ALEXANDRIA, August 27, 1862-11.20 a.m.

In view of Burnside's dispatch, just received, would it not be advisable to thrown the mass of Sumner's corps here to move out with Franklin to Centreville or vicinity? Lower Rappahannock in a dangerous position. They would do better service in front of Washington.

GEO. B. McCLELLAN,

Major-General.

Major-General HALLECK, Washington, D. C.

ALEXANDRIA, August 27, 1862-12.05 p.m.

My aide has just returned from General Franklin's camp; reports that Generals Franklin, Smith, and Slocum are all in Washington. He gave the order to the next in rank to place the corps in readiness to move at once. I learn that heavy firing has been heard this morning at Centreville, and have sent to ascertain the truth. I can find no cavalry to send out on the roads. Are the works garrisoned and ready for defense?

GEO. B. McCLELLAN,

Major-General.

Major-General HALLECK, Washington.

ALEXANDRIA, August 27, 1862-12.20 p.m.

What bridge exist over Bull Run? Have steps been taken to construct bridges for the advance of troops to re-enforce Pope or to enable him to retreat if in trouble?

There should be two gunboats at Aquia Creek at once. Shall I push the rest of Sumner's corps here, or is Pope so strong as to be reasonably certain of success? I have sent to inspect the works near here and their garrisons.

As soon as I can find General Casey or some other commanding officer, I will see to the railway, &c. It would be well to have them report to me, as I do not know where they are. I am trying to find them, and will lose no time in carrying out your orders. Would like to see Barnard.

GEO. B. McCLELLAN,

Major-General.

Major General H. W. HALLECK, Washington.

ALEXANDRIA, August 27, 1862-1. 15. p.m.

Franklin's artillery have no horses except for four guns without caissons. I can pick up no cavalry. In view of these facts, will it not be well to push Sumner's corps here by water as rapidly as possible, to make immediate arrangements for placing the works in front of Washington in an efficient condition of defense? I have no means of knowing the enemy's force between Pope and ourselves.

Can Franklin without his artillery or cavalry, effect any useful purpose in front?

Should not Burnside take steps at once to evacuate Falmouth and Aquia, at the same time covering the retreat of any of Pope's troops who may fall back in that direction?

I do not see that we have force enough in hand to form a connection with Pope, whose exact position we do not know. Are we safe in the direction of the valley?

GEO. B. McCLELLAN,

Major-General.

Major-General HALLECK, Washington.

ALEXANDRIA, August 27, 1862-1.35 p.m.

I learn that Taylor's brigade, sent this morning to Bull Bridge, is either cut to pieces or captured; that the force against them had many guns and about 5,000 infantry, receiving re-enforcements every minute; also that Gainesville is in possession of the enemy. Please send some cavalry out toward Dranesville, via Chain Bridge, to watch Lewinsville and Dranesville, and go as far as they can. If you will give me even one squadron of good cavalry here, I will ascertain the state of the case. I think our policy now is to make these works perfectly safe, and mobilize a couple of corps as soon as possible, but not to advance them until they can have their artillery and cavalry. I have sent for Colonel Tyler to place his artillerymen in the works.

Is Fort Marcy securely held?

GEO. B. McCLELLAN,

Major-General.

General HALLECK.

ALEXANDRIA, August 27, 1862-2.30 p.m.

Sumner has been ordered to send here all of his corps that are within reach. Orders have been sent to Couch to come here from Yorktown with the least possible delay.