Numbers 40. HEADQUARTERS ARMY OF VIRGINIA, Warrenton, August 24, 1862-3.45 p. m.

Major-General HALLECK:

I arrived at Warrenton last night. The enemy had left two hours previously. Milroy's brigade, the advance of Sigel's corps, came upon the enemy late yesterday afternoon near Great River [Run], about 4 miles from Warrenton Sulphur Springs, and near the month of it. A sharp action took place, which lasted till after dark, the enemy being driven across Great River, but destroying the bridge behind him. Early this morning General Buford reached Waterloo Bridge, which was defended by a considerable force of the enemy and one piece of artillery. He took possession of the bridge and destroyed it. Sigel's force advanced again on the left this morning, and when last heard from was pursuing the enemy in the direction of Waterloo Bridge. His column was being shelled from the opposite bank of the river, which is still too deep to be forded. The enemy has made no advance against Rappahannock Station since we left, though yesterday morning, whilst we were withdrawing our forces from the opposite side of the river, he brought forward his columns of infantry and attempted to carry the heights which we were leaving by storm. He was, however, repulsed with considerable loss. We have had a continuous engagement, principally with artillery, along the whole line of the river for 8 or 10 miler during the last three days. No force of the enemy has yet been able to cross except that now inclosed by our forces between Sulphur Springs and Waterloo Bridge, which will undoubtedly be captured, unless they find some means, of which I know nothing, of escaping across the river between these places.

Early to-morrow, after clearing this side of the river, I shall move back a considerable part of this force to the neighborhood of Rappahannock Station. By that time the river will doubtless be fordable again. I shall leave a corps of observation here to watch the crossings at Waterloo and Sulphur Springs. The forces arriving form Washington and Alexandria will be assembled, I think, on Licking River [Run], between Germantown and the railroad, with a reserve for the force at Warrenton somewhere between Warrenton Junction and this place, until you are ready to begin a forward movement. I cannot form an estimate of the forces of the enemy. He has been developed in heavy force by simultaneous reconnaissances along a line of 9 miles from the railroad crossing of the Rappahannock as far, at least, as Sulphur Springs. I should like to have some idea of the forces which are coming here and your plans of organizing them, that they may be assigned and posted in some order. Our losses during the last three days have been guide heavy, among the killed being Brigadier-General Bohlen, commanding a brigade in Sigel's corps. The whole tendency of the enemy, since he appeared in front of us at Rappahannock Station, has been toward our right, but how far his movements in that direction will extend I am not yet able to say. I shall to-morrow remove my headquarters to some central point, probably Warrenton Junction.

JNO. POPE,

Major-General, Commanding.