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the vapours 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. 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 On the other hand, water has a considerable opaque masses. 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 so soon as fresh water. So that, in consequence of all these circumstances, the ocean is not subject to such variations of heat as the land, and, by imparting its temperature to the winds and by its currents, it diminishes the rigour of climate on the coasts and in the islands, which are never subject to such extremes of heat and cold as are experienced in the interior of continents, though they are liable to fogs and rain from the evaporation of the adjacent seas. On each side of the equator to the 48th degree of latitude, the surface of the ocean is in general warmer than the air above it. The mean of the difference of the tem
perature at noon and midnight is about 10.37, the greatest deviation never exceeding from 00.36 to 2016, which is much cooler than the air over the land.
On land the temperature depends upon the nature of the soil and its products, its habitual moisture or dryness. From the eastern extremity of the Sahara desert quite across Africa, the soil is almost entirely barren sand; and the Sahara desert itself extends over an area of 194,000 square leagues, equal to twice the area of the Mediterranean Sea, and raises the temperature of the air by radiation from 90° to 100°, which must have a most extensive influence. On the contrary, vegetation cools the air by evaporation and the apparent radiation of cold from the leaves of plants, because they absorb more caloric than they give out. The graminiferous plains of South America cover an extent ten times greater than France, occupying no less than about 50,000 square leagues, which is more than the whole chain of the Andes, and all the scattered mountain-groups of Brazil. These, together with the plains of North America and the steppes of Europe and Asia, must have an extensive cooling effect on the atmosphere if it be considered that in calm and serene nights they cause the thermometer to descend 120 or 14°, and that in the meadows and heaths in England the absorption of heat by the grass is sufficient to cause the temperature to sink to the point of congelation during the night for ten months in the year. Forests cool the air also by shading the ground from the rays of the sun, and by evaporation from the boughs. Hales found that the leaves of a single plant of helianthus three feet high exposed nearly forty feet of surface; and, if it be considered that the woody regions of the river Amazons, and the higher part of the Orinoco, occupy an area of 260,000 square leagues, some idea may be formed of the torrents of vapour which rise from the leaves of the forests all over the globe. However, the frigorific effects of their evaporation are counteracted in some measure by the perfect calm which reigns in the tropical wildernesses. The innumerable rivers, lakes, pools, and marshes interspersed through the continents absorb caloric, and cool the air by evaporation; but, on account of the chilled and dense particles sinking to the bottom, deep water diminishes the cold of winter, so long as ice is not formed.
In consequence of the difference in the radiating and absorbing powers of the sea and land, their configuration greatly modifies
the distribution of heat over the surface of the globe. Under the equator only one-sixth part of the circumference is land; and the superficial extent of land in the northern and southern hemispheres is in the proportion of three to one. The effect of this unequal division is greater in the temperate than in the torrid zones, for the area of land in the northern temperate zone is to that in the southern as thirteen to one, whereas the proportion of land between the equator and each tropic is as five to four. It is a curious fact, noticed by Mr. Gardner, that only one twenty-seventh part of the land of the globe has land diametrically opposite to it. This disproportionate arrangement of the solid part of the globe has a powerful influence on the temperature of the southern hemisphere. But, besides these greater modifications, the peninsulas, promontories, and capes, running out into the ocean, together with bays and internal seas, all affect temperature. To these may be added the position of continental masses with regard to the cardinal points. All these diversities of land and water influence temperature by the agency of the winds. On this account the temperature is lower on the eastern coasts both of the New and Old World than on the western; for, considering Europe as an island, the general temperature is mild in proportion as the aspect is open to the Atlantic Ocean, the superficial temperature of which, as far north as the 45th and 50th degrees of latitude, does not fall below 48° or 51° of Fahrenheit, even in the middle of winter. On the contrary, the cold of Russia arises from its exposure to the northern and eastern winds. But the European part of that empire has a less rigorous climate than the Asiatic, because it does not extend to so high a latitude.
The interposition of the atmosphere modifies all the effects of the sun's heat. The earth communicates its temperature so slowly, that M. Arago has occasionally found as much as from 14° to 18° of difference between the heat of the soil and that of the air two or three inches above it.
