WASHINGTON, May 29, 1863.

Major-General HEINTZELMAN,

Commanding, Washington:

GENERAL: It is reported that the enemy's cavalry is collecting in force on the Upper Rappahannock. I think General Stahel should be cautioned to be exceedingly vigilant against raids to cut the Orange and Alexandria Railroad and into Alexandria. If made, they will be quick and rapid. The utmost vigilance must be kept up at all the posts. Staff and other officers must be detailed to see that the troops are continually on the alert.

Very respectfully, your obedient servant,

H. W. HALLECK,

General-in-chief.

FAIRFAX COURT-HOUSE, May 29, 1863-10 a. m.

Major General S. P. HEINTZELMAN,

Commanding Defenses of Washington:

One of my most reliable informants, whom I keep outside of the lines, and whose communications can be fully trusted, makes the following report to me:

There is no strong force of the enemy concentrated this side of the Blue Ridge. Only Mosby's men are scouting through the country in every direction in small parties. On Tuesday last, Mosby sent 9 men to Dranesville and 5 men down the Little River turnpike. On Wednesday, Mosby was collecting a party of men, with the intention of making a raid in some direction not ascertained.

(This information coincides with the reports furnished by my scouting expeditions, and the body so gathered was probably the one encountered and routed at Thoroughfare Gap yesterday.)

My informant further report that Dr. [Jesse] Ewell, a relative of General Ewell, and who has a son [John S. Ewell] in the quartermaster's department of the rebel army, and who lives near Aldie, on the road from Sudley Springs, has been making arrangements this week for the pasture of a large number of cavalry horses, particularly with a man named [S. A.] Buckner, at Sudley Springs. He wanted Buckner to pasture 2,000 horses.

It is the current conversation and belief that Stuart is to be this side of the Blue Ridge within a week. All the events and circumstances indicate such to be the fact.

There is unusual activity and expectation among the rebels in the section of country this side the Blue Ridge. He also reports Hood's division of infantry at Culpeper Court-House. This information was brought by exchanged rebel prisoners captured by men some time ago, and who have returned to their homes this week.

They also say that the last rebel pickets were passed at Orleans, which is near Flint Hill, on the Thumb Run, at about 8 miles from Salem, in a southwesterly direction. They further say that the rebel army is being concentrated on and near the Rappahannock for an immediate aggressive movement, and that Longstreet's division and Garnett's brigade are expected in the section of country between the Blue Ridge and Bull Run.

In Richmond last week it was discussed and determined that a forward movement should be made at once, as the Union Army is now weekend by a large number of regiments who have gone home, whose