II Under the second, and more modern acceptation of the word climate, we have to notice the leading features of that great variety of condition and temperature, which pervades the atmosphere of the globe in different regions.

These have been sometimes treated under four very obvious divisions. The cold and humid; cold and dry; warm and humid; hot and dry. 1. A cold and humid climate is such as pervades the eastern shores of Canada, Newfoundland, and Siberia, where the atmosphere, loaded with fogs, yields little encouragement to settlers, and the few vegetable productions only seem to increase the general gloominess of the scene, being confined to a few hardy shrubs. The fenny districts of more temperate regions, as those of some parts of the eastern shores of England, partake of this climate in a modified degree. 2. A cold and dry climate prevails in most of the northern countries of Europe and Asia, in the winters of Upper Canada, &c. and this seems not to be inimical even to delicate human constitutions, while the finest streams of water and abundant vegetation attend it. 3. Warm and humid climates pervade large districts of both the Old and New Worlds; as, Guinea, Demerara, Panama; Hindostan, particularly Bengal, Zanguebar, and Senegal. Here vegetation luxuriates, and is, at the same time, vigorous; the verdant plains are diversified by gigantic trees; and, while the miasmatic vapors render the atmosphere uncongenial to man, the noblest and most ferocious of animals, reptiles of immense size, and beautiful specimens of the feathered creation abound. 4. Hot and dry climates are those of the African and Arabian deserts, where plants, animals, and man, alike languish under burning suns, water is evaporated, and the earth is sand, or iron, and the heavens brass.' These four climates do not, however, as Mr.

Myers observes, always exist according to the full import of the terms by which they are designated. They are subject to various modifications, particularly of two distinct kinds. The one results from the alternation of two different climates in the same region; the other, from the greater or less prevalence of either of the four elements. Thus, when heat, dryness, and humidity are duly combined, they render the climate comparatively temperate. In Egypt, for instance, the combinations of heat and humidity, during the inundation of the Nile, and of heat and dryness during the rest of the year, temper a climate, which, without these alternations, would be insupportable. In Holland the cold humidity of the autumn is succeeded by frost, which increases the salubrity of the climate, that would otherwise not be so healthy. In some places, however, the changes take place so rapidly, or the difference of temperature is so great, that it renders the climate more pernicious to the constitution, than though only one of the kinds existed. The inhabitants of Astrachan and some other cities experience the heats of Africa in summer, and the colds of Siberia in winter. The same constitution of the atmosphere is also agreeably modified, in some instances, by the solar heat; for the dry heat which renders the great desert of Sahara almost inaccessible, becomes a pleasant temperature at either Madrid or Marseilles. The fatal effects of humid heat are less powerful as we advance from the equator; and on the contrary the cold, either dry or humid, is more supportable as we quit the depths of the polar regions and approach towards the tropics. Bergen and Brest have the same winter constitution of the atmosphere, which is rendered humid and varied by the contiguity of the western ocean, but the annual temperatures of the two places are widely different.

The varieties of season connected with this question, we cannot enter into in this place; further than to observe, that in the torrid zone the winter is generally a wet, the summer a dry, season; and that this constitutes the great division of their year: but these are in direct opposition to those seasons, as they would result from the position of the sun in the ecliptic. The rain always accompanies the sun, so that, when this luminary is in the northern signs of the zodiac, the countries on that side of the equator have their rainy season. The vertical rays of the sun continually rarify the atmosphere in these regions, the air of colder districts rushes in to restore the equilibrium, the vapors become condensed, and a deluge of rain is the consequence. Those parts of the torrid zone, where there is scarcely any evaporation, have no rain; and in other places the mountains so modify the monsoons as to produce two rainy seasons in the year.' See SEASON.

Professor Leslie insists that all the varieties of climate are reducible to these two causes-distance from the equator, and height above the level of the sea. Latitude and local elevation form, indeed,' says he, 'the great bases of the law of Climate, and any other modifications have only a partial and very limited influence.'

