Manchester Memoirs, Vol. Iv. (1910), No. I. 11 



The erratic movements and irregular orbits of the 

 three outer Jovian satellites recently discovered have, 

 however, presented a new problem for solution in con- 

 nexion with the nebular theory of the evolution of satellites, 

 as it was found that the orbital motion of the outermost 

 one was in a retrograde direction. 



An attempt has been made to explain the anomaly by 

 assuming that Jupiter at an earlier period of his history 

 performed a semi-revolution about his polar axis, and that 

 all the inner satellites turned over, in like manner, in 

 opposition to the orbital direction of their erratic outer 

 member. 



An insuperable objection to this ingenious hypothesis 

 is the absence of any causal connexion between the 

 assumed inversions of the axial motions of planets, 

 together with their satellites, and their orbital revolutions, 

 and, consequently, leaves untouched the problem of the 

 retrograde orbital motion of a satellite, which it is the 

 precise object of the hypothesis to explain. The fallacy 

 involved in the scheme will at once be apparent when 

 applied to the orbital rotationsof all the planets which are 

 clearly independent of the positions of their axes of 

 rotation in relation to the plane of the ecliptic. And here 

 it may be useful to apply Newton's ' First rule of 

 reasoning in philosophy,' as laid down in the " Principia " 

 that, ' we are to admit no more causes of natural things 

 than such as are both true and sufficient to explain their 

 appearances ; for Nature does nothing in vain, and more 

 is in vain when less will serve, for Nature is pleased with 

 simplicity, and affects not the pomp of superfluous 

 causes.' 



I have already said that when a comet is ejected from 

 a planet opposite to the orbital motion its direction would 



