[Inclosure.]

HEADQUARTERS CHIEF ENGINEER OF DEFENSES,
Washington, July 21, 1864.

Major General J. G. BARNARD,

Chief Engineer:

GENERAL: We respectfully report the following as the result of our examination of the ground lately occupied by the rebels, made in obedience to your orders: The rebel force approached Washington by two roads from Rockville-the Seventh-street road an the turnpike road through Tennallytown. The force on the Tennalytown road was McCausland's brigade of cavalry and mounted infantry, consisting of five regiments, three of which were the Twenty-second Virginia Mounted Infantry, Twenty-second and Fourteenth Virginia Cavalry; in all, about 1,500 men, including Jackson's battery of artillery, five guns. This force arrived about 8 o'clock on Monday morning and left during Tuesday night and were picketed along the road both sides of the fences, extending from Mr. Hawn's house to the old church. The main force, under General Early, passed down the Seventh-street road, the rear, consisting of Echols' and Wharton's brigades, reached and encamped on Mr. Bathcelor's place about 6 o'clock Monday evening. Most of this force was encamped along the Seventh-street road between Batchelor's and Blair's. Two small cavalry camps to the wastwoard, near the Sligo, were formed apparently as a rendezvous for roving bands of cavalry in search of horses, &c., and for observations on their extreme left.

The road marked on the accompanying sketch,* passing by Judge Dunlop's house near Rock Creek, was used as a line of communication between McCausland's and the main force, and by General Williams' brigade of infantry on the retreat Tuesday afternoon. Mr. Batchelor, who lives on the Seventh-street road about two miles above Blair's, states that the rebels commenced passing his house toward Washington about 9 o'clock Monday morning (July 11.). All of the cavalry had not passed until abut 12 m., when they were followed by a body of sappers and miners, or infantry with pickets and shovels, and that the infantry and artillery continued to pass constantly until 6 o'clock in the evening, when the rear guard, consisting of the brigade of General Echols and Wharton, encamped or bivouacked near his hours eon the side toward Washington. The rear guard remained near his house till Tuesday evening about 7 o'clock, when they were the first to commence the retreat, carrying the wagons with them. Mr. Batchelor states that he remained on his front porch all night while the rebel were retreating; that in the retreat there were only three halts of about fifteen minutes each, and that the rear guard, consisting of about 2,000 cavalry, had not all passed until 5 o'clock Wednesday morning. Mr. Batchelor estimates the artillery at twenty pieces. Mr. Davis, who keeps the toll-gate at Silver Spring, saw the artillery pass his place to blair's mrs. Barnes, who lives at the upper end of Blair's place, was at home all the time the rebel,s were in the neighborhood. The first rebel killed by our pickets fell and was buried near her house. The rebels bivouacked around her house. She frequently heard their conversa-

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*Not found.

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