10° or 12°, as appears from American Meteorological Tables *. The causes of this remarkable difference are many. The highest part of North America lies between the 40th and 50th degree of north latitude, and the 100th and 110th degree of longitude west from London; for there the greatest rivers originate. The very height, therefore, makes this spot colder than it otherwise would be. It is covered with immense forests, and abounds with large swamps and morasses, which render it incapable of receiving any great degree of heat; so that the rigour of winter is much less tempered by the heat of the earth than in the old continent. To the east lie a number of very large lakes; and farther north, Hudson's Bay; about 50 miles on the south of which there is a range of mountains, which prevent its receiving any heat from that quarter. This bay is bounded on the east by the mountainous country of Labrador and by a number of islands. Hence the coldness of the north-west winds and the lowness of the temperature. But as the cultivated parts of North America are now much warmer than formerly, there is reason to expect that the climate will become still milder when the country is better cleared of woods, though perhaps it will never equal the temperature of the old continent.

Islands are warmer than continents in the same degree of lati tude; and countries lying to the windward of extensive mountains or forests are warmer than those lying to the leeward. Stones or sand have a less capacity for heat than earth has, which is always

• For the following statement of the extremes of heat and cold at Montreal and Three Rivers in Canada, I am indebted to an ingenious officer, who kept a register for eight years, from the year 1776 to 1784, inclusive.

"In the warmest summer the thermometer was not observed to rise higher than 91°, though it has been said to have risen so high as 96° and even 98° at Quebec ; but where these observations were made, the thermometer was generally from 80° to 84° in the warmest summers, and the average of the ordinary summers was about 70o.

"In the severe frosts the thermometer sunk to 45o below 0. This happened three times within this period, viz. on the 23d and 25th February 1782, and os the 10th February 1784.

"In the ordinary winters, at Three Rivers, the mercury stood at from 10° to 25° below 0, and in thr severe winters from 250 to 35o below 0

"The summer observations were taken at about nine o'clock in the morning, and three in the afternoon. The winter, before sun-rise, and about nine, and sometimes ten o'clock at night.

somewhat moist; they heat or cool, therefore, more rapidly, and to a greater degree. flence the violent heat of Arabia and Africa, and the intense cold of Terra del Fuego. Living vegetables alter their temperature very slowly, but their evaporation is great; and if they be tall and close, as in forests, they exclude the sun's rays from the earth, and shelter the winter snow from the wind and the sun. Woody countries, therefore, are much colder than those which are cultivated,

1

[Thomson's Chemistry.

The atmosphere is also liable to elevations and depressions ana. logous to those of the Sea, and perhaps these changes may have some little effect on the winds and on the weather; but their influence must be very inconsiderable, since the addition of two or three feet to the height of the atmosphere at any part can scarcely be expected to be perceptible. The height of an aerial tide must be nearly the same with the observed height of the principal tides of the sea and the variation of atmospherical pressure, which is measured between the difference of the actual form and the spheroid of equilibrium, must be equivalent to the weight of a column of about ten feet of air, or only of an inch of mercury. A periodical variation five times as great as this, has, indeed, been observed near the quator, where the state of the atmosphere is the least liable to accidental disturbances: but this change cannot in any degree be referred to the effect of the moon's action, since it happens always about the same hour of the day or night. The atmosphere is also affected by a general current from east to west, like that of the sea, and there is reason, from astronomical observations, to suppose that a similar circumstance happens in the atmosphere of Jupiter, on account of the actions of his satellites, which must be considerably more powerful than that of the moon. [Young's Nat. Phil.

CHAP. XXXV.

ZONES AND CLIMATES.

ZONES and Climates are artificial divisions of the earth's surface for the purpose of defining its temperature in particular parts; and they have hence an intimate connection with the temperature of the atmosphere, upon which indeed they are mainly though not altogether dependent.

Geographers have divided the surface of the globe into sixty climates, of which thirty are northward of the equator, and thirty southward; by these climates the length of the day, from sun. rising to sun.setting, is shewn, in its increase and decrease proceeding from the equator to the poles; from the equator to the arctic, or northern polar circle, twenty-four climates are traced out, and through each a difference of half an hour arises in the length of the day and night; and in like manner from the equator to the antarctic, or southern polar circle. The six climates which lie between the polar circles and the poles, both the northern and the southern, differ from each other by one entire month progressively.

