The word Climate, or κλίμα, being derived from the verb κλίνειν, to incline, was applied by the ancients to signify that obliquity of the sphere with respect to the horizon from which results the inequality of day and night. The great astronomer and geographer Ptolemy distinguished the surface of our globe, from the equator to the arctic circle, into climates or parallel zones, corresponding to the successive increase of a quarter of an hour in the length of midsummer-day. Within the tropics, these zones are nearly of equal breadth; but, in the higher latitudes, they contract so much, that it was deemed enough to reckon them by their doubles, answering consequently to intervals of half an hour in the extension of the longest day. To compute them is an easy problem in spherical trigonometry. As the sine of the excess of the semidurnal arc above a quadrant is to the radius, so is the tangent of the obliquity of the ecliptic, or of 23° 28', to the cotangent of the latitude. The semidurnal arcs are assumed to be 91° 52' 1", 93° 45', 95° 37' 47", 97° 30', &c.; and the following table, extracted from Ptolemy's great work, will give some general idea of the distribution of seasons over the surface of our globe:

| Latitude | Length of Midsummer-Day | |----------|-------------------------| | I. | 12h 00m | | II. | 12h 00m | | III. | 12h 00m | | IV. | 12h 00m | | V. | 12h 00m | | VI. | 12h 00m | | VII. | 12h 00m | | VIII. | 12h 00m | | IX. | 12h 00m | | X. | 12h 00m | | XI. | 12h 00m | | XII. | 12h 00m | | XIII. | 12h 00m |

These numbers are calculated on the supposition that the obliquity of the ecliptic was 23° 51' 20", to which, accord- Climate. ing to the theory of Laplace, it must have actually approached in the time of Ptolemy. They seem to be affected by some small errors, especially in the parallels beyond the seventeenth, as the irregular breadth of the zone abundantly shows; but they are, on the whole, more accurate than those given by Varrenius.

Ptolemy describes the general appearances which the heavens will present on each parallel, and assigns the corresponding lengths of the shadow of the gnomon at both solstices. He justly maintains, in opposition to the more ancient opinion, that the equatorial region is habitable, since the action of the sun, not continuing long vertical, is there mitigated; but he will not venture to describe the inhabitants, because no person, he says, having yet penetrated so far south, the reports circulated respecting them appeared to be merely conjectural. He therefore passes over the first parallel to the second.

This second parallel, then, according to Ptolemy, runs through the isle of Taprobana, supposed to be Ceylon, in the latitude of 4° 15'. The third parallel, in the latitude of 8° 25', traverses the gulf of Adalitus. The fourth parallel crosses the Adalitic gulf, in latitude 12° 45'. The fifth parallel passes through the isle of Merie, in Upper Egypt, at latitude 16° 27'. The sixth parallel runs through the territory of the Napati, in latitude 20° 15'. All these climates or parallels lying below the tropic, the inhabitants are therefore Amphibians, or see the sun pass twice over their heads in the course of the year. The seventh parallel, at the latitude of 23° 51', and consequently bordering the tropic, runs through Syene, in Upper Egypt. The eighth parallel, in latitude 27° 12', traverses Ptolemais, in the Thebaid. The ninth zone, corresponding to a day of fourteen hours of length, passes through Lower Egypt, at the latitude of 36° 12'. The tenth parallel, in latitude 33° 18', runs through the middle of Phoenicia. The eleventh parallel, at the thirty-sixth degree of latitude, passes through the isle of Rhodes. The twelfth parallel, in latitude 38° 25', crosses Smyrna. The thirteenth parallel traverses the Hellespont, in latitude 40° 56'. The fourteenth parallel, in latitude 43° 4', runs through Marseilles. The fifteenth parallel passes through the middle of the Pontic Sea, in latitude 45° 1'. The sixteenth parallel runs through the sources of the Ister or Danube, in latitude 46° 51'. The seventeenth parallel, corresponding to a day of sixteen hours in length, traverses the mouths of the Boryatines, in latitude 48° 32'. The eighteenth parallel, at the latitude of 50° 4', crosses the Palus Marotis. The nineteenth parallel passes through the most southern part of Britain, in latitude 51° 40'. The twentieth parallel crosses the mouths of the Rhine, in latitude 52° 56'. The twenty-first parallel passes through the mouths of the Tainas, in latitude 54° 30'. The twenty-second parallel, at the fifty-fifth degree of latitude, traverses the country of the Brigantes, in Great Britain, that is, the southern and larger portion of this island, reckoning from the Frith of Forth. The twenty-third parallel, in the fifty-sixth degree of latitude, passes through the middle of Great Britain. The twenty-fourth parallel, at the latitude of 57°, runs through Cataraconium, in Great Britain. The twenty-fifth parallel, corresponding to a day of eighteen hours, runs through the southern parts of Little Britain, in latitude 58°. The twenty-sixth parallel, corresponding to a day of 18½ hours in length, traverses the middle of Little Britain, in latitude 58° 30'. It should be observed that the latitudes of the places in our own island are most inaccurately given by Ptolemy, and generally advanced about two or three degrees farther north than their true position. By Little Britain he meant undoubtedly that part of Scotland which lies on the north side of the Friths of Forth and Clyde, and forms almost a peninsula.

The high zones become so narrow, that Ptolemy separates the twenty-sixth to an interval of half instead of a quarter of an hour in the length of the day; but he thinks it superfluous to extend this subdivision farther into such remote and inhospitable countries. Resuming the calculation, however, he places the parallel where midsummer day is prolonged to nineteen hours, in the latitude of 61°, or the north of Little Britain. The parallel of 19½ hours would pass through the Ebudes, Hebrides, or Western Isles, in latitude 62°. The parallel of twenty hours runs through the island of Thule, in the latitude of 63°. The parallel of twenty-one hours would traverse the unknown Scythian nations, in latitude 64°. The parallels of twenty-two and twenty-three hours would run through the latitudes of 65° and 66°. He places in latitude 66° 8' 40' the arctic circle itself, where the sun does not set during the whole of midsummer day. Within this circle the inhabitants are Perissians, or have the sun lingering above the horizon during part of the summer, and the shadow of the gnomon successively projected in every direction. In the latitude of 67°, the sun continues almost a whole month above the horizon; in the latitude of 69°, it shines two months; in the latitudes 73°, 78°, and 84°, that luminary displays his presence for three, four, and five months. At the pole itself, the sun appears, during the space of six months, describing circles parallel to the horizon.

its modern acceptation, signifies that peculiar condition of the atmosphere in regard to heat and moisture which prevails in any given place. The diversified of this character which it displays has been generally referred to word. The combined operation of several different causes, which are chiefly reducible, however, to these two: distance from the equator, and height above the level of the sea. Latitude and local elevation form, indeed, the great bases of the law of climate; but as we shall soon see, other causes are not to be neglected in the estimation of differences of climate.

If we dig into the ground, we find the temperature to become gradually more steady, till we reach a depth of perhaps forty or fifty feet. When this perforation is made during winter, the ground gets sensibly warmer till the limit is attained; but in summer, on the contrary, it grows always colder, till it has reached the same limit. At a certain depth, therefore, under the surface, the temperature of the ground remains quite permanent.

It would be a hasty conclusion, however, to regard this limit of temperature as the natural and absolute heat of our globe. If we dig on the summit of a mountain, or any very elevated spot, we shall discover the ground to be considerably colder than in the plain below; or, if we make a similar perforation on the same level, but in a more southern latitude, we shall find greater warmth than before. The heat thus obtained at some moderate depth is hence only the mean result of all the various impressions which the surface of the earth receives from the sun and the atmosphere.

The method employed hitherto for ascertaining the temperature at different depths under ground, consists in digging a hole, and burying a sluggish thermometer for several hours, or the space of a whole night. The celebrated naturalist and accurate observer, Saussure, in the month of October 1785, made an interesting set of observations on the banks of the Arve, near Geneva. By digging downwards on successive days, he reached at last the depth of 31 feet. While the surface of the ground had retained a heat of 60°3 by Fahrenheit's scale, the temperature of the earth at the depth of 4 feet was 60°8, at 16 feet 56°, at 21 feet 53°6, and at 28 feet 51°8. A thermometer buried 31 feet deep was found, when taken up in summer, to stand at 49°5, and when raised in winter, to indicate 52°2. Notwithstanding this great depth, therefore, it had still felt the vicissitude of the seasons, having varied 2°7 in the course of the year. The extreme impressions must have taken six months to penetrate to the bulb, since the temperature was lowest in summer and highest in winter. But this plan of observing is clumsy and imperfect, there not being sufficient time to allow the mass of earth to regain its proper degree of heat, and too much for the instrument to retain its impression unaltered before it can be raised up and observed. In order to throw distinct light on a subject so curious and important, Robert Ferguson, Esq. of Raith, a gentleman whose elegant mind is imbued with the love of science, caused a series of large mercurial thermometers, with stems of unusual length, to be planted in his spacious garden at Abbotshall, about 50 feet above the level of the sea, and near a mile from the shore of Kirkcaldy, in latitude 56° 10'. The main part of each stem having a very narrow bore, had a piece of wider tube joined above it; and to support the internal pressure of the column of mercury, the bulbs were formed of thick cylinders. The instruments, inclosed for protection in wooden cases, were then sunk beside each other to the depths of one, two, four, and eight feet, in a soft gravelly soil, which turns, at four feet below the surface, into quicksand, or a bed of sand and water. These thermometers were carefully observed from time to time by Mr Charles Norval, the very intelligent gardener at Raith; and we have had access to a register of their variations for nearly three years. It thence appears that, in this climate, and on naked soil, the frost seldom or never penetrates one foot into the ground. The thermometer at that depth fell to 33° of Fahrenheit on the 30th December 1815, and remained at the same point till the 12th February 1816; but in the ensuing year it descended no lower than 34°, at which it continued stationary from the 23rd December 1816 to 1st January 1817. At the same depth, of one foot, it reached the maximum 58° on the 13th July 1815, but in the following year it rose only to 54° on the 21st July; and in the year 1817 it mounted to 56° about the 5th July. This thermometer, in the space of three years, travelled, therefore, over an interval of 25°, the medium being 45½°, and attained its highest and lowest points about three weeks after the solstice of summer and of winter.

The thermometer planted at the depth of two feet sunk to 36° on the 4th February 1816; but it stood at 38° about the beginning of January 1817. It rose to 56° on the 1st of August 1815; but in the next year it reached only 53° on 24th July; and, in 1817, it again reached 56° on 10th July. At the depth of two feet, the extreme variation was therefore 20°; and the maxima and minima took place about four or five weeks after either solstice.

The thermometer of four feet depth had sunk to 39° about the 11th February 1816, and was stationary at 40° near the 3d February 1817. It rose to 54° on the 2d August 1815, and stood at 52° during the greater part of August and September in the years 1816 and 1819. It ranged, therefore, only 15°, the mean being 46½°, and the extreme points occurring near two months after either solstice.

