288 THE PRINCIPLES OF SCIENCE. 



ments. Newton remarked upon the regularity and uni- 

 formity of these motions, and contrasted them with the 

 eccentricity and irregularity of the cometary orbits 8 . 

 Could we, in fact, look down upon the system from the 

 northern side, we should see all the planets moving round 

 from west to east, the satellites moving round their 

 primaries and the sun, planets, and all the satellites 

 rotating in the same direction, with some exceptions on 

 the verge of the system. Now in the time of Laplace 

 eleven planets were known, and the directions of rotation 

 were known for the sun, six planets, the satellites of Jupiter, 

 Saturn's ring, and one of his satellites. Thus there were 

 altogether 43 motions all concurring, namely : 



Orbital motions of eleven planets . .11 

 Orbital motions of eighteen satellites . .18 

 Axial rotations . . . . . -14 



43 



The probability that 43 motions independent of each 

 other would coincide by chance is the 42nd power of ^, so 

 that the odds are about 4,400,000,000,000 to i in favour 

 of some common cause for the uniformity of direction. This, 

 probability, as Laplace observes*, is higher than that of 

 many historical events which we undoubtingly believe. 

 In the present day, the probability is much increased by 

 the discovery of additional planets, and the rotation of 

 other satellites, and it is only slightly weakened by the 

 fact that some of the outlying satellites are exceptional in 

 direction, there being considerable evidence of an acci- 

 dental disturbance in the more distant parts of the 

 system. 



Hardly less remarkable than the uniformity of motion 



8 ' Principia,' bk. ii. General scholium. 



t ' Essai Philosophique,' p. 55. Laplace appears to count the rings of 

 Saturn as giving two independent movements. 



