124 THE WORLD MACHINE 



Yet clearer is Simplicius, one of the last of the great 

 Alexandrian school, who, in his commentary on Aristotle, attri- 

 butes the " non-falling of heavenly bodies " to the fact that 

 " the rotatory force predominates over the actual falling force 

 or downward attraction." That this was no mere guess, and that 

 Simplicius had perfectly definite ideas of the physical fact of 

 which he is speaking, is evidenced in his illustration from the 

 familiar instance that " water in a phial is not spilled when 

 the -movement of rotation is more rapid than the downward 

 movement of the water." l 



Moreover, it is clear that Aristotle and Hipparchus as well, 

 knew of the acceleration of falling bodies, but neither they nor 

 any one apparently seems to have thought of the simple expedient 

 of measuring the rate of the fall, and hence finding out the rate 

 of the acceleration. 



This is the more notable in an observing genius like 

 Hipparchus, who, unlike Aristotle, had that exact and measuring 

 mind which is essentially the type of the modern creators of 

 experimental science. It was Hipparchus who discovered the 

 precession of the equinoxes, a fact which required not merely 

 the most delicate observations, but a mind intensely alive. He 

 was a mathematician, and he constructed the first table of 

 chords. Apparently it was he who first worked out the 

 wonderful idea of parallax and applied it to the measure of 

 distance. The highly ingenious theorem, determining the 

 distance of the sun from the diameter of the earth's shadow, 

 generally attributed to him, has already been noted. 



It was Hipparchus, probably, of all the ancients, who 

 determined most accurately the distance of the moon, and it 

 was he furthermore who understood the variations in its 

 distance, who first drew a map of its path in space. His ideas 

 on the grandeur of the sun were the same as those of Aristarchus ; 

 and after all it made very little difference in the problem that 

 was before them whether the diameter of the sun was six or 

 eight times that of the earth or a hundred and six or eight 

 times. The physical difficulty of conceiving the revolution of 

 a larger body around the smaller was merely intensified by our 

 modern methods, it was in no material way changed. More- 

 over, Hipparchus must have known of the ideas of Eratosthenes, 

 who made the diameter twenty-seven times and the distance 

 1 Simplicius, Scholia. 



