UNIVERS' 

 S^c. 



CHAPTER VI 



ERATOSTHENES AND THE EARLIEST 

 MEASURES OF THE EARTH 



ON a leisurely summer afternoon, as you lie and look up into 

 the cloudless blue, one may agree that it is very charming and 

 very well done ; so much so that one may easily wish all life 

 like that and usually does ; but it hardly seems very inform- 

 ing nor even so very stimulative to investigation. For example, 

 who, without a hint from history, would ever believe that there 

 is in the sky a simple way to compute the circumference, and 

 hence the diameter, the area, and the solid bulk of this globe ! 

 yes, and to measure the obliquity of the ecliptic, or tip of 

 the earth's axle to the plane of its orbit round the sun as well. 



Yet Eratosthenes of Cyrene, the myriad-minded keeper of 

 the Alexandrian Library under Ptolemy Euergetes scholar, 

 poet, critic, geographer, physiographer, mathematician, in- 

 ventor, mechanician, astronomer, wit and sceptic found a way 

 and did it. I have told of this method elsewhere ; as I gaze 

 into the non-committal emptiness above me, the marvel of it 

 lingers still. 



Watching the sun as it nears the height of the arching blue 

 overhead, I note that a post near where I lie casts a slanting 

 shadow, as a sundial does. I reflect that if I go southward 

 the length of this noonday shadow grows less and less, and I 

 know that there is a place where, in the longest day of the year, 

 it will disappear entirely. This is when the sun is sheerly in 

 the zenith, and has reached its summer station, where it will 

 seem to stand still for a few days, then again recede ; the sol- 

 stitium of the Latins. If I can find out just how far to the 

 south this point lies, it is evident that I hold in my hands the 

 solution of the problem. I have only to take out my watch, 

 or my jack-knife, hang it on a string and make a plummet of 

 it, and holding it over the farthest edge of the shadow cast 



73 