The most obvious effects of latitude appear, ir the degree of obliquity given to the solar rays, as

Ab B represents the hemisphere, and the dotted semicircle CD the upper limit of the atmosphere; Ea and Eb are the extreme rays of a pencil of light falling upon the earth at a and b. And E'a and E' c, the extreme rays of an equal pencil, when the sun has a less elevation, as at the winter solstice. It will be evident that the rays reach the surface of the earth in a much more attenuated state in the one case than in the other; and that, as the number of rays in the two pencils is the same, their density will be inversely as the areas of the elliptic spaces they cover. These are as the transverse axes ab, ac, since the two conjugate axes are equal. The pencil Ea has also a much greater space to pass through the terrestrial atmosphere, than Ea, by which its influence is still further weakened. These causes unite, therefore, in diminishing the solar influence, as the altitude of that luminary decreases; and as this altitude is greatest at the equator, where the sun is vertical, and decreases as the latitude increases, the propriety of the appellations given to the different zones becomes evident, as well as the effects which such a circumstance must have upon the general temperature of the different regions upon which the sun shines. But, in addition to what we call the height of the sun, the distance of that luminary from the earth must always be taken into consideration in the estimate of different climates. The ratio of these distances at the summer and winter solstices is nearly as 30 to 29; and the number of rays that fall on the same space are inversely as the squares of these distances; and, consequently, in this case, as 900 to 841.-Hence the solar influence in winter is to that in summer, in reference to the distance of the sun only, as 900 to 841, or very nearly as 1.0702 to 1. length of the day, or the continuation of the sun above the horizon, certainly increases the heat of particular regions, as the shortness of the night also affords less time for a dispersion of the heat accumulated. Refraction and reflection also modify the action of the sun's rays. M. Bouguer has calculated that of 10,000 rays which fall perpendicularly upon the atmosphere, only 8123 reach the surface of the earth; and, if their angle of incidence be 50°, not more than 7624 fall on its surface; that the number was reduced to 2031 when the angle was 7°; and to

The

5° only when the direction was horizontal. And the earth itself, as absorbing a number of these rays and returning them to the air by reflection, becomes a great source of heat; distance from the earth must therefore be a source of cold; thus we find that, as you ascend in the atmosphere, the cold increases. In the vicinity of Paris, the temperature of the earth being 47°, at the estimated height of 11,084 feet it was found to be 21°, or 11° below congelation, by M Charles, who ascended in a balloon. And lord Mulgrave, at the bottom of Hackluyt-hill, lat. 80°, found the temperature of the air 50°; but on the top, at the height of 1503 feet, only 42°. Hence we find, that the highest mountains, even under the equator, have their tops continually covered with Snow. M. Bouguer found the cold of Pinchina, one of the Cordelieres, immediately under the line, to extend from 7° to 9° below the freezing point every morning before sun-rise; and hence at a certain height, which varies in almost every latitude, it constantly freezes at night all the year round, though in the warm climates it thaws in the same degree the next day. This height M. Bouguer calls the lower term of congelation : between the tropics he places it at the height of 15,577 feet, English measure. And thus while the base of these mountains rests on burning sand, about half way up, in the plains of Quito, we found a temperate zone of the most delightful character. As the hot winds from below ascend the sides of the mountains, they become so cooled by the expansion of the air, that they do not affect the snow on the summits; and the cold winds which sweep over their snowy crests, and descend to the lower regions, are condensed as they proceed, and acquire a temperate warmth before they reach these fertile plains.

A great source of cold is evaporation. The same cause which makes the condensation of vapor a source of heat, makes evaporation the source of cold; as it absorbs the fire in the latter instance, which it gives out in the former: the heat thus absorbed is called latent heat; it producing, in that state, no sensation of warmth. At a certain height above the lower term of congelation it never freezes, not because the cold decreases, but because the vapors do not ascend so high; this height M. Bouguer calls the upper term of congelation, and under the equator he fixes it at the height of 28,000 feet. Kirwan has given us the following mean height of the upper and lower terms of congelation, for the latitude of every 5°, in feet.

