The armament proposed for the several batteries is given in the following communication:

OFFICE OF CHIEF ENGINEER EXPEDITIONARY CORPS,

Hilton Head, S. C., December 5, 1861.

Brigadier General THOMAS W. SHERMAN,

Commanding Expeditionary Corps, Hilton Head, S. C.:

SIR: Should it be determined to attempt the reduction of Fort Pulaski from Tybee Island, I recommend the following armament for the batteries, inclusive of pieces held in reserve to replace those dismounted or otherwise rendered unserviceable:

Ten 10-inch sea-coast mortars; ten 13-inch sea-coast mortars; eighty heavy rifled guns of the best kind, to be used some against the barbette guns of the fort and some against the walls; eight columbiads for firing solid principally, some of them to fire shells, in case it be found practicable to drop them in or explode them over the fort. The northers should each have 900 rounds of shell, the guns and columbiads the same number of rounds of solid shot, and the columbiads 300 rounds of shell besides. It would be well to have a 15-inch columbiad, if one can be obtained.

Very respectfully, your obedient servant,

W. A. GILLMORE,

Captain, and Chief Engineer Expeditionary Corps.

The project set forth int he foregoing communications received General Sherman's sanction at once, with some slight modifications as to the number and caliber of the mortars to be used, and was forwarded to Washington and approved there. Information was in due time received that orders to prepare and forward the ordnance and ordnance stores had been issued. For months, therefore, preceding the fall of Pulaski, its reduction from Big Tybee, favored by a thorough investment, formed one of General Sherman's approved plans, awaiting only the action of others in sending the necessary supplies for its completion. The Forty-sixth Regiment New York Volunteers, Colonel R. Rosa, was sent to occupy Big Tybee Island early in December.

Operations for investing the place by the erection of batteries on the Savannah River above the work were set on foot about the middle of January, 1862. It was known to General Sherman before that time that gunboats of medium draught could enter the river above Fort Pulaski without encountering any batteries on the south side through Warsaw Sound, Wilmington Narrows (or Freeborn's Cut), and Saint Augustine Creek, and on the north side through New River, Wall's Cut, and either Wright or Mud Rivers.

Wall's Cut is an artificial channel, narrow but deep, connecting New and Wright Rivers, and has for years been used in making the inland water passage between Charleston and Savannah. This cut the enemy had obstructed by an old hulk and numerous heavy piles, as ascertained about the 1st of January by Lieutenant J. H. Wilson, of the Topographical Engineers. These obstructions had all been removed by a detachment of our engineers troops, under Major Beard, Forty-eight Regiment New York Volunteers, secretly sent from Hilton Head by General Sherman for that purpose. The piles were sawed off on a level with the bottom of the stream, and the hulk was swung around against the side of the cut, leaving ample room for the passage of transports and gunboats. The opening of Wall's Cut, which required four days and four nights to effect, was reported to the Navy on the 14th January, in order that the gunboats might enter the Savannah River and cover us in the erection of our investing batteries. At this time the enemy's gunboats were daily passing up and down the river.

Mud River is navigable at high spring tide for vessels of 8 1/2 to 9 feet draught. Wright River Bar has about 11 1/2 feet of water at ordinary high tide. The Wright River passage rendered it necessary to approach to within about 2 miles of Fort Pulaski.