They ate breakfast and went to the chart-room. Though the ship was far out in the ocean, it was still many hours’ sail from the location given for the derelict. The captain began to study the ship’s logbook, as the sailing record is called.
“See here, Henry,” he remarked after a. moment. “This logbook might interest you.”
Henry looked at the book, and saw entered there a detailed record of what was done on shipboard, not only from hour to hour but even every few minutes. Glancing back, he saw that his own rescue was noted down, and the recovery of his suit-case, and the exact time the executive officer came aboard, as well as the time when the Iroquois got under way.
“Captain Hardwick,” he said presently, “what does this entry about the log mean? I see it is written down every hour.”
“That’s the way we keep our dead reckoning,’ said the captain. “When we can see sun or stars, we know exactly where we are. But when it’s cloudy we have to figure our position by dead reckoning. We know by our compass which way we are heading. We can tell by the number of revolutions of our propeller how fast we ought to be moving. We have an apparatus fastened to the taffrail that drags in the sea a good many fathoms behind us, and that turns like a propeller. It turns the line with it. The