The preceding may give a sufficient idea of the sort of foundation which Descartes builds upon, and his manner of raising the structure. He proceeds through what he supposes to be explanations of all the phenomena of light, of the formation of planets and comets, and of all the varieties of conformation which are seen in the solar system. Why comets have tails and planets none; how the primary particles of other vortices find their way into ours, so that we can see the fixed stars; how the planet/ obtained their first motions of projection : how the spots on the PIM are formed, and so on, are all explained by the powers of the two specie,' of particles : an hypothesis on their nature being always ready when wanted. A reader who has looked into this book of Descartes's ' l'rinclpia' begins to understand two things better than before : first, the satire on philosophical explana tions contained at the end of :Value's ' 31alade Imaginairc; written a few years after the death of Descartes; next, the declaration of Newton, hypotheses nom finger As the hypothesis of vortices is um/ally represented, it has a certain reasonableness of appearance, which no doubt makes many wonder why it should be so universally contemned. If a fluid mass were whirled round the sun, it would carry the planets with it : and the supposition of minor vortices, ono round each. planet which has a satellite, is perfectly consistent with the laws of hydrostatics. When Newton proposed to refute the system of De cartes, he was obliged to have recourse to numerical considerations : he could not bet admit that a planet, in one of Descartes's vortices, would have an orbit ; but he showed, from the nature of fluid motion, that it could not have the orbit which, from the time of Kepler, it was known to have. The quality of a phenomenon is known before its amount is measured ; and it is natural to expect, in the history of philosophy, that explaita tions which serve to account for the nature of a phenomenon, but are irreconcilable with its amount, should precede those which are drawn from consideration of both. The possibility of the planetary motions
finding their proximate cause in the rotation of a fluid Imes which fills the solar system, is a thing which did suggest itself, and ought to have suggested itself, to the inquirers of the time which elapsed be tween Copernicus and Newton. Descartes says expressly, putandum cot, non tantum Solis et Fixarum, sod totius edam cmli inateriain fluidam ease : quod jam rulyo °nines astronomi eoncedunt, videut phmnomena planetarum viz aliter posse explicari." No mechanical difficulty stood in the way in their time; and those who had seen particles of dust whirled about by the air would have no difficulty in Imagining the hypothesis of a vortex. Now wo find this fault with the common notion of Descartee'e system : the disparagement which belongs to it as a whole—to the primary and secondary particles which, though obtained from the same original particles, yet have different laws of motion, and to the gratuitous deduction of everything from this fancy—is conveyed to their readers by writers who only present the most rational extract which could be made, namely, the idea of a vortex. This is the sort of syllogism on which such writers proceed : "Descartes's system is ridiculous; all I know of that system is its vortices; therefore 1 must laugh at the vortices." Yet not only was Newton obliged to have recourse to his most powerful weapons to refute these vortices, but it is not at all a settled point that his refuta tion is sound ; that is, his mathematical refutation. His remark that comets could not find their way through the vortex is much more to the purpose, though Descartes has his way out of this difficulty, as out of every other.