Sometimes the temperature of the upper air is higher than that of the lower air, particularly when a large mass of vapors is condensed by electrical agency; for no part of the heat given out by that cause being lost by communication with air much colder, that which surrounds the vapors so condensed, must be heated to a ccnsiderable degree. The clouds, by absorbing the sun's rays, are more heated than the clear air would be. These, and other circumstances, render the true height of the terms of congelation, at any time, subject to considerable uncertainty.

Of evaporation, the following facts may be observed: 1. That, in our climates, evaporation is about four times as great from the 21st of March to the 21st of September, as from the 21st of September to the 21st of March. 2. That, other circumstances being the same, it is greater in proportion as the difference between the temperature of the air, and that of the evaporating surface is greater; and so much the smaller, as the difference is smaller; and therefore smallest, when the temperature of the air and evaporating liquor are equal. The former part of this proposition however requires some restriction; for, if air be more than 15° colder than the evaporating surface, there is scarcely any evaporation; but, on the contrary, it deposits its moisture on the surface of the liquor. 3. The degree of cold produced by evaporation is always much greater when the air is warmer than the evaporating surface, than that which is produced when the surface is warmer than the air. Hence, warm winds, as the Sirocco and Harmatan, are more drying than cold winds. 4. Evaporation is more copious when the air is less loaded with vapors, and is therefore greatly promoted by cold winds flowing into warmer countries. 5. Evaporation is greatly increased by a current of air or wind flowing over the evaporating surface, because unsaturated air is constantly brought into contact with it. Hence, calm days are hottest, as has commonly been remarked. 6. Tracts of land covered with trees or vegetables emit more vapor than the same space covered with water. Mr. Williams (Philadelphia Transactions) found this quantity to amount to one-third more. Hence, the air about a wood or forest is made colder by evaporation from trees and shrubs, while the plants themselves are kept in a more moderate heat, and secured from the burning heat of the sun, by the vapors perspired from the leaves. Thus, we find the shade of vegetables more effectual to cool us, as well as more agreeable, than the shade from rocks and buildings: and from cause the clearing away of woods lessens the vapors, and consequently diminishes the quantity of rain, and increases the temperature. Several parishes in Jamaica which used to produce fine crops of sugar canes, are now dry for nine months in a year, and are turned into cattle-pens through the clearing away of the woods. Hence again, water is most plentiful in those countries where woods abound, and the best springs are there found. Since the woods in the neighbourhood of the American towns have been cut down, many streams have become dry; and others have been reduced so low, as to

the same

cause great interruptions to the miller. It appears probable, that the climates of European countries were more severe in ancient times than they are at present. Cæsar says, that the vine could not be cultivated in Gaul, on account of its winter-cold. The rein-deer, now found only in the zone of Lapland, was then an inhabitant of the Pyrenees. The Tiber was frequently frozen over, and the ground about Rome covered with snow for several weeks together, which very rarely happens in our times. The Rhine and the Danube, in the reign of Augustus, were generally frozen over for several months of winter. The barbarians who overran the Roman empire a few centuries afterwards, transported their armies and waggons across the ice of these rivers.

Drainage of the ground, and removal of forests, however, cannot be reckoned among the sources of the increased warmth of the Italian winters. Chemical writers, says Dr. Ure, have omitted to notice an astronomical cause of the progressive amelioration of the climates of the northern hemisphere. In consequence of the apogee portion of the terrestrial orbit being contained between our vernal and autumnal equinox, our summer half of the year, or the interval which elapses between the sun's crossing the equator in spring, and in autumn, is about seven days longer than our winter half year. Hence, also, one reason for the relative coldness of the southern hemisphere.

