Unknown to McDOWELL, Centreville, July 21, 1861

: 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 reached Fairfax Station. The other two must be there in a few minutes. Wesuppose 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 your 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