We cannot mistake this, if we observe how anxiously they have varied the figure of the top, or point of each obelisk; sometimes it is a very sharp one; sometimes a portion of a circle, to try to get rid of the great impediment that perplexed them, the penumbra.
The projection of the pavements, constantly to the northward, so diligently levelled, and made into exact planes by large slabs of granite, most artificially joined, have been so substantially secured, that they might serve for the observation to this day; and it is probable, the position of this city and the well were coeval, the result of intention, and both the works of these first astronomers, immediately after the building of Thebes. If this was the case, we may conclude, that the fact of the sun illuminating the bottom of the well in Eratosthenes's time was a supposed one, from the uniform tradition, that once it had been so, the periodical change of the quantity of the angle, made by the equator and ecliptic, not being then known, and therefore that the quantity of the celestial arch, comprehended between Alexandria and Syene, might be as erroneous from another cause, as the base had been by assuming a wrong distance on the earth, in place of one exactly measured.
There is at Axum an obelisk erected by Ptolemy Evergetes, the very prince who was patron to Eratosthenes, without hieroglyphics, directly facing the south, with its top first cut into a narrow neck, then spread out like a fan in a semicircular form, with a pavement curiously levelled to receive the shade, and make the separation of the true shadow from the penumbra as distinct as possible.
