A HISTORY OF SCIENCE 



reference to the plane of the sun's orbit that is to say, 

 the plane of the ecliptic no longer seemed to cut the 

 sphere of the fixed stars at precisely the point where 

 the two coincided in former centuries. The plane of 

 the ecliptic must therefore be conceived as slowly re- 

 volving in such a way as gradually to circumnavigate 

 the heavens. This important phenomenon is described 

 as the precession of the equinoxes. 



It is much in question whether this phenomenon was 

 not known to the ancient Egyptian astronomers ; but 

 in any event, Hipparchus is to be credited with dem- 

 onstrating the fact and making it known to the West- 

 ern world. A further service was rendered theoretical 

 astronomy by Hipparchus through his invention of the 

 planosphere, an instrument for the representation of 

 the mechanism of the heavens. His computations of 

 the properties of the spheres led him also to what was 

 virtually a discovery of the method of trigonometry, 

 giving him, therefore, a high position in the field of 

 mathematics. All in all, then, Hipparchus is a most 

 heroic figure. He may well be considered the greatest 

 star-gazer of antiquity, though he cannot, without in- 

 justice to his great precursors, be allowed the title 

 which is sometimes given him of " father of systematic 

 astronomy." 



CTESIBIUS AND HERO: MAGICIANS OP ALEXANDRIA 



Just about the time when Hipparchus was working 

 out at Rhodes his puzzles of celestial mechanics, there 

 was a man in Alexandria who was exercising a strangely 

 inventive genius over mechanical problems of another 

 sort ; a man who, following the example set by Archi- 



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