(almost insurmountable) in distinguishing the division of the penumbra.
There certainly is one error very apparent, in measuring the base betwixt Syene and Alexandria; that is, they were not (as supposed) under the same meridian; for though, to my very great concern afterwards, I had no opportunity of fixing the longitude at this first visit to Syene, as I had done the latitude, yet on my return, in the year 1772, from an eclipse of the first satellite of Jupiter, I found its longitude to be 33° 30'; and the longitude of Alexandria, being 30° 16' 7", there is 3° 14' that Syene is to the eastward of the meridian of Alexandria, or so far from their being under the same meridian as supposed.
It is impossible to fix the time of the building of Syene; upon the most critical examination of its hieroglyphics and proportions, I should imagine it to have been founded some time after Thebes, but before Dendera, Luxor, or Carnac.
It would be no less curious to know, whether the well, which Eratosthenes made use of for one of the terms of the geodesique base, and his arch of the meridian, between Alexandria and Syene, was coeval with the building of that city, or whether it was made for the experiment. I should be inclined to think the former was the case; and the placing this city first, then the well under the tropic, were with a view of ascertaining the length of the solar year. In short, this point, so material to be settled, was the constant object of attention of the first astronomers, and this was the use of the dial of Osimandyas; this inquiry was the occasion of the number of obelisks raised in every ancient city in Egypt.
