With Frank eager to go and his mother anxious to have his health improved, there was not much doubt that in the end the doctor's invitation would be accepted. That was the way it turned out, and when the doctor's steam-launch started from the naval depot wharf two days later, Frank was in the little cabin with the doctor, and his trunk was stowed away somewhere up forward.
"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?"