808 EPOCHS IN THE HISTORY OF THE CONTEMPLATION OP 



justice on " the deep consideration of Nature by Aiiaxagoras, 

 from whom we hear, not without astonishment, that the 

 moon ( 467 ) if its force of rotation ceased would fall to the earth 

 as a stone discharged from a sling." I have already, in my first 

 volume, when treating of the fall of aerolites, ( 468 ) noticed 

 similar expressions of the Clazomenian, and of Diogenes of 

 Apollonia, respecting the tf cessation or interruption of the 

 force of rotation." Of the attracting force which the centre 

 of the earth exerts on all heavy masses removed from it, 

 Plato had a clearer idea than Aristotle ; who was, indeed, 

 like Hipparchus, acquainted with the acceleration of bodies 

 in falling, but who did not correctly apprehend its cause. 

 In Plato, and according to Democritus, attraction is 

 limited to bodies which have affinity with each other ; or in 

 other words, to the tending together of homogeneous ele- 

 mentary substances. ( 469 ) But at a later period, probably in 

 the 6th century, the Alexandrian John Philoponus, a pupil of 

 Ammonius Ilermese, ascribes the movements of cosmical bo- 

 dies to a primitive impulse, and combines with this idea that 

 of the fall of bodies, or the tendency of all substances, heavy 

 or light, to come to the ground. ( 47 ) But the idea which 

 Copernicus divined, and which Kepler enunciated more 

 clearly in his fine work " de Stella Martis," even applying 

 it ( 471 ) to the ebb and flood of the Ocean, we find invested 

 with new life, and rendered more fruitful (1666 and 1674) 

 by the sagacity of the ingenious Eobert Hooke. The 

 Newtonian theory of gravitation came next, and presented 

 the grand means of transforming the whole of physical 

 astronomy into a system of celestial mechanics. ( 472 ) 



Copernicus, as we perceive not only from his dedication 

 to the Pope, but also from several passages in the book 