The circumstances which have been enumerated, and many more, concur in disturbing the regular distribution of heat over the globe, and occasion numberless local irregularities. Nevertheless the mean annual temperature becomes gradually lower from the equator to the poles. But the diminution of mean heat is most rapid between the 40th and 45th degrees of latitude both
in Europe and America, which accords perfectly with theory: whence it appears that the variation in the square of the cosine of the latitude (N. 127), which expresses the law of the change of temperature, is a maximum towards the 45th degree of latitude. The mean annual temperature under the equator in America is about 8140 of Fahrenheit: in Africa it is said to be nearly 830. The difference probably arises from the winds of Siberia and Canada, whose chilly influence is sensibly felt in Asia and America, even within 180 of the equator.
The isothermal lines are nearly parallel to the equator, till about the 22nd degree of latitude on each side of it, where they begin to lose their parallelism, and continue to do so more and more as the latitude augments. With regard to the northern hemisphere, the isothermal line of 59° of Fahrenheit passes between Rome and Florence in latitude 43°; and near Raleigh in North Carolina, latitude 36°: that of 50° of equal annual temperature runs through the Netherlands, latitude 51°; and near Boston in the United States, latitude 4240: that of 41° passes near Stockholm, latitude 5910; and St. George's Bay, Newfoundland, latitude 48°: and lastly, the line of 32°, the freezing point of water, passes between Ulea in Lapland, latitude 66°, and Table Bay, on the coast of Labrador, latitude 54°.
Thus it appears that the isothermal lines, which are nearly parallel to the equator for about 220, afterwards deviate more and more. From observations made during the numerous voyages in the Arctic Seas, it is found that the isothermal lines of Europe and America entirely separate in the high latitudes, and surround two poles of maximum cold: one, in 79° N. lat. and 120° E. long., has a mean temperature of 2o Fahrenheit; and the other, whose temperature was determined by Sir David Brewster to be 31° Fahrenheit, from the observations of Sir Edward Parry is near Melville Island. The pole of the earth's rotation, whose mean temperature is probably not below 15° Fahrenheit, is nearly midway between the two; and the line which joins these points of maximum cold is almost coincident with that diameter of the polar basin which bisects it, and passes through its two great outlets into the Pacific and Atlantic Oceans, a most remarkable feature, and strongly indicative of the absence of land, and of the prevalence of a materially milder
climate in the polar Ocean, probably not under 15° Fahrenheit.* It is believed that two corresponding poles of maximum cold exist in the southern hemisphere, though observations are wanting to trace the course of the southern isothermal lines with the same accuracy as the northern.
The isothermal lines, or such as pass through places where the mean annual temperature of the air is the same, do not always coincide with the isogeothermal lines, which are those passing through places where the mean temperature of the ground is the same. Sir David Brewster, in discussing this subject, finds that the isogeothermal lines are always parallel to the isothermal lines; consequently the same general formula will serve to determine both, since the difference is a constant quantity obtained by observation, and depending upon the distance of the place from the neutral isothermal line. These results are confirmed by the observations of M. Kupffer of Kasan during his excursions to the north, which show that the European and the American portions of the isogeothermal line of 320 of Fahrenheit actually separate, and go round the two poles of maximum cold. This traveller remarked, also, that the temperature both of the air and of the soil decreases most rapidly towards the 45th degree of latitude.
It is evident that places may have the same mean annual temperature, and yet differ materially in climate. In one, the winters may be mild and the summers cool; whereas another may experience the extremes of heat and cold. Lines passing through places having the same mean summer or winter temperature are neither parallel to the isothermal, the geothermal lines, nor to one another, and they differ still more from the parallels of latitude. In Europe, the latitude of two places which have the same annual heat never differs more than 80 or 9°; whereas the difference in the latitude of those having the same mean winter temperature is sometimes as much as 180 or 190. At Kasan, in the interior of Russia, in latitude 55°-48, nearly the same with that of Edinburgh, the mean annual temperature is about 3706; at Edinburgh it is 470-84. At Kasan the mean summer temperature is 640-84, and that of winter 20.12; whereas