From Secretary Stanton To General Mcclellan.

Washington, May 18, 1862.

GENERAL: Your despatch to the President, asking reinforcements, has been received and carefully considered.

The President is not willing to uncover the capital entirely; and it is believed that, even if this were prudent, it would require more time to effect a junction between your army and that of the Rappahannock by the way of the Potomac and York rivers than by a land march. In order, therefore, to increase the strength of the attack upon Richmond at the earliest moment, General McDowell has been ordered to march upon that city by the shortest route. He is ordered, keeping himself always in position to save the capital from all possible attack, so to operate as to put his left wing in communication with your right wing, and you are instructed to co-operate so as to establish this communication as soon as possible by extending your right-wing to the north of Richmond.

It is believed that this communication can be safely established either north or south of the Pamunkey River.

In any event, you will be able to prevent the main body of the enemy's forces from leaving Richmond and falling in overwhelming force upon General McDowell. He will move with between thirty-five and forty thousand men.

A copy of the instructions to General McDowell are with this. The specific task assigned to his command has been to provide against any danger to the capital of the nation.

At your earnest call for reinforcements, he is sent forward to co-operate in the reduction of Richmond, but charged, in attempting this, not to uncover the city of Washington; and you will give no order, either before or after your junction, which can put him out of position to cover this city. You and he will communicate with each other by telegraph or otherwise as frequently as may be necessary for efficient cooperation. When General McDowell is in position on your right, his supplies must be drawn from West Point, and you will instruct your staff-officers to be prepared to supply him by that route.

The President desires that General McDowell retain the command of the Department of the Rappahannock and of the forces with which he moves forward.

By order of the President: EDWIN M. STANTON, Secretary of War.

MAJOR-GENERAL GEORGE B. McCLELLAN, Commanding Army of the Potomac, before Richmond.