100 DISSERTATION SECOND. [part ii. 



an accurate knowledge of the nature of that tendency, and of 

 the equality, at least in this instance, between action and re- 

 action. Then, again, the idea of an animal force or som< 

 other equally unintelligible power being necessary to carry 

 on the circular motion, and to prevent the bodies from mov- 

 ing directly toward each other, is very strange ; considering 

 that Kepler knew the inertia of matter, and ought, therefore, 

 to have understood the nature of centrifugal force, and it^ 

 power to counteract the mutual gravitations of the two bo- 

 dies. In this respect, the great astronomer who was laying 

 the foundation of all that is known of the heavens, was not 

 so far advanced as Anaxagoras and Plutarch ; — so slow and 

 unequal are the steps by which science advances to perfec- 

 tion. The mutual gravity of the earth and moon is not sup- 

 posed by Kepler to have any concern in the production of 

 their circular motions ; yet he holds the tides to be pro- 

 duced by the gravitation of the waters of the sea toward the 

 moon.' 



The length to which Galileo advanced in this direction, 

 and the point at which he stopped, are no less curious to be 

 remarked. Though so well acquainted with the nature of 

 gravity on the earth's surface, — the object of so many of his 

 researches and discoveries, and though he conceived it to 

 exist in all the planets, nay, in all the celestial bodies, and to 

 be the cause of their round figure, he did not believe it to be 

 a power that extended from one of those bodies to another. 

 He seems to have thought that gravity was a principle 

 which regulated the domestic economy of each particular 

 body, but had nothing to do with their external relations ; so 

 that he censured Kepler for supposing, that the phenomena 

 of the tides are produced by the gravitation of the waters of 

 the ocean to the moon. 2 



1 Aslronomia Stella Martis. Introd. Parag. Q. 

 ►ial. Ho. Tom. IV. p. 325, Edit, dc Padova. 



