The temperature of the earth (as a general rule) is greatest on the equator and diminishes gradually towards each pole, but this is by no means invariable, for two places of the same latitude may be very different in climate, and a system of lines passing round the earth from east to west, would each pass through regions of various temperatures, or what is the same thing, lines made to follow the same temperature would have to make various curves and contortions; such lines have been constructed and are called "isothermal" lines. The causes of this are to be found in the various currents of the ocean, the tides and winds, and in the proximity to the ocean of snow-clad mountains and arid plains. The variation of latitude in these lines sometimes reaches as much as seven degrees. Europe contains about 3,900,000 square miles of surface, and is separated from Asia by the Caucasus, Caspian Sea, River Ural, and Uralian Mountains. It is about 3000 miles long and 2400 broad, about two-thirds being plain and table-land and one-third mountain land. The chief mountain ranges are those which run through Norway and Sweden in a north-westerly direction, and the mountain system along the south part from Portugal to Turkey. This last includes the Pyrenees, which runs from the Bay of Biscay to the Mediterranean, the highest peak of which is Mont Maladeta (11,500 feet); the Alps, the highest point of which is Mont Blanc (15,748 feet); the Apennines, the highest point of which is Mont Viso (12,586 feet); the Carpathian Mountains, extending from Presburg in Hungary towards the sources of the Waag and March, the highest point being Mont Lemnitz (7962 feet); and the Balkan range, which may be considered a continuation of the Alps eastward, runs as far as the Black Sea, together with many inferior ranges and branches. The climate of Europe embraces a range from the temperate to extreme cold. It is bounded by the Arctic Ocean on the north, Asia eastward, the Mediterranean Sea southward, and the Atlantic Ocean to the westward; it contains two great inland seas, the Black Sea and the Caspian Sea. There are three great volcanoes in Europe, Hecla, Vesuvius, and Etna, but the plains of Auvergne contain many which are extinct. Asia contains the highest mountains and the most extensive table-lands in the world. It is somewhat square, being bounded northwards by the Arctic Ocean, westward by Europe, southward by the Indian Ocean, and eastward by the North Pacific, and contains about 17,500,000 square miles; the greater part is made up of two extensive tracts of elevated land called table-lands, although these are often varied by valleys and mountain chains of great extent, yet as a whole, they are from 5000 to 10,000 feet above the level of the sea. The eastern table-land is that of Thibet and the Great Desert Gobi, and the western that of Persia.