Porter reports himself 21,000 strong, which can only be by the addition of new troops. He is ordered to-night to join you as quickly as possible. I am for sending you all than can be spared, and I hope others can follow Porter very soon.

A. LINCOLN.

Camp near Rockville, Md., September 11, 1862. (Received 6 p. m.)

Major General H. W. HALLECK,

General-in-Chief:

GENERAL: At the time this army moved from Washington, it was not known what the intentions of the rebels were in placing their forces on this side of the Potomac. It might have been a feint to draw away our troops from Washington, for the purpose of throwing their main army into the city as soon as we were out of the way, or it might have been supposed to be precisely what they are now doing. In view of this uncertain condition of things, I left what I conceived to be a sufficient force to defend the city against any army they could bring against it from the Virginia side of the Potomac. This uncertainty, in my judgment, exists no longer. All the evidence that has been accumulated from various sources since we left Washington goes to prove most conclusively that almost the entire rebel army in Virginia, amounting to not less than 120,000 men, is in the vicinity of Frederick City. These troops, for the most part, consist of their oldest regiments, and are commanded by their best generals. Several brigades joined them yesterday, direct from Richmond, two deserters from which say that they saw no other troops between Richmond and Leesburg. Everything seems to indicate that they intend to hazard all upon the issue of the coming battle. They are probably aware that their forces are numerically superior to ours by at least 25 per cent. This, with the prestige of their recent successes, will, without doubt, inspire them with a confidence which will cause them to fight well. The momentous consequences involved in the struggle of the next few days impels me, at the risk of being considered slow and overcautious, to most earnestly recommend that every available man be at once added to this army.

I believe this army fully appreciates the importance of a victory at this time, and will fight well; but the result of a general battle, with such odds as the enemy now appears to have against us, might, to say the least, be doubtful; and if we should be defeated the consequences to the country would be disastrous in the extreme. Under these circumstances, I would recommend that one or two of the three army corps now on the Potomac, opposite Washington, be at once withdrawn and sent to re-enforce this army. I would also advise that the force of Colonel Miles, at Harper's Ferry, where it can be of but little use, and is continually exposed to be cut off by the enemy, be immediately ordered here. This would add about 25,000 old troops to our present force, and would greatly strengthen us.

If there are any rebel forces remaining on the other side of the Potomac, they must be so few that the troops left in the forts, after the two corps shall have been withdrawn, will be sufficient to check them; and, with the large cavalry force now on that side kept well out in front to give warning of the distant approach of any very large army, a part of this army might be sent back within the entrenchments to assist in repelling an attack. But even if Washington should be taken while these armies are confronting each other, this would not, in my judgment, bear