196 



A Short History of Astronomy [CH. vii. 



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 yords, 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. 



" There is there- 

 fore a conflict be- 

 tween 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." * 

 FIG. 63. Kepler's idea of gravity. , 



From the Epitome. Ihe 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 f 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 



* Epitome, Book IV., Part 2. 



f Introduction to the Commentaries on the Motions of Mars. 



