“An air lock!” cried Ned. “Did you see that, boys?”
“Just like the little old Sea Lion we used in the China Sea!”
“Here he comes again with a line!” announced Harry. “Now what?”
The boys heard a hammering and thumping near one of the sea cocks.
CHAPTER XIX
MACKINDER AGAIN
Even as the five boys glanced at each other with startled and wondering looks, the thumping ceased abruptly. In a short space it was resumed. Instinctively the boys gathered near the spot.
While they stood there trying to determine the cause for the strange procedure, the noise ceased. They heard a tapping at one of the portholes. Jimmie rushed across the compartment to investigate.
“Hey, fellows, come over here a minute!” he called out.