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Rh That principle has been used recently for steering ships. A large gyrostat has its axis floated horizontally in mercury. If the axis points East and West, then the rotating Earth is always trying to lift the West end, but it only makes the axis turn round. And this is also more or less true for any other position except that when the axis is due North and South. Hence the axis will be always seeking that due North and South position, as the magnetic needle seeks the Magnetic Poles. This has been known a long time, but only recently has the tendency been used to make a good practical compass. This has at last been done, and the most wonderfully convincing proof of the rotation of the Earth is thus provided—we are steering ships by means of it, and entrusting thousands of lives to its efficacy.

Perhaps it will interest you to know what was the nature of the invention which has made a practical success of this compass. You have seen a workman use a plumb-line to get buildings vertical, and you know that a plumb-line is very like a pendulum. If you set it swinging it will go on swinging for some time, and be no use as a plumb-line. To bring it to rest the workman sometimes puts the bob in a bucket of water, which quickly stops the swinging. This kind of stopping is called "damping" the swing, though it has nothing to do with the water wetting the bob; it merely means that the inconvenient swinging is stopped. Now when a gyro-compass is set up, unless it is pointed exactly North from the first (which is unlikely), it will swing to and fro to find the North, just as a plumb-bob swings to and fro to find the vertical. But the swinging of the