The general commanding wishes you to communicate, as soon as possible, with General Bayard, and direct him to co-operate with General Pleasonton in endeavoring to capture this cavalry, which is probably Stuart's. They came in from Union, and it is said Stuart intended going to Middleburg.

R. B. MARCY,

Chief of Staff.

[Indorsement.]

HEADQUARTERS DEFENSES OF WASHINGTON, October 31, [1862]-11.10 p. m.

Will General Sigel please communicate this order to General Bayard Acknowledge.

S. P. HEINTZELMAN,
Major-General, Commanding.

HEADQUARTERS DEFENSES OF WASHINGTON, October 31, 1862-1.30 a. m.

Major-General SIGEL,

Fairfax Court-House:

In compliance with order from General McClellan, the commanding general directs that General Bayard shall at once report, if possible, to Headquarters Army of the Potomac for orders.

Please communicate this to General Bayard, and report whether he is able to comply with this direction, and at what time he does so. Acknowledge.

RICH'D B. IRWIN,

Captain, Aide-de-Camp, and Acting Assistant Adjutant-General.

FAIRFAX COURT-HOUSE, October 31, 1862.

Generals BANKS and HEINTZELMAN:

The following received and respectfully forwarded:

HEADQUARTERS CAVALRY DIVISION, Near Aldie, October 31.

Major-General SIGEL:

GENERAL: Yesterday morning I passed through Hopewell Gap, and thence to Middleburg. A few enemy's pickets were driven in at the time my column advanced through Hopewell Gap. Colonel Wyndham took the Aldie road to Middleburg. A detachment of my force was sent from Hopewell to White Plains. They ascertained that there is a strong picket of the enemy at Thoroughfare Gap. Their advanced pickets were seen by this party going to White Plains. In the afternoon a patrol was sent out toward Upperville, 6 miles from Middleburg. They encountered a company of cavalry, who, after a few shots, retreated.

From all information received, I am of the opinion that there is no infantry between the mountains, except at Snickersville, Piedmont, and at Warrenton. Strong detachments of cavalry are patroling the country from Warrenton to Middleburg, but they usually return to Warrenton. I received orders during the night from General McClellan to co-operate with his cavalry. I am going to have an interview this morning with General Pleasonton.

Very respectfully,
GEO. D. BAYARD,

Brigadier-General, Commanding.

F. SIGEL,

Major-General.