"That was for heating cannon-balls," the doctor answered, "so that they could fire hot shot into a hostile ship." "And this thing looks like an oven big enough to supply a city." "It is an oven," the doctor explained. "This is the fort's oven. You know at one time there were nearly three thousand people here, prisoners and garrison, and all their bread was baked in this brick oven. That is the reason it is almost as big as a house." "And this great machine in the bastion?" Frank asked. "It looks something like a steam-engine; but it is rusty enough to fall to pieces." "Ah, I am glad you reminded me of that!" the doctor explained. "I must caution you about the water-tanks. That big machine is a condenser, Frank. So many people required not only a great deal of bread, but a great deal of water, too, and no fresh water is to be had out of this coral rock. So this big condenser was put up. It pumped water out of the Gulf and converted it into steam, and when the steam condensed into water again the water was fresh. This old machine used to run day and night at one time. "Then," the doctor went on, "they had to have places to store the water, of course. For that purpose they built a system of water-tanks under the entire fort. Under every one of these lower casemates there is a great stone tank twenty or thirty feet square and ten or twelve feet deep; and they are all connected, so that now when they are not full you could go under the whole fort through the tanks. I suppose there is not another series of water-tanks like them in the whole country. They have not been used for years, but a little rain-water still flows in from the roofs, so that the water is always two or three feet deep in them. It is a dark, slimy place down in the tanks. "And that is what I want to caution you about," he continued. "You see in the stone floor of every one of these lower casemates there is a trap about two and a half feet square, covered with a square stone with an iron ring in the centre. Those traps lead down to the tanks. Sometimes one of the covers is lifted and is carelessly left off. I want you to be very careful about them if you come into the casemates at night, for it would be an ugly thing to tumble into the tanks. Here, I will lift this cover and let you look down." [Illustration: "UGH!" FRANK EXCLAIMED; "WHAT A GHOSTLY PLACE!"] "Ugh!" Frank exclaimed, drawing back from the black hole. "What a ghostly place! I suppose that stagnant water is full of all sorts of creatures, too!" The dinner bell called them back to their quarters, and Frank found that the quarantine establishment comprised about eighteen persons, including the doctors and nurses, engineers, boatmen, and laborers; and these, with the ordnance sergeant and the light-house-keeper, were the sole inhabitants of the big fort.