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much affects the climate, and as a general rule the western sides of continents and large islands, are warmer than the eastern. Certain portions of the globe, which from their nearness to the equator would be extremely warm, are rendered tolerably cool by their elevated situations. This is the case with much of the tropical land in America, which is so raised that it rivals even European climates in mildness and agreeable temperature. The air of these elevated tropical districts is remarkably pure and transparent, and the winds which sweep over the plains, are cooled by their passage down the snow-capped mountains, which rear their bright summits to the skies. The vast expanse of table-land, forming the empire of Mexico is of this character, being elevated 7000 feet above the level of the ocean. This land in many parts has the fertility of a cultivated garden. The plains of Columbia in South America, and indeed all along the ridge of the Andes, are similarly situated. The chart which we have given represents the direction of the isothermal lines, or lines connecting places which have the same mean annual heat. It will be evident that places may thus be situated on the same isothermal line, which have very unequal mean temperatures of summer and winter. We need only refer to the table on page 157, to be convinced of this. Thus, the mean annual temperature of London, and Cambridge, Mass. is the same, 50°36'; 'but the mean temperature o the warmest month at London is 64°40', while at Cambridge it is 72°86', and of the coldest month, at London 37°76, at Cambridge 29.84, London therefore has a colder summer and a warmer winter than Cambridge. The reason of this, is undoubtedly, the insular situation of the former, for as a general rule the extremes of temperature are experienced in large inland tracts, and little felt in islands remote from continents. The difference between the mean temperature of summer and winter is nothing at the equator, and increases continually with the latitude. When the mean annual temperature is low the difierences between the ́extremes of the seasons is great, and the contrary.

The effect of climate upon the plants and animals is very marked.

geographical distribution of Each, generally has its pe

culiar climate where it thrives best, and beyond certain limits it ceases to exist. The successive zones of vegetation, as we recede from the equatorial regions, have sometimes been supposed to be represented by the different altitudes upon the mountains under the equator, as it is evident we have in ascending from the valleys to their snow-capped summits, every variety of temperature. The analogy fails however in one essential point, for as we ascend the mountains the pressure of the atmosphere is continually diminished and it is evident that less nutriment is thus afforded for the growth of the plant. The influence which the variations of climate alluded to, must have upon vegetation is very evident, thus in many parts of Siberia, wheat and rye are raised upon a soil which is constantly frozen at a depth of three feet, while in Iceland, where the mean temperature of the year is much warmer, and the winter's cold but inconsiderable, it is not possible to raise any of the ceralia or common grains, as the low summer temperature does not suffer them to ripen. It is for the same reason that the vine does not flourish in England, for although it can endure a tolerably great degree of cold, yet it requires a hot summer to make the fruit ripen, and yield a drinkable wine. There is ne subject connected with meteorology which requires a more careful, and studied investigation than that of climates. So many causes influence the temperature of the air, and some of them are so variable, that no labor short of a well conducted series of observations, extending through a long course of years can give a satisfactory result. In the brief account we have given, we have been able to present little else than the leading facts, and must refer the reader to the writings of Leslie, De Candolle, Mirbel, and Humbolt, for further information.

TABLE OF TEMPERATURES.

157

TABLE

Exhibiting the mean temperature of various places compiled principally from the observations of Baron Alex. Von Humboldt.

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