l88 CELESTIAL MEASURINGS AND WEIGHINGS. 



of which had perished in the lapse of that interval. The 

 Chaldean astronomers at the later epoch above men- 

 tioned are reported to have arrived at an estimate not 

 very remote from the truth. But the first estimate which 

 has been handed down to us, accompanied with a state- 

 ment of the process by which it was arrived at, is that of 

 Eratosthenes (b.c. 250) ; who, measuring the shadow 

 cast by a vertical rod on the day of the summer solstice 

 at Alexandria, and coupling it with the fact reported to 

 him, that at Syene in Upper Egypt on the same da}^, 

 the bottom of a well received the full sunshine, con- 

 cluded a difference of latitude between the two places, 

 equal to one 50th part of the circumference of a meridian. 

 Hence, imagining the two places to lie pretty nearly 

 north and south of one another, he concluded the cir- 

 cumference of the earth to be fifty times the distance 

 from Alexandria to Syene, which on the most probable 

 interpretation of his estimate of that distance in the 

 itinerary measures of the time, affords an approximation 

 to what we now know to be the truth, by no means con- 

 temptible ; falling within about a sixth part of the real 

 circumference, and if the deviation of the mutual direc- 

 tion of the places from the true meridian be allowed for, 

 within much less. 



(13.) Thus we see that with very coarse and rude 

 means of observation and measurement, it is not difiicult 

 to arrive at what may be termed a respectable estimate 

 (as contrasted with a mere guess) of the size of our own 

 globe ; which is our first step outwards into those distant 

 regions which will next engage our attention. We need 



