PRELUDE TO THE EPOCH OF NEWTON. 387 



stated as follows : " A physical speculation, in which it is demonstrated 

 that the vehicle of that Virtue which urges the planets, circulates 

 through the spaces of the universe after the manner of a river or whirl- 

 pool (vortex), moving quicker than the planets." I think it will be 

 found, by any one who reads Kepler's phrases concerning the moving 

 force, — the magnetic nature, — the immaterial virtue of the sun, that 

 they convey no distinct conception, except so far as they are inter- 

 preted by the expressions just quoted. A vortex of fluid constantly 

 whirling round the sun, kept in this whirling motion by the rotation 

 of the sun himself, and carrying the pAinets round the sun by its revo- 

 lution, as a whirlpool 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 

 inertia of bodies, and of putting them and keeping them in motion, 

 the only material properties with which he had any thing to do. Kep- 

 ler's physical 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 spe- 

 cies," and by the fickleness and variety of his phraseology on the sub- 

 ject, 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 ven- 

 ture 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 differ- 

 ent 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 motions of the stars as a mechanical problem, subject to 

 the same conditions as other mechanical problems, and capable of the 

 same exactness of solution. And there was an especial inconsistency 

 in the circumstance of the Theory of Vortices being put forwards by 

 Descartes, who pretended, or was asserted by his admirers, to have 

 been one of the discoverers of the true Laws of Motion. It certainly 

 shows both great conceit and great shallowness, that he should have 

 proclaimed with much pomp this crude invention of the ante-mechan- 

 ical period, at the time when the best mathematicians of Europe, as 

 Borelli in Italy, Hooke and Vvallis in England, Huyghens in Holland, 



