It was in imagination, but his mental calculation was not far wrong, for within a few yards of where it might be expected, and not ten from where he was now sculling, he saw something roll up as it were to the surface, there was a gleam of white in the sunlit water, and then it was disappearing again, when, acting upon the impulse of the moment, Harry loosened his hold of the oar, took two steps forward over the thwarts, and leaped into the sea.