xii.] THE INDUCTIVE OR INVERSE METHOD. 249 



urged by some common force from which the sun is 

 exempt. It may be said that the. rotation of the earth is 

 proved in like manner, because it is immensely more pro- 

 bable that one body would revolve than that the sun, 

 moon, planets, comets, and the whole of the stars of the 

 heavens should be whirled round the earth daily, with a 

 uniform motion superadded to their own peculiar motions. 

 This appears to be mainly the reason which led Gilbert, 

 one of the earliest English Copernicans, and in every way 

 an admirable physicist, to admit the rotation of the earth, 

 while Francis. Bacon denied it. 



In contemplating the planetary system, we are struck 

 with the similarity in direction of nearly all its movements. 

 Newton remarked upon the regularity and uniformity of 

 these motions, and contrasted them with the eccentricity 

 and irregularity of the cometary orbits. 1 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 satellites rotating in the same direction, 

 with some exceptions on the verge of the system. In the 

 time of Laplace eleven planets were known, and the direc- 

 tions 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, 2 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 



Principia, bk. ii. General scholium. 



8 Essai Philosophiqiie, p. 55. Laplace appears to count the rings of 

 Siturn as giving two independe: 



ient movements. 



