps is in the vicinity of Thoroughfare Gap, accompanied by the Eighth Pennsylvania Cavalry. In consequence of this, the general commanding directs that you withdraw the whole of your force in that vicinity to your present position by the shortest safe route, keeping up communication with the pickets of the Second Corps. Colonel T. C. Devin, who has just returned, says that Hampton's brigade camped with their wagon train at Salem last night. He also directs me to say that you can hold your command in readiness to advance to-morrow morning. I have the honor to be, very respectfully, your obedient servant,

A. J. ALEXANDER, Assistant Adjutant-General.

FREDERICK, MD., June 20, 1863-3. 10 p. m.

Major-General HOOKER, Headquarters Army of the Potomac:

Dr. H. Seller, a refugee from Williamsport, left Frederick at 2. 30 this p. m., for Baltimore, to communicate with General Schenck. From him I learn the following, which is the most reliable information I have yet obtained: He left Williamsport night before last, at which time no forces were crossing. He has every reason to believe that none had crossed the river at that point. The whole number that crossed at Williamsport is between 3, 000 and 5, 000. Six hundred and fifty cavalry went up first. The infantry are Georgians, North Carolinians, and Virginians-General Rodes in command. Some few companies of cavalry crossed at Dams Nos. 4 and 5, below and above Williamsport, it is said, for Harper's Ferry. A regiment of cavalry or less, a major in command, is occupying Sharpsburg. Infantry reported crossing at Shepherdstown, but very doubtful; some 2, 000 are reported to be there. In short, a division of infantry, under General Rodes, and some 1, 200 cavalry, under Jenkins, together with mounted infantry, comprise all the forces at or near Williamsport, the majority of the infantry being at Charleston. The main body of Lee's army is not in the vicinity or within supporting distance of this force. no artillery has crossed the river. The scouts sent to Elk Ridge will return to-morrow.

BABCOCK.

HEADQUARTERS ARMY OF THE POTOMAC,
June 20, 1863-5. 20 p. m.

Commanding Officer Cavalry Corps:

Your dispatch of 12. 30 of this date has been received and laid before the major-general commanding, who authorizes you, in accordance with your request, to move to-morrow with your