Mok, the second African, had been stationed to watch the lake approach, and he had deserted! Now the hot thought flashed upon the captain that the rascal had been a spy.

The Rackbirds had known that there were shipwrecked people in these caves.
How could they help knowing it, if they had killed Davis and the others?
But, cowardly hounds as they were, they had been afraid to attack the place until they knew how many people were in it, what arms they had, and in what way the place could best be assailed.

This Mok had found out everything.

If the boy could swim across the lake, that black man could do it, and he had gone out through the cleft, and was probably now making his report to the gang.
All this flashed through the captain's brain in a few seconds.

He set his teeth together.