CHARLESTON, S. C., September 24, 1862.

September 16.-I inspected, with General Commanding Pemberton and Brigadier-General Jordan, my chief of staff, and Colonels Gonzales, chief of artillery, and Lay, Confederate States inspector-general, four new sand batteries en barbette near the west end of Sullivan's Island, bearing on the floating boom which is being laid across the channel to Fort Sumter. These batteries are not yet finished and only two 10-inch columbiads are in position, one not being serviceable yet. The magazines are not yet constructed. The boom is composed of railroad iron, strongly linked together with heavy iron bands, and is protected and buoyed up by pieces of timber of the length of the bars, bound together by iron rings. The bars are 4 feet under water and the whole is anchored every sixth section with an anchor. About one-fourth of this boon is laid. It has been tested, I am informed, by running against it a loaded vessel towed by a steamboat, but it resisted well, parting the tow-line, a 2-inch hawser. I am told it will all be completed in a week or ten days. It is proposed also to lay another one about 100 yards in rear of this one if sufficient chains and anchors can be had; also a rope obstruction about the same distance in front of the other, the object of which is to entangle the propellers of the enemy's steamers while under the fire of the heavy guns of the fort. The armament of the four sand batteries is to consist of seven 10-inch columbiads, one 8-inch columbiad, and two 42-pounder rifled guns. Fort Sumter has forty-six heavy guns (above 32-pounders) and Fort Moultrie nine heavy guns, bearing at once on these obstructions. There will, besides, be two strong gunboats, armed each with four guns, to assist in their defense. I am informed they will be finished in one month. They are considered by me perfectly indispensable in the defense of this harbor.

We then visited the Neck Battery, on Morris Island, erected to defend the approach to Fort Sumter. It is not yet entirely completed, requiring about two weeks longer to finish it. Its gorge ought to be entirely closed to prevent surprise. A few rifled guns ought also to be put to bear on the Main Channel.

On returning I inspected the small work (Fort Ripley) now being built on cribs in the bay about half way between Caslte Pinckney and Fort Johnson. It is intended for five heavy guns en barbette, and is nearly ready for its armament. Its foundation ought to be protected outside to the high-water mark with debris. A series of these small works in shallow water nearer to the entrance of the harbor would be very advantageous.

I did not visit Castle Pinckney, the armament of which is nine 24-pounders and one 24-pounder rifled, as I am acquainted already with this work, and considered it nearly worthless, capable of exerting but little influence on the defenses of Charleston.

September 17.-I inspected this day, with General Pemberton and Colonels Gonzales and Lay, the defensive lines on James Island from the Wappoo to near Secessionville, a distance of about 3 miles. They very properly arranged and located, with the exception of Fort Pemberton, on the Stono, and of some of the redoubts. A simple system might, I think, have been originally devised requiring a smaller number of men to defend it. However, the present one ought to answer our purpose with a proper force of about one and a half men to the running yard of development. Each redoubt and redan has at least one heavy gun now in position. The lines between Dill's Creek and the Wappoo are not yet entirely completed, requiring about fifteen days more. Fort