After having crossed the tropics, one of our two travellers will enter the _North Temperate Zone_, bounded by the Arctic Polar Circle--the other, the _South Temperate Zone_, bounded by the Antarctic Polar Circle. Having passed the polar circles, they will find themselves speedily arrested by ice and snow which never melt--by eternal ice and snow. These inhospitable regions compose the two _frigid zones_, which cover, like two immense hoods (forming the 0·82 parts of the terrestrial surface), the one, the northern hemisphere, the other, the southern.[19] In their progress through these various climates, our two travellers will arrive at a very curious comparative result,--that the southern hemisphere is colder than the northern. This difference becomes especially recognisable below the 50th degree of south latitude; so that, after passing the Antarctic Circle, the ice opposes the voyager's course with nearly insurmountable obstacles; while, in the northern hemisphere, the whaler frequently penetrates to Spitzbergen, situated much nearer to the Pole than to the Polar Circle. This is a general fact; we confine ourselves to putting it forward. * * * * * Let us now suppose that our two travellers, always ready to compare the results of their inquiries, accomplish the ascent of a very lofty mountain situated under the Equator, such as Chimborazo. In the course of their ascent, they will traverse the same climates and the same zones which had marked the stages of their journey from the Equator to the Poles: at their starting-point they will find themselves in the Torrid zone, then will come the Temperate and the Frigid zones, the latter rendered inaccessible by glaciers and eternal snows. These vertical zones of the mountain are characterised by vegetables and animals whose types are found in the corresponding horizontal zones of the terrestrial surface. But what is most remarkable is, that there exists between the northern and the southern slopes of the mountain the same difference as between the southern and northern hemispheres: the line of the eternal snows descends much lower on the northern than on the southern slope, in the same manner as, in the southern hemisphere, the polar ice advances much nearer the Equator than in the northern. Such is the general view-point which we must adopt for the clearer comprehension of the details of observation. Of course, when speaking of the limit of the eternal snows, we refer only to the _lower_ limit, that is to say, to the greatest elevation attained by the snow-line in the course of a single year. As for the _upper_ limit, it entirely escapes us; for the summits of the loftiest mountains do not reach the atmospheric strata which, by virtue of their refraction, cannot contain any vesicular, aqueous, or condensable vapour.