KEPLER 189 



it took on a very definite form. The work which appeared in 

 1609, in which he announces the first two of his laws, con- 

 tains an introduction which came perilously near to a complete 

 anticipation of the ideas of Newton. 



Every natural substance, 1 he says, by nature remains in 

 repose when it is isolated and outside the sphere of activity 

 of bodies which have an affinity for it. He anticipates Galileo. 

 Gravitation, he goes on, is an affection or quality with which 

 bodies are endowed in order to hold them together. This 

 faculty or property is general ; but the globe of the earth does 

 not attract a stone any more than the stone attracts the earth. 

 Bodies are not drawn towards the earth's centre because it is 

 the centre of the world, but as the centre of things of the same 

 species and of the same family. If the earth were not round, 

 bodies would not be drawn towards its central point, but would 

 tend in different directions. If the earth and the moon were 

 not held in their orbits by some force, the earth would mount 

 towards the mogn by the fifty-fifth part of their distance, and 

 the moon would descend the remainder of the distance in 

 order to be united to it. The sphere of the moon's activity 

 extends as far as the earth and attracts the waters of the torrid 

 zone. The moon passes to the zenith ; the waters follow it 

 and mount with it, more notably in the deep and open oceans, 

 with less liberty in the Mediterranean and in gulfs. 



If this action of the moon extends to the earth, it follows 

 equally that the force of the earth's attraction extends to the 

 moon. No portion of matter which exists on the earth can 

 lift itself up and escape from its powerful grasp. There is 

 nothing intrinsically light or weightless. All bodies are material ; 

 lightness is only a less degree of weight. Even the moon cannot 

 slip its leash. 



Surprising in the last degree it is to meet with these perfectly 

 just ideas in a work appearing a generation before Newton was 

 born. But Kepler does not stop here ; he has shown that the 

 planets which have the swiftest movement are those nearest 

 the sun ; the farther they are from the sun, the slower their 

 motion. What is their motor force ? It can reside only in 

 the planets and the sun itself. These two recognised effects, 

 diminution of the speed with the increase of distance, must have 

 one and the same cause. But the centre of the world where 



1 Quoted from Bailly, Histoire de ? Astronomic Moderne (1779), ii. 41. 



