"How do you manage to keep your course under water, captain?" asked Frank.
"Simple," was the reply. "Gyroscopic compass."
At that moment the man on watch at the bell receiver reported the sound of a ship's propellers above. Captain Nicholson turned his place at the periscope over to his first officer and listened himself.
The steady rhythmic beat was well off the port bow.
At Captain Nicholson's command, the main ballast tank was emptied until the conning tower was well awash. Then the commander, Frank and Jack went up to have a look around, for the airship, as well as for the vessel.
"Those sky pilots," said the commander, "maintain that they can see us and get us with bombs at any depth. However, I see nothing of our friend. Looks like he had lost his bet this time."
They returned below, and Frank put his eye to the periscope.
Almost instantly he made out the outline of large vessel of some kind. He reported this to Captain Nicholson, who brushed him quickly aside and peered into the periscope himself.
"Merchant vessel of some kind," he said aloud. "We haven't time to fool with him now. May be able to get him as we come back."
Once more now the three made their way to the bridge. The clouds had gradually thickened and it was very dark.