On this particular night the task had fallen to Rob and Merritt. There was nothing very arduous about it, the only duty involved being to keep ears and eyes open. Both lads had been engrossed in books dealing with their favorite subjects when Merritt called Rob's attention to the odd sound he had noticed.
"Maybe my ears are not quite so sharp as yours, old boy," said Rob, after an interval of listening. "I've got a slight cold, anyhow, and perhaps that's why I don't hear so readily."
"Possibly so."
"You are sure you weren't mistaken?"
"Think I'm hearing things?" indignantly responded Merritt. "No, siree, I'm willing to bet. Hark! There it is again!"
"By Hookey! I heard it that time, too. What can it be?"
"Hush!"
The noise was a most peculiar one. It seemed to be a sort of scraping on the outside of the submarine's hull. The diving craft was anchored at some distance from the shore, so as to be more readily prepared for a projected run the following day. This made the noise all the more inexplicable, as, had the craft been in the shed, it might have been caused by the inventor or the ensign paying a night visit to see that all was well, which they sometimes did.
"Perhaps it's a log bumping against the side."
"No; it appears to come from under the water."