to open the conning tower and let in fresh air as soon as the top was above the surface.
With a bound the Advance reached the top. Tom frantically worked the worm gear that opened the tower. In rushed the fresh, life-giving air, and the treasure-hunters filled their lungs with it.
And it was only just in time, for Mr. Sharp was almost gone. He quickly revived, as did the others, when they could breathe as much as they wished of the glorious oxygen.
"That was a close call," commented Mr. Swift. "We'll not go below again until I have provided for all emergencies. I should have seen to the air tanks and the expanding one before going below. We'll sail home on the surface now."
The submarine was put about and headed for her dock. On the way she passed a small steamer, and the passengers looked down in wonder at the strange craft.
When the Advance reached the secluded creek where she had been launched, her passengers had fully recovered from their terrible experience, though the nerves of Mr. Swift and Mr. Damon were not at ease for some days thereafter.
"I should never have made a submerged test without making sure that we had a reserve supply of air," remarked the aged inventor. "I will not
