Strabo, Geography
H.C. Hamilton, Esq., W. Falconer, M.A., Ed.

But taking this for granted, and proving, as he imagines, that, according to Eratosthenes, Babylon is east of Thapsacus rather more than 1000 stadia, he draws from this false hypothesis a new argument, which he uses to the following purpose; and says, If we suppose a right line drawn from Thapsacus towards the south, and another from Babylon perpendicular thereto, a right-angled triangle would be the result; whose sides should be, 1. A line drawn from Thapsacus to Babylon; 2. A perpendicular drawn from Babylon to the meridian of Thapsacus; 3. The meridian line of Thapsacus. The hypotenuse of this triangle would be a right line drawn from Thapsacus to Babylon, which he estimates at 4800 stadia. The perpendicular drawn from Babylon to the meridian of Thapsacus is scarcely more than 1000 stadia; the same amount by which the line drawn [from the Caspian Gates] to Thapsacus exceeds that [from the common frontier of Carmania and Persia] to Babylon. The two sides [of the triangle] being given, Hipparchus proceeds to find the third, which is much greater than the perpendicular¹ aforesaid. To this he adds the line drawn from Thapsacus northwards to the mountains of Armenia, one part of which, according to Eratosthenes, was measured, and found to be 1100 stadia; the other, or part unmeasured by Eratosthenes, Hipparchus estimates to be 1000 stadia at the least: so that the two together amount to 2100 stadia. Adding this to the [length of the] side upon which falls the perpendicular drawn from Babylon, Hipparchus estimated a distance of many thousand stadia from the mountains of Armenia and the parallel of Athens to this perpendicular, which falls on the parallel of Babylon.² From the parallel of Athens³ to that of Babylon he shows that there cannot be a greater distance than 2400 stadia, even admitting the estimate supplied by Eratosthenes himself of the number of stadia which the entire meridian contains;⁴ and that if this be so, the mountains of Armenia and the Taurus cannot be under the same parallel of latitude as Athens, (which is the opinion of' Eratosthenes,) but many thousand stadia to the north, as the data supplied by that writer himself prove.

But here, for the formation of his right-angled triangle, Hipparchus not only makes use of propositions already overturned, but assumes what was never granted, namely, that the hypotenuse subtending his right angle, which is the straight line from Thapsacus to Babylon, is 4800 stadia in length. What Eratosthenes says is, that this route follows the course of the Euphrates, and adds, that Mesopotamia and Babylon are encompassed as it were by a great circle formed by the Euphrates and Tigris, but principally by the former of these rivers. So that a straight line from Thapsacus to Babylon would neither follow the course of the Euphrates, nor yet be near so many stadia in length. Thus the argument [of Hipparchus] is overturned. We have stated before, that supposing two lines drawn from the Caspian Gates, one to Thapsacus, and the other to the mountains of Armenia opposite Thapsacus, and distant therefrom, according to Hipparchus's own estimate, 2100 stadia at the very least, neither of them would be parallel to each other, nor yet to that line which, passing through Babylon, is styled by Eratosthenes the southern side [of the third section]. As he could not inform us of the exact length of the route by the mountains, Eratosthenes tells us the distance between Thapsacus and the Caspian Gates; in fact, to speak in a general way, he puts this distance in place of the other; besides, as he merely wanted to give the length of the territory between Ariana and the Euphrates, he was not particular to have the exact measure of either route. To pretend that he considered the lines to be parallel to each other, is evidently to accuse the man of more than childish ignorance, and we dismiss the insinuation as nonsense forthwith.