"What kind of signals?"
He didn't know the answer to that. "Anyway, since it's underwater, if it sends out anything it must be sound impulses. Otherwise we wouldn't hear it wail. And what good is sound if not for signals?" added Rick.
"Sonar," Scotty reminded.
The boys were familiar with sonar because of the Spindrift work on the Submobile. Very high frequency sound impulses were sent out, and the echoes were timed or used in other ways. It was the way in which bottom tracings were made by surface craft, and the way in which Navy ships detected submarines. It could be used for locating schools of fish.
"It could be sonar of some kind," Rick agreed. "But what good would it do anyone to stick a sonar device on an island like this?"
And there speculation stopped again, the question still unanswered.
They dove to the wreck and continued the hard labor of taking the aft end of the ship apart. When they finally got the new area cleared of rotted boards and timbers it was only to find a cabin already filled with sand.
Rick borrowed the spear from Scotty's gun and thrust it down into the sand. It slid in easily, meeting no obstruction. He probed with it but found nothing except more sand.
Discouraged, he wrote on his belt slate, "Mybe no bottm. Flr of cbn my be gne."
Scotty nodded. He lifted his hands in a gesture of inquiry. Now what?