8 NEWTON S PR1NCIPIA. 



, 



accompanied his statement with so little proof beyond the 

 agreement with the phenomena, which the Ptolemaic 

 hypothesis could equally boast of*, that for more than 

 half a century afterwards it had no general acceptance. 

 Bacon himself rejecting it ; when Galileo, by his telescopic 

 discoveries, especially of the phases of Venus and the sa 

 tellites of Jupiter, and by his yet more important dis 

 coveries in the laws of motion, may be said first to have 

 proved the truth of the Copernican system. Afterwards 

 the satellites of Saturn, added to Kepler s observation of 

 Mercury s transit over the sun, afforded most important 

 confirmation. The great discoveries of this eminent man 

 followed close after those of Galileo: First) the motions 

 of the planets were found to be in ellipses with the sun in 

 one focus ; secondly, lines drawn to the sun from them were 

 found to describe areas proportional to the times of their 

 revolution ; and, thirdly, the relation was established be 

 tween the squares of those times and the cubes of the 

 distances of the bodies from the focus. 



How near this brought scientific men to the cause 

 or law of the whole is manifest, especially when we 

 regard the connexion thus established between the re 

 volving bodies and the great luminary in the centre. 

 Although Kepler himself erroneously mingled with the 

 influence which this law of motion led him to ascribe to 

 the sun, a transverse force which he deemed necessary to 

 maintain the projectile motion of the planets round the 

 centre ; yet others formed more correct ideas of the matter. 

 It seems to have been Huygens, who, fourteen years 

 before the &quot;Principia&quot; was published, first showed the 

 true nature of centrifugal forces. Several years earlier, 

 however, Borelli, in treating of the motion of Jupiter s 



* It is certain that its greater simplicity was, before Galileo s time, 

 the only argument in favour of the Copernican theory against the Ptolemaan. 



