SCIENCE AND PROGRESS 



Egypt, lay on the tropic that is to say, on the day 

 of the summer solstice the gnomon cast no shadow, 

 the rays of the sun shone to the bottom of a deep 

 well. On that day the sun 

 was in the zenith from 

 Syene. Syene, he also com- 

 puted, was on the same 

 meridian as Alexandria . So 

 on this day he measured 

 the arc included between 

 the sun and a point in the 

 zenith from Alexandria. 

 The figure shows his idea. 



The arc A B, described by 

 the angle a (it is exagger- 

 ated in the drawing) is, to 

 all intents and purposes, the 

 same as the arc D C. Era- 

 tosthenes estimated these 

 at 7 12', or -g^ the circum- 

 ference of a circle. He had 

 then but to find the dis- 

 tance, D C, between Syene and Alexandria. This 

 the bematists, or surveyors of the Ptolemies, had 

 set at about 5000 stadia. He computed the cir- 

 cumference of the earth, therefore, at fifty times 

 this, or 250,000 stadia roughly, 30,000 miles. 

 r ;iin, the result was vitiated, though in no 

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