HKB
The difference of length between the northern and southern sides is caused, he tells us, by the Euphrates, which after running south some distance shifts its course almost due east.
Of the two remaining sides, he describes the western first, but whether we are to regard it as one single straight line, or two, seems to be undecided. He says,—From Thapsacus to Babylon, following the course of the Euphrates, there are 4800 stadia; from thence to the mouth of the Euphrates[561] and the city of Teredon, 3000[562] more; from Thapsacus northward to the Gates of Armenia, having been measured, is stated to be 1100 stadia, but the distance through Gordyæa and Armenia, not having yet been measured, is not given. The eastern side, which stretches lengthwise through Persia from the Red Sea towards Media and the north, does not appear to be less than 8000 stadia, and measured from certain headlands above 9000, the rest of the distance through Parætacena and Media to the Caspian Gates being 3000 stadia. The rivers Tigris and Euphrates flowing from Armenia towards the south, after having passed the Gordyæan mountains, and having formed a great circle which embraces the vast country of Mesopotamia, turn towards the rising of the sun in winter and the south, particularly the Euphrates, which, continually approaching nearer and nearer to the Tigris, passes by the rampart of Semiramis,[563] and at about 200 stadia from the village of Opis,[564] thence it flows through Babylon, and so discharges itself into the Persian Gulf. Thus the figure of Mesopotamia and Babylon resembles the cushion of a rower’s bench.—Such are the words of Eratosthenes.
In the Third Section it is true he does make some mistakes, which we shall take into consideration; but they are nothing like the amount which Hipparchus attributes to him. However, we will examine his objections. [In the first place,] he would have the ancient charts left just as they are, and by no means India brought more to the south, as Eratosthenes thinks proper. Indeed, he asserts that the very arguments adduced by that writer only confirm him the more in his opinion. He says, “According to Eratosthenes, the northern side of the third section is bounded by a line of 10,000 stadia drawn from the Caspian Gates to the Euphrates, the southern side from Babylon to the confines of Carmania is a little more than 9000 stadia. On the western side, following the course of the Euphrates, from Thapsacus to Babylon there are 4800 stadia, and thence to the outlets of the river 3000 stadia more. Northward from Thapsacus [to the Gates of Armenia] is reckoned 1100 stadia; the rest has not been measured. Now since Eratosthenes says that the northern side of this Third Section is about 10,000 stadia, and that the right line parallel thereto drawn from Babylon to the eastern side is computed at just above 9000 stadia, it follows that Babylon is not much more than 1000 stadia east of the passage of [the Euphrates] near Thapsacus.”