The city lies about a hundred miles, to use round figures, above the multitudinous mouths of the Mississippi. It lies less than half a dozen miles west of the so-called Lake Pontchartrain, which is an inlet from the gulf, with two other bodies of water, Mississippi Sound and Lake Borgne, lying between. But the passes into Lake Borgne and from that body of water into Lake Pontchartrain, are shallow and difficult, as the British discovered in 1814 in their attempt to approach New Orleans by the "back door," as it were. On the other hand, the Mississippi has five principal mouths, with some others that carry less water. Thus it was, or seemed to be, impossible for any Federal fleet to blockade the entrances to that stream and cut off commerce between New Orleans and the outer world. But above and beyond all these considerations, was the desire of the Federal authorities to conquer control of the Mississippi itself throughout its entire length. That would be not only to split the Confederacy into halves, cutting off a large part of its food resources, but also to make of the great river a convenient highway for the transportation of Federal food supplies, troops, ammunition and all else that is needful in war, to such points as might have need of them. Thus the reduction of the defenses of New Orleans, and the conquest of that city became a matter of supreme strategic importance. To this task Farragut was assigned with a fleet that, in our time, could not possibly force its way past a single well-defended fort, or successfully meet an adversary afloat. He had in his fleet, first of all--in Navy Department estimation--twenty-one schooners, each carrying a mortar intended to throw shells high in air and drop them into the Confederate defensive works. These proved to be utterly useless, as Farragut had from the beginning believed that they would be. He had besides, six sloops of war, sixteen gunboats, and eight other ships. His flagship, the _Hartford_, was a wooden vessel, carrying twenty-two Dahlgren nine-inch guns besides howitzers in the tops. The others were similarly armed. All were under-powered, and could make only eight knots an hour where there was no current. In such a stream as the Mississippi four knots constituted the limit of their performance. There were transports also, carrying an army of about 15,000 men under command of General Benjamin F. Butler. This force was intended to occupy the city after Farragut should have captured it, but until he should do so it was only an incumbrance to his expedition. He got rid of it for a time by landing the troops on one of the islands that separate Mississippi Sound from the gulf, and leaving them there until such time as he should have need of them.