424 SPECIAL RESULTS IN THE URANOLOGICAL 



tion into one another; and to processes of formation by 

 solidification or by rarefaction. The revolution of the 

 celestial sphere, "which keeps the Earth steadfast in the 

 centre/' is, indeed, already mentioned by Empedocles as an 

 active moving cosmical force. As in these first remote pre- 

 ludes, as it were, to physical theories of an aether, the fiery 

 air, and even fire itself, represent the expansive force of heat, 

 so there was connected with this upper sethereal region the 

 idea of an impetus of revolution tearing away rocky frag- 

 ments from the Earth. Hence Aristotle (Meteorol. i. 339, 

 Bekker) terms the aether " the for-ever-moviug body" as 

 it were, the immediate substratum of motion, and seeks 

 etymological reasons "for this assertion ( 686 ). Therefore we 

 find in the biography of Lysander, " that the intermission of 

 the rotative force causes the fall of heavenly bodies;" as 

 also in another place, where Plutarch is evidently alluding 

 to the opinions of Anaxagoras, or of Diogenes of Apollonia 

 (de Facie in Orbe Lunse, p. 923), he puts forward the state- 

 ment, " that the Moon, if its force of revolution ceased, 

 would fall to the Earth, like the stone in the sling" ( 68 7) . We 

 see in this comparison of the sling, how the idea of a cen- 

 trifugal force of rotation or revolution, which Empedocles 

 recognised in the (apparent) revolution of the celestial 

 sphere, gradually came to have associated with it the corre- 

 sponding, or counterpart, idea of a centripetal force. This 

 force was more clearly and specifically indicated by the most 

 sagacious of all the elucidators of Aristotle, Simplicius (page 

 491, Bekker). He proposes to explain the "non -falling" 

 of the heavenly bodies by the " force of revolution prevail- 

 ing over the proper falling force, or downward traction." 

 These are the first presentiments or anticipations respecting 