As climates describe the length of days, so zones describe the degree of heat prevailing on different parts of the earth. Both ancient and modern geographers agree in dividing the earth into five zones in number and three in quality, namely, the torrid, the temperate north and south, and the frigid both north and south. The torrid zone extends from the equator to the tropic of cancer north ard, and to the tropic of capricorn southward, twentythree degrees and an half each (very nearly), making forty-seven degrees in all. The two temperate zones extend themselves from the two tropics to the polar circles on each side the equator, being forty-three degrees cach, and eighty-six degrees in the whole.The two frigid zones embrace the regions from the polar circles to the poles, extending, in each direction, over twenty-three and au

half degrees, in the whole forty.degrees. Thus northward or southward,

In the whole........ 90 degrees, which is the

distance from the equator to either pole; but considering the northern and southern regions in one account, the general division of the globe into zones will stand thus :

The torrid zones......47 degrees.

earth from north to south, which geographers have divided into two hemispheres, viz. the northern and southern.

The ancients, having very imperfect knowledge of the globe, considered the two temperate zones as the only habitable parts of the earth, conceiving the heat of the torrid, and the cold of the frigid zones to be equally insupportable. This opinion is now well known to be erroneous; mankind having been ascertained to exist within the arctic circle; and some species of quadrupeds and birds even as high up as eighty degrees; while the attempts made by Captain Cook to penetrate within the antarctic circle, when he proceeded something beyond the seventy-first degree, proves that, although there is no known land lying much more southward than the sixtieth degree of south latitude, yet birds of curious kinds inhahit the expanse of ice which, in every direction, stops all advance toward the south-pole. At the same time that the discovery of the West Indies and of the American continent, had clearly established that the population of mankind either is or may be rendered equal within the region of the torrid zone to that of the best inhabited countries of Europe.

The climates of different parts of the earth's surface are unquestionably owing in great measure to their position with respect to the sun. At the equator, where the sun is always nearly vertical, any given part of the surface receives a much greater quantity of light and heat, than an equal portion near the poles; and it is

also still more affected by the sun's vertical rays, because their passage through the atmosphere is shorter than that of the oblique. rays. As far as the sun's mean altitude only is concerned, it appears from Simson's calculations, that the heat received at the equator in the whole year, is nearly twice and a half as great as at the poles; this proportion being nearly the same as that of the meridan heat of a vertical sun, to the heat derived, at the altitude 2340, in the middle of the long annual day at the poles. But the difference is rendered still greater, by the effect of the atmosphere, which interrupts a greater portion of the heat at the poles than elsewhere. Bouguer has calculated, upon the supposition of the similarity of the affections of heat and light, that in latitude 45°, 80 parts out of 100 are transmitted at noon in July, and 55 only in December. The heat intercepted by the atmosphere is perhaps not wholly, but very nearly, lost with respect to the climate of the neighbouring places. It is obvious that, at any individual place, the climate in summer must approach in some degree to the equatorial climate, the sun's altitude being greater, and in winter to the climate of the polar regions.

While the earth is becoming warmer at any particular spot, the heat thrown off by radiation into the atmosphere, and thence into the empty space beyond it, together with that which is transmitted to the internal parts of the earth, must be less than the heat received from the sun; and when the earth is growing colder, more heat must pass off than is received: but whenever the heat of the surface is stationary, neither increasing nor diminishing, as at the times of the greatest and least heat, it is obvious that the heat re ceived from the sun must be precisely equal to the heat which is thrown off. Now this quantity may be estimated by the degree of refrigeration in the night; and hence Mr. Prévost has very ingeniously deduced the proportion of the sun's heat arriving at the surface of the earth in the latitude of Geneva, in July, and in December; which he finds to be as 7 or 8 to 1; and this result agrees very well with a calculation deduced from the length of the day, the sun's altitude, and the interception of his rays by the atmosphere.

In London the temperature generally varies, in the course of the day and night, somewhat more than 5o, and less than 20°. In January, the mean diurnal variation of temperature is 6o, in