HEADQUARTERS OF THE ARMY, July 21, 1861-9 p. m.

General McDOWELL,
Centreville,

Besides three regiments sent you by General Runyon from the reserve, four regiments have crossed the river to-day. Two of the latter we know have reach Fairfax Station. The other two must be there in a few minutes. We suppose you to have rallied your army at Centreville, or, at the worst, you will rally at Fairfax Court-House and Fairfax Station. We know that you and you experienced officers will do all that is proper and possible. A company of regulars has also gone over. Additional re-enforcements shall follow early to-morrow. We are not discouraged.

WINFIELD SCOTT.

FAIRFAX, July 21, 1861.

General McDOWELL:

I have learned from my scouts that large trees are felled across the turnpike on road from here to Alexandria. Things are looking ugly here.

McCUNN.

FAIRFAX COURT-HOUSE, July 21, 1861-9.10 p. m.

Colonel E. D. TOWNSEND:

We are reliably informed that the enemy's cavalry will attack us on the left to-night.

Send instructions.

D. A. WOODBURY,

Colonel, Commanding [Fourth Michigan Infantry].

FAIRFAX STATION, July 21, 1861-11.5.

Lieutenant-General SCOTT:

Orders have arrived that no more regiments are to come here from Alexandria to-night.

I have placed myself in best position. Have removed obstructions of slide from railroad track.

I have no communication from General McDowell.

I am guarding the roads lest a surprise.

Colonel Woodsbury telegraphed me that he expects an attack from cavalry . What shall I do?

McCUNN,

Thirty-Seventh New York Volunteers.

WASHINGTON, July 21, 1861-11.45 p. m.

Colonel McCUNN,

Fairfax Station:

General McDowell is at Fairfax Court-House, where he will try to make a stand. Communicate whit him there, and also let Colonel Woodbury know.

WINFIELD SCOTT.