COLONEL: I have the honor to report that on the 11th instant, being then at martinsburg, Va., in command of the troops at that place, in there detained to Camp Chase, ohio, by rail, under guard of 1 lieutenant and 10 men. There were 29 prisoners, and as the enemy were advancing, I sent an additional guard of a lieutenant and 30 men to go with them as far as Sir John's Run, and then to return, that being the dangerous portion of the road.

On the night of the same day, the enemy having crossed the Potomac into Virginia, the railroad communication west was cut off, but I have every reason to believe that the prisoners were safely transported. For the above reason neither of the guards were able to rejoin their commands.

On the 11th instant reports reached me, through scouts and others, that the enemy were crossing the Potomac into Virginia at or about Williamsport and Cherry Run in force; also, that they were passing to the west of martinsburg, between it and North Mountain, thus cutting of our retreat in that direction.

It being ordered by major-General Wool that the place should be held to the last extremity, at noon on the 11th instant I sent out one section of Captain Phillips' battery and four companies of the Sixty-fifth Illinois, together with half a company of cavalry and two teams, with axes, &c., the whole under command of Colonel Cameron, if the Sixty-fifth Illinois, with orders to proceed out upon the Williamsburg [Williamsport road, as far as practicable, and to obstruct the roads, tear up the brigades, and in every way possible, retard the advance of the enemy.

At night-fall, it having been well ascertained that the enemy were between us and North Mountain, and were in very large force near Falling Waters, on the Williamsport road, some 7 miles from Martinsburg, and were still crossing, it became evident that with the small force at my disposal the position could not longer be held.

Colonel Cameron's party wa accordingly recalled, and every exertion made to convey the public property to Harper's Ferry, that being the only line of retreat left open.

The railroad agent had, the previous day, sent off some 11 empty cars, in defiance of my orders for them to be retained, but I had detained the train up from Harper's Ferry that day, consisting of but 6 cars, and I caused all the surplus arms, clothing, ammunition, and camp equipage to be conveyed to the railroad depot, to be sent thence by rail to Harper's Ferry that day, consisting of but 6 cars, and I caused all the surplus army, clothing, ammunition, and camp equipage to be conveyed to the railroad depot, to be sent thence by rail to Harper's Ferry, as but one of the regiments under my command was provided with transportation. This was done mostly by the men themselves, the transportation (being divided as equally as possible between the several regiments) being wholly insufficient for the purpose.

The railroad train was loaded to the extent of its capacity and sent to Harper's Ferry, where it arrived in safety. The transportation was then employed to haul the most valuable property remaining, and the troops and wagons took up their line of march at 2 o'clock on the morning of the 12th.

But little public property was abandoned, consisting mostly of tents and cap equipage, which could not be conveyed with the means at disposal.

Upon the march, the pickets of the enemy were encountered at Halltown, but they were driven back to Charlestown, the command arriving safely at Harper's Ferry on the afternoon of the 12th.