25. Thus, says Eratosthenes, we have given you a description of the northern side; as for the southern, we cannot take its measure along the sea, on account of the Persian Gulf, which intercepts [its continuity], but from Babylon through Susa and Persepolis to the confines of Persia and Carmania there are 9200 stadia. This he calls the southern side, but he does not say it is parallel to the northern. 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.
26. 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 Euphrates561 and the city of Teredon, 3000562 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.
27. 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.”
28. We answer, that if the Caspian Gates and the boundary line of Carmania and Persia were exactly under the same meridian, and if right lines drawn in the direction of Thapsacus and Babylon would intersect such meridian at right angles, the inference would be just.565 For then the line [from the common frontier of Carmania and Persia] to Babylon, if produced to the meridian of Thapsacus, would appear to the eye equal, or nearly equal, to that from the Caspian Gates to Thapsacus. Consequently, Babylon would only be east of Thapsacus in the same proportion as the line drawn from the Caspian Gates to Thapsacus exceeds the line drawn from the frontier of Carmania to Babylon.566 Eratosthenes, however, does not tell us that the line which bounds the western coast of Ariana follows the direction of the meridian; nor yet that a line drawn from the Caspian Gates to Thapsacus would form right angles with the meridian of the Caspian Gates. But rather, that the line which would form right angles with the meridian, would be one which should follow the course of the Taurus, and with which the line drawn from the Caspian Gates to Thapsacus would form an acute angle. Nor, again, does he ever say that a line drawn from Carmania to Babylon would be parallel to that drawn [from the Caspian Gates] to Thapsacus; and even if it were parallel, this would prove nothing for the argument of Hipparchus, since it does not form right angles with the meridian of the Caspian Gates.
29. 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 perpendicular567 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.568 From the parallel of Athens569 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;570 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