Jackson's large force changes the aspect of matters along the Baltimore and Ohio Railroad. I regard Milroy at Cheat Mountain in measure insulated, and in view of an advance of Loring via.

Moorefield and Philippi, perhaps too much so. With the aid of General Kelley, now in Cumberland, whose advice and knowledge of this road in my rear to Big Cacapon necessarily re-enforced.

Should I advance, I can cross 4,000 infantry at Little Cacopn Bridge, fall on Romney, via Winchester turnpike by a mountain road, and retake at any pike. I have hesitated to do so only that I am not massed and ready for mountains and have nothing there worth capturing. Should a merely defensive course be adopted, my successor here should be a soldier and disciplinarian. This command is more like an armed mob than an army.

Resolved to a brief statement, this force can join Williams along the railroad at some risk of encountering a much larger one. If the railroad be guarded at once by re-enforcement from Ohio it can take Bath, and a portion of Banks' column then threatens Martinsburg. It can take Martinsburg should Banks cross; it can take Winchester and hold the Blue Mountain Pass, or fall on Leesburg and join McCall. In the latter case the enemy's left would endeavor to cut it off at Leesburg, but only by exposing himself by a flank march to your right.

Colonel Dunning man. If Colonel Grover, of Utah celebrity, were appointed brigadier-general he might relieve me.

Trusting that I have not exceeded the terms of your instructions, I am, respectfully.

F. W. LANDER,

Brigadier-General.

HEADQUARTERS DIVISION, AT FREDERICK, MD., January 18, 1862.

Major-General McCLELLAN,.

Commander-in-Chief U. S. A.:

GENERAL: It gives me pleasure to acknowledge the receipt of your letter of the 16th instant,* proposing certain inquiries in regard to the number and disposition of forces necessary to guard the Potomac and the reconstruction of the Baltimore and Ohio Railway.

I. In regard to the disposition and number of forces necessary to guard the Potomac in the event of an advance of the main army:

It is not practicable in my judgment to substitute cavalry altogether for the infantry now guarding the river, but to a large extent it will make a more available and effective defense than now exists, as it will enable us to keep a more perfect observation of the enemy's movements; to concentrate suddenly upon points threatened, and the avoid the unhappy effects of forced marches that infantry will be compelled to make, usually without any satisfactory service or results. The exceptions to an exclusively cavalry guard will occur at those points where villages lie on the bank of the river, or at the dams which support the canal, or the most practicable fords, the defense of which demands the long range and effective fire of the rifled cannon and musket. With such exceptions cavalry can replace infantry with advantage.

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*Not found.

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