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find some cause for it, but nothing has yet been added to our knowledge by these attempts.

The motions of the planets are governed by their gravitation. Were there only a single planet moving round the sun it would be acted on by no force but the sun's attraction. By purely mathematical calculation it is shown that such a planet would describe an ellipse, having the sun in one focus. It would keep going round and round in this ellipse forever. But in accordance with the law, the planets must gravitate towards each other. This mutual gravitation is far less than that of the sun, because in our solar system the planets are of much smaller mass than the central body. In consequence of this mutual attraction the planets deviate from the ellipse. Their orbits are very nearly, but not exactly, ellipses. Still, the problem of their motion is one of pure mathematical demonstration. It has occupied the ablest mathematicians of the world since the time of Newton. Every generation has studied and added to the work of the preceding one. One hundred years after Newton, Laplace and Lagrange showed that the ellipses near which the planets move gradually change their form and position. These changes can be calculated thousands, tens of thousands, or even hundreds of thousands of years in advance. Thus it is known that the eccentricity of the earth's orbit round the sun is now slightly diminishing, and that it will continue to diminish for about forty thousand years. Then it will increase so that in the course of many thousands more of years it will be greater than it now is. The same is true of all