The Seventh New York Cavalry, Lieutenant-Colonel Babbitt; the Sixth New York Cavalry, Colonel Devin; the Tenth New York Cavalry, Colonel Lemon; Pierce's regiment of Pennsylvania cavalry, now in Philadelphia, and Swain;s regiment of New York cavalry, now in New York, will report to Major-General Dix for assignment by him to duty under Colonel Dixon S. Miles, Second Infantry, in guarding the railways, &c.

Upon the arrival of any three of these arrival of any these regiments General Dix will cause the Tenth Maine, the Sixtieth New york, and First District of Columbia Volunteers to be relieved from duty with his division and ordered to report for duty until further orders to Brigadier General James S. Wadsworth, military governor of the District of Columbia, who will concentrate them near Fort Corcoran under the command of the senior officer present.

General Wadsworth will report the execution of this order.

The Quartermaster's Department will furnish necessary transportation for the movement indicated.

By command of Major General McClellan:

S. WILLIAMS,

Assistant Adjutant-General.

HEADQUARTERS THIRD CORPS,
Hampton, March 29, 1862-3 p. m.

Major-General McCLELLAN:

General Porter's division returned to camp at 6 p. m . Brigade went about 1 mile beyond the Half-way House, nearly 4 miles beyond Big Bethel. His skirmishers, some of Berdan's sharpshooters, went near to Howard's Branch, at Howell's Mill, where was seen a breastwork. Three deserters who came in yesterday morning from Yorktown report that General Magruder was there with nearly all his force, about 8,000 men, having left a guard of but 500 at Yorktown and in the batteries at Shipping Point. Our cavalry drove in their pickets, but captured none, their horses being too fleet. General Smith's division followed the road near James River from Newport News, encamped for the night at Watt's Creek, and returned to this camp yesterday morning. He reported a strong force at Young's Mill. From the best information I can obtain General Magruder has between 15,000 and 20,000 men under his command.

S. P. HEINTZELMAN,

Brigadier-General.

WARRENTON JUNCTION,

March 29, 1862-1 a. m.

General S. WILLIAMS, Seminary:

Express just received from General Howard. He drove the enemy across the Rappahannock Bridge, and is now in camp on this bank of and near the rappahannock River. The enemy blew up the bridge in his retreat. There was skirmishing during the march and a few shots exchanged by the artillery, without any loss on our side. Their loss, if any, is not known. General Howard will return to this camp to-morrow morning.

E. V. Sumner,

Brigadier-General, Commanding Second Army Corps.