The thermometer whose bulb was planted eight feet deep descended to 42° on the 16th February 1816, but stood at 49½° on the 11th February 1817. It rose to 51½° on the 11th September 1815, fell to 50° on the 14th September 1816, and mounted again to 51° on the 20th September 1817. This thermometer had, therefore, a range of only 9½°, the medium temperature being 46½°, and the extremes of heat and cold occurring nearly three months after the solstice of summer and of winter.

These observations are quite satisfactory, and exhibit very clearly the slow progress by which the impressions of heat or cold penetrate into the ground. It will not be far from the truth to estimate the rate of this penetration at an inch every day. The thermometers hence attained their maximum at different periods, though in a tolerably regular succession. The mean temperature of the ground, however, seemed rather to increase with the depth; but this anomaly evidently proceeded from the coldness of the two successive summers, and particularly that of 1816, which occasioned such late harvests and scanty crops. Thus the thermometer of one foot indicated the medium heat of only 43° during the whole of the year 1816. But it will be satisfactory to exhibit the leading facts in a tabular form. The following are the mean results for each month, only those for December 1817 are supplied from the corresponding month in 1815.

| Month | 1816 | 1817 | |-------------|------|------| | January | 33°-0 | 36°-3 | | February | 33°-7 | 36°-0 | | March | 35°-0 | 36°-7 | | April | 39°-7 | 38°-4 | | May | 44°-9 | 43°-3 | | June | 51°-6 | 50°-0 | | July | 54°-0 | 52°-5 | | August | 50°-0 | 52°-5 | | September | 51°-6 | 51°-3 | | October | 47°-0 | 49°-3 | | November | 40°-8 | 43°-8 | | December | 33°-7 | 40°-0 |

If the thermometers 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 greenstone 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 Vauncluse, and forms almost immediately a respectable and translucent river, has been observed not to vary in its temperature by the tenth part of a degree, through all the seasons of the year. It is, therefore, an object highly important for scientific travellers to notice the precise heat of springs in favourable situations, as they issue from their rocky beds. Such observations would generally afford the medium temperature of any climate. It is only requisite to exclude the superficial and the thermal springs, which are not difficult to distinguish.

It must, however, be recollected that at considerably greater depths a higher temperature is found. This was first observed in very deep mines, and was by some ascribed to the heat of the miners' candles and of their bodies, or to the sudden evolution of the latent heat of air extricated by its condensation in such confined spaces. But the temperature has been found to be too high for such causes; and the great increase of heat observed in the waters of Artesian wells, according to their profundity, as was ably pointed out by Arago, is a strong proof of the existence of a source of temperature within the earth itself, independent of solar influence. That this cause, however, does not affect the temperature of the outer crust of our earth is obvious from the steadiness of mean temperature in different places, which Arago has shown, over Europe at least, to have been for some centuries, as nearly as we can judge from the freezing of certain rivers, lakes, and seas in winter, about what it is still found to be. It is, no doubt, the effect of radiation, and of the movements of the atmosphere, and of the ocean which have combined to produce the steadiness of the mean temperature.

From a comparison of meteorological observations made Mayer's rule at distant points on the surface of our globe, the celebrated astronomer Professor Mayer of Göttingen has endeavoured to discover an empirical law which connects the various re- of any place. Round the pole, the mean temperature he assumes This table, and the formula on which it is calculated, though once supposed to afford a pretty accurate idea of the mean temperature of different latitudes at the level of the sea, can now only be received as an ingenious though erroneous speculation: for the researches of later observers have shown that it accords not with observations either in high latitudes, or in regions 70° or 80° W. or E. of the meridians of Paris and London. At the time the astronomer Mayer published his formula, the mean temperature of scarcely three or four points on the globe were accurately ascertained; his formula is therefore founded on very imperfect data, and the subsequent labours of travellers and navigators have demonstrated the fallacious grounds from which it was deduced. We may also remark that Mayer was misled by Bouguer's too high estimate of the equatorial mean temperature, which is assumed to be 24° Réaumur; but the more accurate determinations of Humboldt make it no more than 22° Réaumur, which is equivalent to 81° Fahrenheit. Among the earliest to afford more correct data we must reckon British navigators and scientific travellers. Humboldt, in discussing the difference of terrestrial temperature, says—"To begin with the extreme north, I shall here mention a man in the first place whom the dangerous occupation of whale fishing has not prevented from carrying on the most refined meteorological observations. Scoresby has for the first time determined the mean temperature of the polar seas, which he has ascertained between the volcanic island of Jan Mayen and that part of East Greenland discovered by himself. Parry, Sabine, and Franklin have for several years been employed in investigating the temperature of the atmosphere and the sea in the polar regions; they have penetrated to Port Bowen and Melville Island, therefore nearly to 75° N. Lat., and they have in this arduous task displayed a perseverance of which we scarcely find a parallel in the history of human exertions and struggles against the elements."

Humboldt also pays a high compliment to the exertions of Captain Weddell, who has overthrown a prejudice that was even sanctioned by the illustrious Cook, viz., that the south polar ice renders the antarctic far less accessible than the arctic seas. Since Humboldt's memoir was written, the discoveries of Sir James Ross far in the antarctic regions have confirmed that observation. The information afforded by the arctic voyages are so important and well established, that we hesitate not to give a summary of their observations on the mean temperature of high northern latitudes.

| Lat. | Centesimal | Dif. | Fahrenheit | Dif. | Lat. | Centesimal | Dif. | Fahrenheit | Dif. | Lat. | Centesimal | Dif. | Fahrenheit | Dif. | |------|------------|-----|------------|-----|------|------------|-----|------------|-----|------|------------|-----|------------|-----| | 0 | 29°00' | -00 | 84°2' | -00 | 30 | 21°75' | -43 | 71°1' | -77 | 60 | 7°25' | -44 | 45°0' | -79 | | 1 | 28°99' | -01 | 84°2' | -02 | 31 | 21°31' | -44 | 70°3' | -79 | 61 | 6°82' | -43 | 44°3' | -77 | | 2 | 28°96' | -03 | 84°1' | -05 | 32 | 20°86' | -45 | 69°5' | -81 | 62 | 6°39' | -43 | 43°5' | -77 | | 3 | 28°92' | -04 | 84°0' | -07 | 33 | 20°40' | -46 | 68°7' | -83 | 63 | 5°98' | -41 | 42°8' | -77 | | 4 | 28°86' | -06 | 83°9' | -11 | 34 | 19°93' | -47 | 67°9' | -84 | 64 | 5°57' | -41 | 42°0' | -74 | | 5 | 28°78' | -08 | 83°8' | -13 | 35 | 19°46' | -47 | 67°0' | -85 | 65 | 5°18' | -39 | 41°3' | -71 | | 6 | 28°68' | -10 | 83°6' | -18 | 36 | 18°98' | -48 | 66°2' | -86 | 66 | 4°80' | -38 | 40°6' | -68 | | 7 | 28°57' | -11 | 83°4' | -20 | 37 | 18°50' | -49 | 65°3' | -87 | 67 | 4°43' | -37 | 40°0' | -67 | | 8 | 28°44' | -13 | 83°2' | -23 | 38 | 18°01' | -49 | 64°4' | -88 | 68 | 4°07' | -36 | 39°3' | -65 | | 9 | 28°29' | -15 | 82°9' | -27 | 39 | 17°50' | -49 | 63°5' | -88 | 69 | 3°72' | -35 | 38°7' | -63 | | 10 | 28°13' | -16 | 82°6' | -30 | 40 | 17°01' | -49 | 62°6' | -89 | 70 | 3°39' | -33 | 38°1' | -60 | | 11 | 27°95' | -18 | 82°3' | -32 | 41 | 16°52' | -49 | 61°7' | -90 | 71 | 3°07' | -32 | 37°5' | -57 | | 12 | 27°75' | -20 | 82°0' | -36 | 42 | 16°02' | -50 | 60°8' | -90 | 72 | 2°77' | -30 | 37°0' | -54 | | 13 | 27°53' | -22 | 81°6' | -40 | 43 | 15°52' | -50 | 59°9' | -91 | 73 | 2°48' | -29 | 36°5' | -52 | | 14 | 27°30' | -23 | 81°1' | -42 | 44 | 15°01' | -51 | 59°0' | -91 | 74 | 2°20' | -28 | 36°0' | -50 | | 15 | 27°06' | -24 | 80°7' | -44 | 45 | 14°50' | -51 | 58°1' | -92 | 75 | 1°94' | -26 | 35°5' | -47 | | 16 | 26°80' | -26 | 80°2' | -47 | 46 | 13°99' | -51 | 57°2' | -92 | 76 | 1°70' | -24 | 35°1' | -43 | | 17 | 26°52' | -28 | 79°7' | -50 | 47 | 13°49' | -50 | 56°3' | -91 | 77 | 1°47' | -23 | 34°6' | -41 | | 18 | 26°23' | -29 | 79°2' | -52 | 48 | 12°98' | -51 | 55°4' | -91 | 78 | 1°25' | -22 | 34°2' | -40 | | 19 | 25°93' | -30 | 78°7' | -54 | 49 | 12°48' | -50 | 54°5' | -90 | 79 | 1°05' | -20 | 33°9' | -36 | | 20 | 25°61' | -32 | 78°1' | -57 | 50 | 11°98' | -50 | 53°6' | -90 | 80 | -86' | -19 | 33°6' | -34 | | 21 | 25°28' | -33 | 77°6' | -60 | 51 | 11°49' | -49 | 52°7' | -89 | 81 | -71' | -17 | 33°3' | -31 | | 22 | 24°93' | -35 | 76°9' | -63 | 52 | 10°99' | -50 | 51°8' | -90 | 82 | -56' | -15 | 33°0' | -27 | | 23 | 24°57' | -36 | 76°2' | -65 | 53 | 10°50' | -49 | 50°9' | -88 | 83 | -43' | -13 | 32°8' | -23 | | 24 | 24°20' | -37 | 75°6' | -67 | 54 | 10°02' | -48 | 50°0' | -87 | 84 | -32' | -11 | 32°6' | -20 | | 25 | 23°82' | -38 | 74°9' | -68 | 55 | 9°54' | -48 | 49°2' | -86 | 85 | -22' | -10 | 32°4' | -18 | | 26 | 23°43' | -39 | 74°2' | -70 | 56 | 9°07' | -47 | 48°3' | -85 | 86 | -14' | -08 | 32°3' | -15 | | 27 | 23°02' | -41 | 73°5' | -72 | 57 | 8°60' | -47 | 47°5' | -84 | 87 | -08' | -06 | 32°2' | -11 | | 28 | 22°61' | -42 | 72°7' | -74 | 58 | 8°14' | -46 | 46°6' | -83 | 88 | -04' | -04 | 32°1' | -07 | | 29 | 22°18' | -43 | 71°9' | -76 | 59 | 7°69' | -45 | 45°8' | -81 | 89 | -01' | -03 | 32°0' | -05 | In Scoresby's *Arctic Regions* the mean temperature of the atmosphere of the Greenland seas, about Lat. 78° N., as deduced from 18 whale-fishing voyages, is for the month of April 14°2 Fahr., for June 31°4, for July 37°, or for the three warm months 26°-27°5, while, from fair data, he calculates the mean annual temperature of that latitude at no more than 17°0. In his "Greenland Voyage" of 1822, alluded to by Humboldt, he found the mean temperature for the five months in which he navigated above Lat. 65° thus: April 32°-22, May 21°-42, June 32°-975, July 34°-30, August 35°-247, giving a mean temperature for the whole of 32°-231.

