CH. IV. ERATOSTHENES— PARALLEL OF LATITUDE. 27 



things : ist, how strongly it throbs ; 2nd, how quickly; 3rd, 

 whether the beats are regular or irregular. Many of the 

 names which Erasistratus and Herophilus gave to parts of the 

 body are still used by anatomists, and the school of medi- 

 cine founded by them in Alexandria was renowned for more 

 than six hundred years. 



Eratosthenes, 276. — 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 Alex- 

 andria, where the king, Ptolemy Evergetes, 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 his two great works were, laying 

 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 

 14J 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. 



