The position, however, at Mobile Bay is essentially different. That bay takes its name from the Mobile River, which is formed by the junction of the Alabama and Tombigbee, which extend each about 300 miles into the interior, approaching at their head waters near the Tennessee River. If the enemy possessed its mouth, and fortified Mobile Point and Dauphine Island, being superior at sea it would be very difficult for us to dispossess him of either, even of Mobile Point; and holding that position, Pensacola would soon fall, as without incurring great expense in the construction of works there it would present but a feeble resistance to a strong force in its rear. If we had a work at Mobile Point only, the enemy might take Dauphine Island, which would afford him great aid in attacking the point, and enable him, even should we succeed in repelling the attack, to render us great mischief there and throughout the whole Gulf. In every view which can be taken of the subject it appears indispensable for us to command the entrance into Mobile Bay, and that decision being taken, I think the considerations which favor the occupation of Dauphine Island by a strong work are conclusive. It is proper to observe that after the repulse before New Orleans in the late war the British forces took possession of Dauphine Island and held it till the peace. Under neither of the reports of the Board of Engineers and Naval Commissioners could any but sloops of war enter the bay or the anchorage between Dauphine and Pelican islands. Both reports give to that anchorage 18 feet at low water and 20-1/2 at high. The only difference between them consists in this, that in the first a bar leading to the anchorage, reducing the depth of water to 12 feet at low tide, was omitted. In neither case could frigates enter, though sloops of war of larger size might. The whole scope, however, of this reasoning turns on a different principle--on the works necessary to defend that bay and, by means thereof, New Orleans, the Mississippi, and all the surrounding country against a powerful invasion both by land and sea, and not on the precise depth of water in any of the approaches to the bay or to the island.