CHAP. XXXV.

ZONES AND CLIMATES.

ZONES and Climates are artificial divisions of the earth's surface for the purpose of defining its temperature in particular parts; and they have hence an intimate connection with the temperature of the atmosphere, upon which indeed they are mainly though not altogether dependent.

Geographers have divided the surface of the globe into sixty climates, of which thirty are northward of the equator, and thirty southward; by these climates the length of the day, from sun. rising to sun-setting, is shewn, in its increase and decrease proceeding from the equator to the poles; from the equator to the arctic, or northern polar circle, twenty-four climates are traced out, and through each a difference of half an hour arises in the length of the day and night; and in like manner from the equator to the antarctic, or southern polar circle. The six climates which lie between the polar circles and the poles, both the northern and the southern, differ from each other by one entire month progressively.

As climates describe the length of days, so zones describe the degree of heat prevailing on different parts of the earth. Both ancient and modern geographers aree in dividing the earth into five zones in number and three in quality, namely, the torrid, the temperate north and south, and the frigid both north and south. The torrid zone extends from the equator to the tropic of cancer northward, and to the tropic of capricorn southward, twentythree degrees and an half each (very nearly), making forty-seven degrees in all. The two temperate zones extend themselves from the two tropics to the polar circles on each side the equator, being forty-three degrees each, and eighty-six degrees in the whole. The two frigid zones embrace the regions from the polar circles to the poles, extending, in each direction, over twenty-three and an

half degrees, in the whole forty.degrees. Thus northward or southward,

In the whole........ 90 degrees, which is the

distance from the equator to either pole; but considering the northern and southern regions in one account, the general division of the globe into zones will stand thus :

The torrid zones......47 degrees.

The temperate....... 86

The frigid

47

180 degrees, the extent of the

earth from north to south, which geographers have divided into two hemispheres, viz, the northern and southern.

The ancients, having very imperfect knowledge of the globe, considered the two temperate zones as the only habitable parts of the earth, conceiving the heat of the torrid, and the cold of the frigid zones to be equally insupportable. This opinion is now well known to be erroneous; mankind having been ascertained to exist within the arctic circle; and some species of quadrupeds and birds even as high up as eighty degrees; while the attempts made by Captain Cook to penetrate within the antarctic circle, when he proceeded something beyond the seventy-first degree, proves that, although there is no known land lying much more southward than the sixtieth degree of south latitude, yet birds of curious kinds inhahit the expanse of ice which, in every direction, stops all advance toward the south-pole. At the same time that the discovery of the West Indies and of the American continent, had clearly established that the population of mankind either is or may be rendered equal within the region of the torrid zone to that of the best inhabited countries of Europe.

The climates of different parts of the earth's surface are unquestionably owing in great measure to their position with respect to the sun. At the equator, where the sun is always nearly vertical, any given part of the surface receives a much greater quantity of light and heat, than an equal portion near the poles; and it is

also still more affected by the sun's vertical rays, because their passage through the atmosphere is shorter than that of the oblique rays. As far as the sun's mean altitude only is concerned, it appears from Simson's calculations, that the heat received at the equator in the whole year, is nearly twice and a half as great as at the poles; this proportion being nearly the same as that of the meridan heat of a vertical sun, to the heat derived, at the altitude 231, in the middle of the long annual day at the poles. But the difference is rendered still greater, by the effect of the atmo. sphere, which interrupts a greater portion of the heat at the poles than elsewhere. Bouguer has calculated, upon the supposition of the similarity of the affections of heat and light, that in latitude 45°, 80 parts out of 100 are transmitted at noon in July, and 55 only in December. The heat intercepted by the atmosphere is perhaps not wholly, but very nearly, lost with respect to the climate of the neighbouring places. It is obvious that, at any individual place, the climate in summer must approach in some degree to the equatorial climate, the sun's altitude being greater, and in winter to the climate of the polar regions.

