gyrostat will be much slower and more persistent than that of the pendulum—so slow and so persistent that most people thought it impossible ever to bring it to rest. But some ingenious Germans have actually found a way, and their invention (which corresponds to the bucket of water with the plumb-bob) has made a practical success of the compass. The gyrostat in spinning round at a great rate makes air currents; and the inventors have led these air currents past a delicately adjusted metal tongue, so that any tendency to swing is resisted. Perhaps it will also interest you to hear how they came to pay attention to the matter at all. They had the notion of going to the North Pole by submarine. They thought that a boat could be sunk so as to travel under the ice to the North Pole and back. But the question arose how they were to steer. The magnetic compass would only take them to the Magnetic Pole, which is several hundred miles away from the true North Pole. The stars would not be visible from under the ice. Hence they had to think of some other plan, and it occurred to them to enquire whether they could not make use of this long-known but never-used principle of the gyrostat setting its axis North and South. Their success has been so complete that their compass is actually superior to the magnetic compass.
But we must now return to Newton and the law of Gravity. Newton proved, then, that the law of Gravity made bodies move in ellipses. An ellipse is called a conic section, because if you take a cone and cut it across you may get an ellipse. But it is not the only possible curve; you may get a parabola
