open to the western 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. 123), which expresses the law of the change of temperature, is a maximum toward the 45th degree of latitude. The mean annual temperature under the line in America is about 810 of Fahrenheit: in Africa it is said to be nearly 83°. 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 22d 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 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 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 22°, afterward deviate more and more. From the observations of Sir Charles Giesecke in Greenland, of Captain Scoresby in the Arctic Seas, and also from those of Sir Edward Parry and Sir John Franklin, 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 America and the other in the north of Asia, neither of which coincides with the pole of the earth's rotation. These poles are both situate in about the 80th parallel of north latitude. The transatlantic pole is in the 100th degree of west longitude, about 5° to the north of Sir Graham Moore's Bay, in the Polar Seas; and the Asiatic pole is in the 95th degree of east longitude, a little to the north of the Bay of Taimura, near the North-east Cape. According to the estimation of Sir David Brewster, from the observations of M. de Humboldt and Captains Parry and Scoresby, the mean annual temperature of the Asiatic pole is nearly 1° of Fahrenheit's thermometer, and that of the transatlantic pole about 3}° below zero, whereas he supposes the mean annual temperature of the pole of rotation to be 4° or 5°. 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.

Sir

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. 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 depend

ing 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 traveler remarked, also, that the temperature both of the air and of the soil decreases most rapidly toward 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 8° or 9°; whereas the difference in the latitude of those having the same mean winter temperature is sometimes as much as 18° or 19°. At Kasan in the interior of Russia, in latitude 55°-48, nearly the same with that of Edinburgh, the mean annual temperature is about 37°.6; at Edinburgh it is 470.84. At Kasan, the mean summer temperature is 64°.84, and that of winter 20.12; whereas at Edinburgh the mean summer temperature is 58°-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, traveling is performed in sledges, and frequently on the ice of the river St. LawThe cold at Melville Island on the 15th of January, 1820, according to Sir Edward Parry, was 55° below the zero of Fahrenheit's thermometer, only 3° above the temperature of the ethereal regions, yet the summer heat in these high latitudes is insupportable.

rence.

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. 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 is invariably the same.

SECTION XXVII.

Influence of Temperature on Vegetation-Vegetation varies with the Lati tude and Height above the Sea-Geographical Distribution of Land Plants-Distribution of Marine Plants-Corallines, Shell-fish, Reptiles, Insects, Birds, and Quadrupeds-Varieties of Mankind, yet Identity of Species.

THE gradual decrease of temperature in the air and in the earth, from the equator to the poles, is clearly indicated by its influence on vegetation. In the valleys of the torrid zone, where the mean annual temperature is very high, and where there is abundance of light and moisture, nature adorns the soil with all the luxuriance of perpetual summer. The palm, the bombax ceiba, and a variety of magnificent trees, tower to the height of 150 or 200 feet above the banana, the bamboo, the arborescent fern, and numberless other tropical productions, so interlaced by creeping and parasitical plants as often to present an impenetrable barrier. But the richness of vegetation gradually diminishes with the temperature the splendor of the tropical forest is succeeded

by the regions of the olive and vine; these again yield to the verdant meadows of more temperate climes; then follow the birch and the pine, which probably owe their existence in very high latitudes more to the warmth of the soil than to that of the air. But even these enduring plants become dwarfish stunted shrubs, till a verdant carpet of mosses and lichens, enameled with flowers, exhibits the last sign of vegetable life during the short but fervent summers at the polar regions. Such is the effect of cold and diminished light on the vegetable kingdom, that the number of species growing under the fine, and in the northern latitudes of 45° and 68°, are in the proportion of the numbers 12, 4, and 1. Notwithstanding the remarkable difference between a tropical and polar Flora, light and moisture seem to be almost the only requisites for vegetation, since neither heat, cold, nor even comparative darkness, absolutely destroy the fertility of nature. In salt plains and sandy deserts alone, hopeless barrenness prevails. Plants grow on the borders of hot springs-they form the oasis wherever moisture exists, among the burning sands of Africa— they are found in caverns almost void of light, though generally blanched and feeble. The ocean teems with vegetation. The snow itself not only produces a red alga, discovered by Saussure in the frozen declivities of the Alps, found in abundance by the author crossing the Col de Bonhomme from Savoy to Piedmont, and by the polar navigators in the Arctic regions, but it affords shelter to the productions of those inhospitable climes against the piercing winds that sweep over fields of everlasting ice. Those interesting mariners narrate, that ander this cold defence plants spring up, dissolve the snow a few inches round, and the part above being again quickly frozen into a transparent sheet of ice, admits the sun's rays, which warm and cherish the plants in this natural hot-house, till the returning summer renders such protection unnecessary.

The chemical action of light is, however, absolutely requisite for the growth of plants which derive their principal nourishment from the atmosphere. They consume carbonic acid gas, vapor, nitrogen, and the ammonia it contains; but it is the chemical agency of light