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Thence he proceeded to Elephantina and Syene, two islands, formerly frontiers of the Roman empire, which is now widened to the Red Sea. Whilst Germanicus spent this summer in several provinces, Drusus was sowing feuds amongst the Germans, and thence reaped no light renown; and, as the power of Maroboduus was already broken, he engaged them to persist and complete his ruin.
This was done by measuring on the ground the distance between Syene, a city exactly under the tropic, and Alexandria situated on the same meridian. The distance was found to be five thousand stadia. The meridional distance of the sun from the zenith of Alexandria, he estimated to be 7 degrees 12', or a fiftieth part of the circumference of the meridian.
He says, that the change in the obliquity of the ecliptic in this interval was only 2' 36". "A gnomon, therefore, at Syene, of the length of twelve inches, if it cast no shadow on the day of the solstice in the time of Eratosthenes, should have cast a shadow in the time of Plutarch of the length only of 9/1000th, or not quite 1/100th part of an inch.
The temple of Isis among the palm groves in Philæ, a rocky island in the Nile near the cataracts of Syênê, was begun in this reign, though not finished till some reigns later. It is still the wonder of travellers, and by its size and style proves the wealth and good taste of the priests.
They would gladly have pushed on without delay, for although the Egyptian authority extended further up the river, Syene was the last town where the governor would concern himself with the affairs of Egypt, or where fugitives from justice were likely to be arrested.
Beside these, there were several bodies of native militia, from Abydos, Syênê, and other cities, which were not formed into legions. The Egyptian cavalry were a first and second Egyptian troop, several bodies of native archers mounted, three troops on dromedaries, and a body of Diocletian's third legion promoted to the cavalry.
Eratosthenes improved upon this. He observed that at the summer solstice at Syene, at noon, the sun cast no shadow, while at the same moment the upright gnomon at Alexandria cast a shadow corresponding to an angle between the gnomon and the sun's rays of 1/50th of four right angles.
Its result, however, could not have been absolutely accurate, because, while correct in principle, its data were defective. In point of fact Syene did not lie precisely on the same meridian as Alexandria, neither did it lie exactly on the tropic. Here, then, are two elements of inaccuracy.
The importance of this discovery will justify us in entering on some details respecting the means which this philosopher employed, and the result which he obtained. It is uncertain whether the well at Syene, in Upper Egypt, which he used for this purpose, was dug by his directions, or existed previously.
Under the portico opened the doors of the apartments, into which the light came softened by the shade of the galleries. In the centre of the court sparkled in the sunshine a pool of water, edged with a margin of Syêné granite.
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