It could be but one thing.

It was the boom of a cannon, and it could come only from a ship, a ship in danger, a ship driven by the storm, knowing nothing of either sea or island, sending forth her signal of distress which was also a cry for help.
It was his ship! The ship of rescue! But he must first rescue _it_! Now he heard the voices of the good spirits, the voices that had been silent all through the afternoon and evening, singing through the storm, calling to him, summoning him to action.

He had not taken off his clothes and he leaped from the couch, snatched up a lighted lantern, stuffed flint and steel in his pocket, and ran out into the wind and rain, of which he was now scarcely conscious.
The boom came to his ears a second time, off to the east, and now distinctly the report of a cannon.

He waited a little, watching, and, when the report came a third time, he saw dimly the flash of the gun, but it was too dark for him to see anything of the ship.

She was outside the reefs, how far he could not tell, but he knew by the difference in the three reports that she was driving toward the island.
It was for him to save the unknown vessel that was to save him, and in the darkness and storm he felt equal to the task.