[Inclosure.]

HEADQUARTERS ARMY OF THE POTOMAC,
June 25, 1862.

General HEINTZELMAN:

If an attack is made in force on your line must receive it in you entrenchments, your pickets only to give you warning, and only supported sufficiently to prevent them from being driven in by a small force. If they are attacked by a large force they must at once fall back to the entrenchments, leaving full play for the artillery and musketry in the entrenchments. One-half of your force should be placed in reserve, to strengthen points most vigorously attacked. The general commanding thinks that the mass of Palmer's and Birney's brigades and De Russy's two guns should at once be moved behind your entrenchments, provided you are confident that the enemy will attack in force in the morning, leaving merely the picket lines with rather more than usual support to hold the ground against any new attack by skirmishers, to observe the enemy.

The general wishes to fight behind the entrenchments if attacked in force. Be sure and have the 8-inch howitzers all in position and well supplied with ammunition before morning; also the four Napoleon guns intended for the redoubts.

Acknowledge at once.

By order of General McClellan:

R. B. MARCY,

Chief of Staff.

HEADQUARTERS POPLAR HILL, VA.,
June 25, 1862.

General S. WILLIAMS,

Adjutant-General Army of the Potomac:

SIR: Much of the time since my arrival at this place has been devoted to an examination of the White Oak Swamp, commencing on the left of Couch's pickets. After crossing and recrossing at different sections I am of the opinion that it affords but a slight defense against an enterprising enemy. Infantry can pass through at most points at this time, when the present rain has added much to the volume of water. Judging from the character of swamps in general, this one should offer greater obstacles and impediments toward its mouth, but the reverse is the case, the approaches being firmer and the bottom of harder material. Perhaps a number of dams would cause a sufficient overflow in the swamp to deter the enemy from attempts at crossing. Two hundred choppers are at work on an abatis and others in the swamp for closing all fords and passes by obstructions.

The slashing of the timber on the right and left, opening of some rifle pits, will strengthen the position. Works in connection with the others in the direction of the entrenched camp would of course strengthen this flank, but I have no force adequate for so long a line. The country across the swamp is open and quite favorable for cavalry. Early information by mounted pickets is of the utmost importance. A large force of this arm should be at my disposal-at least one regiment.

Have been over the lines held by General Neglee. A small work or rifle pit across the railway should be made near the screen of timber. Have directed this and the slashing of timber in the vicinity of the lines.