Vegetation, instead of being universal, would have been confined to a narrow zone; and the whole human race would have been driven together into one limited habitable space, to interfere with, incommode, and destroy each other. The arrangement is best as it is.
We find very important modifications of temperature, occasioned not only by astronomical influences, but by local causes and geographical characteristics. For while, as a general rule, the nearer we approach the equator, the warmer we shall be, yet temperature is greatly affected by mountains, seas, currents of air or water, by radiation, by forests, and by vegetation. It is found, in fact, that the lines of temperature, (the happy conception of Humboldt,) when they are traced upon the map, are anything but true zones or circles.
The line of the greatest mean warmth is not coincident with the equator, but falls to the north of it. This line at 160° W. Long, from Greenwich is 4° below the geographical equator; at 80° it is about 6° north, sweeping along the coast of New Granada; at 20° it comes down and touches the equator; at 40° E. Long., it crosses the Red Sea about 16° north of the equator, and at 120° it falls at Borneo, several degrees below it;--and the points of the greatest heat, in this line, are in Abyssinia, nearer the tropic of Cancer than to the equator. On the other hand, the greatest mean cold points, according to the opinions of Humboldt, Sir David Brewster, and others, do not coincide, as would seem natural, with the geographical poles, but they are both to be found in the northern hemisphere, in Latitude 80°, 95°E. Long. and 100° W. Long. from Greenwich. The western is ascertained to be 4-1/2° colder than the eastern or Siberian. If this be the fact,--but it is not positively admitted,--an open sea at the pole may be considered as probable, on the ground of its having a higher mean temperature than is found at 80°. Kaemptz places one of these cold points at the north of Barrow's Straits,--the other near Cape Taimur, in Siberia. Burghaus, in his Atlas, transfers the American cold pole to 78° N. Lat. It is perhaps too early to determine rigorously the true temperature of these points.
A noticeable fact also is this,--that places in the same latitude rarely receive the same amount of heat. Quebec, in British America, and Drontheim, in Norway, enjoy about the same quantity, while the former is in 47° and the latter in 68° N. Lat. The mean winter temperature of Pekin, 39° 45' N. Lat., is 5° below the freezing-point; while at Naples, which is north of Pekin, it seldom, if ever, goes below it, and Paris, 500 miles farther north, has a mean winter temperature of 6° above the freezing-point. The city of New York, about 11° south of London, has a winter temperature of much greater severity. The mean temperature of the State of New York, as determined by a long series of observations, is 44° 31'.
The mean temperature of countries is found to be very stable, and but very small variations have been detected in modern times. But that there have been important climatic changes, since the Christian era, cannot be doubted, unless we doubt history. Not many centuries ago, it was a common thing for all the British rivers to freeze up during the winter, and to remain so for several months. If space permitted, an interesting statement could he made of the changes which have taken place in vegetation in Greenland, and throughout certain northern parts of Europe,--also in Palestine, Greece, and other southern countries,--while we know that the earth's inclination upon its axis has been unchanged.
Mrs. Somerville remarks, that, though the temperature of any one place may be subject to very great variations, yet it never differs from the mean state more than a few degrees.
Without this atmospheric covering of ours, it is considered that the temperature of the earth at its surface would be the same as that of the celestial spaces, supposed to be at least 76° below zero, or possibly, says Humboldt, 1400° below! Human life, without our atmosphere, could not exist for a single moment.
It is computed, that, if the annual heat received by the earth on its surface could be equally distributed over it, it would melt, in the course of a year, a stratum of ice 46 feet thick, though it covered the whole globe, and as a consequence the amount of unradiated heat would render it uninhabitable.
The relative position of the sun affects temperature, rather than its distance. In winter the earth is three millions of miles nearer the sun than in summer, but the oblique rays of the former season reach us in less quantity than the more direct The distribution of land and water, the nature of the soil, the indentation of bays, the elevation of land above the sea-level, insularity, etc., all, as we have already suggested, have a modifying influence on temperature.
The atmosphere possesses also a reflecting and refracting power, arising from its varying density, and, perhaps, in the latter case, somewhat from its lenticular outline.