Thus at Jakutsk (62° N. lat.), which is situated but six degrees farther to the north than Edinburgh (55° 58´), the mean temperature of the coldest month is -40°, and mercury a solid body during one-sixth part of the year; while at Irkutsk (52° 16´ N. lat.), situated but little farther to the north than Oxford (51° 46´), the thermometer frequently falls to -30°, or even -40°; temperatures which are of course quite unheard of on the banks of the Isis. For these dreadful winters in the heart of Siberia, and under comparatively low degrees of latitude, there are various causes. The land is, in the first place, an immense plain slanting to the north; moreover, it is situated at such a distance from the Atlantic, that beyond the Ural the western sea-winds, which bring warmth to our winters, assume the character of cold land-winds; and, finally, it merges in the south into the high Mongolian plateau, which, situated 4000 feet above the level of the sea, has of course but little warmth to impart to it in winter; so that, from whatever side the wind may blow at that season, it constantly conveys cold. But in summer the scene undergoes a total change. Under the influence of the sun circling for months round the North Pole, floods of warmth are poured into Central Siberia, and rapidly cause the thermometer to rise; no neighboring sea refreshes the air with a cooling breeze; whether the wind come from the heated Mongolian deserts, or sweep over the Siberian plains, it imbibes warmth on every side. Thus the terrible winter of Jakutsk is followed by an equally _immoderate_ summer (58° 3´), so that rye and barley are able to ripen on a soil which a few feet below the surface is perpetually frozen. [Illustration: 87. IRKUTSK.]