28 SCIENCE OF THE GREEKS. PT. T. 



This discovery led him on to try and measure the cir- 

 cumference of the earth. Having found a line straight 

 round the earth from east to west, he knew that if he drew 

 a line at right angles to it, that is exactly north and south, 

 he should have a line which would describe a circle round 

 the earth from pole to pole, as the equator marks a circle 

 round the earth midway between the two poles. This 

 second line he drew from Alexandria, and it passed right 

 through Syene, now called Assouan, one of the southern 

 cities of Egypt, and thus he knew that Alexandria and 

 Syene were on the same meridian of longitude. 



Now he found that at Syene the sun was exactly over- 

 head at midday, at the time of the summer solstice. He 

 knew this by means of a gnomon, or upright pillar (B, Fig. 2), 

 which was used by the Greeks to measure the height of the 

 sun in the sky. At Syene this pillar cast no shadow at noon 

 of the summer solstice, proving that the sun shone straight 

 down upon the top of it ; and this was further proved by the 

 sun shining down to the bottom of a deep well, which it 

 would not do unless it were directly overhead. But at 

 Alexandria the gnomon did cast a shadow, because, as 

 Alexandria was further north and the earth is round, the 

 sun there was not directly overhead. Now, as light 

 travels in straight lines (see p. 21), a line drawn from 

 the extreme point of the shadow cast by the pillar or 

 gnomon up to the top of the pillar itself would, if carried 

 on, run straight into the sun, and thus the angle between 

 this line and the pillar showed at what angle the sun's rays 

 were falling at Alexandria. By measuring this angle, Eratos- 

 thenes found that Alexandria was ^th of the whole circum- 

 ference of the earth north of Syene, where the rays were 

 perpendicular. You can form an idea of this from the 

 accompanying diagram, Fig. 2. 



