The currents of the air exercise as favourable an influence upon the climate of Europe as do those of the ocean. The south-westerly winds predominating on the coasts pass over the warm gulf-stream, and, on reaching Europe, they part with the heat stored up by them between the tropics. The north-westerly, northerly, and even north-easterly winds, which blow during a portion of the year, are less cold than might be expected, for they, too, have to cross the warm waters of the gulf-stream. And lastly, there is the Sahara, which elevates the temperature of a portion of Europe. The increase in temperature due to the combined influence of winds and ocean currents amounts to 40° 50°, and even 60°, if we compare Europe with other parts of the world lying under the same latitudes. Nowhere else, not even on the western coast of North America, do the isothermals, or lines of equal annual temperature, ascend so high towards the arctic regions. The inhabitants of Europe, though they may live 900 to 1,200 miles farther away from the equator, enjoy as mild a climate as do those of America, and the decrease of temperature on going northward is far less rapid than in any other part of the globe. This uniformity of temperature constitutes one of the most characteristic features of Europe. The whole of it lies within the temperate region bounded by the isothermal lines of 32° F. and 68° F., whilst in America and Asia that privileged zone has only half this extent. {17} [Illustration: Fig. 4.—THE ISOTHERMAL ZONE OF EUROPE. Scale 1 : 60,000,000. Erhard.]