Lecture I--The Ptolemaic Era

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Eratosthenes, again, contributed his mite to the treasure of human science--his one mite; and yet by that he is better known than by all the volumes which he seems to have poured out, on Ethics, Chronology, Criticism on the Old Attic Comedy, and what not, spun out of his weary brain during a long life of research and meditation. They have all perished,--like ninety-nine hundredths of the labours of that great literary age; and perhaps the world is no poorer for the loss. But one thing, which he attempted on a sound and practical philosophic method, stands, and will stand for ever. And after all, is not that enough to have lived for? to have found out one true thing, and, therefore, one imperishable thing, in one's life? If each one of us could but say when he died: "This one thing I have found out; this one thing I have proved to be possible; this one eternal fact I have rescued from Hela, the realm of the formless and unknown," how rich one such generation might make the world for ever!

But such is not the appointed method. The finders are few and far between, because the true seekers are few and far between; and a whole generation has often nothing to show for its existence but one solitary gem which some one man--often unnoticed in his time--has picked up for them, and so given them "a local habitation and a name."

Eratosthenes had heard that in Syene, in Upper Egypt, deep wells were enlightened to the bottom on the day of the summer solstice, and that vertical objects cast no shadows.

He had before suggested, as is supposed, to Ptolemy Euergetes, to make him the two great copper armillae, or circles for determining the equinox, which stood for centuries in "that which is called the Square Porch"--probably somewhere in the Museum. By these he had calculated the obliquity of the ecliptic, closely enough to serve for a thousand years after. That was one work done. But what had the Syene shadows to do with that? Syene must be under that ecliptic. On the edge of it. In short, just under the tropic. Now he had ascertained exactly the latitude of one place on the earth's surface. He had his known point from whence to start on a world-journey, and he would use it; he would calculate the circumference of the earth--and he did it. By observations made at Alexandria, he ascertained its latitude compared with that of Syene; and so ascertained what proportion to the whole circumference was borne by the 5000 stadia between Alexandria and Syene. He fell into an error, by supposing Alexandria and Syene to be under the same meridians of longitude: but that did not prevent his arriving at a fair rough result of 252,000 stadia--31,500 Roman miles; considerably too much; but still, before him, I suppose, none knew whether it was 10,000, or 10,000,000. The right method having once been found, nothing remained but to employ it more accurately.