In Sir Edward Parry's two winter sojourns in the arctic regions we have still more decisive proofs of the fallacies of Mayer's formula. The following table of mean temperature is from the account of his third voyage, p. 71.

| Months | Melville Island, Lat. 74° | Wister Island, Lat. 66° | Igloolik, Lat. 69° | Port Bowen, Lat. 73° | |--------|--------------------------|------------------------|------------------|--------------------| | October | -6°46 | +9°51 | +9°79 | +10°85 | | November| -23°60 | +4°75 | -22°37 | -5°00 | | December| -24°79 | -15°94 | -30°80 | -19°05 | | January | -33°09 | -25°96 | -20°07 | -22°91 | | February| -35°19 | -27°97 | -23°41 | -27°32 | | March | -21°10 | -14°64 | -22°75 | -28°37 | | Mean | -24°04 | -11°71 | -18°27 | -16°30 |

The mean temperature for all the months of the year at Port Bowen and its vicinity, was found to be no more than +10°01. In the perilous sojourn of four successive winters by Sir John Ross in Boothia Felix, Lat. 70°, we have the results of observations made continuously for each month of two successive years, 1830, 1831, and the annual mean temperature for each is +2°38 and +4°91.

In Captain Sir John Franklin's journeys to the shores of the American Arctic Ocean, the mean temperature of Fort Franklin, Lat. 65°-12, during two years is thus given—

| Season | Temperature | |--------------|-------------| | Spring | +14°43 | | Summer | +50°40 | | Autumn | +20°00 | | Winter | -16°81 | | Mean | +11°46 |

The mean temperature for each month of 1826 at Fort Franklin is thus given in his second expedition—

| Month | Temperature | |-----------|-------------| | January | -23°78 | | February | -12°70 | | March | -8°26 | | April | +15°21 | | May | +30°35 | | June | +48°00 | | July | +52°10 | | August | +51°09 | | September | +39°18 | | October | +24°07 | | November | -3°01 | | December | -7°42 |

A comparison of all these observations shows an increasing degree of cold as the longitude increases from the meridian of western Europe.

These observations on mean temperature in North America and its seas, compared with observations made in Europe, sufficiently show that some other coefficients than latitude and elevation above the sea must affect mean temperature. In fact, ever since meteorological observations began to be made with comparable thermometers, this became obvious; but it has been very apparent within the last few years, when we had obtained accurate registers of the heat of various parts of Asia, and of certain intertropical countries. The comparison of numerous such registers from various parts of the new and of the old continent, together with his own very extensive and accurate investigations of atmospheric temperature in various parts of the northern Temperate and Torrid Zones, proved to the illustrious Alexander von Humboldt, that no empirical formula will enable us to ascertain the law which regulates the mean temperature of the surface of our globe; and that we must collect the facts by multiplied and accurate observation in each locality. His philosophical views on this subject were first published in an admirable essay in the third volume of *Mémoires de la Société d'Arcueil*. He there showed that the very striking difference in mean temperature observed in large tracts of country under the same latitude, and at the same elevation above the sea, could not arise from the trifling influence of mere local peculiarities, but must depend on more general causes; as the form of continents, the nature of their surface, and especially on their size, position, and proportion to the adjacent seas.

As above two-thirds of the surface of our globe are covered by water, and as a transparent body like water and an opaque one like the land absorb and radiate the solar heat with very different facility, when falling on them at similar angles, it is obvious that the relative proportion of land and water at any part of the earth's surface must greatly modify the mean temperature of that region. The radiation of heat from rough and smooth surfaces towards a cloudless sky is very different; and the opaque, rough surface of the land will cool more rapidly than the surface of the sea by radiation. The comparatively smaller portion of the ocean covered with ice and snow than of the land will also produce some difference of mean temperature in places under the same latitude.

The difference between land and water in receiving and transmitting heat constitutes the difference between an insular and a continental climate. Water is not so soon heated by the summer sun as land, therefore the air over it is cooler in summer; and as the land cools faster than the sea in winter, the warmer sea air mitigates the cold of that season. Compare, for instance, the mildness of the climate of the British Isles with that of Sweden or the north of Germany, under the same parallels. This effect is further increased by the oceanic currents that carry the waters of warmer regions to our shores, in that remarkable deviation of the Gulf stream that sets across the Atlantic from Newfoundland to the western shores of Europe.

Laying aside hypothetical speculation, this distinguished philosopher undertook a very careful discussion of his own observations, and those of other meteorologists. His first difficulty was in obtaining the true mean temperature of any locality. He found that the usual method of taking half the sum of the maxima and minima of the day and night, or of the summer and the winter, would not give the true daily or annual mean temperature. But repeated observation led him to the important conclusion, "that the thermometer at the moment of sunset in every season indicates very nearly the daily mean temperature." Humboldt's observations on this point were chiefly made between 46° and 48° N. Lat.; and there is strong reason to believe that the same holds good in other latitudes—a circumstance which will mightily abridge the labour and greatly facilitate the acquisition of those important results. Humboldt finds that in western Europe the mean temperature of latitudes 30°, 40°, 50°, 60° are respectively 70°5, 63°1, 50°2, and 40°6 Fahr.; but in eastern America, under the same parallels, are 60°9, 54°5, 42°, 38°. Over the tropical Atlantic the lines of equal temperature are nearly parallel to the equator. On the other hand, proceeding east from Europe the lines of equal temperature bend toward the south; showing that Europe enjoys a higher mean temperature than either eastern America or eastern Asia.

Humboldt conceived the beautiful idea of representing on a chart of the world these variations of mean temperature, ### Climate

Climate, by lines passing through places possessing the same annual mean temperature, which he denominates **isothermal lines**.

A specimen of such isothermals is given in Plate CLXXIII.

He has given a summary of his observations on mean temperature in the following table for the northern hemisphere, divided into 6 isothermal bands. The degrees of heat are reduced from Humboldt’s centigrade scale to that of Fahrenheit. This table not only gives the annual mean temperature, but that of the four seasons, considering December, January, and February as the three winter months. The prefixed asterisk (*) indicates those places where the mean temperature has been most accurately observed. The maxima and minima of the warmest and coldest months are also given:

#### Table of Climate Data

| Isothermal Bands | Positions | Distribution of Heat in the different Seasons | Maximum and Minimum | |------------------|-----------|-----------------------------------------------|---------------------| | | | Mean Temp. of the Year | Mean Temp. of Winter | Mean Temp. of Spring | Mean Temp. of Summer | Mean Temp. of Autumn | | | | Mean Temp. of Warmest Month | Mean Temp. of Coldest Month | | Latin Grade 20°W** | Nain | 57° 8' | 61° 20°W | 0 | 34°24 | 2 | 6 | 4 | 8 | | | Ennestsiae | 68° 30' | 20° 47' | +8°6 | 24°96 | 4 | 6 | 8 | 10 | | | Hospice | 66° 30' | 21° 47' | +2°6 | 26°96 | 3 | 5 | 8 | 10 | | | Gothenland | 63° 30' | 23° 16' | 0 | 40°6 | 4 | 6 | 8 | 10 | | | North Cape | 70° 1 | 25° W | 0 | 32°0 | 2 | 4 | 6 | 8 | | | Ule | 65° 3 | 25° E | 0 | 25°8 | 3 | 5 | 7 | 9 | | | Uome | 63° 5 | 20° 21' | 0 | 23°36 | 2 | 4 | 6 | 7 | | | St Petersberg | 39° 0 | 10° 19' | 0 | 30°84 | 3 | 5 | 7 | 9 | | | Dronethelm | 63° 24' | 20° 24' | 0 | 30°62 | 3 | 5 | 7 | 9 | | | Moscow | 56° 45' | 37° 32' | 2 | 97° | 0 | 4 | 6 | 8 | | | Assoc | 60° 34' | 11° 18' | 0 | 30°48 | 2 | 4 | 6 | 8 | | Latin Grade 21°N** | Upsal | 59° 51' | 17° 31' | 0 | 42°6 | 3 | 5 | 7 | 9 | | | Stockholm | 58° 20' | 18° 31' | 0 | 42°6 | 3 | 5 | 7 | 9 | | | Quebec | 48° 17' | 11° 51W | 4 | 97°14 | 3 | 5 | 7 | 9 | | | Christiania | 59° 55' | 10° 48E | 0 | 42°6 | 3 | 5 | 7 | 9 | | | Convent of Peysenberg | 47° 47' | 10° 34E | 2 | 96°6 | 3 | 5 | 7 | 9 | | | Copenhagen | 55° 41' | 12° 35E | 0 | 45°6 | 3 | 5 | 7 | 9 | | | Kendal | 56° 17' | 2° 46W | 0 | 42°6 | 3 | 5 | 7 | 9 | | | Malouin Islands | 51° 25' | 69° 59W | 0 | 42°6 | 3 | 5 | 7 | 9 | | | Prague | 50° 14' | 2° 24E | 0 | 49°6 | 3 | 5 | 7 | 9 | | | Gottingen | 61° 32' | 9° 33E | 0 | 46°6 | 3 | 5 | 7 | 9 | | | Zurich | 47° 22' | 8° 32E | 0 | 35°6 | 2 | 4 | 6 | 8 | | | Edinburgh | 55° 57' | 3° 10W | 0 | 45°6 | 3 | 5 | 7 | 9 | | | Warsaw | 52° 21' | 26° 8E | 0 | 41°8 | 2 | 4 | 6 | 8 | | | Coire | 46° 50' | 9° 30W | 0 | 37°6 | 2 | 4 | 6 | 8 | | | Dublin | 63° 21' | 1° 15W | 0 | 41°8 | 2 | 4 | 6 | 8 | | | Berlin | 46° 5 | 7° 25E | 2 | 959 | 2 | 4 | 6 | 8 | | | Geneva | 46° 12' | 6° 5E | 0 | 30°6 | 2 | 4 | 6 | 8 | | | Mainheim | 49° 29' | 8° 22E | 0 | 35°6 | 2 | 4 | 6 | 8 | | Latin Grade 50°N** | Vienna | 48° 12' | 11° 22E | 0 | 30°54 | 2 | 4 | 6 | 8 | | | Clermont | 45° 46' | 3° 5E | 2 | 92° | 0 | 2 | 4 | 6 | | | Barsa | 47° 29' | 19° 17E | 0 | 49°6 | 2 | 4 | 6 | 8 | | | Cambridge(U.S) | 42° 25' | 71° 3W | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Paris | 43° 50' | 2° 20E | 2 | 86°44 | 2 | 4 | 6 | 8 | | | London | 51° 30' | 0° 0W | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Arkelskirk | 51° 21' | 2° 22W | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Amsterdam | 52° 22' | 4° 50E | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Bruxelles | 50° 50' | 2° 0E | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Franeker | 42° 33' | 6° 22W | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Philadelphia | 39° 56' | 7° 16W | 0 | 30°86 | 2 | 4 | 6 | 8 | | | New York | 40° 40' | 7° 53W | 0 | 30°86 | 2 | 4 | 6 | 8 | | | Cincinnati | 39° 6 | 8° 43W | 0 | 97°6 | 2 | 4 | 6 | 8 | | | St Ma | 48° 39' | 2° 0W | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Nantes | 47° 13' | 1° 32W | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Pekin | 39° 54' | 16° 27E | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Milan | 45° 28' | 9° 11E | 2 | 96°8 | 2 | 4 | 6 | 8 | | | Bordeaux | 44° 50' | 8° 34W | 0 | 94°8 | 2 | 4 | 6 | 8 | | Latin Grade 60°N** | Marseilles | 43° 17' | 5° 22E | 0 | 94°6 | 2 | 4 | 6 | 8 | | | Montpellier | 43° 33' | 5° 52E | 0 | 94°6 | 2 | 4 | 6 | 8 | | | Rome | 43° 53' | 8° 11E | 0 | 94°6 | 2 | 4 | 6 | 8 | | | Toulon | 43° 7 | 5° 50E| 0 | 94°8 | 2 | 4 | 6 | 8 | | | Namaskan | 32° 45' | 0° 40E | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Napoleon | 31° 28' | 0° 50E | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Funchal | 32° 37' | 6° 56W | 0 | 94°6 | 2 | 4 | 6 | 8 | | Lat Grade greater than 60°N** | Algiers | 36° 48' | 3° 1E | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Cairo | 30° 2 | 11° 18E | 0 | 94°6 | 2 | 4 | 6 | 8 | | | Vera Cruz | 19° 11' | 9° 21W | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Havana | 23° 1 | 8° 23W | 0 | 94°8 | 2 | 4 | 6 | 8 | | | Cumuna | 10° 13' | 6° 15W | 0 | 94°8 | 2 | 4 | 6 | 8 |

