"'Farnum,' take a half-hour's run below the surface, then come back above surface."
"That will be a longer experience than I have yet had for one time," remarked Mr. Trahern, with a smile, as he interpreted the signal to Captain Jack.
"We have run for hours below, with safety, sir," Benson answered.
Two minutes later the section of middies that had just come up from a brief trip under water were below again.
"I think you'll find, gentlemen, that it will seem like the longest half hour you can remember," announced young Captain Benson. "My friends and I have spent many long hours under the surface, though we have never yet gotten over the terrible monotony of such a trip. Twenty-four hours under, I think, would make a lunatic of the bravest or the most stolid man."
As they ran along, in the silence and the darkness, the young midshipmen began to look curiously at one another.
"Did you misunderstand the time, Mr. Benson?" asked one of the midshipmen, at last. "It's surely more than a half hour since we made the last dive."
"Almost twelve minutes," Jack corrected, quietly.
"Whew-ew-ew!" whistled several of the naval cadets. Not one of them was a coward, yet, in their experience, the thought that they had put in barely more than a third of the ordered time under water made some of them fidget.
"Say, this gives us some idea how long a whole hour would be," remarked one of the midshipmen.