572 COSMOS. 



the Moon would fall to the Earth like a stone in a sling, if its 

 centrifugal force ceased." * Thus we see in this simile, after the 

 assumption of a centrifugal revolution which Empedocles per- 

 ceived in the apparent rotation of the celestial sphere, a centri- 

 petal force gradually arise as an ideal antithesis. This force 

 was specially and most distinctly described by the acute inter- 

 preter of Aristotle, Simplicius (p. 491, Bekker). He explains 

 the non-falling of the celestial bodies thus : " that the centri- 

 fugal force predominates over the proper fall-force, the draw- 

 ing downwards" These are the first conjectures respecting 

 active central forces ; and the Alexandrian, Johannes Philo- 

 ponus, a disciple of Ammonius Hermea, probably of the sixth 

 century, as it were, recognizing also the inertia of matter, 

 first ascribes " the motion of the revolutionary planets to a 

 primitive impulse" which he ingeniously (T)e creatione Jlfundi, 

 lib. i. cap. xii.), unites with the idea of the " fall, a tendency 

 of all heavy and light bodies towards the Earth." We have 

 thus endeavoured to show how a great phenomenon of nature 

 and the earliest purely cosmical explanation of a fall of aerolites 

 essentially contributed in Grecian antiquity, step by step, but 

 certainly not by mathematical reasoning, to develop the germ 

 which, fostered by the intellectual labours of the following 

 centuries, led to Huygens' discovery of the laws of circular 

 motion. 



Commencing from the geometrical relations of the periodic 

 (not sporadic) falling stars, we direct our attention especially 

 to what recent observations as to the divergence or point of 

 departure, of the meteors, and their entirely planetary velocity, 



3 The remarkable passage alluded to in the text in Plutarch, 

 De facie in orbe Lunce, p. 923, is literally translated, 

 " However, the motion of the Moon and the violence of the 

 revolution itself prevents it from falling, just as things placed 

 in a sling are prevented from falling by their motion in a 

 circle." 



