The abundance of fur-bearing animals in the less rigorous parts of the tundras has induced the hardy Russians to colonize and build towns on these confines of the Frozen World. Yakutsk, on the river Lena, in 62° 1′ 30″ N., may be regarded, perhaps, as the coldest town on the Earth. The ground is perpetually frozen to the depth of more than 400 feet, of which three feet only are thawed in summer, when Fahrenheit’s thermometer frequently marks 77° in the shade. Yet in winter the rigour of the climate is so extreme that mercury is constantly frozen for two and occasionally even for three months. * * * * * From the data set forth in the preceding pages, the reader will conclude that, as indeed results from physical laws, the line of perpetual snow will be found to descend lower and lower on advancing to the Pole. By the line of perpetual snow we mean, of course, the limit above which a continual frost endures. Now, this limit varies according to climate. The lower the temperature, the lower the snow-line; the higher the temperature, the higher the snow-line. In the Tropics it does not sink below the summits of the loftiest mountains. Thus, at 1° from the Equator, where the mean temperature at the sea-level is 84°.2, the snow-line must be sought at the elevation of 15,203 feet; in 51° 30’ lat., the latitude of London, it is usually found at about 5900 feet; in lat. 80°, where the mean temperature is 33°.6, it sinks to 457 feet. These figures, however, represent its _normal_ elevations; but temperature, as we all know, is greatly affected by local circumstances, and therefore the perpetual snow-line varies greatly in height. Owing to causes already explained, the snow-line in the Circumpolar Regions sinks to a very low level; and, therefore, many mountainous regions or elevated table-lands, such as Spitzbergen, Greenland, and Novaia Zemlaia, which, in a more temperate climate, would bloom with emerald slopes and waving woods, are covered with huge glaciers and fields of ice, with apparently interminable reaches of untrodden snow.