great reef.  The group of keys—­Loggerhead, Bird, Long, Middle, East, North, Bush, Sand, and Garden—­are all within seven miles of each other, Garden, Bird, Bush, and Long being in close proximity,—­within swimming-distance, if the swimmer be not nervous in regard to sharks.  From these central keys a great sandy shoal spreads away on all sides, cut up, however, by several deep channels admitting vessels of the largest draught.  To the east and south the reef is two miles wide and rarely over four feet deep, covered at intervals with great fields of branch corals, while here and there clusters of enormous heads of astrea, porites, etc., have collected.  The edge of the reef is formed of dead coral rock, often beaten up by the waves into a continuous wall several miles in extent, and a few steps beyond this the water deepens quickly, until at the length of a vessel from it no bottom is visible.

The one opening in this barrier on the side of our approach, so formidable in a gale, is the passage through which the skill of Sandy had safely brought us, being, as its name explains, five feet deep and not many more in width, and used only at odd times by the few pilots and fishermen of the reef who know the secret of its approach.  But how old Sandy found it when completely covered by the waves, with only the tops of certain trees to steer by, is one of the mysteries.

Our object in visiting this desolate part of the country was to capture turtles.  Here is the ground of the green and loggerhead turtles, and, according to Sandy, the hawksbill, from which the shell of commerce is taken, is also occasionally found.

The squall was now a fast-disappearing pillar in the west.  The anchor-chain ran merrily out, and we rounded to in the narrow harbor of Garden Key.  The boys manned the pump, while Sandy and the writer pulled for the shore, and the dingy soon crunched into the white, sandy beach of the coral island which during the war was the Botany Bay of America.  Surely Dry Tortugas has been maligned:  instead of dry we find it very wet, a key of sand thirteen acres in extent, hardly one foot above the tide, and entirely occupied by probably the largest brick fort in the world.

Fort Jefferson was commenced long before the war, and is now a monument of the ineffectual military methods of thirty years ago.  The work is a six-sided, two-tiered fort of majestic proportions, its faces pierced with over five hundred guns.  How many millions of dollars have been expended in its erection it would be difficult to conjecture.  The question why so important a work was built here is often asked, and we have heard the answer given that it was encouraged by the Key West slave-owners, through their representatives, to give employment to their slaves, who were engaged as laborers by the government.  Garden Key, however, is the key of the gulf, and, as a prospective coaling-station in case of war, it was undoubtedly a spot to be held at all odds, and at the outbreak of the war it formed a convenient spot for the confinement of certain prisoners, as many as three thousand being kept there at one time.  Now the great fort figures as a picture of desolation and is slowly falling to decay, deserted save by the memories of the great conflict, a lighthouse-keeper, and a guard.