This table not only gives the annual mean temperature, but that of the four seasons, considering December, January, and February as the three winter months. The prefixed asterisk (*) indicates those places where the mean temperature has been most accurately observed. The maxima and minima of the warmest and coldest months are also given:

- **Mean Temp. of Warmest Month**: Shows the average maximum temperature during the warmest month. - **Mean Temp. of Colddest Month**: Shows the average minimum temperature during the coldest month.

Each column provides detailed data for each city listed, including latitude (Lat.), longitude (Long.), height in feet (Height), and various temperature measurements throughout the year, winter, spring, summer, and autumn. Humboldt's isothermal lines are drawn on what is termed an equatorial projection; and a first glance will show that they are neither parallel to each other, nor to the lines marking degrees of latitude. In Europe their summit is convex; but over Asia and America their summits are concave, and distinctly indicate the existence of two meridians of extreme cold in the northern hemisphere (the extremities of which will represent the poles of greatest cold), on both sides of which the mean temperature is higher than on those meridians. The poles of greatest cold would seem to be about 70° or 80° to the W. and the E. of the meridians of Paris and London, and about the 80° of N. latitude. This equatorial projection appears to have led Berghaus and others into the erroneous supposition that the isothermal lines formed isolated bands around each pole of greatest cold; whereas they form continuous bands, including both poles of cold, as is well seen in the isothermal charts of Professor Dove of Berlin, which are on the polar projection. (See Plate CLXXIV., which is copied from his work.)

The influence of oceanic currents on mean temperature was also pointed out by Humboldt; especially of his "Peruvian current," which carries the gelid waters from the frozen shores of the southern Victoria Land to the western coasts of Chile and Peru, and shows itself by the lower temperature of its waters far to the west of the Galapagos Islands, in the equatorial Pacific Ocean; and of the longer known "Gulf stream" of the Atlantic, which brings the warm waters of the torrid zone along the eastern coasts of North America; and, where met by the polar current off the banks of Newfoundland, transports a portion of their warmth to the western shores of Europe, and often reflects on the western isles of Scotland, and the Orkneys, the seeds of the Dolichos or Mucuna urens, of the Mimosa scandens, and of other West Indian plants. The Gulf stream gives warning to the judicious mariner of his approach to the continent of America, by the superior temperature of its waters; and Humboldt mentions, that the Peruvian current has been found, off the coast of Peru, to have a temperature of no more than from 60° to 67° Fahr., while the sea just beyond the current raised the thermometer to 80°. Such currents, when extensive, must materially affect the heat of the superincumbent air; and thus we can understand how mean temperature must be locally affected by the direction of great oceanic currents.

It is well known that the heat of the atmosphere diminishes as we ascend mountains, and that, in every climate, there is a point where frost will be perpetual. The mean temperature of any place must therefore be materially affected by its level above the sea. This point of perpetual congelation may, in general terms, be said to diminish in altitude from the equator to the poles. At one time it was imagined that this diminution was uniformly according to latitude; and philosophers sought out a mode of calculating the point of perpetual congelation for each degree of latitude. The best known of these methods was that of Kirwan of Dublin. He conceived that if the point of congelation at the equator and its mean temperature were known, as well as the mean temperature of the given latitude, we could discover the point of congelation at that latitude by a simple equation. Thus, take the excess of both mean temperatures above the freezing point of water—and as this corrected mean temperature at the equator is to the point of perpetual congelation, so is the corrected mean temperature of the latitude to its point of congelation. But this ingenious speculation supposes a uniform depression of that point from the equator to the poles—which is not found to be the fact—and has led to as erroneous conclusions as the formula of other philosophers, who believed that the question might be determined by diminishing the mean temperature of any place by 1° Fahr. for every 300 feet of elevation. Upon these erroneous principles it has even been imagined that we could determine the height of mountains in any latitude, by the computed height of perpetual congelation in that latitude. Thus, when it was first announced that several peaks of the Himalaya range attained an elevation of from 25,000 to 27,000 feet, the accuracy of the observations was called in question; because in that latitude, it was contended that the snow line was not higher than 11,500 feet, and but a small part of these summits was veiled in perpetual snow; consequently their real heights could not possibly be more than a mile higher than the lower limit of snow, or in all from about 16,000 feet. But trigonometric measurements by several observers have since put their vast elevation beyond all doubt; and, what is still more remarkable, it has been proved that the snow line on their southern declivities lies 4000 feet lower than on their northern flanks. The explanation of this singular fact is supplied by Humboldt. The first cause is, that the air from the Indian ocean comes to the southern declivities of the Himalayas loaded with moisture which it deposits on the lofty summits with which it first comes into contact. But, in the second place, on the northern side of this vast aerial rampart, the powerful radiation of heat from the dry elevated table-lands of central Asia, situated below the almost parallel chains of the Himalayas, the Zungling and the Himmelgebirge ranges of Klaproth, causes streams of dry warm air to ascend on the northern flanks of those mountains, and thus elevates the limit of perpetual snow. The happy effect of these circumstances has been pointed out by Humboldt—"Millions of men of Tibetan origin occupy populous towns, in a country where fields and towns would, during the whole year, have been buried in snow if these table-lands had been less continuous and less extensive."

Places which have the same annual mean temperature may differ materially in the difference of the mean temperature of their winter and summer. This, as well as the influence of the cold meridians, is well seen in the following table of Humboldt, in which he compares Europe with eastern America:

| Isothermal Lines | Atlantic Region, Long. 1° W. and 17° E. | Transatlantic Region, Long. 9° to 12° W. | |-----------------|----------------------------------------|----------------------------------------| | Mean Temperature | Winter | Summer | Diff. | Winter | Summer | Diff. | | 68 | 50° | 80° | 21° | 55° | 80° | 25° | | 59 | 44° | 73° | 29° | 39° | 78° | 39° | | 50 | 35° | 69° | 34° | 30° | 71° | 41° | | 41 | 24° | 60° | 36° | 14° | 66° | 52° | | 32 | 14° | 53° | 39° | 1° | 55° | 54° |

The table shows the increase of the difference between the winters and summers, from the parallels of 26° and 30° to the parallels of 55° and 65° N. Lat. The increase of difference is more rapid in the transatlantic region; but it is remarkable that in both the divisions of the annual temperature between winter and summer is such that upon the isothermal line of 32° the difference of the two seasons is about double of that upon the isothermal line of 68°.

If, instead of the mean temperature of the seasons, we take that of the coldest and warmest month, and if we take places lying in the same climatic region and compare them with those in other regions, the difference becomes still more conspicuous; as, for instance, the region of western Europe with the region of eastern America, or of eastern Asia. The general principle in each region is that the differences increase from the equator to the pole; but, in certain situations this ratio is modified by the direction of the prevailing winds, and of oceanic currents, and by an insular climate. Thus, with us the breezes from the S.W., sweeping over the Atlantic, moderate the heat of our summer, and the severity of our winter; the oceanic current from Newfoundland certainly tends also to mitigate the vigour of our cold season; and from these causes, the climate of Britain is Climate, more temperate than that of the same latitudes of continental Europe. The influence of similar causes in other places is indicated in the last column of this table.

| Places | Lat. | Mean Temperature | Diff. | Observations | |------------|------|------------------|-------|--------------| | Camana | 10°27' | 80°1° | 84°4° | 4°3° | Uninterrupted trade winds Monsoons, radiation from sand | | Pondicherry| 11°53' | 76°1° | 91°4° | 15°3° | Same Region, Interior | | Manilla | 14°36' | 76°0° | 86°6° | 18°9° | Monsoons | | Vera Cruz | 19°11' | 79°0° | 81°7° | 9°6° | North winds in winter | | C. Frangla | 19°46' | 77°0° | 86°0° | 9°0° | Uninterrupted trade winds | | Havana | 23°10' | 79°0° | 83°8° | 13°8° | North winds in winter | | Funchal | 32°37' | 64°0° | 76°6° | 12°6° | Insular climate | | Natcher | 51°38' | 46°9° | 78°3° | 31°4° | Transatlantic Region, Interior | | Cincinnati | 39°6' | 79°4° | 74°4° | 44°8° | Eastern Asiatic Region | | Pekin | 39°54' | 24°8° | 84°2° | 59°4° | Transatlantic R. Eastern Coast | | Philadelphia| 39°56' | 29°8° | 77°9° | 47°2° | Same Region, Interior | | New York | 40°40' | 25°3° | 80°5° | 55°5° | Same system of climate | | Rome | 41°53' | 42°1° | 77°9° | 34°9° | Mediterranian Region | | Milan | 45°28' | 33°8° | 55°2° | 21°4° | Same Region, Interior | | Bota | 47°20' | 27°7° | 71°6° | 43°9° | Interior | | Paris | 48°50' | 32°1° | 69°8° | 34°7° | Near the Western Coast | | Quebec | 46°47' | 14°9° | 73°4° | 30°4° | Transatlantic R. Eastern Coast | | Dublin | 53°21' | 37°6° | 60°3° | 22°7° | Western Europe, Insular Climate | | Edinburgh | 55°58' | 38°3° | 59°4° | 21°1° | The same | | Warsaw | 52°15' | 27°1° | 70°3° | 43°2° | Interior of Europe | | Petersburg | 55°56' | 4°6° | 55°7° | 51°1° | Eastern Europe | | North Cape | 71°0' | 22°1° | 46°6° | 24°5° | Insular and Coast Climates |

