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.A line drawn from the frontiers of Carmania to Babylon would form
with the meridian an angle of about 500. One from the Caspian Gates
to Thapsacus would form with the parallel merely an angle of about 30° 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.Namely, 1000 stadia, by the hypothesis of Hipparchus, or 800 according to Eratosthenes. 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.