With a view to such formidable invasion, of which we should never lose sight, and of the great objects to which it would be directed, I think that very strong works at some point within the Gulf of Mexico will be found indispensable. I think also that those works ought to be established at the bay of Mobile--one at Mobile Point and the other on Dauphine Island--whereby the enemy would be excluded and the complete command of that bay, with all the advantages attending it, be secured to ourselves. In the case of such invasion, it will, it is presumed, be deemed necessary to collect at some point other than at New Orleans a strong force, capable of moving in any direction and affording aid to any part which may be attacked; and, in my judgment, no position presents so many advantages as a point of rendezvous for such force as the mouth of that bay. The fortification at the Rigolets will defend the entrance by one passage into Lake Pontchartrain, and also into Pearl River, which empties into the Gulf at that point. Between the Rigolets and Mobile Bay there are but two inlets which deserve the name, those of St. Louis and Pascagola, the entrance into which is too shallow even for the smallest vessels; and from the Rigolets to Mobile Bay the whole coast is equally shallow, affording the depth of a few feet of water only. Cat Island, which is nearest the Rigolets, is about 7-1/2 miles distant from the coast and 30 from the Rigolets. Ship Island is distant about 10 miles from Cat Island and 12 from the coast. Between these islands and the coast the water is very shallow. As to the precise depth of water in approaching those islands from the Gulf, the report of the topographical engineers not having yet been received, it is impossible to speak with precision; but admitting it to be such as for frigates and even ships of the line to enter, the anchorage at both is unsafe, being much exposed to northwest winds. Along the coast, therefore, there is no motive for such strong works on our part--no town to guard, no inlet into the country to defend--and if placed on the islands and the entrance to them is such as to admit large ships of war, distant as they are from the coast, it would be more easy for the enemy to assail them with effect.