While the earth is becoming warmer at any particular spot, the heat thrown off by radiation into the atmosphere, and thence into the empty space beyond it, together with that which is transmitted to the internal parts of the earth, must be less than the heat received from the sun; and when the earth is growing colder, more heat must pass off than is received: but whenever the heat of the surface is stationary, neither increasing nor diminishing, as at the times of the greatest and least heat, it is obvious that the heat re. ceived from the sun must be precisely equal to the heat which is thrown off. Now this quantity may be estimated by the degree of refrigeration in the night; and hence Mr. Prévost has very ingeniously deduced the proportion of the sun's heat arriving at the surface of the earth in the latitude of Geneva, in July, and in December; which he finds to be as 7 or 8 to 1; and this result agrees very well with a calculation deduced from the length of the day, the sun's altitude, and the interception of his rays by the atmosphere.

In London the temperature generally varies, in the course of the day and night, somewhat more than 50, and less than 20°. In January, the mean diurnal variation of temperature is 6o, in

July 10°, and in September, 18°. Hence, says Mr. Kirwan, we may understand the reason of the great frequency of colds in spring and in autumn.

Some philosophers have supposed the earth to become progressively warmer in the course of ages, while others have imagined that its heat is exhausted. Both these opinions appear in general improbable. The greater heat the earth receives by day, the more it throws off, both by day and by night; so that in the course of a few ages the heat must probably have attained its maximum. Local changes may indeed arise from local circumstances; thus, the climate of America is said to have become considerably warmer, since a large part of its surface has been cleared from its dense forests by human labour; and to judge from the descriptions of the ancients, it appears that even in Europe the winters were formerly much colder than they are at present. If, however, Dr. Herschel's opinion of the variation of the heat of the sun be confirmed, it will introduce a great uncertainty into all theories upon the subject: since in these calculations the original heat of the sun has always been supposed unalterable.

The sea is less heated than the land, partly because a greater quantity of water evaporates from it, and partly because the sun's rays penetrate to a considerable depth, and have less effect on the surface, while the water is also mixed, by the agitation of its waves and currents, with the colder water below. It is also more slowly cooled than the land, since, when the temperature of the superficial particles is depressed, they become heavier, and sink to the bottom. For similar reasons, the sea is colder than the land in hot climates, by day, and warmer in cold climates, by night. These circumstances, however, nearly balance each other, so that the mean temperatures of both are equal, that of the sea being only less variable. Although the process of evaporation must cool the sea, yet when the vapours are condensed without reaching the land, their condensation must compensate for this effect by an equal extrication of heat.

There is another cause which perhaps contributes in some de. gree, in temperate climates, to the production of cold; that is, the alternation of freezing and thawing. Mr. Prévost observes that congelation takes place much more suddenly than the oppo. site process of liquefaction; and that of course the same quantity

of heat must be more rapidly extricated in freezing than is ab. sorbed in thawing; that the heat, thus extricated, being disposed to fly off in all directions, and little of it being retained by the neighbouring bodies, more heat is lost than is gained by the alter. nation: so that where ice has once been formed, its production is in this manner redoubled. This circumstance must occur wherever it freezes, that is, on shore, in latitudes above 35°; and it appears that from about 30° to the pole, the land is somewhat colder than the sea, and the more as it is further distant from it; and nearer the equator the land is warmer than the sea: but the process of congelation cannot by any means be the principal cause of the difference, and it is probable that the different capacity of earth and water for heat is materially concerned in it.

Since the atmosphere is very little heated by the passage of the sun's rays through it, it is naturally colder than the earth's sur face; and for this reason, the most elevated tracts of land, which are the most prominent, and the most exposed to the effects of the atmosphere, are always colder than situations nearer the level of the sea. The northern hemisphere is somewhat warmer than the southern, perhaps because of the greater proportion of land that it contains, and also in some measure on account of the greater length of its summer than that of the southern; for al though, as it was long ago 'observed by Simpson, the different distance of the sun compensates precisely for the different velocity of the earth in its orbit, with respect to the whole quantity of heat received on either side of the equinoctial points, yet Mr. Prévost. has shown, that in all probability the same quantity of heat must produce a greatet effect when it is more slowly applied; because the portion lost by radiation from the heated body is greater, as the temperature is higher. Since, therefore, on account of the eccentricity of the earth's orbit, the north pole is turned towards the sun seven or eight days longer than the south pole, the north. ern winters must be milder than the southern: yet the southern summers, though shorter, ought to be somewhat warmer than the northern: but in fact they are colder, partly perhaps from the much greater proportion of sea, which in some degree equalises the temperature, and partly for other reasons. The comparative intensity of the southern summer and winter is not exactly known;