The subject of isotherms has, since the publication of Humboldt's papers, been able taken up by Professor Dove of Berlin, who has added much to our knowledge of the circumstances by which climate is modified. We have already noticed his polar projection of isothermal lines; and he has also given us a series of normal and abnormal isotherms in his very valuable publications on the distribution of terrestrial temperature. He has contributed not only extended annual isothermal charts, and for the four seasons, but also a series of isotherms for every month of the year. His observations on monthly mean temperature are the result of the discussion of registers kept at 700 stations. His fifth memoir contains those made at 230 new stations; the diurnal variations were collected from 29 stations; and since the publication of his fifth dissertation he has availed himself of the observations made at St Helena, the Cape of Good Hope, Madras, Australia, Hobarton in Van Diemen's Land, and Philadelphia in North America. The intervals between the stations have been filled up from the journals of 27 scientific voyages in the intervening seas. The scientific labour of Professor Dove has been immense, and the results highly interesting. He has also given illustrations of the annual variations of atmospheric pressure, and pointed out their bearings on climate. Dove's valuable charts were published by the Academy of Sciences of Berlin. A translation of this work by Mrs Sabine, with very valuable remarks by Colonel Sabine, has appeared in Britain, under the auspices of the British Association.

M. Dove has pointed out more clearly than had been previously done, the importance to determination of climate, of attention to the annual variations of atmospheric pressure; and expresses his surprise, that while daily barometric observations have been needlessly marked, so little attention has been paid to the annual fluctuations of the instrument. The diurnal variation had exhibited great regularity and distinctness in tropical America, and had attracted attention in Europe; but it seems strange that no remark was excited by the fact that barometric pressure was not found to diminish from winter to summer with increasing heat; and the annual variations scarcely excited attention.

The observations of Prinsep in Hindustan pointed out that there was a great difference between barometric variations in India and in tropical America; proving that there was in India a very marked annual variation; but it was erroneously supposed that this phenomenon did not extend beyond the tropics, and that it was an immediate consequence of the monsoons. The laudable investigations ordered by the Russian government made us acquainted with the meteorology of Siberia; and it was found, that north of the Himalaya, the supposed limit of the monsoon variations, the annual barometric variations are seen on a vast scale, extending even to the shores of the Icy Sea, a greatly diminished atmospheric pressure takes place in summer over the whole continent of Asia, and must produce an influx of dewsair from all sides. This is the cause of the prevailing W. winds of Europe, the N. winds of the Frozen Ocean, and the E. winds that prevail on the E. coasts of Asia, and the S.W. monsoon of Hindustan. These facts, and their bearings on climate, have been discussed by Dove with much ability in several memoirs in the Berlin Transactions for 1852, and in Pogendorff's Annalen (58 and 77). The observations made by the British in India, the Cape of Good Hope, in Australia, and especially at Hobarton in Van Diemen's Land, he considers as of particular value; as enabling us to generalize atmospheric phenomena by a comparison of the southern with the northern hemisphere. The following is an abstract of his deductions from his multiplied investigations on this subject:

1. Both in the north temperate and torrid zones the elasticity of aqueous vapour in the air increases with the temperature. In the region of the monsoons this increase is greatest near their northern limit, as in China and Hindustan. No such increase has been observed in the S. hemisphere. The curve of this elasticity has, however, a less convex summit in the region of monsoons than just beyond it; the elasticity continuing nearly the same throughout the rainy monsoon. Near the equator, the convex curve of the N. hemisphere is first flattened, and then transformed into the concave curve of the S. hemisphere; but in the Atlantic this transition takes place somewhat farther N. of the equator. The annual variation in the torrid zone is generally considerable at all places where equatorial currents prevail when the sun's altitude is greatest, and polar currents when the sun's altitude is least. It is inconsiderable wherever the direction of the wind is either comparatively constant throughout the year, or where its changes are opposite to that above described. In the last mentioned regions, the rate of decrease of the mean annual tension of aqueous vapour, with increasing distance from the equator, is more rapid than in the first class.

2. Over Europe and Asia the pressure of dry air decreases from the colder to the warmer months; and everywhere in the temperate zone has its minimum in the warmest month.

3. On comparing the annual variation of pressure of dry air in northern Asia and Hindustan with that of the Indian Ocean and Australia, we must conclude that something more takes place than a simple periodical transfer of the same mass of air in the direction of the meridian between the north and south hemisphere. From the extent of the phenomenon in the northern hemisphere, we must infer, that with the diminished pressure a lateral overflow takes place; and that this is the case is proved by the fact that, at Sitka, on the N.W. coast of America, the pressure of the dry air increases from winter to summer. It is improbable that the overflow takes place only to the east, but probably also to the west; and on this supposition the small amount of diminution of pressure of the dry air from winter to summer in Europe may arise, not solely from the moderate amount of the difference of temperature in the hotter and colder seasons, but also from the lateral afflux of air in the upper regions of the atmosphere tending to compensate the pressure lost by expansion by heat. As at the northern limit of the monsoons at Chusan and Pekin the annual variation of pressure of dry air is not considerable, while, at the northern limit of the trade winds at Madeira and the Azores in the Atlantic it is very small, it is probable that there is in the torrid zone also a lateral overflow in the upper regions of the atmosphere, from the region of the monsoons to that of the trade winds.

4. From the combined action of the variations of aqueous vapour and of dry air we now derive immediately the periodic variations of the whole atmospheric pressure. As the height of the barometric column is the result of this combination, it may be considered as the result of two forces, one the pressure of dry air, the other the elasticity of vapour; and we can understand that, as with increasing heat the air expands and rises higher, and as its upper portion overflows laterally, while at the same time the increase of heat augments evaporation, and thus increases the quantity of aqueous vapour in the atmosphere, so it follows that the periodic variations of barometric pressure should not everywhere bear a simple obvious ratio to the periodic changes of temperature.

5. Throughout Asia, the increase with heat of the elasticity of vapour is never sufficient to compensate the diminished pressure of dry air; and the annual variation of barometric pressure is, therefore, everywhere represented, in accordance with the law of pressure of dry air, by a curve having its lowest point in July. The Russian observations at Yakoutsk, Udkoi, and Minsk, show this to be true up to the Okhotsk Sea on the east, and to the Frozen Ocean on the north. A tendency to these conditions is perceptible on the meridian of Petersburg, and becomes more conspicuous on approaching the Ural range. On the Caspian, and in the Caucasus, it is very distinctly marked; its limits run south from the western shores of the Euxine to Syria, Egypt, and Abyssinia, where it is found also to prevail. Towards the confines of Europe the general maximum is in September or October, the barometric pressure increasing rapidly from July to those autumnal months, towards the latter part of which a slight convexity or secondary minimum is observed; but beyond the Urals the curves become uniformly concave, with a single summer minimum and winter maximum, which holds throughout the rest of Asia. The little difference between the curve at Madras and Manila, and the yet considerable curve at Nangasaki in Japan, show that the region in question extends beyond the eastern coast of Asia; at higher latitudes, it is bounded by Kamtschatka. The observations made by the British at Aden show that its western limit extends far in the direction of Africa.

6. In middle and western Europe the annual barometric variation is a decrease from the month of January to spring, and the minimum is in April; it then rises slowly but steadily to September, and rapidly sinks to November, when it usually has its minimum. In summer the increased evaporation more than counterbalances the loss by expansion, probably from a lateral overflow in the upper regions received from Asia. At Sitka the whole annual curve is convex, a result only found in Europe at considerable elevations, where it is the consequence of the expansion of the whole superincumbent mass of the atmosphere.

7. The region of great annual variation on the Asiatic side of the globe, when the monsoons prevail, extends much farther north in the northern than in the southern hemisphere; the variation has its maximum at Pekin, while at Hobarton, at nearly the same distance from the south side of the equator, it is trifling. The reverse is the case generally in the region of the trade winds, and in the Atlantic; for then the annual variation, though nowhere great, is decidedly more on the south than on the north of the equator, as is shown by the results of observations at the Cape of Good Hope, at St Helena, and Ascension Isle, Rio Janeiro, and Pernambuco, compared with the West Indies and the southern parts of the United States. Hence we find but small differences in annual variation between places in the southern Atlantic and southern Pacific in the same latitudes; while we find great inequalities between the N. Pacific and its eastern coasts and the very trifling variation observed in the north Atlantic. The explanation of this latter fact is evidently from the lateral overflow already noticed.

8. It is known that during the eruption of the Soufrière in St Vincent in 1812, volcanic ashes fell on the decks of vessels in large quantities more than 300 miles to the eastward of the volcano, and consequently were carried in the upper currents of the air in a direction against that of the trade winds; and in the eruption of the Coseguina, in central America, in January 1855, volcanic ashes not only fell in Jamaica, a distance of 800 miles to the east, but also fell on the decks of ships in the Pacific Ocean 700 miles seaward of the volcano. The inference is, therefore, that in the higher regions of the atmosphere between the tropics, the aerial currents are not always in the same direction as the lower currents or trades, but sometimes is in the opposite, sometimes in the same direction. M. Dove has applied his comparison of the barometric relations of the region of the monsoons to that of the trade winds. Thus, if we suppose the upper portion of the air over Asia and Africa to flow off laterally, and if this take place suddenly, it checks the course of the upper or counter-current above the trades, and breaks into the lower current. An E. wind coming into a S.W. current must necessarily produce a rotatory movement of the air in a direction opposite to the apparent movement of the sun. A rotatory storm from S.E. to N.W. in the trade current would, on this theory, be the result of the encounter of two masses of air impelled against each other at many places in succession, as explained by Dove in his memoir on the "Law of Storms." Thus, the West Indian hurricanes, and the Chinese typhoons, occur near the lateral confines on each side of the great atmospheric expansion. The rarer occurrence of rotatory storms in the S. than the N. tropical Atlantic, arises from the more equal distribution of the periodic diminution of atmospheric pressure in the former than in the latter.

