Be pleased to keep the general advised of everything of importance relating to yourself and the enemy. The commanding general further directs that, by means of your cavalry, you ascertain what direction the forces of the enemy that have crossed to the north side of the Potomac have taken.

S. WILLIAMS,

Assistant Adjutant-General.

CAMP NEAR FAIRFAX COURT-HOUSE, June 24, 1863.

Major-General HOOKER, Commanding Army of the Potomac:

GENERAL: In accordance with your request, I present the following reasons for moving our army at. once to the neighborhood of Harper's Ferry:

1. The whole of Lee's army is reported to be on the Potomac, above that place, part of it across the river, and threatening an advance upon Harrisburg.

2. There we can protect Washington as well, and Baltimore better than here, and preserve our communications and routes of supply.

3. It is the shortest line to reach Lee's army; will enable us to operate on his communications, if he advances; to throw overwhelming forces on either portion of his army that he allows the river to divide; and is too strong a position for him to attack us in, even if we make heavy detachments.

4. It will enable us to pass South Mountain without fighting for the passes, if we wish to move upon him, and will thus destroy any advantages these mountains would give as a protection to his right flank.

5. It will prevent Lee from detaching a corps to invade Pennsylvania with, as it would expose the rest of his army to our attack in superior force.

6. These opinions are based upon the idea that we are not to try and go round his army, and drive it out of Maryland, as we did last year, but to paralyze all its movements by threatening its flank and rear if it advances, and gain time to collect re-enfrocements sufficient to render us the stronger army of the two, if we are not so already. Respectfully submitted.

G. K. WARREN,

Brigadier General of Vols., Chief Engineer, Army of the Potomac.

MARYLAND HEIGHTS, MD., June 24, 1863.

(Copy received, War Department, 9. 50 a. m.)

Major-General BUTTERFIELD:

There is a ford over the Shenandoah at Harper's Ferry; another at Key's Ferry, 6 miles above Harper's Ferry; another at Berry's, 18 miles above Harper's Ferry. I am satisfied Lee, with Longstreet's force, is at or near Berryville, as I telegraphed last night. Ewell, with his entire corps, has gone toward Hagerstown. I shall know more this morning.

DAN. TYLER,

Brigadier-General.