There are two principal magazines, one in the eastern battery and one in the western bomb-proof lately completed. Both appear to be serviceable and well protected.

The western bomb-proof, which has been under construction for the past seven months, during which time it has been thrice altered, is very near completion, and is of excellent character. It is of large capacity, and had it been at the other end of the work would, in my opinion, have sufficed for the wants of the position. It would e well to have it and all other bomb-proofs on the island whitewashed for sanitary reasons and to increase the light. The eastern bomb-proof is only commenced, is of irregular form, following the interior slope of the northeastern outworks and flanks. It is to be regretted that its galleries are not wider. I would recommend that the sand of the traverses in rear of gun-chambers Nos. 3 and 4 in the outwork be used for covering the bomb-proof, which could then be raised high enough to overlook the battery, and, if furnished with a banquette, to cover it with an infantry fire. This would give more room, and, as I understand, it is in contemplation to raise the traverse.

Two feet would expose the infantry none the more. Moreover it would shorten the work most materially. At present the eastern part of the work is so full of nooks and corners as to be exceedingly inconvenient, and it would be hard to manage men in the hurry and confusion of an action. Should a bombardment from Long Island be successful in silencing the guns an attack may be looked for from that direction.

The new work progressing at Battery Marshall was determined upon early in November, but has not been prosecuted with energy. It was undertaken after a representation that the battery needed protection from an enfilade and reverse fire from the southwest. it has apparently been worked upon for about two weeks by, say, 400 negro laborers. Sand in sufficient quantities has been thrown up to form a very heavy parapet for about half its extent. The trace appears to have a very great development, and as commenced will require a long time for completion. The northwestern fronts it appears are to be as high as the sea-face of the main work on the southwest or northeast of the inclosure. They are also to be as thick.

With a view of dispensing with unnecessary labor, as well as to afford protection, I respectfully suggest that the portions of the inclosed work under progress of construction should be lowered by 4 feet where they look to the northwest, and that they should be decreased in thickness to 8 feet on the superior slope. No heavy fire can [come] from that direction, and its defenders would be protected from fire from sea or Long Island if the faces were so defiladed. I would recommend that the gate be placed on this front. This work, whatever its merits, is needed much, and I would recommend that it be pushed to completion rapidly and with as much economy of labor as possible, not impairing its strength.

Battery Marshall is garrisoned by one company of the First South Carolina [Regular] Infantry and four companies of the Twentieth South Carolina Volunteers, all under Lieutenant-Colonel Dantzler, of the Twentieth. He is temporarily absent, being succeeded by Captain McMichael. The garrison appeared good soldiers and attentive to their duties.