Pliny[143] says, that, in his time, the city of Syene was situated so directly under the tropic of Cancer, that there was a well, into which the sun shone so perpendicular, that it was enlightened by its rays down to the bottom. Strabo[144] had said the same. The ignorance, or negligence, in the Geodesique measure in this observation, is extraordinary; Egypt had been measured yearly, from early ages, and the distance between Syene and Alexandria should have been known to an ell. From this inaccuracy, I do very much suspect the other measure Eratosthenes is said to have made, by which he fixed the sun’s parallax at 10 seconds and a half, was not really made by him, but was some old Chaldaic, or Egyptian observation, made by more instructed astronomers which he had fallen upon.
The Arabs call it Assouan, which they say signifies enlightened; in allusion, I suppose, to the circumstance of the well, enlightened within by the sun’s being stationary over it in June; in the language of Beja its name signifies a circle, or portion of a circle.
Syene, among other things, is famous for the first attempt made by Greek astronomers to ascertain the measure of the circumference of the earth. Eratosthenes, born at Cyrene about 276 years before Christ, was invited from Athens to Alexandria by Ptolemy Evergetes, who made him keeper of the Royal Library in that city. In this experiment two positions were assumed, that Alexandria and Syene were exactly 5000 stades distant from each other, and that they were precisely under the same meridian. Again, it was verified by the experiment of the well, that, in the summer solstice at mid-day, when the sun was in the tropic of Cancer, in its greatest northern declination, the well[145] at that instant was totally and equally illuminated; and that no style, or gnomon, erected on a perfect plane, did cast, or project, any manner of shadow for 150 stades round, from which it was justly concluded, that the sun, on that day, was so exactly vertical to Syene, that the center of its disk immediately corresponded to the center of the bottom of the well. These preliminaries being fixed, Eratosthenes set about his observation thus:—
On the day of the summer solstice, at the moment the sun was stationary in the meridian of Syene, he placed a style perpendicularly in the bottom of a half-concave sphere, which he exposed in open air to the sun at Alexandria. Now, if that style had cast no shade at Alexandria, it would have been precisely in the same circumstance with a style in the well in Syene; and the reason of its not casting the shade would have been, that the sun was directly vertical to it. But he found, on the contrary, this style at Alexandria did cast a shadow; and by measuring the distance of the top of this shadow from the foot of the style, he found, that, when the sun cast no shadow at Syene, by being in the zenith, at Alexandria he projected a shadow; which shewed he was distant from the vertical point, or zenith, 7⅕°=7° 12´, which was 1/50th of the circumference of the whole heavens, or of a great circle.
This being settled, the conclusion was, that Alexandria and Syene must be distant from each other by the 50th part of the circumference of the whole earth.
Now 5000 stades was the distance already assumed between Alexandria and the well of Syene; and all that was to be done was to repeat 5000 stades fifty times, or multiply 5000 stades by 50, and the answer was 250,000 stades, which was the total of the earth’s circumference. This, admitting the French contents of the Egyptian stadium to be just, will amount to 11,403 leagues for the circumference of the earth sought; and as our present account fixes it to be 9000, the error will be 2403 leagues in excess, or more than one-fourth of the whole sum required.
This observation surely therefore is not worth recording, unless to shew the insufficiency or imperfection of the method; it cannot deserve the encomiums[146] that have been bestowed upon it, if justice has been done to Eratosthenes’ geodesique measures, which I do not, by any manner of means, warrant to be the case, because the measure of his arch of the meridian seems to have been conducted with a much greater degree of success and precision than that of his base.
On the 22d, 23d, and 24th of January, being at Syene, in a house immediately east of the small island in the Nile (where the temple of Cnuphis is still standing, very little injured, and which [147]Strabo, who was himself there, says was in the ancient town, and near the well built for the observation of the solstice) with a three-foot brass quadrant, made by Langlois, and described by [148]Monsieur de la Lande, by a mean of three observations of the sun in the meridian, I concluded the latitude of Syene to be 24° 0´ 45´´ north.
And, as the latitude of Alexandria, by a medium of many observations made by the French academicians, and more recently by Mr Niebuhr and myself, is beyond possibility of contradiction 31° 11´ 33´´, the arch of the meridian contained between Syene and Alexandria, must be 7° 10´ 48´´, or 1´ 12´´ less than Eratosthenes made it. And this is a wonderful precision, if we consider the imperfection of his instrument, in the probable shortness of his radius, and difficulty (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.