miles to the vicinity of Coleman's, near Horse-Pen Run, and during the night of the 10th of September bivouacked,after a march of five miles, on the road to Good Hope, Church, near Broad Run, its farther progress being delayed by a severe storm. On the night of the 11th of September the column encamped beyond Red Hill, after a march of ten miles, and reached one mile and a half of aldie, at a covered point at the intersection of the Aldie pike and the Carolina road, on the night of the 12th of September. Here the command was concealed, and at daybreak of the 13th of September the town was entered by the mounted men, the roads in the vicinity having been ambuscaded during a portion of the night. Nothing was, however, accomplished. Information having been gained that Mosby, with a force, had gone down the Aldie pike the night of, but before, the arrival of the columna Aldie it was deemed best to move after him. The column reached Chantilly after a march of sixteen miles, and encamped on the night of the 14th, ambuscading all the roads in the vicinity.

On the morning of the 15th of September it resumed its march toward Fairfax, all indications and reports of scout kept on the Centerville road and roads to left of the turnpike tending to show that Mosby, whit a large force, but in divided parties, was on the left of the turnpike and between Vienna and Frying Pan. The scouts were driven from Flint Hill, but those at Fairfax reported that Mosby had been seen to pass through the Court-House toward Centerville a short time previous with two men. I dispatched five men to the Centerville road, about three miles distant, to intercept the party, fearing that more men might fail of an approach. Near Germantown three of this number returned and reported a fight with Mosby, in which two of the men had lost their horses and had taken to the woods, and that large parties of guerrillas were now on the right. On the return of the other men it was definitely ascertained that Mosby, or a person resembling him, had been wounded and had escaped. Mosby had certainly been in vicinity of Fairfax just previous to the action and had gone toward Centerville. People on the road had seen him, and from the description of his person and recognition of his picture by parties engaged, there seems to be some color for the report that he was in the action and was wounded, as he or the person in question was seen before riding off to throw up his hands and give signs of pain. This could be observed, as the action was at very close quarters. I dispatched a squadron to the scene shortly after and moved to Fairfax Court-House, sending a party of thirty dismounted men thorough Vienna to Lewinsville. The regiment reached camp at Falls church after a march that day of fifteen miles form Chantilly.

During this scout the weather was almost constantly rainy, and violent storms prevented speedy movement which would perhaps have brought the column to aldie in time for some success, and prevented a movement toward Middleburg and Rector's Cross-Roads, as intended. After the third day the men and horses subsisted on the country as directed. Twelve horses were taken from suspected rebel citizens, and all suspected houses searched on the line of march.

I am, very respectfully, your obedient servant,

H. S. GANSEVOORT,

Colonel Thirteenth New york Cavalry.

Lieutenant E. Y. LANSING,

Acting Assistant Adjutant-General.