My headquarters are on the hill southwest of Warrenton, on Sulphur Springs road.

Very respectfully, general, your obedient servant,

IRVIN MCDOWELL,

Major General, Commanding Third Corps, Army of Virginia.

Numbers 4. HEADQUARTERS THIRD CORPS, ARMY OF VIRGINIA,

Warrenton, August 26, 1862.

Major-General SIGEL, &c.:

GENERAL: I have the honor to acknowledge the receipt of your communication of this date, and I beg now to inquire whether the strength reported by you is effective and reliable. Please inform me also who commands your cavalry.

Have three days' rations cooked and your men prepared in every way for the march early to-morrow morning.

Very respectfully, general, your obedient servant,

IRVIN MCDOWELL,

Major General, Commanding Third Corps, Army of Virginia.

Numbers 5. HEADQUARTERS THIRD CORPS, ARMY OF VIRGINIA,

Warrenton, August 27, 1862-7.30 a. m.

Major-General SIGEL:

Push immediately a strong advance along the turnpike from Warrenton to Gainesville, for the purpose of taking possession of the position of Buckland Mills, on Broad Run, and get your corps in hand as soon as possible to follow this advance. No wagons but for ammunition will accompany your corps on this road. Your baggage trains will immediately proceed to Catlett's. Detach three batteries from your troops to report to Major-General Kearny, commanding divisions, who will be moving by the way of Greenwich to your support.

Further instructions will be given as to the route by which these batteries are to join General Kearny; until they do they will be kept with your command.

By command of Major-General McDowell:

ED. SCHRIVER,

Colonel and Chief of Staff.

Numbers 6. HEADQUARTERS THIRD ARMY CORPS,

Buckland Mills, August 27, 1862-11.30 p. m.

It being understood that a large division of the enemy under Longstreet left Salem at 4 p. m. for the enemy's position in the direction of Manassas, through Thoroughfare Gap, and is now on the march, the following preliminary movements of the left wing of the army will be immediately made:

Major-General Sigel's corps will without delay be concentrated at or near Hay market and Gainesville.

A division of the Third Corps will be left at buckland Mills to operate against the flank of the enemy's column or march to Hay Market, as shall be found most expedient. King's and Ricketts' divisions will march to Gainesville, and start at 2 o'clock in the morning to attack