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 31° 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 2°.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 30 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 line, and in the northern latitudes of 45° and 68°, are in the proportion of the numbers 12, 4, and 1. Notwith

standing 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 Africathey 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

that enables them to absorb, decompose, and consolidate these substances into wood, leaves, flowers, and fruit. The atmosphere would soon be deprived of these elements of vegetable life, were they not perpetually supplied by the animal creation; while in return, plants decompose the moisture they imbibe, and having assimilated the carbonic acid gas, they exhale oxygen for the maintenance of the animated creation, and thus preserve a just equilibrium. Hence it is the powerful and combined influences of the whole solar beams that give such brilliancy to the tropical forests, while with their decreasing energy in the higher latitudes, vegetation becomes less and less vigorous.

By far the greater part of the hundred and ten thousand known species of plants are indigenous in Equinoctial America. Europe contains about half the number; Asia with its islands, somewhat less than Europe; New Holland with the islands in the Pacific, still less; and in Africa there are fewer vegetable productions than in any part of the globe of equal extent. Very few social plants, such as grasses and heaths, that cover large tracts of land, are to be found between the tropics, except on the sea-coasts and elevated plains: some exceptions to this, however, are to be met with in the jungles of the Deccan, Khandish, &c. In the equatorial regions, where the heat is always great, the distribution of plants depends upon the mean annual temperature; whereas in temperate zones the distribution is regulated in some degree by the summer heat. Some plants require a gentle warmth of long continuance, others flourish most where the extremes of heat and cold are greater. The range of wheat is very great: it may be cultivated as far north as the 60th degree of latitude, but in the torrid zone it will seldom form an ear below an elevation of 4500 feet above the level of the sea, from exuberance of vegetation; nor will it ripen above the height of 10,800 feet, though much depends upon local circumstances. Colonel Sykes states that in the Deccan wheat thrives 1800 feet above the level of the sea. The best wines are produced between the 30th and 45th degrees of north latitude. With regard to the vegetable kingdom, elevation is equivalent to latitude, as far as temperature