FORT GAINES, August 4, 1861.

Honorable ROBERT H. SMITH, M. C.,

Richmond, Va.:

SIR: Within two or three months the enemy will probably be prepared to operate against this coast, and will no longer be restrained by climate. That the whole power of the North will be aimed at the cotton ports can hardly be doubted. Light-draft gun-boats are under construction for the invasion of Southern waters, and some of those boats will probably be found plated with iron. Their land forces will move forward substantially as already projected, and their fleets are bound to co-operate in order to prevent the concentration of our forces on the Northern frontier. The demonstration by the fleet may serve as a feint, but is sure to become a real attack if the coast be found feebly guarded. The possibility of any great disaster, like the loss of a cotton port, should be avoided by excess of vigilance and preparation.

I will speak of Mobile and its defenses only. At present, if Fort Morgan be captured, the city is lost, and if the city be taken the forts must fall. They are mutually dependent. Fort Morgan, with proper vigilance, is safe from surprise and sudden assault, and has little to fear, I think, from bombardment by sea. But it is unprepared to sustain a siege, and I greatly fear could not close the channel against a determined effort made by common war steamers to enter the bay. It has about seventy guns mounted, forty-five of which bear on the channel, and of these two only are 10-inch columbiads in barbette and two 8-inch in casemate, with a narrow field of fire. The rest are 32 and 24 pounders, not very formidable to modern ships of war. We have thirteen vacant traverse circles for columbiads, but neither the guns nor carriages. As these guns in barbette are intended to traverse a full circle, their fire would be formidable against the land or water.

If Fort Morgan were to be besieged the enemy would make his landing our of range to the eastward, and, establishing his batteries on that side, would bombard the place at his leisure. The wharf and every landing would be covered by his guns, and the fort would be cut off from all relief by way of the bay. Against relief by land he would erect his opposing batteries. In order to obviate this difficulty I have proposed to the Department to construct lines across the peninsula, some two miles east of the fort, with two redoubts, one on each shore and half a mile within the lines, all to be heavily armed with cannon and manned at the proper time by 3,000 men. The peninsula is about three-quarters of a mile wide. Then the enemy cannot land between the redoubts and the fort, for he would be exposed to the fire of all three. Neither can he place himself between the redoubts and the lines, and he is thus pushed forward a gunshot from the lines and wholly out of range of the fort and wharf. Our front is then equal to his, our communication with the town is open, and there is no reason why we should not have as many men and as many guns and munitions as he. But in order to insure all this another condition must be satisfied, viz, that we must have a naval force in the bay strong enough to dispose of anything that might slip in at night past the fort or to capture armed launches which might be hauled across the peninsula.

Anticipating that iron clad steamers will be brought against us, I see no way of stopping them but by stretching a formidable chain, sustained by rafts, across the main ship channel, under the guns of the fort. The channel is a mile wide and forty or fifty feet deep. Over toward Fort Gaines is a 9-foot channel a mile and three-quarters wide, which might