in consequence of their depending on several causes, which very variously affect the atmospheric equilibrium of these causes electrical changes in the atmosphere are the most frequent and influential.

63. There are also several other winds of a local and peculiar character; such as the Sirocco, in the south of Spain and Italy; the Harmattan, blowing periodically from the Sahara towards the Atlantic; the Simoon and Samiel, peculiar to the burning deserts of Africa and Arabia.

64. The velocity of wind varies from one to a hundred miles an hour. At 10 miles an hour, the wind is called a breeze; at 20, a gale; at 50, a storm; and at 80, a hurricane.

65. Climate.-It has been estimated that, if the entire surface of the earth were divided into 100 equal parts, the torrid zone would contain about 40; each of the temperate about 26; and each of the frigid about 4 only. These zones were subdivided by the ancient geographers into "climates:" their breadths vary with the latitude; and their number and situation are regulated by the length of the longest day. There are 24 from the equator to each of the polar circles, determined by the difference of half an hour in the longest day in each; and 6 from each polar circle to the poles, determined by the difference of a month. But this artificial division of the earth is now little regarded. The climate or mean temperature of a place cannot be determined, except in a general sense, by its distance from the equator. To know it accurately, we must be made acquainted with several other particulars of a local nature. Humboldt has brought to light some general

truths by means of certain isothermal lines, or lines of equal heat, which he has traced on the surface of the globe these we shall consider presently under another head.

66. The distribution of heat on the earth's surface is greatly modified by winds, by the geological structure of soils, by the distribution of land and water, by mountains and plains, by seas and rivers, by the elevation of the place above the level of the sea, and even in some measure by the degree of cultivation. All soils are not heated equally soon; and while one quickly parts with its acquired heat, another retains it for a long time. Clayey and marshy grounds, and those impregnated with salt, cool the atmosphere; but light, sandy, calcareous soils increase heat. The nearness of the sea affects temperature in all climates, by equalizing it. Hence the extremes of heat and cold are not so great in islands as on continents, under the same parallels of latitude. So, also, the intersection of continents by arms of the sea, tends to mitigate alike the fervour of summer and the rigour of winter. Mountainous regions are colder than their latitude would lead us to expect; while regions in the neighbourhood of burning deserts, or in sheltered valleys, are warmer. The Alps contribute to render the climate of Italy delightful, by sheltering it from the cold north winds; but the Altaian mountains, south of Siberia, cut it off from the tropical heats, and render the cold of that country very intense. Had there been a great mountain chain in Sahara, that desert would have been a fertile plain; since the snow-clad summits would have cooled the atmosphere, and the country would have been watered by

the mountain streams. The great cold of Central and Southern Russia is owing principally to the absence of a mountain-chain, which would shelter them from the chilling blasts of the Arctic Ocean.

67. A powerful cause in determining the temperature of a place is its elevation above the level of the sea. A person in the middle of the torrid zone, if elevated 16,000 feet above this level, would find himself, as far as cold and climate are concerned, transported to the frozen regions of the frigid zone. Terrestrial heat is chiefly caused by the sun's reflected rays, and hence is greater in valleys and plains than on mountains and elevated surfaces.

68. The cultivation of the soil gradually affects its temperature. A region shrouded in primeval forests, or covered with swamps and marshes, will have a different temperature when cleared and drained. In hot climates, forests tend to cool the air; but in frigid regions to elevate the temperature, by defending the soil from chilling blasts. The draining of marshes promotes the salubrity of a region.

69. Climate depends in some degree upon moisture as well as upon temperature. The humidity of any climate is indicated by its annual amount of rain, by the difference of its quantities at different seasons of the year, by its dews and fogs. Rain is generally more copious at the equator than at the poles, at the sea coast than on land, and in mountains and high grounds than in plains. In the torrid zone, a small thick rain falls every day on that side of the equator on which the sun is, but generally ceases during the night. Yet in this zone there are tracts where rain seldom or never falls; such as the Sahara of

Africa, the low coasts of Caraccas, and the desert shore of Peru between 15° and 30° S. lat.

More

rain falls in summer than in winter in all latitudes; but in the temperate zones, the rains of winter are more frequent than those of summer, though less in quantity.

70. Climate of Europe.-The climate of the great divisions of the earth's surface is determined by these general principles. The whole of Europe, (with the exception of parts of Lapland, Sweden, Norway, and Northern Russia,) being situated within the temperate zone, suffers but very little from the extremes of heat and cold. The south-western extremity is the hottest region; the heat diminishes rapidly as we advance eastward. The west of Europe has generally a moist atmosphere. The whole continent admits of a two-fold division in respect of climate first, into an Oceanic, an Asiatic, and an African side; and secondly, into a southern, a middle, and a northern zone. The zones determine generally the nature, times and duration of the seasons: the sides regulate in great measure the vegetable productions. The average temperature of Europe exceeds that of Asia and America under the same latitudes; probably because its general elevation is less than of those countries; because its surrounding seas are warmer than the oceans which encompass them; and because the gulph-stream in the Atlantic not only brings a warm current towards Europe from the torrid zone, but also repels from its shores the ice of the Arctic Ocean.

71. Climate of Asia.-Nearly all the circumstances which unite in giving a mild climate to

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Europe, are reversed in the case of Asia. thern regions lie within the Arctic circle. The north winds, unobstructed by mountains, blow over plains of ice; and their cooling influence is not counterbalanced by hot deserts of sand in the southern portion of the continent; there being no land under the equator, in the direction of the length of Asia, except the narrow strips of Sumatra, Borneo, Celebes, and Gilolo. The position of the mountain ranges, again, and the general elevation of the whole continent above the level of the sea, diminish the temperature; nor are there any great seas on the western side to equalise it. Consequently, Asia experiences the extremes of temperature; the winter is excessively cold, and the summer is excessively hot.

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72. Climate of Africa.-By far the greater part of Africa lies within the torrid zone, and the tropical climate of these regions influences those adjacent, with the exception only of that strip of Barbary which is protected by the Atlas Mountains from the hot winds of the desert, and of a portion of Hottentot-land protected by mountains near the Cape. The year divided into the dry and rainy seasons. The regions which have no rain form a zone of considerable breadth; one principal cause of the high temperature of the whole continent. The well-watered regions abound in luxuriant vegetation; but the united heat and moisture are exceedingly unwholesome to man.

73. Climate of America.-The climate of America is nearly as remarkable for cold as that of Africa for heat. Many causes produce this result: a very principal one is the extraordinary elevation of the land above the level of the sea. The vast mountain

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