PIERRE SIMON LAPLACE 145 



If in the zones abandoned by the atmosphere of the Sun, there are 

 any molecules too volatile to be united to each other, or to the planets, 

 they ought in their circulation about this star to exhibit all the ap- 

 pearances of the zodiacal light, without opposing any sensible re- 

 sistance to the different bodies of the planetary system, both on ac- 

 count of their great rarity and also because their motion is very nearly 

 the same as that of the planets which they meet. 



An attentive examination of all the circumstances of this system 

 renders our hypothesis still more probable. The primitive fluidity of 

 the planets is clearly indicated by the compression of their figure, con- 

 formably to the laws of the mutual attraction of their molecules ; it 

 is moreover demonstrated by the regular diminution of gravity, as 

 we proceed from the equator to the poles. This state of primitive 

 fluidity to which we are conducted by astronomical phenomena, is 

 also apparent from those which natural history points out. But in 

 order fully to estimate them, we should take into account the immense 

 variety of combinations formed by all the terrestial substances which 

 were mixed together in a state of vapour, when the depression of 

 their temperature enabled their elements to unite; it is necessary 

 likewise to consider the wonderful changes which this depression ought 

 to cause in the interior and at the surface of the earth, in all its 

 productions, in the constitution and pressure of the atmosphere, in 

 the ocean, and in all substances which it held in a state of solution. 

 Finally, we should take into account the sudden changes, such as great 

 volcanic eruptions, which m^ust at different epochs have deranged the 

 regularity of these changes. Geology, thus studied under the point 

 of view which connects it with astronomy, may, with respect to several 

 objects, acquire both precision and certainty. 



One of the most remarkable phenomena of the solar system is the 

 rigorous equality which is observed to subsist between the angular 

 motions of rotation and revolution of each satellite. It is infinity to 

 unity that this is not the effect of hazard. The theory of universal 

 gravitation makes infinity to disappear from this improbability, by 

 shewing that it is sufficient for the existence of this phenomenon, that 

 at the commencement these motions did not differ much. Then, 

 the attraction of the planet would establish between them a perfect 

 equality ; but at the same time it has given rise to a periodic oscilla- 

 tion in the axis of the satellite directed to the planet, of which oscilla- 