9. It is sufficiently obvious, that the irregular distribution of the land and the sea, which produces the abnormal variations in the form of the isothermal lines, is also the principal cause of the movements of the atmosphere. Thus the monsoons are but a modification of the trade winds, for the cause of which we must look beyond the N. tropic. The form of the Asiatic continent produces the great thermic expansion of the summer air over the interior of the old world, and presents all the characteristic marks of the region of calms, being a centre to which all the adjacent masses of air are drawn. Thus the subtropical atmospheric zone does not, as it were, uninterruptedly encircle the globe. The region over which the heated air ascends, does not therefore move N. or S. parallel with the sun's change of declination, but has rather a fluctuating movement, the fixed point of which is the West Indies, and the maximum of oscillation is over India. The northern excursion of this aerial movement is greater, as we have said, in the northern than in the southern hemisphere. The atmospheric relations of Europe, especially in summer, are therefore essentially of a secondary nature, and we must regard the small alteration in the atmospheric pressure over Europe in the course of the year as a secondary result, of which the explanation would have been exceedingly difficult, without a knowledge of the observations on atmospheric pressure from Asia and Australia. The horary variations of the barometer attracted the attention of Humboldt, and he gave the results in the volume of his travels published in 1825. 1st, He proved that they are perceptible everywhere, to the height of 12,000 feet above the sea. 2d, They consist of two ascending and two descending movements daily; the maximum in the morning is between 8½ and 10½ o'clock; the minimum of the afternoon between 3 and 5; the maximum of the evening between 9 and 11, and the minimum of the night between 3 and 5 o'clock. 3d, In the temperate zone, the periods of maxima and minima are nearer to the passing of the sun over the meridian in winter than in summer by 1 or 2 hours. 4th, In the torrid zone the times of maxima and minima are the same at the level of the sea, and at 8500 feet above it; but this does not appear to hold in the temperate zone. 5th, The oscillations are very small about the time of maxima and minima. 6th, For 15° N. and S. of the equator in the Atlantic and adjacent regions, gales, tempests, and earthquakes do not interrupt the periodicity of these diurnal variations, but on the Indian continent and coasts the rainy monsoons do so, though they do not on the Indian Ocean. 7th, The diurnal variations are different in different months, and decrease as the latitude augments. The morning maxima is a little higher than that of the evening. 8th, No observation yet made indicates a sensible influence of the moon on the oscillations of the atmosphere. Those attributed to the moon seem to be owing to the sun, not as a gravitating but as a calorifying body.

The following Tables show the Horary Variations between Lat. 25° S., and Lat. 55° N. from the Level of the Sea to 8500 feet of elevation.

### Torrid Zone

| Places of Observation | Minima of the Night | Maxima of the Morning | Minima of the Day | Maxima of the Evening | Mean extent of Oscillations in 100ths of a millimetre | Observers | |-----------------------|--------------------|----------------------|------------------|----------------------|-------------------------------------------------|----------| | Equatorial Atlantic Ocean | 4½ | 10½ | 4½ | 10½ | ... | Lamanon and Monges | | Equatorial America, between Lat. 23° N. and 12° S. to 1500 toises of height | 4½ | 9½ | 4½ | 11 | 2·55 | Humboldt and Bonpland | | Payta (Peru), Lat. 5° 5' S. | 3 | 9 | 3½ | 11½ | 3·40 | Duperrey | | Guayaquil, Lat. 10° 36' N. | ... | 9½ | 3½ | 10 | 2·44 | Boussingault & Rivero | | Bogota, Lat. 4° 35' N., height 1366 toises | 4 | 9 | 4 | 10 | 2·29 | ... | | Indian and African Seas, Lat. 10° N. 25° S. | 4 | 8½ | 4 | 11 | ... | Horsburgh | | Equatorial Pacific Ocean | 3½ | 9½ | 4 | 10½ | ... | Langsdorff and Horner | | Sierra Leone, Lat. 8° 30' N. | 5 | 9½ | 3½ | 10 | ... | Sabine | | Mysore, Lat. 14° 11' N., height 400 toises (rainy season) | 5 | 10½ | 4 | 10½ | ... | Kater | | Pacific Ocean, between Lat. 24° 30' N. and 25° S. | 3½ | 9½ | 3½ | 9½ | ... | Simonoff | | Macao, Lat. 22° 12' N. | 5 | 9 | 5 | 10 | ... | Richelet | | Calcutta, Lat. 22° 34' N. | 6 | 9½ | 6 | 10 | ... | Balfoué | | Equinoctial Brazil, at Rio Janeiro, (Lat. 22° 54' S.) and at the missions of the Coroatos Indians | 3 | 9½ | 4 | 11 | 2·34 | Dorta, Freycinet, Eschwege |

### Temperate Zone

| Places of Observation | Minima of the Night | Maxima of the Morning | Minima of the Day | Maxima of the Evening | Mean extent of Oscillations in 100ths of a millimetre | Observers | |-----------------------|--------------------|----------------------|------------------|----------------------|-------------------------------------------------|----------| | Las Palmas (Great Canary), Lat. 28° 8' N. | ... | 10½ | 4½ | 11½ | 1·10½ | De Buch | | Cairo, Lat. 30° 3' | 5½ | 10 | 5 | 10½ | 1·75 | Couelle | | Toulouse, Lat. 43° 34' (mean of 5 years) | ... | 8½ | 5½ | 11 | 1·20 | Marque Victor | | Chambery, Lat. 45° 34', height 13 toises | ... | 10 | 2 | ... | 1·00 | Billiet | | Clermont-Ferrand, Lat. 45° 46', height 210 toises | ... | 8½ | 4 | 10 | 0·94 | Ramond | | Strasbourg, Lat. 48° 34', (mean of 6 years) | ... | 9½ | 3 | 9½ | 0·80 | Herren Schneider | | Paris, Lat. 48° 50', (mean of 9 years) | ... | 9½ | 3 | ... | 0·72 | Arago | | La Chapelle, near Dieppe, Lat. 49° 55' | ... | 9½ | 3 | ... | 0·65 | Nell de Bréauté | | Königsberg, Lat. 54° 42', (mean of 8 years) | ... | 8½ | 2½ | 10 | 0·20 | Sommer & Bessel |

The variations of the superficial temperature of our earth are produced by the influence of solar heat. These superficial impressions are all produced, either directly or through the intervention of the atmosphere, by the action of the solar rays. It may be calculated from experiment, that the entire and unimpaired light of a vertical sun will communicate one centesimal degree of heat every hour to a sheet of water of a foot in thickness. Consequently, since the surface of a sphere is four times that of its generating circle, such a sheet of water, spread over Climate.

the whole of the globe, would receive six degrees of heat every day. But the very inferior capacity of the atmosphere for heat, being estimated as equal to that of a body of water about twelve feet in depth, if the aerial mass finally received and retained all the calorific impressions, it would every day have its temperature raised half a degree, and therefore augmented to $182\frac{1}{2}$ in the course of a whole year. This annual accession of heat, however, is quickly dispersed by the mobility of the fluid medium, and gradually absorbed into the earth, or more quickly diffused through the waters of the ocean, which, besides, occupy at least three-fourths of the whole surface of our globe. The luminous matter communicated by the incessant shining of the sun, whether received on the ground or intercepted in its passage through the air, would hence be capable of communicating one centesimal degree of heat to the body of the earth in the space of 1323 years; a quantity too small, perhaps, to be yet perceived, though its influence may be afterwards detected by very delicate observations.

It is easy to demonstrate, from the laws of optics, that the quantity of light which falls on a horizontal surface must be proportional to the sine of its obliquity. Hence the aggregate light received under the equator at either equinox is to what would accumulate during 24 hours, if maintained at its highest intensity, as the diameter to the circumference of a circle. This daily accession of heat, confined to the mass of atmosphere, would, therefore, in that climate and season, amount to 633,000 parts of a degree. At the pole itself, during the complete circuit of the sun in midsummer's day, the measure of heat would be about a fourth part greater, or 797,000 parts; the continued endurance of the sun above the horizon more than compensating for the feebleness of his oblique rays.

In general, the quantity of light received at any place from the sun in the space of one day is denoted by the product of the sine of the semidurnal arc, or the distance from noon to the time of sunset, into the cosines of the latitude and declination, joined to the product of that arc itself into the sines of the latitude and declination; the latter part of the expression being considered as additive or subtractive, according as the declination lies on the same or on the opposite side of the latitude. Hence, at Edinburgh, in the latitude of $56^\circ$, the heat collected during one day at the summer solstice is 307,000 parts, but at the solstice of winter only 166,000.

If a current of air from the equator, having the ordinary temperature of $27^\circ$ C., were supposed to travel to the pole, from which an equal and contrary current would consequently flow towards the equator, each journey would transport $55^\circ$ of heat. Between two and three such journeys performed every year would therefore be sufficient to disperse the whole accumulation of $138^\circ$. This only requires the existence of a wind advancing northwards at the rate of 46 miles every day. It is not necessary even that the wind should either continue permanent or blow directly north. The same effect would be produced if it were to blow indifferently to every point of the compass, and only at the rate of three miles an hour; a supposition which agrees tolerably with actual observation.

The circulation excited in the body of our atmosphere thus prevents the heat shed by the sun on different parts of the earth's surface from an excessive accumulation. In proportion as the equatorial regions grew warmer from the predominance of illumination, the polar wind would rush with more rapidity, till it had tempered the excess. This balance of the accession, and the consequent dispersion, of heat, has probably been long attained, and it now regulates the gradation of climates in successive latitudes.

The equilibrium of temperature preserved over the globe by the circulation of the atmosphere is not, however, very quickly produced. Hence the remarkable increase of heat which takes place during the summer months in the higher latitudes. But within the arctic circle, 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 the sea in those dreary retreats, absorb, in the act of thawing or passing again to the liquid form, all the surplus heat collected during the continuance of a nightless summer. The rigour of winter, when darkness resumes her tedious reign, is likewise mitigated by the warmth evolved as congelation spreads over the watery surface.