In the article CLIMATE (Supplement to the Encyclopædia Britannica) the following simple rule is given for determining the change of temperature produced by sudden rarefaction, or condensation of air. Multiply 25 by the difference between the density of air and its reciprocal, the product will be the difference of temperature on the centigrade scale. Thus, if the density be twice, or one half 25° × (2—1)

37° cent. 67.5° Fahrenheit, indicates the change of temperature by doubling the density or rarity of air. Were it condensed thirty times, then, by this formula, we have 749° for the elevation of temperature, or 25° (30°). But M. Gay Lussac says, that a condensation of air into one-fifth of its volume is sufficient to ignite tinder; a degree of heat which he states at 300° centigrade = 572 Fahrenheit (Journal of Science, vol. vii. p. 177). This experimental result is incompatible with professor Leslie's formula, which gives only 112.5° for the heat produced by a condensation into one-fifth.

The sea exercises an important equalising influence on the temperature of the globe. In the tropical regions a large extent of ocean spreads coolness on every side, and affords a perpetual succession of refreshing breezes. Islands are always, comparatively, of more temperate climates than continents, and those scattered over the expanse of the Pacific, may be said to enjoy almost a perpetual spring. The districts which are surrounded on every side by tracts of continent, experience no mitigation of heat, and are often utterly consumed by the droughts of summer: insular tracts also, and those situated along the sea-coast, experience much less rigorous winters than the interior of continents. The greatest cold in our hemisphere is said to occur

when any country has a wide extent of sea to the south, and of land to the north. Thus Greenland, in lat. 60°, exhibits a more rigorous climate than Lapland, in lat. 72°. From the like cause, the north-east extremity of Asia suffers a cold almost equally intense; and the same combination of circumstances renders the climate of North America, under the same parallel, much colder than that of Europe.

Nor is the influence of winds in general, and the trade-winds in particular, here to be forgotten. Blowing from east to west across the sands of Africa, the latter produce, on its western coast, a most intense heat, much greater than is experienced on the eastern. In passing the Atlantic, they are considerably cooled; and though, in traversing South America, their temperature is again raised, yet, before reaching the opposite coast, they meet the tremendous snow-clad Andes, which stop their progress, and diffuse a wide coolness.

Again, the mountain ranges of the earth not only present and retain on their sides a refreshing coolness; but, by the mighty rivers to which they give rise, diffuse a great amelioration of the temperature through extensive regions. They are particularly of this character, and give rise to the largest rivers in the torrid and burning climes of the earth. In the temperate climates, and those approaching to the poles, mountains are of moderate elevation, are almost always barren, and give rise to few considerable streams. Professor Mayer, from a comparison of observations, constructed the following empirical rule for finding the relation between the latitude and the mean temperature, in centesimal degrees, at the level of the sea. Multiply the square of the cosine of the latitude by the constant number 29, the product is the temperature. The variation of temperature for each degree of latitude is hence denoted centesimally with very great precision, by half the sine of double the latitude.

that the annual accumulation would increase the average temperature of those regions. But this is not the case. The perpetual motion and currents of the atmosphere preserve a maximum temperature, which is but little varied; for, as the air of the equatorial regions becomes warmer, the northern winds have the greater tendency to rush in upon it with rapidity, and check the ex

cess.

'But within the arctic circle,' observes an intelligent writer, another powerful agent of nature is constantly tempering the inequality of the seasons. The vast beds of snow, or fields of ice which cover the land and sea in these dreary retreats, absorb, in the act of thawing or passing again into their liquid form, all the surplus heat collected during a nightless summer. The rigor of winter, when darkness resumes her tedious reign, is likewise mitigated by the warmth evolved as congelation spreads over the watery surface.'

Experiments have been made, both at home and abroad, to ascertain the comparative temperature of the earth below its surface, as compared with that of the atmosphere; and they have differed very little.

