320 A Short History of Astronomy [CH. xi. 



account for the observed motions of the bodies of the solar 

 system with a tolerable degree of accuracy by means of the 

 law of gravitation. 



Newton's problem ( 228) was therefore approximately 

 solved, and the agreement between theory and observation 

 was in most cases close enough for the practical purpose 

 of predicting for a moderate time the places of the various 

 celestial bodies. The outstanding discrepancies between 

 theory and observation were for the most part so small as 

 compared with those that had already been removed as to 

 leave an almost universal conviction that they were capable 

 of explanation as due to errors of observation, to want 

 of exactness in calculation, or to some similar cause. 



250. Outside the circle of professed astronomers and 

 mathematicians Laplace is best known, not as the author of 

 the Mecanique Celeste^ but as the inventor of the Nebular 

 Hypothesis, 



This famous speculation was published (in 1796) in his 

 popular book the Systhne du Monde already mentioned, 

 and was almost certainly independent of a somewhat similar 

 but less detailed theory which had been suggested by the 

 philosopher Immanuel Kant in 1755. 



Laplace was struck with certain remarkable characteristics 

 of the solar system. The seven planets known to him when 

 he wrote revolved round the sun in the same direction, the 

 fourteen satellites revolved round their primaries still in 

 the same direction,* and such motions of rotation of sun, 

 planets, and satellites about their axes as were known 

 followed the same law. There were thus some 30 or 40 

 motions all in the same direction. If these motions of the 

 several bodies were regarded as the result of chance and 

 were independent of one another, this uniformity would be 

 a coincidence of a most extraordinary character, as unlikely 

 as that a coin when tossed the like number of times should 

 invariably come down with the same face uppermost. 



These motions of rotation and revolution were moreover 

 all in planes but slightly inclined to one another ; and the 



* The motion of the satellites of Uranus (chapter xn., 253, 255) 

 is in the opposite direction. When Laplace first published his theory 

 their motion was doubtful, and he does not appear to have thought 

 it worth while to notice the exception in later editions of his book. 



