The Adventures of Captain Horn by Frank R. Stockton

That morning, when the party in the cavern had had their breakfast, with some hot tea made on a spiritlamp which Mrs. Cliff had brought, and had looked cautiously out at the sunlit landscape, and the sea beyond, without seeing any signs or hearing any sound of wicked men, there came a feeling of relief. There was, indeed, no great ground for such a feeling, but as the Rackbirds had not come the day before nor during the night, perhaps they would not come at all. It might be they did not care whether the black man ran away or not. But Captain Horn did not relax his precautions. He would take no chances, and would keep up a watch day and night.

When, on the night before, the time had come for Ralph's watch to end, his sister had awakened him, and when the captain, in his turn, was aroused, he had not known that it was not the boy who had kept watch during his sleep.