An attempt to seize New Orleans and the lower part of the Mississippi will be made only by a great power or a combination of several powers, with a strong naval and land force, the latter of which must be brought in transports which may sail in shallow water. If the defenses around New Orleans are well posted and of sufficient strength to repel any attack which may be made on them, the city can be assailed only by a land force, which must pass in the direction above suggested, between the Rigolets and the bay of Mobile. It becomes, therefore, an object of high importance to present such an obstacle to such an attempt as would defeat it should it be made. Fortifications are useful for the defense of posts, to prevent the approach to cities and the passage of rivers; but as works their effect can not be felt beyond the reach of their cannon. They are formidable in other respects by the body of men within them, which may be removed and applied to other purposes. Between the Rigolets and the bay of Mobile there is a chain of islands, at the extremity of which is Dauphine Island, which forms, with Mobile Point, from which it is distant about 3-1/4 miles, the entrance into the bay of Mobile, which leads through that part of the State of Alabama to the towns of Mobile and Blakeley. The distance between Dauphine Island and the Rigolets is 90 miles. The principal islands between them are Massacre, Horn, Ship, and Cat islands, near to which there is anchorage for large ships of war. The first object is to prevent the landing of any force for the purposes above stated between the Rigolets and the bay of Mobile; the second, to defeat that force in case it should be landed. When the distance from one point to the other is considered, it is believed that it would be impossible to establish works so near to each other as to prevent the landing of such a force. Its defeat, therefore, should be effectually provided for. If the arrangement should be such as to make that result evident, it ought to be fairly concluded that the attempt would not be made, and thus we should accomplish in the best mode possible and with the least expense the complete security of this important part of our Union, the great object of our system of defense for the whole.