THE LAW OF GRAVITATION. 481 



that the orbits of the phmets were not circuhir, as would seem to be 

 required by his hypothesis, but elliptical, the sun being at one of the 

 foci ; also, the ever- varying radius vector always passed over equal spaces 

 in equal times, hence the motion of the planet in its orbit was not 

 uniform, as his hypothesis would require, but ever- varying ; and this 

 variation too was evidently not fortuitous or uncertain but increased or 

 diminished in the exact ratio to the varying distance of the planet from 

 the sun required by the law just mentioned, of equal spaces in equal 

 times. These facts, apparently so inconsistent with his hypothesis, 

 Kepler accounted for by supposing that each of the planets was ani- 

 mated by an intelligent spirit, by whose agency the motion of the planet 

 was, in part at least, determined. We have seen an allusion to this 

 theory in the quotation above given, on the subject of gravity. He re- 

 garded each of the heavenly bodies, and the earth as one of them, as 

 literally a huge animal, and in one of his works describes with some 

 minuteness the habits of that particular animal on whose body it is our 

 lot to live. 



Kepler's hypothesis of an emanation from the sun of a corporeal 

 nature by whose revolution the planets were propelled in their orbits 

 was received with more or less favor for a time, but was soon super- 

 seded by another memorable hypothesis no more reasonable or plausible 

 and yet from the time of its announcement until the publication of 

 'The Principia' demonstrated its fallacy, it was adopted by most men 

 of science and may be said to have been the accepted theory on the 

 subject. We refer to the Vortices of Descartes. This distinguished 

 jDhilosopher, born 1596, rose to eminence about the time of Kepler's 

 death, which occurred in 1630. By the force of his genius, illustrated 

 not only by that achievement for which his name will ever be held in 

 honored remembrance — the invention of analytical geometry — but by 

 the abundance and ability of his labors in every department of science 

 and philosophy, Descartes, for more than half a century, occupied a 

 position in the learned world scarcely inferior to that which for ages 

 preceding had been held by Aristotle. 



As to the cause of planetary motion, Descartes assumed the ex- 

 istence, throughout the limits of our system, of a subtle transparent 

 fluid in ceaseless revolution about the sun as its center, and that the 

 planets floated in this fluid and were consequently carried round by the 

 sun in its motion, just as in a whirlpool a cork or floating body is car- 

 ried round by the motion of the water. To account for the difference 

 in the times of revolution of different planets, he supposed that the 

 velocity of the revolution of the fluid, at different distances from the 

 sun was different. To account for the revolution of the satellites of 

 the planets, he assumed that in the nighborhood of each planet this 

 fluid revolved about the planet as a center. To this purely fanciful 



VOL. LIX. — 34 



