The sea exercises an important equalizing influence on the temperature of the globe. In the tropical regions a large extent of ocean spreads coolness on every side, and affords a perpetual succession of refreshing breezes. Islands are always, comparatively, of more temperate climates than continents, and those scattered over the expanse of the Pacific may be said to enjoy almost a perpetual spring. The influence of the winds is also very important; particularly that of the trade-winds. Blowing from east to west across the sands of Africa, the latter produce, on its western coast, a most intense heat, much greater than is experienced on the eastern. In passing the Atlantic they are considerably cooled; and though their temperature is again raised in traversing South America, yet, before reaching the opposite coast, they meet the tremendous snow-clad Andes, which stop their progress and diffuse a wide coolness. Again, the mountain ranges of the earth not only present and retain on their sides a refreshing coolness, but, by the mighty rivers to which they give rise, diffuse a great amelioration of the temperature through extensive regions. They are particularly of this character, and give rise to the largest rivers in the torrid and burning zones of the earth. In the temperate climate, and those approaching to the poles, mountains are of moderate elevation, are almost always barren, and give rise to few considerable streams. It appears probable that the climates of European countries were more severe in ancient times than they are at present. Cæsar says that the vine could not be cultivated in Gaul on account of its winter cold. The reindeer, now found only in the zone of Lapland, was then an inhabitant of the Pyrenees. The Tiber was frequently frozen over, and the ground about Rome covered with snow for several weeks together, which very rarely happens in our time. The Rhine and the Danube, in the time of Augustus, was generally frozen over for several months of winter. The barbarians who overran the Roman empire a few centuries afterwards, transported their armies and wagons across the ice of these rivers. Though the fact is well established, the causes of this change of climate do not seem to be satisfactorily explained. CHAPTER XV.――MINERALS.