Page:A short history of astronomy(1898).djvu/252

the treatise On the Magnet (De Magnete) published in 1600 by our countryman William Gilbert of Colchester (1540–1603). He suggested that the planets might thus be regarded as connected with the sun, and therefore as sharing to some extent the sun's own motion of revolution. In other words, a certain "carrying virtue" spread out from the sun, with or like the rays of light and heat, and tried to carry the planets round with the sun.

Fig. 63.—Kepler's idea of gravity. From the Epitome.

"There is therefore a conflict between the carrying power of the sun and the impotence or material sluggishness (inertia) of the planet; each enjoys some measure of victory, for the former moves the planet from its position and the latter frees the planet's body to some extent from the bonds in which it is thus held, . . . but only to be captured again by another portion of this rotatory virtue."^[1]

The annexed diagram is given by Kepler in illustration of this rather confused and vague theory.

He believed also in a more general "gravity," which he defined^[2] as "a mutual bodily affection between allied bodies tending towards their union or junction," and regarded the tides as due to an action of this sort between the moon and the water of the earth. But the speculative ideas thus thrown out, which it is possible to regard as anticipations of Newton's discovery of universal gravitation, were not in any way developed logically, and Kepler's mechanical ideas

1. Epitome, Book IV., Part 2.
2. Introduction to the Commentaries on the Motions of Mars.