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must grip the frame firmly, because when the gyroscope turns over it gives you quite a shock and you must be careful not to drop it. The reason why it turns over may be put in this way: that it insists on spinning in the same direction in which the holder is turning. If he turns in the opposite direction, then the somersault of the gyroscope will practically make it spin in the other direction. You can verify this for yourself by turning your watch face downwards and imagining that you can see the hands through the back; you would see them moving contrary to their usual direction. People who go to the Southern hemisphere find the Sun going round the wrong way; that is because they have themselves turned upside down, pointing their feet in the direction of the heads of northern folk; and it would be the same if they remained at home, but the Earth turned a somersault.

Suppose now that the Earth did turn a somersault; the Moon would be going round us in the direction opposite to our rotation, like Phœbe does round Saturn. If, now, the Earth flung off a new moon, we should have two moons, going round in opposite directions. We explain the existence of Phœbe in this way: she is Saturn's eldest child, born at a time when he was turning on his axis in the direction which Phœbe now indicates. Subsequently he turned a somersault, and the other eight or nine children, all born after the somersault, naturally adopt the new direction. The only thing still to be explained is what made him turn the somersault, and Mr. Stratton has found a sufficient explanation of this in the action of the tides. We must therefore