BROOKE'S STATION, October 29, 1861.

Honorable J. P. BENJAMIN, Secretary of War:

An intelligent soldier sent from Evansport across the river reports 15,000 of the enemy there, with eighty pieces of field artillery, to prevent us from crossing, and the batteries are to be attacked as soon as their vessels can be prepared. He hard nothing of the expedition from below, mentioned by Van Camp. He crossed back, this morning at Mathias Point.

THE. H. HOLMES,

Major-General.

CENTREVILLE, October 29, 1861.

Honorable J. P. BENJAMIN,:

MY DEAR SIR: Your not of the 27th instant has been received, with its inclosure. The note in cipher was addressed to me-that is, to Thomas John Rayford, a name I adopted before leaving Washington, for purposes of cipher correspondence with Mrs. Greenhow, by whom the note probably was written. As you will perceive from the translation inclosed, the subject-matter is unimportant. I say Mrs. G. probably wrote the note, but it is quite possible she did not, and that it is a shallow device of the enemy to entice into a correspondence which shall fall into their hands. This is the best light to view it, as a correspondence with her or further use of that cipher is useless. This cipher I arranged last April. Being my first attempt, and hastily devised, it may be deciphered by any expert, as I found after use of its for a time. I accordingly would have discarded it long since had Mrs. G. escaped detection, and had, indeed, arranged a cipher to send her just as she was arrested. The War Department at Washington came into possession of one of her letters in this cipher, and by its aid ought to have worked out the key. That does not matter, as of course I used it with but the lady, and with her it has served our purpose, including the one great service of saving General Bonham from a disastrous surprise on the 17th of July. I hear from another source that a reward is offered for the key. I am inclined to furnish it through a person in Washington, and let the friend get the consideration, for, I repeat, the possession of the key can do them no possible good now, nor can it prejudice any one. My suspicion has been excited by the way the value of the key is dwelt upon in this note and the desire to get at it on part of enemy, for I cannot doubt that an expert could unravel it.

I know not who wrote the letter signed A. M. H. The place of attack he indicates is one that Dr. Van Camp has just come here to inform us has actually been determined on as the place of descent by the Annapolis armada. Callan, clerk of Senate Military Committee, is informant. It is doubted here, however, but the army has been put in order for such an exigency.

Las night I telegraphed information sent me that Cape Fear River, smithville, &c., were the real points of attack. This came from one (Washington, 24th instant) with capacity and with to make a most efficient emissary. Circumstances have placed her en rapport with me lately, and I expect a good deal of timely, acute observation of useful character from her, but as I cannot be altogether certain of her eighth, all with be received with caution, and nothing communicated to her, as was my course, I may also say, with Mrs. G. The person in question communicates the name of an alien just from Portsmouth, Va., one E. .