In the caves below the observatory at Paris, in 49° of north latitude, and about eighty-five feet below the surface, Fahrenheit's thermometer constantly stands between 52° and 54°, and scarcely ever varies 2°; while at the surface the differencé of temperature, between summer and winter, sometimes exceeds 90°. In the salt mines at Wieliczka, near 50° of latitude, from the depth of 320 to that of 745 feet, the thermometer stands at about 50°. At Cairo, in Egypt, latitude 30°, at the bottom of Joseph's well, the depth of which exceeds 210 feet, the thermometer stands at 70°. In the mines of Mexico, at 20° of latitude, the temperature at the depth of 1650 feet, was 7410; thus it augments in approaching the equator.'-Lacroix's Geo. Physique.

Mr. Leslie reports some very interesting experiinents on this subject, made in the garden of his friend, Robert Ferguson, esq. of Abbots-hall, in a gravelly soil, lat. 56° 10′, and proving how slow is the transmission of heat through the body of the earth. The thermometers were sunk to the depth of one, two, four, and eight feet, and the transmission from the surface appears to have been about an inch per day. The first of these thermometers never sunk lower than 33°, and indicated a mean temperature of 45°-5, which shows that the frost seldom penetrates to that depth. The nature of the soil, however, and external circumstances, must have a great influence on this penetration, as has been proved by experiment. In the same paper it is stated that, in the neighbourhood of Edinburgh, after a long continuance of rigorous weather, the frost was found to have penetrated thirteen inches into the ground in a ploughed field, but only eight inches in one piece of pasture ground, and four inches in another. But, in some of the streets of that city, the frost had descended even below two feet, so as to begin to affect the waterpipes. The greater density and solidity of the pavement had no doubt conducted the frigorific impressions more copiously downwards, while

On this subject, Mr. Leslie remarks, that, If the thermometer had been sunk considerably deeper, they would, no doubt, have indicated a mean temperature of 47.7°. Such is the permanent temperature of a copious spring which flows at a short distance, and about the same elevation, from the side of a basaltic, or green-stone rock. Profuse fountains and deep wells, which are fed by percolation through the crevices of the strata, furnish the surest and easiest mensuration of the temperature of the earth's crust. The body of water which bursts from the caverns of Vaucluse, and forms almost immediately a respectable and translucid river, has been observed not to vary in its temperature, by the tenth part of a degree (centigrade) torough all the seasons of the year. It is therefore an object highly important for scientific travellers, to notice the precise heat of springs in favorable situations, as they issue from their rocky beds. Such choice observations would accurately fix the medium temperature of the climate. It is only requisite to exclude the superficial and the thermal springs, which are not difficult to distinguish.' See the article CLIMATE. Supplement to the Encyclopædia Britan

nica.

the sur

The mean temperature of the air, near face of the earth, has also been ascertained at various places. At Paris and Cairo it was found to correspond nearly with the numbers before stated. At St. Petersburgh in lat. 60° the mean temperature is about 39°. At Wadso, in Lapland, in 70° of latitude, it was found to be about 36°; and in the island of Mageröe, near the North Cape, the mean temperature of the year is stated, by M. Von Buch, to be nearly 32°; the mean for every month in the same situation, is inserted at page 312 of this volume. According

to M. Humboldt, the hottest places are on the southern shores of the Caribbean sea, and the gulf of Guayaquil, in the great equinoxial ocean, between two and three degrees of south latitude. There the mean heat is 81.5°; and the thermometer sometimes rises to 106°. At Belbeis, in Egypt, the thermometer has risen to more than 125 in the shade; but this was occasioned by the hot wind, denominated Sirocco. At St. Petersburgh, on the contrary, the cold is sometimes so intense as to congeal mercury. It has also been observed, at the same place, to rise above 90°.

In the first number of the Edinburgh Philosophical Journal, some facts are stated by Mr. Bald apparently incompatible with the idea of the interior temperature of the earth being deducible from the latitude of the place, or the mean temperature at the surface.

The following table presents at one view the temperature of air and water in the deepest coal-mines in Great Britain.