TABLE of the Height of the Upper Line of Congelation in the different▾ Latitudes of the Northern Hemisphere.

From this method of estimating the diminution of temperature, which agrees remarkably well with observation, we see that the heat diminishes in an arithmetical progression. Hence it follows, that the heat of the air at a distance from the earth, is not owing to the ascent of hot strata of air from the surface of the earth, but to the conducting power of the air.

3. This rule, however, applies only to the temperature of the air during the summer months of the year. In winter the upper strata of the atmosphere are often warmer than the lower. Thus on the 31st of January 1776, the thermometer on the summit of Arthur's Seat stood six degrees higher than a thermometer at Hawkhill, which is 684 feet lower*. Mr. Kirwan considers this

Roy. Phil. Trans. 1777, p. 777.

superior heat, almost uniformly observed during winter, as owing to a current of warm air from the equator, which rolls towards the north pole during our winter *.

4. Such, then, in general, is the method of finding the mean annual temperature over the globe. There are, however, several exceptions to these general rules, which come now to be mentioned.

That part of the Pacific Ocean which lies between north lat. 52o and 66", is no broader at its northern extremity than 42 miles, and at its southern extremity than 1300 miles: it is reasonable to suppose, therefore, that its temperature will be considerably influenced by the surrounding land, which consists of ranges of mountains. covered a great part of the year with snow; and there are besides a great many high, and consequently cold, islands scattered through it. For these reasons, Mr. Kirwan concludes, that its temperature is at least four or five degrees below the standard. But we are not yet furnished with a sufficient number of observations to determine this with accuracy.

It is the general opinion that the southern hemisphere, beyond the 40th degree of latitude, is considerably colder than the corresponding parts of the northern hemisphere. Mr. Kirwan has shown that this holds with respect to the summer of the southern hemisphere, but that the winter in the same latitudes is milder than in the northern hemisphere †.

Small seas surrounded with land, at least in temperate and cold climates, are generally warmer in summer and colder in winter than the standard ocean, because they are a good deal influenced by the temperature of the land. The Gulph of Bothnia, for instance, is for the most part frozen in winter; but in summer it is sometimes heated to 70°, a degree of heat never to be found in the opposite waters of the Atlantic . The German sea is above three degrees colder in winter, and five degrees warmer in summer than the Atlantic §. The Mediterranean sea is, for the greater part of its extent, warmer both in summer and winter than the Atlantic, which therefore flows into the former. The Black Sea is colder than the Mediterranean, and flows into it.

The eastern parts of North America are much colder than the opposite coast of Europe, and fall short of the standard by about

* Irish Trans. viii. p. 375. † Irish Trans. viii. p. 417. Kirwan's Temperature of Lat. p. 53.

Mem. Stock. 1776.

H Ibid.

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 latitude; 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 940, though it has been said to have risen so high as 96o and even 98o 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 70°.

"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 on the 10th February, 1784.

"In the ordinary winters, at Three Rivers, the mercury stood at from 10° to 25° below 0, and in the severe winters from 25° 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. Hence 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.

[Thomson's Chemistry.

The atmosphere is also liable to clevations and depressions analogous 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 equator, 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 obser vations, 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 dependant.

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 sunrising 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 northward, 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 each, 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 an