Rose soon appeared on deck, and she naturally began to question Harry concerning their position and prospects. He was confessing his ignorance, as well as lamenting it, when his companion's sweet face suddenly flushed. She advanced a step eagerly toward the open window of Spike's state-room, then compressed her full, rich under-lip with the ivory of her upper teeth, and stood a single instant, a beautiful statue of irresolution instigated by spirit. The last quality prevailed; and Mulford was really startled when he saw Rose advance quite to the window, thrust in an arm, and turn toward him with his own sextant in her hand. During the course of the passage out, the young man had taught Rose to assist him in observing the longitude; and she was now ready to repeat the practice. Not a moment was lost in executing her intention. Sights were had, and the instrument was returned to its place without attracting the attention of the men, who were all busy in getting up purchases, and in making the other necessary dispositions for discharging the flour. The observations answered the purpose, though somewhat imperfectly made. Mulford had a tolerable notion of their latitude, having kept the brig's run in his head since quitting Yutacan; and he now found that their longitude was about 83 o west from Greenwich. After ascertaining this fact, a glance at the open chart, which lay on Spike's desk, satisfied him that the vessel was anchored within the group of the Dry Tortugas, or at the western termination of the well-known, formidable, and extensive Florida Reef. He had never been in that part of the world before, but had heard enough in sea-gossip, and had read enough in books, to be at once apprised of the true character of their situation. The islets were American; the light-house was American; and the haven in which the Swash lay was the very spot in the contemplation of government for an outer man-of-war harbour, where fleets might rendezvous in the future wars of that portion of the world. He now saw plainly enough the signs of the existence of a vast reef, a short distance to the southward of the vessel, that formed a species of sea-wall, or mole, to protect the port against the waves of the gulf in that direction. This reef he knew to be miles in width.