162 LAPLACE. 



at considerable distances from each other, revolving all around the cen- 

 tral sun iu the direction of the original movement of the nebula; how 

 these planets ought also to Lave movements of ro<^ation operating in 

 similar directions; how, finally, the satellites, when any of such are 

 formed, cannot fail to revolve ui^on their axes and around their respect- 

 ive primaries, iu the direction of rotation of the planets and of their 

 movement of revolution around the sun. 



We have just found, conformably to the principles of mechanics, the 

 forces with which the particles of the nebula were originally endued, in 

 the movements of rotation and revolution of the com^mct and distinct 

 masses which these particles have brought into existence by their con- 

 densation. But we have thereby achieved only a single step. The 

 primitive movement of rotation of the nebula is not connected with the 

 simple attraction of the particles. This movement seems to imply the 

 action of a primordial imi)ulsive force. 



Laplace is far from adopting, iu this respect, the almost universal 

 opinion of philosophers and mathematicians. He does not suppose that 

 the mutual attractions of originally immovable bodies must ultimately 

 reduce all the bodies to a state of rest around their common center of 

 gravity. He maintains, on the contrary, that three bodies, in a state of 

 rest, two of which have a much greater mass than the third, would con- 

 centrate into a single mass only in certain exceptional cases. In gen- 

 eral, the two most considerable bodies would unite together, while the 

 third would revolve around their common center of gravity. Attraction 

 would thus become the cause of a sort of movement which would seem 

 to be explicable solely by an impulsive force. 



It might be supposed, indeed, that iu explaining this part of his sys- 

 tem Lajilace had before his eyes the words which Kousseau has placed 

 in the mouth of the Yicar of Savoy, and that he wished to refute them. 

 " Xewton has discovered the law of attraction," says the author of 

 Emilc; " but attraction alone would soon reduce the universe to an im- 

 movable mass. With this law we must combine a projectile force in 

 order to make the celestial bodies describe curve-lines. Let Descartes 

 reveal to us the physical law which causes his vortices to revolve ; and 

 let Newton show us the hand which launched the planets along the 

 tangents of their orbits." 



According to the cosmogonic ideas of Laplace, comets did not origi- 

 nally form part of the solar system. They are not formed at the expense 

 of the matter of the immense solar nebula. We must consider them as 

 small wandering nebula, which the attractive force of the sun has 

 caused to deviate from their original route. Such of those comets as 

 penetrated into the great nebula at the epoch of condensation and of 

 the formation of planets fell into the sun, describing spiral curves, and 

 must by their action have caused the planetary' orbits to deviate more 

 9r less from the plane of the solar equator, with which they would 

 otherwise have exactly coincided. 



