MARCH 15, 1863.-Scout from Harper's Ferry to Leesburg, Va.

Report of Brigadier General Benjamin S. Roberts, U. S. Army, commanding Defenses of the Upper Potomac.

HARPER'S FERRY, VA., March 18, 1863.

COLONEL: I have the honor to inform the general commanding that in the snow-storm of Saturday I detachment 200 of the Fourteenth Pennsylvania Cavalry at 10 at night for the surprise of Leesburg and the capture of bushwhackers who have recently ambuscaded and fired upon pickets, and by surprise of a few men captured and wounded, as is supposed, mortally 1. Colonel [james N.] Schoonmaker command the party, and at 4 in the morning had reached and surrounded the town; he found no Confederates in or about it. On returning, be captured 5 bushwhackers, of White's band, as they allege. One of the five was riding the horses of one of the two pickets captured a week since by being burned the house where this act of treason was committed.

The prisoner sent to-day, showing a paper purporting to be a leave of absence of eight days from his command, to visit London Country to procure a servant and to recruit, as well as the 4 sent yesterday by my provost-marshal, should not be treated as prisoners of war. They are bushwhackers and outlaws in every sense of the word.

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

B. S. ROBERTS,

Brigadier-General, Commanding.

Colonel W. H. CHESEBROUGH,

Assistant Adjutant-General, Baltimore, Md.

MARCH 15, 1863.-Affair near Dumfries, Va.

REPORTS.

Numbers 1.-Brigadier General Alfred Pleasonton, U. S. Army, commanding Cavalry Division.

Numbers 2.-Captain John Southworth, Eighth Illinois Cavalry.

Numbers 1. Report of Brigadier General Alfred Pleasonton, U. S. Army, commanding Cavalry Division.

HDQRS. FIFTH CAVALRY DIV., ARMY OF THE POTOMAC,
The patrol of the Eighth Illinois was captured at night between Dumfries and Occoquan. Lieutenant-Colonel Clendenin had written orders not to send a less number that a platoon on this patrol. He will be arrested and charges preferred accordingly.

It is recommended taht the rebel partisans and bushwhackers be cleared out from the vicinity of Occoquan and Brensville by a command from this division. One brigade and a couple of Guns would be sufficient. it is reported the enemy have one or two guns at Brentsville.

[A. PLEASONTON.]

Brigadier General S. WILLIAMS,

Assistant Adjutant-General.