NEBULAR THEORY. 361 



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 rota- 

 tion and revolution of the compact and distinct masses 

 which these particles have brought into existence by 

 their condensation. 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 impulsive force. 



Laplace is far from adopting, in this respect, the almost 

 universal opinion of philosophers and mathematicians. 

 He does not suppose that the mutual attractions of orig- 

 inally immovable bodies must ultimately reduce all the 

 bodies to a state of rest around their common centre 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 concentrate into a single mass only 

 in certain exceptional cases. In general, the two most 

 considerable bodies would unite together, while the third 

 would revolve around their common centre 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 in explaining this 

 part of his system Laplace had before his eyes the words 

 which Rousseau has placed in the mouth of the vicar of 

 Savoy, and that he wished to refute them : " Newton 

 has discovered the law of attraction," says the author 

 of Emile, "but attraction alone would soon reduce the 

 universe to an immovable 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 



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