NEAR BRISTOE STATION,

May 18, 1863.

Major-General STAHEL,

Fairfax Court-House:

GENERAL: I have the most reliable information that Mosby, with less than 100 men, passed Gainesville and through Thoroughfare Gap last night. I have force enough at my command, and can catch him if you deem best. If you desire, will give particulars of plan.

Respectfully, &c.,

W. D. MANN,

Colonel, Commanding.

P. S.-Mosby spent part of last night at Greenwich.

BALTIMORE,

May 18, 1863.

Brigadier General W. W. AVERELL,

U. S. Volunteers:

GENERAL: Having reported for duty with this army corps, I am directed by the general commanding to communicate to you the following instructions:

You will proceed to Weston, in Western Virginia, or wherever else you may find Brigadier General B. S. Roberts, and relieve him of his command of the Fourth Separate Brigade of this army corps. On assuming command, you will establish your present headquarters at Weston or Buckhannon, or such other point as you may find it best to select south of the baltimore and Ohio Railroad, drawing your supplies from the depot at Clarksburg. Your command, however, is intended to be, as far as it can be properly made so, a mobile force, and your service will be to keep that region of West Virginia between the railroad and the Kanawha line clear of the enemy, preventing his invasions, and supporting and co-operating with Brigadier-General Kelley, commanding on the line of the railroad, and with Brigadier-General Scammon, commanding on the Kanawha and Gauley Rivers.

You may also be called upon in emergency to follow the enemy, or to cross the mountains east of you, to aid in any movement in the direction of the Valley of Virginia.

On your left you will find it necessary to guard the passes and approaches by way of the Cheat River Mountains.

Keeping these objects in view, it is left to your discretion to station your troops at such points as you may deem most advisable, keeping the body of them, however, together, where it may become necessary and best to concentrate, covering your line of supplies.

You will inspect your command, and report, at as early a day as possible, its exact condition and wants, with a view to having it supplied and put in the most effective condition. It is designed as soon as practicable, by re-enforcements, if they can be obtained, by new organization, and by all means of improvement, to convert or exchange the whole or greater portion of your troops, so as to make yours a force of cavalry, with light artillery and with little or no infantry.

I am, very respectfully, your obedient servant,

WM. H. CHESEBROUGH,

Assistant Adjutant-General.