Scouts report Sigel between Warrenton and Waterloo. Burnside is at Warrenton.

Most respectfully, your obedient servant,

T. L. ROSSER,

Colonel, Commanding Brigade.

General J. E. B. STUART,

Commanding Cavalry.

P. S. - Upon the expedition I lost 3 or 4 horses only, having none of my men injured or captured. I captured about 30 prisoners.

T. L. R.

OCTOBER 29, 1862. - Capture of Confederate pickets opposite Williamsport, Md.

REPORTS.

Numbers 1. - Brigadier General William W. Averell, U. S. Army, commanding Cavalry Brigade.

Numbers 2. - Brigadier General John R. Kenly, U. S. Army, commanding Maryland Brigade.

Numbers 1.

Report of Brigadier General William W. Averell, U. S. Army, commanding cavalry Brigade.

WILLIAMSPORT, October 29, 1862 - 4.30 p. m.

SIR: My brigade arrived in the vicinity of this place about 2.30 p. m. After a short consultation with General Kenly, concluded to capture enemy's pickets on other side of river before crossing. Lieutenant McMachan, of the First Maryland Cavalry, with 15 men, crossed above, in order to get behind the pickets. After he had safely crossed, without being discovered, a flag of truce appeared, covering 6 persons, among whom was the late Colonel Brien. Just as the business of the flag was concluded, and it had started on its return, Lieutenant McMachan dashed upon the picket of 6 men and brought it off before its reserve could interfere. He did not interfere with the flag or the persons under it. General Hampton stated by the flag "that it was not to affect any military operations only on the pike, and, on that, it should only cover the party carrying it." The prisoners belong to the First North Carolina Cavalry, Hampton's brigade. They were reviewed yesterday by General Stuart, they say, this side of Martinsburg. Hampton's brigade is composed of infantry, cavalry, and artillery. There are four regiments of cavalry; they do not know how much infantry; of artillery, one says they have several batteries; another says but one. That force, I have not the slightest doubt, is between me and Martinsburg, and is much stronger than mine. Had I another regiment of cavalry, or were my horses in better condition, I would not hesitate to attack, and will, anyhow, if the general commanding desires it. With regard to the main body of the enemy, the prisoners know nothing since Saturday; they say they do not know that it has moved.

I am, colonel, very respectfully, your obedient servant,

WM. W. AVERELL,

Brigadier-General, Volunteers.

[Lieutenant] Colonel OLIVER D. GREENE,

Chief of Staff, Sixth Corps, Army of the Potomac.