PRELUDE TO THE EPOCH OF NEWTON. 139 



the rotation of the sun himself, and carrying the 

 planets round the sun by its revolution, as a whirl- 

 pool carries straws, could be readily understood ; 

 and though it appears to have been held by Kepler 

 that this current and vortex was immaterial, he 

 ascribes to it the power of overcoming the iner- 

 tia of bodies, and of putting them and keeping 

 them in motion, the only material properties with 

 which he had anything to do. Kepler's physi- 

 cal reasonings, therefore, amount, in fact, to the 

 doctrine of vortices round the central bodies, and 

 are occasionally so stated by himself; though by 

 asserting these vortices to be "an immaterial 

 species," and by the fickleness and variety of his 

 phraseology on the subject, he leaves this theory 

 in some confusion; a proceeding, indeed, which 

 both his want of sound mechanical conceptions, 

 and his busy and inventive fancy, might have led 

 us to expect. Nor, we may venture to say, was it 

 easy for any one at Kepler's time to devise a more 

 plausible theory than the theory of vortices might 

 have been made. It was only with the formation 

 and progress of the science of mechanics that this 

 theory became untenable. 



(Descartes.} But if Kepler might be excused, 

 or indeed admired, for propounding the theory of 

 vortices at his time, the case was different when 

 the laws of motion had been fully developed, and 

 when those who knew the state of mechanical 

 science ought to have learned to consider the mo- 



