spring be on elevated ground, the temperature is reduced by computation to what it would be at the level of the sea, assuming that the heat of the soil varies according to the same law as the heat of the atmosphere, which is about a degree of Fahrenheit's thermometer for every 656 feet. From a comparison of the temperature of numerous springs with that of the air, Sir David Brewster concludes that there is a particular line passing nearly through Berlin, at which the temperature of springs and that of the atmosphere coincide; that in approaching the Arctic Circle the temperature of springs is always higher than that of the air, while proceeding towards the equator it is lower. He likewise found that the isogeothermal lines are always parallel to the isothermal lines, consequently the same general formulæ will suffer 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 American portions of the isogeothermal line of 32° Fahrenheit actually separate, and go round the two poles of maximum cold. This traveller reinarked also, that the temperature both of the air and of the soil decreases most rapidly towards the 45° of latitude. The temperature of the ground at the equator is lower on the coasts and islands than in the interior of the continents; the warmest part is in the interior of Africa, but the temperature is obviously affected by the nature of the soil, especially if it be volcanic.

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 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 47°.84. At Kasan, the mean summer temperature is 64°-84, and that of winter 2012, whereas at Edinburgh the mean summer temperature is 58° 21, and that of winter 38° 66. Whence it appears that the difference of winter temperature is much greater than that of the 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 Petersburg; 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 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.

SECTION XXVII.

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 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 a hundred and fifty or two hundred 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 existvery 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, enamelled with flowers, exhibits the last signs of vegetable life during the short but fervent summers at the polar regions. Such is the effect of cold on the vegetable kingdom, that the numbers 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. But notwithstanding the remarkable difference between a tropical and polar Flora, moisture seems to be almost the only requisite for vegetation, since neither heat,

ence in

cold, nor even darkness destroy the fertility of nature; in salt plains and sandy deserts alone hopeless barrenness prevails. Plants grow on the borders of hot springsthey form the oases, wherever moisture exists, among the burning sands of Africa-they are found in caverns 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, under this cold defence, plants spring up, dissolve the snow a few inches round, and that the part above, being again quickly frozen into a transparent sheet of ice, admits the sun's rays, which warm and cherish the plant in this natural hot-house, till the returning summer renders such protection unnecessary.

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. 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 60° 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 the exuberance of vegetation; nor will it ripen above the height of 10800 feet, though much depends upon local circumstances. The best wines are produced between the 30° and 45° of north latitude. But with regard to the vegetable kingdom, elevation is equivalent to latitude, as far as temperature is concerned. In ascending the mountains of the torrid zone, the richness of the tropical vegetation diminishes with the height; a succession of plants similar, though not identical with those found in latitudes of corresponding mean temperature takes place; the lofty forests lose by degrees their splendor, stunted shrubs succeed, till at last the progress of the lichen is checked by eternal snow. On the volcano of Teneriffe, there are five successive zones, each producing a distinct race of plants. The first is the region of vines, the next that of laurels, these are followed by the districts of pines, of mountain broom, and of grass; the whole covering the declivity of the peak through an extent of 11200 feet of perpendicular height.

Near the equator the oak flourishes at the height of 9200 feet above the level of the sea, and on the lofty range of the Hymalaya the primula, the convallaria, and the veronica blossom, but not the primrose, the lily of the valley, or the veronica which adorn our meadows; for although the herbarium collected by Mr. Moorcroft on his route