596 HISTORY OF GEOLOGY. 



Yet the analogy of the progress of knowledge on 

 other subjects, points out very clearly the necessity 

 of such a science. When phenomenal astronomy 

 had arrived at a high point of completeness, by the 

 labours of ages, and especially by the discovery of 

 Kepler's laws, astronomers were vehemently de- 

 sirous of knowing the causes of these motions ; and 

 sanguine men, such as Kepler, readily conjectured 

 that the motions were the effects of certain virtues 

 and influences, by which the heavenly bodies acted 

 upon each other. But it did not at first occur to 

 him and his fellow-speculators, that they had not 

 ascertained what motions the influences of one body 

 upon another could produce; and that, therefore, 

 they were not prepared to judge whether such 

 causes as they spoke of, did really regulate the 

 motions of the planets. Yet such was found to 

 be the necessary course of sound inference. Men 

 needed a science of motion, in order to arrive at a 

 science of the heavenly motions : they could not ad- 

 vance in the study of the mechanics of the heavens, 

 till they had learned the mechanics of terrestrial 

 bodies. And thus they were, in such speculations, 

 at a stand for nearly a century, from the time of 

 Kepler to the time of Newton, while the science of 

 mechanics was formed by Galileo and his succes- 

 sors. Till that task was executed, all the attempts 

 to assign the causes of cosmical phenomena were 

 fanciful guesses and vague assertions; after that 

 was done, they became demonstrations. The science 



