HDQRS. DEPARTMENT NORTHEASTERN VIRGINIA, Arlington, June 24, 1861.

Lieutenant Colonel E. D. TOWNSEND,
Assistant Adjutant-General, Headquarters Army:

COLONEL: I have the honor to submit the following, in answer to your letter of June 21, in reference to a column to co-operate from this position with a movement to be made with a portion of Major-General Patterson's command "to sweep the enemy from Leesburg towards Alexandria." For a better understanding of what follows, I have the honor to transmit a map, which I have had prepared, showing the position of our troops and of that of the enemy in front of it. There is at Manassas Junction and the places in its front or immediate vicinity a force of from 23,000 to 25,000 infantry and about 2,000 cavalry and supply of well-provided artillery. The advanced position of this force is at Centreville, Germantown, Fairfax Court-House, and Fairfax Station, the numbers and proportion at each varying from time to time. How much of a force is beyond Manassas Gap, in the valley, and could be brought within the valley, and could be brought within the operations here contemplated, I have no means of judging. There is nothing to hinder their coming, and unless they are kept engaged by our troops around Harper's Ferry, re-enforcements, in case of serious operations from that section would have to be guarded against, as would also those from places to the south of Manassas, on the line of the railroad to Richmond or Lynchburg, which would be pressed forward whenever it should become known we were moving upon them or they upon us in any force.

I have not learned that the troops in our front are fully provided with transportation, through I am satisfied they are not so deficient as we have supposed, and not as much so as we are at this time ourselves, for the Quartermaster-General, after supplying yesterday transportation for two regiments to move each about six miles, had but three horses left.

We have in this department, good, bad, and indifferent, twenty regiments of infantry, giving an aggregate of less than 14,000; four companies of cavalry, giving about 250; one battery of regular artillery of six rifled guns; one battery of volunteers artillery, smooth-bores-an excellent company, but not accustomed to their guns, and hardly fit for service in the field. There are three companies of regular artillery, but they are in the earthworks, and not available for field service. The General-in-Chief was pleased to say he wished I would fight this project of a combined movement to sweep away the enemy from Leesburg towards Alexandria with him step by step. I take advantage of this permission, if, indeed, I do not obey a command, to say that it seems to me the distance between General Patterson's force and this one is so great, and the line of march each has to take is such (a flank exposed), that, in my view, the force to move from each position should be constituted without reference to material support from the other. I am thought by those for whose judgment I have great respect, and who have been on the ground, to hazard something in having my advanced position so near Falls Church, when it is thrown forward from the right of the line, Fort Corcoran, and there are means of re-enforcing it promptly by the Georgetown turnpike and the railroad to Alexandria.

What would be our position if a movement is made to the right, at this the right bank of the Potomac, towards Leesburg? In the first place, as we are for any such purpose without means of wagon transportation, we should be obliged to repair and use the railroad; but whether this was done or not, we should march with the left flank of