ikt. iv.} DISSERTATION SECOND. 137 



The same astronomer was perhaps the first person who 

 conceived that there must be always a law capable of 

 being expressed by arithmetick or geometry, which con- 

 nects snch phenomena as have a physical dependence on 

 one another. His conviction of this truth, and the delight 

 which he appears to have experienced in the contempla- 

 tion of such laws, led him to seek, with great eagerness, 

 for the relation between the periodical times of the planets, 

 and their distances from the sun. He seems, indeed, to 

 have looked towards this object with such earnestness, 

 that, while it was not attained, he regarded all his other 

 discoveries as incomplete. He at last found, infinitely to 

 his satisfaction, that in any two planets; the squares of the 

 times of the revolution are as the cubes of their mean dis- 

 tances from the sun. This beautiful and simple law had a 

 value beyond what Kepler could possibly conceive ; yet a 

 sort of scientifick instinct instructed him in its great im- 

 portance. He has marked the year and the day when it 

 became known to him; it was on the ftth of May, 1618; 

 and perhaps philosophers will' agree, that there are few 

 days in the scientifick history of the world which deserve 

 so well to be remembered. 



These great discoveries, however, were not much at- 

 tended to by the astronomers of that period, or by those 

 who immediately followed. They were but little con- 

 sidered by Gassendi, — they were undervalued by Riccioli, 

 — and were never mentioned by Descartes, it was an 

 honour reserved for Newton to estimate them at their true 

 value. 



Indeed, the discoveries of Kepler were at first so far 

 from being duly appreciated, that they were objected to, 

 not for being false, but for offering to astronomers, in the 

 calculation of the place of a planet in its orbit, a problem 

 too difficult to be resolved by elementarv geometry. To 



