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southward, bearing these icy masses past its shores. similar cause is also assigned, by M. Baer, for the temperate climate of the western shores of the Old World; that being preserved from extreme cold by the protection afforded by Nova Zembla, and a submarine ridge extending from thence to Spitzbergen, which forms a barrier to the ice accumulated on, and borne down by the great rivers of Siberia.

The effect produced on climate by the relative position of land and water, is exhibited on a large scale in the difference of climate observable in the northern and southern hemispheres. The space occupied by the waters of the ocean is much greater in the southern than in the northern hemisphere, and consequently, a much more equable temperature prevails in the former than in the latter. Thus, in Tasmania, or Van Diemen's Land, which is situated about the same distance south of the equator, as Rome is to the north of that line, the winters are milder than at Naples, and the summers not warmer than at Paris.

It is, however, a remarkable fact, that in high latitudes in the southern hemisphere, the cold is much greater than in similar parallels in the northern hemisphere. Thus, in the island of Georgia, which is in the same latitude as Yorkshire, the ground is covered with snow at all seasons of the year. This greater severity of climate has been supposed to arise, partly from the absence of any large surface of land in the temperate region of the southern hemisphere, capable of reflecting the heat of the sun, and partly from the occurrence of some yet undiscovered elevated land within the south polar circle; to which may be added, the want of any protecting ridge to screen it from the cold of the latter regions, and the floating ice-bergs.

The elevation of land above the level of the sea, is another leading cause of the diversity of climate. We have already seen that the air becomes rarefied, or less dense, as we ascend above the level of the sea; we also find a gradual decrease of temperature, in proportion to the elevation of land above that level, (this decrease being about a degree of Fahrenheit

for every 656 feet,) until we attain a point where perpetual congelation takes place. This point, which is called the snow-line, or line of perpetual snow, varies in different latitudes, and even in similar latitudes under different conditions; but, generally speaking, a gradual decrease in the elevation of the line above the sea level takes place, as we approach the frozen regions of the polar circles. The limit of the snow-line within the tropics, is about 16,000 or 17,000 feet above the level of the sea; and in the northern hemisphere, this line descends to the level of the sea, at about latitude 80°. In the southern hemisphere, as we have just seen, the shores of the island of Georgia, situated in latitude 56°, are covered with perpetual frost; it will therefore be evident that the same rules will not apply to both hemispheres; little, however, is yet known on this subject relative to the southern hemisphere, but the following table will convey a general idea of the snow-line in the northern hemisphere, though, owing to local variations, this can only be regarded as an approximation to the truth :

In considering the above table, we shall perhaps feel some surprise, at finding the snow-line higher at the distance of twenty degrees from the equator, than at the equator itself. This, however, is readily explained by the circumstance that, at the equator, the sun is never more than twelve hours above the horizon; whereas, near the tropics, the longest days are thirteen hours and a-half in length; and,

as the sun at that period is vertical, or nearly so, the summer heat, on which the line of perpetual snow depends, is greater than directly under the equator.

The gradual decrease of temperature at increasing elevations above the level of the sea, has a marked and very beneficial effect, in diversifying the climates over the surface of the globe, and in fitting it for the production and maintenance of an almost endless variety of plants and animals. All countries situated between the tropics are subject to an excessively high temperature; and in such localities, when the land is nearly level with the sea, the heat is extreme; and unless furnished with most abundant supplies of rain, they present a highly parched and arid appearance; such regions being more destitute of vegetation, and consequently of animals, than almost any others on the surface of the globe. Of this description is the Sahara, or Great Desert of Africa. Where tracts of land occur, similar in latitude and elevation, but abundantly supplied with rain, the most luxuriant fertility prevails, enlivened by innumerable tribes of animated beings, but usually insalubrious, and unfitted for the abode of man. Such are the swampy and pestilential shores of Comayagua, or Honduras. At an elevation of about 3,000 feet above the sea, however, a totally different climate prevails; thus, the elevated lands of Central America, at no great distance from the last-mentioned district, are described by Colonel Galindo, as presenting the most delightful climate imaginable, where "perpetual spring and verdure ever exist, and realize the dream of the ancient European poets, who, without experiencing it, imagined a similar climate for their favourite Elysian fields, or the island of Calypso." At still greater elevations we arrive at the line of perpetual snow. The mountains of Central America, or Guatemala, however, scarcely attain that height above the sea; but the Andes present a vast number of snowcapped summits within the torrid zone.

