"Now for a quick run to 'Tugas, let us hope!" the doctor exclaimed, as the launch began to cut through the clear water of Key West Harbor. "Eight-thirty; we ought to be there by four-thirty this evening, unless the weather changes. We can't risk any rough seas in this launch, you know. If a storm should come up, we'd have to take refuge on one of the four or five keys between here and 'Tugas. Do you know that Fort Jefferson is the hardest place in the United States for a traveller to reach, Frank, unless he has a government boat to travel in?" The Gulf was as smooth as a pond, as it often is in summer, and every minute Frank could see fish darting through the transparent water, and great turtles and sea-fans and brain-stones on the bottom. He was as much excited over it as if he had been starting for China. "There's something ahead," he exclaimed, about the middle of the afternoon, "that I should say was a city growing right out of the water if I didn't know that it must be the great fort. But there can hardly be any fort as big as that. Is that really it, doctor?" "That is really the fort," the doctor answered, "and I think we will be there now inside of an hour." "But it seems to stand right in the water!" Frank exclaimed. "I don't see any land around it at all!" "It would take good eyes to see any land around it," the doctor replied, with a twinkle in his own eyes. "You see, the island was only five acres in extent, and they built a fort covering seven acres, so the foundations were laid right out in the water." When they were near enough to see plainly, Frank did not try to conceal his delight. "What an immense building!" he exclaimed. "I never imagined there was such a big building in the world. No wonder it cost thirty millions! And there are roofs and chimneys inside the walls, and palm-trees waving over the top. I didn't know they had chimneys on a fort, doctor, and palm-trees?" "They do in this one," the doctor laughed. "The roofs and chimneys belong to the officers' quarters and barracks, and the palm-trees have been growing ever since the fort was dismantled, thirty years ago." Once inside the great walls, they were in a large yard grown up with palms and bushes; and crossing this, they entered the officers' quarters, where Dr. Murray had his office and living-rooms. Such big rooms, too, with great open fireplaces, and broad halls with iron staircases. "Now make yourself at home, Frank," the doctor told him. "The whole place is open to you, and you can go anywhere you like." For an hour or more he wandered alone among the open casemates, dodging around conical piles of cannon-balls, patting the immense but long-silent columbiads. Then the doctor joined him for a short time before dinner in the vaulted casemates. "There are so many rusty machines here, doctor!" Frank exclaimed. "This looks like a little furnace. What do you suppose it was for?"