Of the light received from the sun, a considerable portion is always detained and absorbed in its passage through the atmosphere. Even a vertical ray, shot through the clearest air, will lose more than the fifth part of its intensity before it reaches the surface of the earth. In most cases, the loss which light will suffer in the shortest transit through the atmosphere may be estimated at one-fourth of the whole. But the oblique rays must undergo a much greater absorption. If, from their slanting course, they have to encounter twice the number of aerial particles, their intensity must be reduced to $\frac{1}{8}$ths, or the square of three-fourths; and if they describe triple the vertical tract, only $\frac{3}{4}$th parts, or the cube of three-fourth, will reach the ground. In general, if the tracts of light follow an arithmetical progression, the diminished force with which it escapes and arrives at the ground will form a decreasing geometrical progression. To determine the train of aerial particles which the oblique rays of the sun must traverse in their passage through the atmosphere, is a nice problem, which requires a skilful application of the integral calculus. Without stopping to engage at present in the details of this intricate investigation, it may suffice to remark, that, in general, the length of the tract is nearly in the inverse ratio of the sine of the sun's altitude. But the following table, to every five degrees, is calculated from rigorous formulas; the length of oblique tract being reduced to the standard of air uniformly dense. These quantities again are diminished in the ratio of the sine of obliquity, to express the calorific action which those enfeebled and slanting rays finally exert at the surface of the earth.

| Sun's Altitude | Measure of Atmospheric Tract | Intensity of the Light transmitted | Calorific Action at the Surface | |----------------|-------------------------------|------------------------------------|--------------------------------| | 90° | 1'000 | .750 | .740 | | 85 | 1'004 | .749 | .747 | | 80 | 1'015 | .747 | .735 | | 75 | 1'035 | .742 | .717 | | 70 | 1'064 | .736 | .691 | | 65 | 1'103 | .728 | .660 | | 60 | 1'154 | .718 | .609 | | 55 | 1'220 | .704 | .577 | | 50 | 1'305 | .687 | .526 | | 45 | 1'413 | .666 | .454 | | 40 | 1'554 | .640 | .411 | | 35 | 1'740 | .606 | .348 | | 30 | 1'995 | .563 | .282 | | 25 | 2'359 | .507 | | | 20 | 2'905 | .434 | .214 | | 15 | 3'841 | .331 | .148 | | 10 | 5'610 | .199 | .086 | | 5 | 10'450 | .050 | .035 | | 0 | 37'850 | .00002 | .004 |

It hence appears that, even when the sky is most serene, only one-half of the sun's light can reach the surface of the earth from an altitude of $25^\circ$, or one-third from that of $15^\circ$, and that, if the obliquity of the rays were increased to $5^\circ$, no more than the twentieth part of them would actually be transmitted. The annual quantity of light which Climate falls may be computed as equivalent at the equator to a uniform illumination from an altitude of $45^\circ$; and, in the mean latitude of $45^\circ$, and at the pole the effects are the same as if the rays had respectively the constant obliquities of $13^\circ 2'$ and $7^\circ 17'$. Therefore, under the most favourable circumstances, of 1000 parts of light transmitted from the sun, only 378 can, at a medium estimate, penetrate to the surface at the equator, 228 in the latitude of $45^\circ$, and 110 at the pole, of their oblique rays; but the shades received by a given portion of the surface are still less, being only 115, 51, and 14. In cloudy weather, the portion of light that can finally reach the ground will seldom amount to the third of those quantities; and when the sky becomes darkened with accumulated vapours, almost every shining ray is intercepted in its passage.

The light which at last gains the surface being there absorbed and converted into heat, is, in this form, profusely delivered to the ambient air, or more feebly conducted downwards into the body of the earth. But the rays which fall on seas or lakes are not immediately arrested in their course; they penetrate always with diminishing energy, till, at a certain depth, they are no longer visible. This depth depends, without doubt, on the clearness of the medium, though probably not one-tenth part of the incident light can advance five fathoms in most translucent water. The surface of the ocean is not, therefore, like that of the land, heated by the direct action of the sun during the day, since his rays are not intercepted at their entrance, but suffered partially to descend into the mass, and to waste their calorific power on a liquid stratum of ten or twelve feet in thickness.

But the surface of deep collections of water is kept always warmer than the ordinary standard of the place, by the operation of another cause, arising from the peculiar constitution of fluids. Although these are capable, like solids, of conducting heat slowly through their mass, yet they transfer it principally in a copious flow by their internal mobility. The heated portions of a fluid being dilated, must continue to float on the surface; while the portions which are cooled, becoming consequently denser, will sink downwards by their superior gravity. Hence the bed of a very deep pool is always excessively cold, since the atmospheric influences are modified in their effects by the laws of statics. The mean temperature of the climate is not communicated by these variable impressions; every change to warmth being spent on the upper stratum, while every transition to cold penetrates to the bottom, which thus experiences all the rigours of winter, without receiving any share of the summer's heat. But, if the beds of profound bodies of water remain perpetually cold, their surface undergoes some variety of temperature, and is generally warmer than the average weekly or monthly heat of the air.

These principles are confirmed by observations made on our own lakes, and strikingly exemplified in those of Switzerland, which have a depth proportioned to the stupendous altitude of their encircling mountains. It appears, from the careful observations of Saussure, that the bottoms of those majestic basins, whether situated in the lower plains, or embosomed in the regions of the upper Alps, are almost all of them equally cold, being only a few degrees above the point of congelation. That accurate observer found the temperature of the Lake of Geneva, at the depth of 1000 feet, to be $42^\circ$, and could discover no monthly variation under 160 feet from the surface. In the course of July, he examined the Lakes of Thun and Lucerne; the former at the depth of 370, and the latter at that of 640 feet, had both the temperature of $41^\circ$, while the superficial waters indicated respectively $64^\circ$ and $68^\circ$ by Fahrenheit's scale. The bottom of the Lago Maggiore, on the Italian side of the Alps, was a little warmer, being $44^\circ$ to the depth of 360 feet, while the surface was almost as high as $78^\circ$. Barlocchi has since found that the Lago Sabatino, near Rome, at the depth of 490 feet, was only $44^\circ$, while the thermometer, dipped at the surface, marked $77^\circ$.

Through the friendship of Mr James Jardine, civil engineer, we are enabled to give the results of his observations on some of the principal Scottish lakes, which, as might be expected from him, were conducted with the most scrupulous accuracy. The instrument which he employed for exploring the temperature at different depths was free from the ordinary objections; being a register thermometer, let down in a horizontal position, which could acquire the impression in not many seconds, and might be drawn up leisurely, without risk of subsequent alteration. It would appear that the variable impressions of the seasons do not penetrate more than 15 or 20 fathoms; that below this depth, an almost uniform coldness prevails. Thus, in the deepest part of Loch Lomond, on the 8th September 1812, the temperature of the surface was $59^\circ$ of Fahrenheit; at the depth of 15 fathoms, $43^\circ$; at that of 40 fathoms, $41^\circ$; and from that point to about 3 feet from the bottom, at 100 fathoms, it decreased only the fifth part of a degree. Again, on the preceding day, the superficial water of Loch Katrine being at $57^\circ$, the thermometer, let down 10 fathoms, indicated $50^\circ$; at the depth of 20 fathoms it marked $48^\circ$; at the depth of 35 fathoms it fell to $41^\circ$; and on the verge of the bottom, at 80 fathoms, it had only varied to $41^\circ$. At the same place, on the 3d September 1814, the heat of the surface was $56^\circ$; at that of 10 fathoms, $49^\circ$; at that of 20 fathoms, $44^\circ$; at that of 30 fathoms, $41^\circ$; and at that of 80 fathoms, $41^\circ$.

Hence it is that, even in the northern latitudes, the deep lakes are never, during the hardest winters, completely frozen over. But if the same water be let into a shallow basin, it will, in a rigorous season, be chilled thoroughly, and converted into ice. This may even happen when spread above the surface of salt water, which is always considerably denser. Thus, frost takes no effect on Loch Ness, nor on the river of that name, which, in a rapid course of a few miles, discharges the surplus water into the sea. But in very severe winters, a sheet of ice appears formed along the shore; the impressions of cold being almost wholly expended on the accumulation of fresh water, since the chilled portions of this, which continually descend, are stopped in their progress by the greater density of the recumbent sea water.

The seas and the ocean itself obey the same law of the distribution of heat, only the difference of temperature experienced by sounding in the Mediterranean is less conspicuous than in the fresh-water lakes. Saussure found that the temperature at the bottom, in the Gulfs of Nice and of Genoa, at the depths of 925 and of 1920 feet, was the same, or $55^\circ$, the heat of the superficial water being about $89^\circ$. But the mean temperature, or that of the body of the land on the same parallel of latitude, is $59^\circ$. The smallness of the diminution here observed may perhaps be attributed to the effect of evaporation in such hot confined bays, the water at the surface being thus rendered saltier, and consequently disposed, by its acquired density, to sink into the colder mass below.

In open seas, and in damper climates, the depression of temperature is greater in the inferior strata. This difference becomes augmented in proportion to the extreme variation of the seasons. Lord Mulgrave, on the 4th September 1773, in the latitude of $65^\circ$ north, drew up water from the depth of 4100 feet, which he found to have the temperature of $40^\circ$, while the thermometer, dipped at the surface, stood, on the 19th June, at $55^\circ$. In the later experiments of Sir James C. Ross, the oceanic temperature at vast depths was found to remain stationary at about $40^\circ$ Fahr. In the antarctic seas, in the year 1840, he found, when the surface temperature was $32^\circ$, at 400 fathoms it was $38^\circ$; at 600 fathoms it was $39^\circ$; and in December of 1841, in the S. Atlantic, when Climate.

the surface was at 53°, at the depth of 1050 fathoms, the register thermometer just indicated 40°, and sank no lower at greater depths. In the following year, he found, a little to the north of Ascension Island, when the surface temperature was 42°, at 600 fathoms it stood just at 40°, and sank no lower when let down to the enormous depth of 4600 fathoms, 27,600 feet, or above 5 miles. In the still deeper soundings of Captain Denham in 1852, at a depth of 46,236 feet, or above 8½ miles, the temperature was still 40°.

A like gradation of temperature is produced by the alternating influence of the seasons in deep and stagnant masses of air. When this active fluid is confined in profound caverns, opening to the sky without being much exposed, and either perpendicular or gently inclined, its lower strata become intensely and permanently cold. The mild air of summer floats motionless at the mouth of the pit; but, in winter, the superior air, cooled many degrees perhaps below the freezing point, and therefore greatly condensed, precipitates itself continually to the bottom.

This fact takes place in most caverns, and in draw-wells which are left uncovered. Saussure found, on the 1st of July, when the thermometer in the shade stood at 78° of Fahrenheit, that a cave in the Monte Testaceo, a small hill in the vicinity of Rome, formed entirely by the enormous accumulation of broken pottery, had the temperature of 50°; and two other caves in the same porous mass were cooled to 44°. On the 9th July, when the external air was at 61°, the cave of St. Marino, at the foot of a sandstone hill about 2080 feet above the level of the sea, indicated only 44½, which is 6° below the mean temperature of the soil in that situation. In the grotto of Ischia, and in the caves of Cesi and of Chiavenna, the thermometer marked likewise 44½; but in the caves of Caprino, on the borders of the Lake of Lugano, it stood at different times of the year at 37° and 42°; and in those of Hergiswil, near Lucerne, the heat of the interior on the 31st of July was only 89¾.

But this phenomenon is still more striking in certain peculiar circumstances. The famous Swedish mine of Dannemora, which yields the richest iron ore in the world, presents an immense excavation, probably two or three hundred feet in depth. On the occasion of some repairs which suspended the usual labours, the basin appeared some years since full of water, with huge blocks of ice floating in it. The silver mine of Königsberg, in Norway, has for its main shaft a frightful open cavern, perhaps three hundred feet deep and thirty feet wide, of which the bottom is covered with perpetual snow. Hence, likewise, on the sides of Etna and of the mountains in Spain, the collected snows are preserved all the year in caves and crevices of the rocks, from which natural stores the muleteers carry down, during summer, to the villages and the cities of the plain, a material so necessary to comfort in those parched climates.

