The Nebular Hypothesis of Laplace. 313 



process of condensation, and hence must arise a movement of 

 rotation round a centre, and in the same direction as the revo- 

 lution of the whole mass round the sun. Henceforth the 

 materials of a ring abandoned by a nebula become a system 

 analogous to the nebula, but of much smaller dimensions. 

 They have given rise to a new nebula moving round the centre 

 of the original nebula, and in the same direction. This new 

 nebula would, by successive condensations, throw off in suc- 

 cession different rings of nebulous matter, and would at last 

 form a planet turning on its own axis, and revolving in the 

 same direction round the sun. The abandoned rings of matter 

 would behave just as those thrown off by the original nebula, 

 and give birth to the satellites of the planet. Some rings may 

 preserve an exceptional regularity, and thus, as in the case of 

 Saturn's appendages, be able to retain their original form." 



" The matter united at a certain distance from a planet to 

 form a satellite, would elongate itself in the direction of a line 

 joining the planet, just as the action of the moon determines 

 an elongation of the sea in the direction of a line joining the 

 earth and moon. This elongation of a satellite while in its 

 fluid state — much greater than in the comparison just made — 

 would give it a tendency always to turn the same points of its 

 surface towards the centre of the planet ; and thus may be 

 simply explained this remarkable fact with reference to the 

 moon, which, as Herschel thought, was also exhibited by 

 Jupiter's satellites." 



" We see that the hypothesis of Laplace on the origin and 

 formation of our planetary system takes due cognizance of all 

 the peculiarities by which it is characterised. Almost complete 

 coincidences in the planes of the orbits of the planets, slight- 

 ness of the eccentricity of their orbits, identity of direction in 

 the movements of rotation and revolution throughout the 

 system — all is explained in the most natural manner, and in 

 conformity with the laws of mechanics." 



" In this hypothesis the body of a planet formed by conden- 

 sations such as we have described, would at first be a liquid 

 mass affecting the form of a spheroid flattened in the direction 

 of its axis of rotation, and surrounded by an atmosphere the 

 residue of the nebula from which it sprung. This liquid mass, 

 continuing to grow cooler, would solidify little by little over 

 its whole surface. The solid crust thus formed would be 

 gradually altered in shape, and at last would split in several 

 parts, by reason of the progressive diminution of the liquid 

 volume in its interior, as the temperature was lowered. In 

 the mean time if the atmosphere contained a great quantity of 

 aqueous vapour its condensation would form enormous masses 

 of water, which would occasion degradations of the surface, 



