ASTRONOMY OF EIGHTEENTH CENTURY. 269 



volved in 10 hours 32 minutes 15 seconds. In February, 1789, La- 

 place published a paper proving further that the theory of gravitation 

 required that Saturn itself should revolve about its axis. In November 

 of the same year Herschel announced that he had observed the ro- 

 tation of the planet. 



The name of Laplace must ever be associated with one of the 

 grandest speculations of modern philosophers. The chief grounds 

 upon which is founded the bold conjecture called the Nebular -Hypothesis 

 may first be briefly stated. All the planets revolve round the sun 

 from west to east, and they revolve in planes that are but slightly in- 

 clined to each other. All the satellites (those of Uranus excepted) 

 revolve round their primaries from west to east. The sun, and all the 

 planets and satellites whose rotation has been made out, revolve on 

 axes from west to east. A calculation which has been made by aid 

 of the doctrine of probabilities shows that the chances against this 

 coincidence having been the result of mere accident are many thou- 

 sands of millions to one. In Laplace's hypothesis these phenomena 

 find their explanation in the circumstances through which, as he con- 

 ceived, the solar system was formed. He supposes that at some re- 

 mote epoch our sun was the centre of an immense nebula extending 

 far beyond the region in which the most distant planet of our system 

 now revolves. But, at the period contemplated by Laplace, no planet 

 was yet in existence. The materials of which the sun and the planets 

 were afterwards formed existed, at first, in a state of uniform diffusion. 

 This diffused matter was at so very high a temperature, that all its con- 

 stituents may be conceived to have existed in the state of gas. As the 

 gaseous matter cooled, some of its particles, condensing into liquids or 

 solids, might collect by gravitative attraction at the centre of the nebu- 

 lous mass, and thus a centre of attraction would be established, towards 

 which the gradually condensing matter would be drawn, and add its 

 effect to the centripetal tendency. Any circumstance whatever which 

 could cause one part of the nebulous mass to differ from another (such, 

 for example, as a radiation of heat greater on one side than on the 

 other) would cause rotatory movement to be set up, and this move- 

 ment would increase in rapidity. There would at length arrive a 

 period in which the velocity of rotation of the outermost parts would 

 become great enough to overcome the attraction of the central nucleus, 

 and as they condensed these parts would separate into a ring, which 

 would continue to revolve with the velocity it possessed at the moment 

 of its separation from the rest. In the planet Saturn we witness the 

 results of such a separation, and it is possible by an experiment on a 

 small scale perfectly to imitate the effects here spoken of. Similar 

 separations of revolving rings we may conceive to have successively 

 taken place ; and these rings while all revolving in nearly the same 

 plane, would be endowed with different velocities ; those nearer to the 

 central mass, corresponding with greater condensation and more rapid 



