PORT ROYAL, S. C., December 27, 1861.

General GEORGE B. McCLELLAN,

Commanding U. S. Army:

DEAR GENERAL: The Ocean Queen arrived yesterday with a mail, but brought no news of any cavalry. Had I cavalry and another light battery this would be a fine opportunity to strike for the railroad and cut off Savannah, and thus prepare ourselves for operations south of it.

It is absolutely necessary to do something here soon, without waiting for our armament for P-L-S--. The rebels are exceedingly active. They are erecting earthworks around Port Royal Island-one I understand to-day, on the Ashepoo, above Otter Island, and another below Thunderbolt, on Skidaway Island, making two on this island. In a word, they are erecting a cordon of earthworks, armed with heavy cannon, a certain distance from the coats, just far enough back to avoid fire from gunboats.

I shall try to organize a dash on the forts about Port Royal Island and also to the railroad, if not running too much risk without cavalry and artillery. It will require much consideration. It would not do to sustain here defeat. We are badly in want of boats, too.

We are quite secure here now. The place is well fortified, and can be held by 2,500 or 3,000 men against any force that can be brought against it. Had I the means, now is the time to do something. These fellows are getting and stronger every day.

Very truly, yours,

T. W. SHERMAN.

HEADQUARTERS EXPEDITIONARY CORPS,
Port Royal, S. C., January 2, 1861.

Major General GEORGE B. McCLELLAN, U. S. A.,

Washington, D. C.:

GENERAL: I ordered a dash on the enemy's batteries opposite Port Royal Island, but no doubt you will have perceived the object of advancing no farther, which the public will not. I want to make no real movement in that direction, yet I want the enemy to think I do, which he evidently does. I want to make a great dash on the north side of Savannah River, thus occupying the road to that city, the whole country between Broad River and Savannah River, and the southern end of the railroad, and at the same time, if found practicable, the islands in this river north of Pulaski.

But to do this I must have a regiment at least of cavalry and a least another strong battery of light artillery. Circumstances are developing themselves wonderfully here. The time the come to act; every day's delay now is a sad loss.

Let me take possession of that country now, and the siege of the south of Savannah will be comparatively light and easy. I think Pulaski need not be shelled. I can cut it off, I think.

Important information has been obtained with regard to the topography of the county about Savannah River which we ever knew before. I hope to lay the whole subject before you in an official and topographical form as soon as it can be prepared. But we are so hard pushed for proper engineers that they have no time for office work.

I think a terrible blow struck here will aid your important work must materially, but we must have cavalry and light artillery. You, general, are to be the service of this country, if is saved. Let me aid you in