It is not, however, by the Mississippi only, or the waters which communicate directly with or approach nearest to New Orleans, that the town is assailable.  It will be recollected that in the late war the public solicitude was excited not so much by the danger which menaced it in those directions as by the apprehension that, while a feint might be made there, the main force, landing either in the bay of Mobile or other waters between that bay and the Rigolets, would be thrown above the town in the rear of the army which had been collected there for its defense.  Full confidence was entertained that that gallant army, led by the gallant and able chief who commanded it, would repel any attack to which it might be exposed in front.  But had such a force been thrown above the town, and a position taken on the banks of the river, the disadvantage to which our troops would have been subjected, attacked in front and rear as they might have been, may easily be conceived.  As their supplies would have been cut off, they could not long have remained in the city, and, withdrawing from it, it must have fallen immediately into the hands of the force below.  In ascending the river to attack the force above, the attack must have been made to great disadvantage, since it must have been on such ground and at such time as the enemy preferred.  These considerations shew that defenses other than such as are immediately connected with the city are of great importance to its safety.

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.