Major General Fitz John Porter will remain at Warrenton Junction till he is relieved by Major-General Banks, when he will immediately push forward with his corps in the direction of Greenwich and Gainesville, to assist the operations on the right wing.

Major-General Banks, as soon as he arrives at Warrenton Junction, will assume the charge of the trains and cover their movement toward Manassas Junction. The trains of his own corps, under escort of two regiments of infantry and a battery of artillery, will pursue the road south of the railroad, which conducts into rear of Manassas Junction. As soon as all the trains have passed Warrenton Junction he will take post behind Cedar Run, covering the fords and bridges of that stream, and holding his position as long as possible. He will cause all the railroad trains to be loaded with the public and private stores now here, and run them back toward Manassas Junction as far as the railroad is practicable. Wherever a bridge is burned, so as to prevent the farther passage of the railroad trains, he will assemble them all as near together as possible, and protect them with his command until the bridges are rebuilt. If the enemy is too strong before him before the bridge can be repaired, he will be careful to destroy entirely the trains, locomotives, and stores before she falls back in the direction of Manassas Junction. He is, however, to understand that he is to defend his position as long as possible, keeping himself in constant communication with Major-General Porter on his right. If any sick now in hospital at Warrenton Junction are not provided for and able to be transported, he will have them loaded into the wagon train of this own corps, even should this necessitate the destruction of much baggage and regimental property, and carried to Manassas Junction. The very important duties devolving upon major-general commanding the Army of Virginia feels assured will be discharged with intelligence, and fidelity.

The general headquarters will be with the corps of Major-General Heintzelman until further notice.

By command of Major-General Pope:

GEO. D. RUGGLES,

Colonel and Chief of Staff.

Numbers 55. HEADQUARTERS ARMY OF VIRGINIA, Bristoe Station, August 27, 1862-6.30 p. m.

Major General FITZ JOHN PORTER,

Warrenton Junction:

GENERAL: The major-general commanding directs that you start at 1 o'clock to-night and come forward with your whole corps, or such part of it as is with you, so as to be here at daylight to-morrow morning. Hooker has had a very sever action with the enemy, with a loss of about 300 killed and wounded. The enemy has been driven back, but is retiring along the railroad. We must drive him from Manassas, and clear the country between that place and Gainesville, where McDowell is. If Morell has not joined you, send word to him to push forward immediately. Also send word to Banks to hurry forward with all speed to take you place at Warrenton Junction. It is necessary, on all accounts, that you should be here by daylight.

I send an officer with this dispatch, who will conduct you to this place. Be sure to send word to Banks, who is on the road from Fayetteville, probably in the direction of Bealeton. Say to Banks also that he