148 GALILEO AND HIS JUDGES. 



was drawing to its close, there came on the scene 

 some thoughtful and able astronomers, who could 

 not only utilise the knowledge of their predecessors, 

 but could also guess, with more or less accuracy, 

 what that law hitherto unknown might be, which 



O ' 



governed the planets and our own Earth in their 

 movements. It was about this time that the Royal 

 Society was founded in London, and a stimulus was 

 thus given to investigation and to experiment. The 

 third law of Kepler, which states that in all the 

 planetary orbits the square of the periodic time of 

 revolution is in a constant proportion to the cube 

 of the mean distance, suggested the existence of 

 another law, not yet discovered, a law of attraction, 

 on which this itself depended. Among the as- 

 tronomers of that day three names deserve special 

 mention, Wren, Hooke, and Hal ley, because each of 

 them guessed with some accuracy at the true doctrine 

 as it is now known to be that the planets 

 are attracted to the Sun by a force which acts in- 

 versely as the square of the distance. Hooke, in 

 particular, deserves the credit of having applied 

 this law to the path of a projectile, under certain 

 circumstances, as well as to the planetary orbits ; 

 but though he thus lighted upon true conclusions, 

 he appears to have been deficient in mathematical 

 skill, and therefore unable to verify his results. It 

 is, however, only just to the memory of Horrox, who 

 was carried off by an early death, to mention that the 

 true theory of the identity of terrestrial and astro- 



