DIAGRAM TO ILLUSTRATE ERATOSTHENES' MEASUREMENT OK THK 



GLOBE 



FIG. i. AF is a gnomon at Alexandria; SB a gnomon at Syene; IS and JK reprc 

 sent the sun's rays. The an^le actually measured by Eratosthenes is KFA, as de- 

 termined by the shadow cast by the gnomon AF. This anj(le is equal t<> the opp., 

 site angle JFL, which measures tin- sun's distance from the zenith; and which is also 

 equal to the angle AES to determine the si/e of which is the real object of the en- 

 tire measurement. 



Fin. .' shows the form of the vnomon actually employed in antiquity. The 

 hemisphere KA beiiiK marked with a scale, it is obvious that in actual practice 

 Eratosthenes required only to set his gnomon in the sunlight at the proper mo- 

 ment, and read off the answer to his problem at a Rlance. The simplicity of the 

 method makes the result seem all the more wonderful. 



