214 A SHORT HISTORY OF SCIENCE 



It should be borne in mind that Kepler's results depend not 

 on a priori theory for their confirmation, but upon actual ob- 

 servations supporting them and interpreted by them. The great 

 further step of showing that the three laws are not independent 

 and empirical, but mathematical consequences of a single me- 

 chanical law still awaited the genius of Newton. 



Kepler's notions in regard to force and motion are still crude. 

 Thus, for example, having in mind an analogy with magnetism, 

 Kepler says in his Epitome of the Copernican Astronomy, 

 (1618-1621): 



'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.' 



Elsewhere he says : 



1 We must suppose one of two things : either that the moving spirits, 

 in proportion as they are more removed from the sun, are more feeble ; 

 or that there is one moving spirit in the centre of all the orbits, 

 namely, in the sun, which urges each body the more vehemently in 

 proportion as it is nearer; but in more distant spaces languishes in 

 consequence of the remoteness and attenuation of its virtue.' 



Whewell. 



He recognized the necessity of a force exercised by the sun, but 

 believed it inversely proportional to the distance instead of to 

 the square of the distance. His notions of gravity are expressed 

 in his book on Mars: 



' Every bodily substance will rest in any place in which it is placed 

 isolated, outside the reach of the power of a body of the same kind. 

 Gravity is the mutual tendency of cognate bodies to join each other 

 (of which kind the magnetic force is), so that the earth draws a stone 

 much more than the stone draws the earth. Supposing that the 

 earth were in the centre of the world, heavy bodies would not seek 

 the centre of the world as such, but the centre of a round, cognate 



