PIERRE SIMON LAPLACE 139 



isfies the first of the five preceding phenomena * ; for it is evident that 

 all bodies thus formed should move very nearly in the plane which 

 passes through the centre of the Sun, and through the direction of 

 the torrent of matter which has produced them: but the four re- 

 maining phenomena appear to me inexplicable on this supposition. 

 Indeed, the absolute motion of the molecules of a planet ought to be 

 in the same direction as the motion of the centre of gravity; but it 

 by no means follows from this, that the motion of rotation of a 

 planet should be also in the same direction. Thus the Earth may 

 revolve from east to west, and yet the absolute motion of each of 

 its molecules may be directed from west to east. This observation 

 applies also to the revolution of the satellites, of which the direction 

 in the same hypothesis, is not necessarily the same as that of the 

 motion of projection of the planets. 



The small eccentricity of the planetary orbits is a phenomenon, 

 not only difficult to explain on this hypothesis, but altogether in- 

 consistent with it. We know from the theory of central forces, that 

 if a body which moves in a re-entrant orbit about the Sun, passes 

 very near the body of the Sun, it will return constantly to it, at the end 

 of each revolution. Hence it follows that if the planets were origi- 

 nally detached from the Sun, they would touch it, at each return to 

 this star; and their orbits, instead of being nearly circular, would 

 be very eccentric. Indeed it must be admitted that a torrent of 

 matter detached from the Sun, cannot be compared to a globe which 

 just skims by its surface; from the impulsions which the parts of this 

 torrent receive from each other, combined with their mutual at- 

 traction, they may, by changing the direction of their motions, in- 

 crease the distances of their perihelions from the Sun. But their 

 orbits should be extremely eccentric, or at least all the orbits would 

 not be q. p. circular, except by the most extraordinary chance. 

 Finally, no reason can be assigned on the hypothesis of BuflFon, why 

 the orbits of more than one hundred comets, which have been al- 



*viz: "The motions of the planets in the same direction, and very nearly 

 in the same plane; the motions of the satellites in the same direction as those 

 of the planets; the motions of the rotation of these different bodies and 

 also of the sun, in the same direction as their motions of projection, and in 

 planes very little inclined to each other ; the small eccentricity of the orbits 

 of the comets and satellites ; finally, the great eccentricity of the orbits of the 

 comets, their inclinations being at the same time entirely indeterminate." 



