CH. iv. ERATOSTHENES PARALLEL OF LATITUDE. 27 



to parts of the body are still used by anatomists, and the 

 school of medicine founded by them in Alexandria was 

 renowned for more than six hundred years. 



Eratosthenes, 276 B.C. We must now turn to the 

 science of geography, which at this time began first really 

 to be studied by a Greek named Eratosthenes, born at 

 Cyrene 276 B.C. Like all men of science of that day, he 

 too came to Alexandria, where the king, Ptolemy Euergetes, 

 made him keeper of the Royal Library. He made a map 

 of all the world that was then known, and described the 

 countries of Europe, Asia, and Libya; but he is chiefly 

 famous for having laid down the first parallel of latitude, and 

 trying to measure the circumference of the earth. He laid 

 down the parallel of latitude in the following manner. He 

 knew that at all places on the equator the day was exactly 

 the same length all the year round, and that the length of 

 the days and nights varied more and more as you went 

 northwards ; therefore he reasoned that, if he could draw a 

 line east and west through a number of places whose longest 

 day was exactly the same length, those places would all be 

 at the same distance from the equator. He began at the 

 Straits of Gibraltar, where the longest day was exactly 14^ 

 hours, and then 'observing all those places whose longest day 

 was also 14^ hours, he drew a line through the south coast 

 of Sicily, across the south of the Peloponnesus, the island of 

 Rhodes, the bay of Issus,and across the Euphrates and Tigris, 

 out to the mountains of India. If you follow this line on 

 a map you will find it is the 36th parallel of north latitude, 

 and that Eratosthenes' observation was perfectly correct. 



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, 