The snow-line, as has been already observed, is subject to variation in different localities, though situated in the

same parallels of latitude. Thus, it appears that an elevated plain, or table-land, in the vicinity of a mountain, tends to elevate the temperature of the mountain, and consequently to raise the snow-line, which will be found to be higher on a mountain so situated, than on one that is isolated, or where the ascent is more sudden and precipitous. As it has just been stated, that the temperature of elevated land is not so high as that of low plains, we might expect the very reverse to be the case. A familiar illustration, if we may be permitted to introduce it, will perhaps afford the clearest explanation of this phenomenon.

The higher temperature on mountains so situated, appears to be owing to the effect produced on the atmosphere by the radiation or the reflection of the sun's rays from the high table-land; an effect very similar to this may be observed in our own dwellings. If the windows of our apartments are situated near the ground, we may, in hot sunny weather, perceive that the temperature of our rooms is increased by the reflection of the sun's rays, more especially if they fall on a stone pavement, for the amount will vary according to the nature of the surface; and should it consist of turf or meadow land, the reflection will be scarcely perceptible. If, then, we ascend into our upper apartments, we shall find the reflection from the stone pavement-partly owing to the greater distance, and partly to the different angle or direction from the ground to the upper window— will, if not wholly without effect, be far less intense, and will communicate little or no heat to the upper rooms. Should, however, a balcony, or a portico, extend in front of our upper windows, this will form no bad representation of a table-land adjoining a mountain; and will, by reflecting the sun's rays directly into our upper rooms, elevate their temperature.

The stupendous Himalaya mountains afford a striking instance of the effect produced by such an elevated tableland. On the southern side of this mighty range, in lat. 30°, the snow-line commences at the elevation of about 12,500

or 13,000 feet above the level of the sea; whilst on the northern side, where, from the aspect, we might be led to expect a greater degree of cold, the snow-line does not descend below 16,000 feet above the sea. Our readers will readily perceive that this is capable of a satisfactory explanation, when they are reminded that the northern declivity of this mountain mass terminates in the immense elevated plains of Tibet, the surface of which exceeds 10,000 feet above the level of the sea; whilst on the southern side, the mountains rise directly to a great elevation, from a flat country scarcely 1,000 feet above the sea, and covered with a close jungle; of all descriptions of surface the least favourable for radiating heat.

In high latitudes, however, where table-lands occur of sufficient elevation to be covered with snow during the greater part of the year, the very reverse takes place; and in such situations, the temperature will be lower on mountains so circumstanced, than on such as are isolated. And it has even been observed that, in sunny calm weather, the temperature on isolated mountains, in very high latitudes, increases with the elevation. Thus, in Spitzbergen, the temperature at a lower station was found to be about 36°; and at the summit of a mountain, 1,542 feet above the sea, it rose to 40°.

Among the causes which produce some effects on the climate of any region, though not so important as the preceding, is the nature of the soil. This is principally owing to the greater or less power any soil possesses of radiating heat. Thus, sandy soils are subject to become rapidly and intensely heated, and when the sun's rays are withdrawn, they readily radiate, or impart to the atmosphere, the heat they have acquired, thus increasing the general temperature. Clayey soils, on the other hand, become slowly heated, and as slowly part with heat. Swampy ground chills the air, and extensive forest tracts have a similar effect. And thus, cultivation not unfrequently effects a change in the climate of any country; for if marshes are