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the sun's light diminishes from the centre to the circumference of the solar disc; but in the moon the gradation is reversed.
Much has been done within a few years to ascertain the manner in which heat is distributed over the surface of our planet, and the variations of climate; which in a general view mean every change of the atmosphere, such as of temperature, humidity, variations of barometric pressure, purity of air, the serenity of the heavens, the effects of winds, and electric tension. Temperature depends upon the property which all bodies possess, more or less, of perpetually absorbing and emitting or radiating heat. When the interchange is equal, the temperature of a body remains the same; but when the radiation exceeds the absorption, it becomes colder, and vice versa. But in order to determine the distribution of heat over the surface of the earth, it is necessary to find a standard by which the temperature in different latitudes may be compared. For that purpose it is requisite to ascertain by experiment the mean temperature of the day, of the month, and of the year, at as many places as possible throughout the earth. The annual average temperature may be found by adding the mean temperatures of all the months in the year, and dividing the sum by twelve. The average of ten or fifteen years will give it with tolerable accuracy; for although the temperature in any place may be subject to very great variations, yet it never deviates more than a few degrees from its mean state, which consequently offers a good standard of comparison.
If climate depended solely upon the heat of the sun, all places having the same latitude would have the same mean annual temperature. The motion of the sun in
the ecliptic, indeed, occasions perpetual variations in the length of the day, and in the direction of the rays with regard to the earth; yet, as the cause is periodic, the mean annual temperature from the sun's motion alone must be constant in each parallel of latitude. For it is evident that the accumulation of heat in the long days of summer, which is but little diminished by radiation during the short nights, is balanced by the small quantity of heat received during the short days in winter and its radiation in the long frosty and clear nights. In fact, if the globe were everywhere on a level with the surface of the sea, and also of the same substance, so as to absorb heat equally, and radiate the same, the mean heat of the sun would be regularly distributed over its surface in zones of equal annual temperature parallel to the equator, from which it would decrease to each pole as the square of the cosine of the latitude; and its quantity would only depend upon the altitudes of the sun, atmospheric currents, and the internal heat of the earth evinced by the vast number of volcanos and hot springs, in every region from the equator to the polar circles, which has probably been cooling down to its present state for thousands of ages. The distribution' of heat, however, in the same parallel is very irregular in all latitudes, except between the tropics, where the isothermal lines, or the lines passing through places of equal mean annual temperature, are parallel to the equator. The causes of disturbance are very numerous; but such as have the greatest influence, according to Humboldt, to whom we are indebted for the greater part of what is known on the subject, are the elevation of the continents, the distribution of land and water over the surface of the globe, exposing different absorbing and radiating powers;
the variations in the surface of the land, as forests, sandy deserts, verdant plains, rocks, &c., mountain-chains covered with masses of snow, which diminish the temperature; the reverberation of the sun's rays in the valleys, which increases it; and the interchange of currents, both of air and water, which mitigate the rigor of climates; the warm currents from the equator softening the severity of the polar frosts, and the cold currents from the poles tempering the intense heat of the equatorial regions. To these may be added cultivation, though its influence extends over but a small portion of the globe, only a fourth part of the land being inhabited.
Temperature does not vary much with latitudes or as with the height above the level of the sea; and the decrease is more rapid in the higher strata of the atmosphere than in the lower, because they are farther removed from the radiation of the earth, and being highly rarefied, the heat is diffused through a larger space. A portion of air at the surface of the earth, whose temperature is 70° of Fahrenheit, if carried to the height of two miles and a half, will expand so much that its temperature will be reduced 50°; and in the ethereal regions the temperature is 90° below the point of congelation.
The height at which snow lies perpetually decreases from the equator to the poles, and is higher in summer than in winter; but it varies from many circumstances. Snow rarely falls when the cold is intense and the atmosphere dry. Extensive forests produce moisture by their evaporation, and high table-lands, on the contrary, dry and warm the air. In the Cordilleras of the Andes, plains of only twenty-five square leagues raise the temperature as much as three or four degrees above what is found at the
same altitude on the rapid declivity of a mountain, consequently the line of perpetual snow varies according as one or other of these causes prevails. Aspect has also a great influence; the line of perpetual snow is much more. elevated on the southern than on the northern side of the Himalaya mountains; but on the whole it appears that the mean height between the tropics at which the snow lies perpetually is about 15207 feet above the level of the sea; whereas snow does not cover the ground continually at the level of the sea till near the north pole. In the southern hemisphere, however, the cold is greater than in the northern. In Sandwich land, between the 54th and 58th degrees of latitude, perpetual snow and ice extend to the sea-beach; and in the island of St. George's, in the 53rd degree of south latitude, which corresponds with the latitude, of the central counties of England, perpetual snow descends even to the level of the ocean. This preponderance of cold in the southern hemisphere cannot be altogether attributed to the winter being longer than ours by so small a quantity as 73 days, even allowing to that its due influence; but it is probably owing to the open sea round the south pole, which permits the icebergs to descend to a lower latitude by ten degrees than they do in the northern hemisphere, on account of the numerous obstructions opposed to them by the islands and continents about the north pole. Icebergs seldom float farther to the south than the Azores; whereas those that come from the south pole descend as far as the Cape of Good Hope, and occasion a continual absorption of heat in melting.
The influence of mountain-chains does not wholly depend upon the line of perpetual congelation; they attract and condense the vapors floating in the air, and send them
down in torrents of rain; they radiate heat into the atmosphere at a lower elevation, and increase the temperature of the valleys by the reflection of the sun's rays, and by the shelter they afford against prevailing winds. But, on the contrary, one of the most general and powerful causes of cold arising from the vicinity of mountains is the freezing currents of wind which rush from their lofty peaks along the rapid declivities, chilling the surrounding valleys such is the cutting north wind called the bise in Switzerland.
Next to elevation, the difference in the radiating and absorbing powers of the sea and land has the greatest influence in disturbing the regular distribution of heat. The extent of the dry land is not above the fourth part of that of the ocean, so that the general temperature of the atmosphere, regarded as the result of the partial temperatures of the whole surface of the globe, is most powerfully modified by the sea; besides, the ocean acts more uniformly on the atmosphere than the diversified surface of the solid mass does, both by the equality of its curvature and its homogeneity. In opaque substances the accumulation of heat is confined to the stratum nearest the surface: but the seas become less heated at their surface than the land, because the solar rays, before being extinguished, penetrate the transparent liquid to a greater depth, and in greater numbers than in the opaque masses. On the other hand, water has a considerable radiating power, which, together with evaporation, would reduce the surface of the ocean to a very low temperature, if the cold particles did not sink to the bottom, on account of their superior density. The seas preserve a considerable portion of the heat they receive in summer, and, from their saltness, do not freeze