"I'm afraid," answered the first officer, "that we're in for a settled spell of cloudy weather and fog." "And I'm morally certain that you're right," said the Captain, with a serious face, as he thought of what was before him. When the celestial bodies--the sun, moon, and stars--cannot be seen, then begins grave trouble for the navigator. As long as these are visible, by observing their altitudes above the horizon with the sextant--an instrument designed for that purpose--and by some simple astronomical calculations, he can ascertain the latitude and longitude of his ship, and thus know just where he is and which way to steer in order to reach his port. But the moment he loses the heavenly bodies he must feel his way into port by "dead reckoning," which consists of measuring the actual distance sailed by means of the log-line, and of ascertaining the direction by the compass. It is a method subject to errors of many kinds, caused by incorrect registering of the log, by deviation of the compass, and by currents. It is like trying to walk through a room in the dark by counting the number of your steps. So it was not remarkable that Captain Jason Argo looked grave. "At noon to-day we made our position 47° west longitude and 46° 30' north latitude," said the Captain, reflectively. "Yes, sir," answered the first officer. "As we are steering, that should have made us seventy-five miles from the easterly edge of the Newfoundland Banks." "To a dot, sir." "And it is now three o'clock. What does the patent log show?" "It is registering nineteen and three-quarter knots an hour." The patent log is an instrument for recording the distance sailed by the ship. It consists of a dial on the outside of a case, inside of which are wheels to turn the hands. Attached to the machinery is a long line, at the end of which is a rotator shaped somewhat like a ship's propeller. This rotator drags through the sea, and makes a certain number of revolutions every mile, twisting the line, and thus turning the hands on the dial, where the number of knots is marked. "It's a new log, isn't it?" "Yes, sir; I received it at Queenstown." "Good. What is its percentage of error?" "Two per cent." Patent logs usually overrate the distance run, and the percentage of error has to be ascertained. "It's running fully two per cent. now, I fancy," said the Captain, stepping to the speaking-tube that ran to the engine-room and calling for the chief engineer. "Hello, Mr. Bargot! How many revolutions are you making a minute?" A jumble of figures returned through the tube. "And that's nineteen knots, isn't it?" said the Captain. "Good." The speed of vessels can be tolerably well calculated from the number of revolutions of the screw. "Now," said the Captain, turning to the third officer, who was also on the bridge, "what was the last record of the chip log?" "Twenty knots, sir." "Common log is not much good at high speeds," commented the Captain.