IN THE HISTORY OP SCIENCE. [141] 



objections were very unsatisfactorily answered, even granting the additional machinery which 

 its defenders demanded. One formidable objection was soon started, and continued to the 

 last to be the torment of the Cartesians. If terrestrial gravity, it was urged, arise from the 

 centrifugal force of a vortex which revolves about the earth's axis, terrestrial gravity ought 

 to act in planes perpendicular to the earth's axis, instead of tending to the earth's center. 

 This objection was taken by James Bernoulli*, and by Huyghens f not long after the publica- 

 tion of Descartes's Principia. Huyghens (who adopted the theory of vortices with modifica- 

 tions of his own) supposes that there are particles of the fluid matter which move about the 

 earth in every possible direction, within the spherical space which includes terrestrial objects ; 

 and that the greater part of these motions being in spherical surfaces concentric with the 

 earth, produces a tendency towards the earth's center. 



This was a procedure tolerably arbitrary, but it was the best which could be done. 

 Saurin, a little laterj, gave nearly the same solution of this difficulty. The solution, identify- 

 ing a vortex of some kind with a central force, made the hypothesis of vortices applicable 

 wherever central forces existed ; but then, in return, it deprived the image of a vortex of all 

 that clearness and simplicity which had been its first great recommendation. 



But still there remained difficulties not less formidable. According to this explanation of 

 gravity, since the tendency of bodies to the earth's center arose from the superior centrifugal 

 force of the whirling matter which pushed them inward as water pushes a light body upward, 

 bodies ought to tend more strongly to the center in proportion as they are less dense. The 

 rarest bodies should be the heaviest ; contrary to what we find. 



Descartes's original solution of this difficulty has a certain degree of ingenuity. Accord- 

 ing to him (Princip. iv. 23) a terrestrial body consists of particles of the third element, and 

 the more it has of such particles, the more it excludes the parts of the celestial matter, from 

 the revolution of which matter gravity arises ; and therefore the denser is the terrestrial body, 

 and the heavier it will be. 



But though this might satisfy him, it could not satisfy the mathematicians who followed 

 him, and tried to reduce his system to calculation on mechanical principles. For how could 

 they do this, if the celestial matter, by the operation of which the phenomena of force and 

 motion were produced, was so entirely different from ordinary matter, which alone had supplied 

 men with experimental illustrations of mechanical principles ? In order that the celestial 

 matter, by its whirling, might produce the gravity of heavy bodies, it was mechanically neces- 

 sary that it must be very dense ; and dense in the ordinary sense of the term ; for it was by 

 regarding density in the ordinary sense of the term that the mechanical necessity had been 

 established. 



The Cartesians tried to escape this result (Huyghens, Pesanteur, p. l6l, and John Ber- 

 noulli, Nouvelles Pensees, Art. 31) by saying that there were two meanings of density and 

 rarity ; that some fluids might be rare by having their particles far asunder, others, by having 

 their particles very small though in contact. But it is difficult to think that they could, as 



* Jac. Bernoulli, Nouvelles Pensees sur le Systeme de 

 M. Descartes, Op. T. I. p. 239 (1686). 



+ De la Came de la Pesanteur (1689), p. 135. 



+ Journal des Savans, 1703. Mem. Acad. Par. 1709. 



Bulfinger, in 1726 (Acad. Petrop.), conceived that by making 

 a sphere revolve at the same time about two axes at right 

 angles to each other, every particle would describe a great 

 circle ; but this is not so. 



