A HISTORY OF SCIENCE 



tical powers of that force were sufficiently open to his 

 observation. The mere experiment of throwing a 

 stone from a sling would, to an observing mind, be 

 full of suggestiveness. It would be obvious that by 

 whirling the sling about, the stone which it held would 

 be sustained in its circling path about the hand in 

 seeming defiance of the earth's pull, and after the stone 

 had left the sling, it could fly away from the earth to 

 a distance which the most casual observation would 

 prove to be proportionate to the speed of its flight. 

 Extremely rapid motion, then, might project bodies 

 from the earth's surface off into space; a sufficiently 

 rapid whirl would keep them there. Anaxagoras 

 conceived that this was precisely what had occurred. 

 His imagination even carried him a step farther to a 

 conception of a slackening of speed, through which the 

 heavenly bodies would lose their centrifugal force, 

 and, responding to the perpetual pull of gravitation, 

 would fall back to the earth, just as the great stone at 

 ^Egespotomi had been observed to do. 



Here we would seem to have a clear conception of 

 the idea of universal gravitation, and Anaxagoras 

 stands before us as the anticipator of Newton. Were 

 it not for one scientific maxim, we might exalt the old 

 Greek above the greatest of modern natural philoso- 

 phers; but that maxim bids us pause. It is phrased 

 thus, "He discovers who proves." Anaxagoras could 

 not prove; his argument was at best suggestive, not 

 demonstrative. He did not even know the laws 

 which govern falling bodies ; much less could he apply 

 such laws, even had he known them, to sidereal bodies 

 at whose size and distance he could only guess in the 



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