The Army of the Potomac is concentrating at Middletown rapidly, and will all be gone to-morrow, excepting a brigade of infantry, a brigade of cavalry, and a battery of rifle guns under General Neill, who is following up the retreat of the rebels from Fairfield toward Waynesborough. Between him and your orders have been issued for your frequent communication with each other.

As your note requires no special action, I do not awake General Meade to reply, as he is now refreshing himself with the first quiet sleep he has had since he came in command, if not for many nights before.

Yours, respectfully,

G. K. WARREN,

Acting Chief of Staff.

[P. S.]-Headquarters to-morrow night will be at Frederick, Md.

HARRISBURG, July 6, 1863.

Colonel PIERCE, Mercersburg:

Send in your prisoners and captured property that are not needed to Mount Union or some other point to the rear. Accounts must be rendered carefully of that property. Get your prisoners well to the rear.

Your brilliant conduct has quite electrified us. Let me know if the enemy are retreating rapidly, or what they are doing. I must know this quickly.

D. N. COUCH,

Major-General.

Send this dispatch through in preference to all others.

MERCERSBURG, via LOUDON, July 6, 1863.

Major-General COUCH:

Most of the prisoners taken were wounded, and the teams all unable to get the wagons farther than this place. Last night I was all night paroling wounded, and getting the wagons so they could be gotten away. I fear they may be cut off yet, as I understand a force is trying to come in our rear. I know nothing of the main army.

L. B. PIERCE,

Colonel, Commanding.

JULY 6, 1863.

Co. L. B. PIERCE, Mercersburg:

It is supposed bridges are destroyed, and the enemy may retreat by way of Hancock. It is most important that this fact should be ascertained at the earliest moment by your scouts. Great results may depend upon your activity in this matter.

By order of Major-General Couch:

WAYNE McVEAGH,

Major, and Aide-de-Camp.