islets, at the Dry Tortugas, and finding them deserted, was beating up, either to go into Key West, or to pass to the southward of the reef again, by the passage through which she had come as lately as the previous day. This was highly encouraging; and could he only get to the boat, and remove the party from the wreck before it sank, there was now every prospect of a final escape. To the southward, also, the mate fancied he saw a sail. It was probably a much smaller vessel than the ship in the north-west, and at a greater distance. It might, however, be the lofty sails of some large craft, standing along the reef, going westward, bound to New Orleans, or to that new and important port, Point Isabel; or it might be some wrecker, or other craft, edging away into the passage. As it was, it appeared only as a speck in the horizon, and was too far off to offer much prospect of succor. Thus acquainted with the state of things around him, Mulford gave his attention seriously to his duties. He was chiefly afraid that the returning tide might lift the boat from the rock on which it had grounded, and that it would float beyond his reach. Then there was the frightful and ever increasing peril of the wreck, and the dreadful fate that so inevitably menaced those it held, were not relief prompt. This thought goaded him nearly to desperation, and he felt at moments almost ready to plunge into the midst of the sharks, and fight his way to his object. But reflection showed him a less hazardous way of making an effort to reach the boat. The sharks’ fins described a semi-circle only, as had been the case of his single attendant during the night, and he thought that the shoalness of the water prevented their going further than they did, in a south-easterly direction, which was that of the boat. He well knew that a shark required sufficient water to sink beneath its prey, ere it made its swoop, and that it uniformly turned on its back, and struck upward whenever it gave one of its voracious bites. This was owing to the greater length of its upper than of its lower jaw, and Mulford had heard it was a physical necessity of its formation. Right or wrong, he determined to act on this theory, and began at once to wade along the part of the reef that his enemies seemed unwilling to approach.