A HISTORY OF SCIENCE 



to it, is the slight eccentricity of the planetary orbits. 

 We know, by the theory of central forces, that if a body 

 moves in a closed orbit around the sun and touches it, 

 it also always comes back to that point at every revo- 

 lution ; whence it follows that if the planets were origi- 

 nally detached from the sun, they would touch it at 

 each return towards it, and their orbits, far from being 

 circular, would be very eccentric. It is true that a mass 

 of matter driven from the sun cannot be exactly com- 

 pared to a globe which touches its surface, for the im- 

 pulse which the particles of this mass receive from one 

 another and the reciprocal attractions which they ex- 

 ert among themselves, could, in changing the direction 

 of their movements, remove their perihelions from the 

 sun; but their orbits would be always most eccentric, 

 or at least they would not have slight eccentricities 

 except by the most extraordinary chance. Thus we 

 cannot see, according to the hypothesis of Buffon, 

 why the orbits of more than a hundred comets already 

 observed are so elliptical. This hypothesis is there- 

 fore very far from satisfying the preceding phenomena. 

 Let us see if it is possible to trace them back to their 

 true cause. 



"Whatever may be its ultimate nature, seeing that it 

 has caused or modified the movements of the planets, 

 it is necessary that this cause should embrace every 

 body, and, in view of the enormous distances which 

 separate them, it could only have been a fluid of im- 

 mense extent. In order to have given them an almost 

 circular movement in the same direction around the 

 sun, it is necessary that this fluid should have envel- 

 oped the sun as in an atmosphere. The consideration 



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