FAIRFAX COURT-HOUSE, July 21, 1861-5.15 o'clock.

General SCOTT:

I am directed to send the accompanying dispatch to you or to deliver in person.

HANSCON.

CENTREVILLE, July 21-4 p. m.

Adjutant-General THOMAS:

General McDowell wishes all the troops that can be sent from Washington to come here without delay. He has ordered the reserve now here under Colonel Miles to advance to the bridge over Bull Run, on the Warrenton road, having driven the enemy before him. Colonel Miles is now about three or four miles from here, directing operations near Blackburn's Ford, and in his absence I communicate.

G. H. MENDELL,

First Lieutenant, Topographical Engineers.

JULY 21, 1861.

General McDowell's army in full retreat through Centreville.

The day is lost. Save Washington and the remnants of this army.

All available troops ought to be thrown forward in one body.

General McDowell is doing all he can to cover the retreat. Colonel Miles is forming for that purpose. He was in reserve at Centreville.

The routed troops will not reform.

B. S. ALEXANDER,

Captain, Corps Engineers.

WASHINGTON, D. C., July 21, 1861-7 p. m.

COMMANDING GENERAL IN BALTIMORE:

Put your troops on the alert. Bad news from McDowell' army not credited by me. Shall write again in an hour. Doubt whether I shall call Delaware regiment here or order it to Baltimore./

WINFIELD SCOTT.

JULY 21, 1861-8 p. m.

Brigadier-General RUNYON:

Of the regiments which crossed the river this morning you are directed to retain two for the defense of Alexandria. It is now known that McDowell has rallied his army at or about Centrevile. Consequently, you will send forward the two regiments to support the rally. You are aware that Taylor's regiment was Saturday morning at Burke's Station. Cannot this regiment be ordered up to McDowell also?

WINFIELD SCOTT.

HEADQUARTERS ARMY, July 21, 1861.

General McDOWELL,
Fairfax Court-House.

Three regiments-Woodbury's, McCunn's, and another, name not known-are at Fairfax Station.

A commissary train is stopped a little way out of Alexandria with a drove of cattle. Call it up, if you mean to risk a stand; but under the circumstances it seems best to return to the line of the Potomac.

WINFIELD SCOTT.