HEADQUARTERS ARMY OF THE POTOMAC.

Middleburg, September 11, 1862-10 p. M.

Major-General BURNSIDE, Ridgeville:

GENERAL: General Sumner is ordered to Urbana to-morrow morning, with Banks' corps on his right at Ijamsville Cross-Roads; Franklin on his left at Licksville Cross-Roads. The Sugar Loaf is in our possession. Our information is that enemy has abandoned Frederick and is moving towrad Hagerstown. if the infromation you gain tends to confirm this, push on toward Frederick by the National pike and the railroad as rapidly as possible. We will do the same from Urbana. Keep your flankers and cavalry well out to the right toward Liberty, Westminster,&c. Be extremely cautions in your advance. Watch your front and flanks withcare, and be careful to communicate fully with headquarters and the troops on your left. It is thought that the enemy has abandoned Frederick and moved toward Hagerstown. In any event occupy Frederick to-morrow if you can possibly do so without too much exposing your command. A similar movement will be made from Urbana in force. Be careful to communicate with that place and Clarksburg before and after you move to-morrow, and base your movements upon what you hear from them.

By command of Major-General McClellan:

R. B. MARCY.

Chief of Staff.

[19.]

SPECIAL ORDERS,
HDQRS. THIRD CORPS, ARMY OF VIRGINIA, Numbers 7.
Unity, Md., September 11, 1862.

On the receipt of this order, Reyunolds' division will resume its march and encamp at Poplar Sprigns. Hatch'd division will follow to Lisbon and encamp at that point, and Ricketts' will march by the most direct route to Cooksville, where eit will encamp. Owen's cavalry will precede Reynolds' division and encamp in rear of it.

By command of Major-General Hooker:

JOS. DICKINSON.

Assistant Adjutant-General.

[19.]

HEADQUARTERS ARMY OF THE POTOMAC.

Camp near Rockville, September 11, 1862-12.30 p. M.

Brigadier-General PLEASONTON, Barnesville:

GENERAL: The general commanding desire to know the position of all of your force-what you have left to watch the Potomac, &c. Please communicate all you know bearing on the possibility and propriety of our throwing a column over the Monocacy at its mouth to cut the retreat of the rebels. Can you, without too much risk, send a small party to communicate with Harper's Ferry by the south side of the Potomac? And can you push a reconnaissance across the mouth of the Monocacy and thence up toward Frederick?

I am, general, very respectfully your obedient servant,
R. B. MARCY,

Chief of Staff.

P. S. - Send reports by telegraph to this point, as well as to Middleburg and Clarksburg, as the general commanding may be at either one of these points to-night.

R. B. M.

[19.]