18. Hipparchus narrates that at the Dnieper and [the north of] Keltica, during the whole of the summer nights there is one continued twilight from sunset to sunrise, but at the winter solstice the sun never rises more than nine cubits above the horizon.[531] He adds that this phenomenon is yet more remarkable in regions 6300[532] stadia north of Marseilles, (these regions he supposes to be peopled by Kelts, but I believe are inhabited by Britons, and 2500 stadia north of Keltica,) where the sun at the winter solstice[533] rises only six cubits above the horizon. That at 9100[534] stadia north of Marseilles it only rises four cubits, and not so much as three in the countries beyond, and which I consider much farther north than Ierne.[535] However, Hipparchus, on the authority of Pytheas, places them south of Britain, and says that the longest day there consists only of 19 hours;[536] while in countries where the sun rises but four cubits above the horizon, and which are situated 9100[537] stadia north of Marseilles, the day has 18 hours. Consequently [according to his hypothesis] the most southerly parts of Britain must be north of these regions. They must therefore be under the same parallel, or almost the same, as the parts of Bactriana next to the Caucasus, which I have shown are, according to the followers of Deimachus, 3800 stadia farther north than Ierne.[538] Now if we add this to the number between Marseilles and Ierne, we shall get 12,500 stadia. But who ever made known to us that, in those parts, I mean, in the vicinity of Bactra, this was the duration of the longest day, or the height which the sun attains in the meridian at the winter solstice? All these things are patent to the eyes of every man, and require no mathematical investigation; therefore they certainly would have been mentioned by numerous writers both amongst the ancients who have left us histories of Persia, and by the later writers too, who have carried them down to our own time. How, too, would their fertility, which I have described above, harmonize with such a latitude? The facts here advanced are sufficient to give an idea of the learned manner in which Hipparchus attempts to controvert the reasoning of Eratosthenes by mere petitiones principii.