"Do you know," he said, "you ought to open a window, or something."
"Why, what's the matter?" asked Tom quickly, looking to see if the odd man was joking.
"Well, of course I don't exactly mean a window," explained Mr. Damon, "but we need fresh air."
"Fresh air!" There was a startled note in Mr. Swift's voice as he repeated the words.
"Yes, I can hardly breathe in the living-room, and it's not much better here."
"Why, there ought to be plenty of fresh air," went on the inventor. "It is renewed automatically."
Tom jumped up and looked at an indicator. He uttered a startled cry.
"The air hasn't been changed in the last hour!" he exclaimed. "It is bad. There's not enough oxygen in it. I notice it, now that I've stopped working. The gage indicates it, too. The automatic air-changer must have stopped working. I'll fix it."
He hurried to the machine which was depended on to supply fresh air to the submarine.
"Why, the air tanks are empty!" the young inventor cried. "We haven't any more air except what is in the ship now!"