The Shadow
Of A Perpendicular Column Of The Height Of 100 Feet Would Have Been 9/10ths
Of An Inch.
" As, however, the ancients do not appear to have constructed
gnomons of such a size, and as gnomons of inferior
Size would have given a
shadow scarcely perceptible, it is probable that Plutarch is mistaken in
his assertion; or, at any rate, that the very small variation which did
take place between his time and that of Eratosthenes (if it were observed
at all) was ascertained by means of the well itself, which would point it
out much more distinctly and accurately than any gnomon the ancients can be
supposed to have used.
We are also indebted to Eratosthenes for the first regular parallel of
latitude, and also for tracing a meridian. His parallel of latitude began
at the Straits of Gibraltar, and passed eastward through Rhodes to the
mountains of India; the intermediate places being carefully set down. His
meridian line passed through Rhodes and Alexandria, as far as Syene and
Meroe. Meroe, on this account, became an object of the greatest interest
and importance to all the succeeding ancient geographers and astronomers,
and they have taken the utmost labour and care to ascertain its latitude
accurately. Strabo informs us, that Eratosthenes constructed a map of the
world; but he does not give such particulars as will enable us to trace the
extent of his geographical knowledge. At the extremity of the world to the
east, bounded by the ocean, Thina was placed in the map of Eratosthenes, in
the parallel of Rhodes; a parallel which passes through the empire of
China, within the Great Wall.
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