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perature at noon and midnight is about 10.37, the greatest deviation never exceeding from 00.36 to 20:16, 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 12° 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 iş 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 811° of Fahrenheit: in Africa it is said to be nearly 83o. The difference probably arises from the winds of Siberia and Canada, whose chilly influence is sensibly felt in Asia and America, even within 18° 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 360 : that of 50° of equal annual temperature runs through the Netherlands, latitude 51°; and near Boston in the United States, latitude 421°: that of 41° passes near Stockholm, latitude 591°; 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 660, and Table Bay, on the coast of Labrador, latitude 54o.
Thus it appears that the isothermal lines, which are nearly parallel to the equator for about 22°, 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 790 N. lat. and 120° E. long., has a mean temperature of 20 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 no rn.
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 32° 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 90; 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 550:48, nearly the same with that of Edinburgh, the mean annual temperature is about 370:6; at Edinburgh it is 470.84. At Kasan the mean summer temperature is 640.84, and that of winter 20.12; whereas
* • Meteorology,' by Sir J. Herschel.
at Edinburgh the mean summer temperature is 580.28, and that of winter 38°•66. Whence it appears that the difference of winter temperature is much greater than that of summer. At Quebec the summers are as warm as those in Paris, and grapes sometimes ripen in the open air : whereas the winters are as severe as in Petersburgh; the snow lies five feet deep for several months, wheel carriages cannot be used, the ice is too hard for skating, travelling is performed in sledges, and frequently on the ice of the river St. Lawrence. The cold at Melville Island on the 15th of January, 1820, according to Sir Edward Parry, was 550 below the zero of Fahrenheit's thermometer; and when Dr. Kane was on the northern coast of Greenland it was 700 below that point; yet the summer heat during the day in these high latitudes is insupportable.
Observations tend to prove that all the climates of the earth are stable, and that their vicissitudes are only periods or oscillations of more or less extent, which vanish in the mean annual temperature of a sufficient number of years. This constancy of the mean annual temperature of the different places on the surface of the globe shows that the same quantity of heat which is annually received by the earth is annually radiated into space; and that would be the case even if the quantity of heat emitted by the sun should vary with his spots, for, if more were received, more would be radiated. Nevertheless, a variety of causes may disturb the climate of a place ; cultivation may make it warmer ; but it is at the expense of some other place, which becomes colder in the same proportion. There may be a succession of cold summers and mild winters, but in some other country the contrary takes place to effect the compensation ; wind, rain, snow, fog, and the other meteoric phenomena, are the ministers employed to accomplish the changes. The distribution of heat may vary with a variety of circumstances; but the absolute quantity lost and gained by the whole earth in the course of a year, if not invariably the same, is at least periodical.