Comet ary Bodies and Saturrts Rings. 749 



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 rotations 

 of 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 

 4 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 cf superfluous 

 causes/' 



I have already said that when a comet is ejected from a 

 planet opposite to the orbital motion its direction would 

 be retrograde to that of the planet from which it was 

 ejected. 



The orbital velocity of Jupiter being eight miles per 

 second, a body ejected from its interior at a much greater 

 velocity (which I will call the critical velocity) would, by 

 the diminished attraction of the planet, conjointly with the 

 action of sdar gravity, revolve with a retrograde motion in 

 an irregular and much enlarged orbit in accordance with the 

 observations (PI. XIV. fig. 1). And if ejected with a velocity 

 much greater than that necessary to retain it within the 

 sphere of the planet's attraction, the body would move in a 

 separate and elliptical orbit as a comet. 



Considering the comparative minuteness of Jupiter's three 

 outer satellites, which are estimated to be less than thirty 

 miles in diameter, and that the orbits of J.YL and J. VII. are 

 both inclined at 30° to the plane of the ecliptic, and have 

 nearly the same periodic times and distances, these small 

 bodies are hardly entitled to rank as satellites, but may 

 rightly be regarded as planetary ejectamenta. Nevertheless, 

 the discovery of them is of great importance, as furnishing 

 another indirect proof of the planetary origin of comets. 



Phil. Mag. S. 6. Vol. 22. No. 131. JS T ov. 1911. 3 D 



