[140] Dr. WHEWELL, ON THE TRANSFORMATION OF HYPOTHESES 



sort of verbal consistency; till the original hypothesis becomes inextricably confused, or breaks 

 down under the weight of the auxiliary hypotheses thus fastened upon it, in order to make it 

 consistent with the facts. 



This often-occurring course of events might be illustrated from the history of the astro- 

 nomical theory of epicycles and eccentrics, as is well known. But my present purpose^ is to 

 give one or two brief illustrations of a somewhat similar tendency from other parts of scien- 

 tific history ; and in the first place, from that part which has already been referred to, the 

 battle of the Cartesian and Newtonian systems. 



3. The part of the Cartesian system of vortices which is most familiarly known to general 

 readers is the explanation of the motions of the planets by supposing them carried round the 

 sun by a kind of whirlpool of fluid matter in which they are immersed : and the explanation 

 of the motions of the satellites round their primaries by similar subordinate whirlpools, turning 

 round the primary, and carried, along with it, by the primary vortex. But it should be 

 borne in mind that a part of the Cartesian hypothesis which was considered quite as important 

 as the cosmical explanation, was the explanation which it was held to afford of terrestrial 

 gravity. Terrestrial gravity was asserted to arise from the motion of the vortex of subtile 

 matter which revolved round the earth's axis and filled the surrounding space. It was main- 

 tained that by the rotation of such a vortex, the particles of the subtile matter would exert a 

 centrifugal force, and by virtue of that force, tend to recede from the center : and it was held 

 that all bodies which were near the earth, and therefore immersed in the vortex, would be 

 pressed towards the center by the effort of the subtile matter to recede from the center*. 



These two assumed effects of the Cartesian vortices — to carry bodies in their stream, as 

 straws are carried round by a whirlpool, and to press bodies to the center by the centrifugal 

 effort of the whirling matter — must be considered separately, because they were modified sepa- 

 rately, as the progress of discussion drove the Cartesians from point to point. The former 

 effect indeed, the dragging force of the vortex, as we may call it, would not bear working out 

 on mechanical principles at all ; for as soon as the law of motion was acknowledged (which 

 Descartes himself was one of the loudest in proclaiming), that a body in motion keeps all the 

 motion which it has, and receives in addition all that is impressed upon it ; — as soon, in short, 

 as philosophers rejected the notion of an inertness in matter which constantly retards its move- 

 ments, — it was plain that a planet perpetually dragged. onwards in its orbit by a fluid moving 

 quicker than itself, must be perpetually accelerated; and therefore could not follow those 

 constantly-recurring cycles of quicker and slower motion which the planets exhibit to us. 



The Cartesian mathematicians, then, left untouched the calculation of the progressive 

 motion of the planets ; and, clinging to the assumption that a vortex would produce a ten- 

 dency of bodies to the center, made various successive efforts to construct their vortices in 

 such a manner that the centripetal forces produced by them should coincide with those which 

 the phenomena required, and therefore of course, in the end, with those which the Newtonian 

 theory asserted. 



In truth, the Cartesian vortex was a bad piece of machinery for producing a central. force : 

 from the first, objections were made to the sufficiency of its mechanism, and most of these 



* Cartes. Princip. iv. 23. 



