If the Portuguese mariners of the generation before Columbus had acquiesced in Ptolemy's views as final, they surely would not have devoted their energies to the task of circumnavigating Africa. But there were yet other theoretical or fanciful obstacles in the way. When you look at a modern map of the world, the "five zones" may seem like a mere graphic device for marking conveniently the relations of different regions to the solar source of heat; but before the great Portuguese voyages and the epoch-making third voyage of Vespucius, to be described hereafter, a discouraging doctrine was entertained with regard to these zones. Ancient travellers in Scythia and voyagers to "Thule"--which in Ptolemy's scheme perhaps meant the Shetland isles[360]--had learned something of Arctic phenomena. The long winter nights,[361] the snow and ice, and the bitter winds, made a deep impression upon visitors from the Mediterranean;[362] and when such facts were contrasted with the scorching blasts that came from Sahara, the resulting theory was undeniably plausible. In the extreme north the ocean must be frozen and the country uninhabitable by reason of the cold; contrariwise, in the far south the ocean must be boiling hot and the country inhabitable only by gnomes and salamanders. Applying these ideas to the conception of the earth as a sphere, Pomponius Mela tells us that the surface of the sphere is divided into five zones, of which only two are fit to support human life. About each pole stretches a dead and frozen zone; the southern and northern hemispheres have each a temperate zone, with the same changes of seasons, but not occurring at the same (but opposite) times; the north temperate zone is the seat of the Oecumene ([Greek: oikoumene]), or Inhabited World; the south temperate zone is also inhabited by the Antichthones or Antipodes, but about these people we know nothing, because between us and them there intervenes the burning zone, which it is impossible to cross.[363] [Footnote 360: Bunbury, _op. cit._ vol. ii. pp. 492, 527. The name is used in different geographical senses by various ancient writers, as is well shown in Lewis's _Astronomy of the Ancients_, pp. 467-481.] [Footnote 361: The Romans, at least by the first century A. D., knew also of the shortness of northern nights in summer. Arma quidem ultra Littora Invernae promovimus, et modo captas Orcadas, ac minima contentos nocte Britannos. Juvenal, ii. 159. See also Pliny, _Hist. Nat._, iv. 30; Martianus Capella, vi. 595; Achilles Tatius, XXXV.] [Footnote 362: The reader will remember Virgil's magnificent description of a Scythian winter (_Georg._, iii. 352):--