Such is the disposition induced in a confined column of air; but in a free atmosphere the gradation of temperature is exactly reversed, the lower strata being invariably warmer than the upper. This most important fact in meteorology and physical geography was thought sufficiently explained, in the infancy of physical science, from the proximity of the heat supposed to be reflected by the surface of the earth. But it were idle to attempt any serious confutation of such crude ideas. The true cause of the cold that prevails in the higher regions of the atmosphere is undoubtedly the enlarged capacity which air acquires by rarefaction. From the unequal action of the sun's rays, and the vicissitudes of day and night, a quick and perpetual circulation is maintained between the lower and the upper strata; and it is obvious that, for each portion of the air which rises from the surface, an equal and corresponding portion must likewise descend. But that which mounts up, acquiring an augmented attraction for heat, has its temperature proportionally diminished; while the correlative mass falling down, carries its share of heat along with it, and, again relaxing its attraction, seems to diffuse warmth below. A stratum at any given height in the atmosphere is hence affected both by the passage of air from below and by the return of air from above, the former absorbing a portion of heat, and the latter evolving it. But the mean temperature at every elevation is on the whole still permanent, and consequently those disturbing causes must be exactly balanced, or the absolute measure of heat is the same at all heights, suffering merely some external modification from the difference of capacity in several portions of the fluid with which it has combined. That temperature is hence inversely as the capacity of air having the rarity due to the given altitude.

It only remains, therefore, to discover the capacity of Investigated air, or its attraction for heat under successive pressures, or at different degrees of rarity. But this problem requires a very nice investigation, and appears incapable of being resolved by any direct procedure. If the elaborate experiments of Dr. Crawford and others on the capacity of air in its ordinary state gave such erroneous results, what hope could be formed of ascertaining even its minute shadings, by any similar plan of operation? But, independently almost of any theory, a simple method occurs for conducting this research. A delicate thermometer, suspended within the receiver of an air-pump, indicates a decrease of temperature as the process of rarefaction advances; and, on stopping this operation at any stage, the thermometer will slowly regain its former state. If now, when the equilibrium is restored, the air be suddenly readmitted, the dilated portion which had remained in the receiver liberates the heat absorbed by it during the progress of rarefaction. The thermometer accordingly rises quickly through a certain space, then becomes for a short while stationary, and afterwards slowly subsides. But the instrument does evidently not measure the whole of the heat thus evolved; a great part of it being spent in warming up to the same point the internal surface of the receiver. This action, however, is merely superficial, since its effect appears to be momentary. Consequently, the internal surface of the receiver, with that of the plate on which it stands, as penetrated by the sudden impression to a certain very minute depth, forms a constant film of matter, which, as well as the body of air itself, draws its supply from the extricated heat. Under the same receiver, therefore, although the air will not seize the whole of the heat disengaged in the act of admission, it must always retain a proportional share of it. A series of experiments, at successive degrees of rarefaction must hence discover, if not the absolute, yet the relative changes of the air's capacity for heat.

To institute this inquiry with the desired success, an excellent and powerful air-pump was used, having a receiver of the very largest dimensions, of an oblong spheroidal form, approaching, however, nearly to the globular, and with a narrow bottom. The apparatus being placed in the middle of a close room, which had a steady temperature, a thermometer with a slender stem, open at top and a small bulb of extreme sensibility, was fixed in a vertical position, a few inches above the centre of the plate. Having replaced the receiver, and allowed it to stand some time, one-fifth of the air was now extracted from under it; and, after a considerable interval, the cock was suddenly opened, to restore the equilibrium; when the mercury of the thermometer, which had been stationary, mounted up very rapidly 30 centesimal degrees, from which point it afterwards slowly descended.

The temperature of the room having been regained, two-fifths of the air in the receiver were then extracted; and after some lapse of time, the external communication being repeated, the thermometer rose instantly 6½ centesimal de- degrees. On extracting three-fifths of the internal air, the corresponding ascent of the thermometer, at the restoration of the equilibrium, was 7° of those degrees; and, when the contents of the receiver had been rarefied five times, the heat evolved, on the re-admission of the air, amounted to 8° degrees. The rate of progressive effect was thus evidently diminishing. On pushing the rarefaction as far as it was really practicable, or till the residual air had become rarefied about 300 times, the change indicated by the thermometer did not reach to more than 8°3.

But, to determine the absolute quantity of heat which is disengaged in the transition of air from a rarer to a denser state, it becomes necessary to ascertain what part of it was consumed on the sides of the receiver. By varying the size of the receiver, and consequently altering the proportion between its surface and its contents, some light may be thrown on this question. Another similar receiver was therefore provided, having half the former dimensions; and with this the same set of experiments was repeated. Its included air being reduced successively to the density of four-fifths, three-fifths, two-fifths, and one-fifth, and then rarefied as much as possible, the thermometer mounted each time through the shorter spaces of 1°8, 3°2, 4°2, 4°8, and finally 5° centesimal degrees. These quantities evidently follow the same proportion as the former, of which indeed they are only three-fifths. But the smaller receiver, having under the fourth part of the surface of the larger, only the eighth part of its contents, exposes comparatively twice the extent of surface. The rise of temperature which its included air exhibits must consequently be the same as what would have obtained within the larger receiver, if, while its capacity remained the same, its surface had been actually doubled. If we suppose the air to hold one part of the heat, while two parts and four parts are respectively expended on the inside of the receivers, the results would correspond with observation; for the whole quantity evolved being in both cases the same, the air under the larger receiver would retain one-third, and, under the smaller receiver, only one-fifth; the impressions being thus in the ratio of five to three. The same conclusion may be obtained somewhat differently. If the heat spent on the inside of the large receiver had been spread over twice the surface, it would have raised the temperature only 1°5; but this mounted really to 1°8, and therefore the difference 3° was the effects of 1°8 derived from the contained air. Of the heat thus shared between the air and the doubled surface, one part was hence retained and five communicated. Consequently, to obtain the true results, it is only necessary to multiply the second set of quantities by five, or the first set by three. If no waste, therefore, took place against the inside of the receiver, the heat evolved in the passage of air from the densities of four-fifths, three-fifths, two-fifths, one-fifth, and extreme rarefaction, to its ordinary state, would be 9, 16, 21, 24, and 25 centesimal degrees.

It is not difficult to discover the law of this progression. They are obviously formed by the successive addition of the odd numbers, 9, 7, 5, 3, and 1; and are, consequently, the excesses of the square of 5 above the squares of 4, 3, 2, and 1. Wherefore, if the square of the density be taken from unity, the remainder, multiplied by 25, will express in centesimal degrees the rise of temperature which accompanies the return of the air to its ordinary state.

The numbers thus obtained, however, do not still express the final results. If the restoration of four parts of the air included under the receiver to their usual density, disengage heat sufficient to raise the temperature of the whole five parts 9 degrees, its real measure must have been 11°4 degrees, or the former augmented in the ratio of 4 to 5. For the same reason, if three-fifths, two-fifths, and one-fifth of the air in the transition of density, evolve portions of heat which would elevate the temperature of the mass 16, 21, and 24 degrees, the actual quantities are 26°8, 52°8, and 120°, or those numbers multiplied by 5, and divided by 3, 2, and 1.

These conclusions are easily reduced to formulae. Let $\theta$ denote the density of the air, and $25 (1 - \theta)$ will express, in centesimal degrees, the elevation of the thermometer which would follow the re-admission of the air, if none of the heat were spent on the inside of the receiver. Consequently, $25 \left( \frac{1 - \theta}{\theta} \right)$, or $25 \left( \frac{1}{\theta} - \theta \right)$, will exhibit on the same scale the whole quantity of heat evolved in the restoration of density. The last formula is extremely simple, implying that 25, multiplied into the difference between the density of air and its reciprocal, will represent the measure of heat due to the change of condition. This result may be either additive or subtractive; it may express the heat emitted in the condensation of air, or the heat absorbed during its opposite rarefaction.

Thus, the heat extricated from air which has its density doubled is $25^2 (2 - \frac{1}{2})$, or $37\frac{1}{2}$; and the same quantity is then of the withdrawn, either when this air recovers its former density, or when air of the ordinary state expands into double its volume. Hence the copious heat extricated by the sudden compression of air. If it were condensed thirty times, the heat discharged would amount to $25 \left( \frac{30}{1} - \frac{1}{30} \right)$, or 749°, which is more than sufficient for the inflammation of fungous or soft substances. On this principle are constructed the pneumatic matches invented by Mollet of Lyons, which produce their effect by the momentary action of a small syringe.

But, to discover the relative capacity or attraction which air of a given density has for heat, it would be necessary to know the extent of the natural scale, or the position of the absolute zero. The conclusions, however, from different data, are not very constant; yet several experiments appear to fix nearly the point from which the infusion of heat commences at 750 centesimal degrees below congelation. On this supposition, therefore, air which is rarefied thirty times has its capacity doubled, the heat contained in it being dilated only fifteen times. For the same reason, air sixty times rarer than ordinary acquires a triple attraction for heat, which, in this union, becomes attenuated only twenty times. But these inferences are merely speculative, and the law of the gradation of temperature in the atmosphere is quite independent of the existence of an absolute term of heat.

The last formula now investigated has been already laid before the public, without any explication, however, or indeed indication, of the process by which it was discovered. The experiments on which it rests were begun many years since, and have been repeated with every precaution. But the mean results only are retained; and, for the sake of simplicity, a few slight modifications have been introduced, to adapt the apparatus to more convenient proportions. Though it was impossible to blow a receiver that should have exactly half the dimensions of another, nothing seemed easier, from the general mode of investigation, than to apply the minute corrections which any small deviations of size or form required. The mixture of obscure and intricate computations has been thus avoided.

Since the absolute quantity of heat contained in every part of any vertical column of the atmosphere has been shown to remain unchanged, this formula must likewise represent the diminution of temperature in the higher strata corresponding to the decreased density of the air at different elevations. The same formula will determine the measure and gradation of this effect. Reckoning the density of the air at the surface of the earth unity, the difference between the density at any given altitude and its reciprocal, being multiplied by 25, will express the mean diminution of tem- Climate.

Examples of the application of the formula.

According to Lasius, the same barometer which, at Goslar, an ancient town seated in the bosom of the Hartz Forest, stands at 29,500 inches, would fall to 26,444 on the top of the Brocken, in that mining district. This gives 896 for the density of the air on the summit, the reciprocal of which is 1:116; but 1:116 - 896 = 22, and 22 x 25 = 5:5, the calculated difference of temperature. The actual difference is very nearly the same, being only 5°2'; as we had once an opportunity ourselves of observing, having found the temperature of a copious spring at Goslar to be eight centesimal degrees, while that of the noted Hecken-Brunnen, or Witch-Well, on the summit of the Brocken, was only 2°8'.

Saussure, whose accuracy always inspires confidence, found that, while at his villa of Conche, near Geneva, the barometer stood at 28:500, another similar instrument fell to 25:165 on the top of the mountain of Nant Bourant. The diminished density of the air at this elevation was, therefore, 890; the difference between which and its reciprocal 1:123 being multiplied by 25°, gives 5°82. But a thermometer buried a whole night at two feet deep in that lofty station marked only 12°75', while it indicated 6°25 more, or 19', a few days afterwards, when sunk to the same depth at Conche. The discrepancy here is thus less than half a degree.

On the top of a higher mountain, the Chapieu, the same observer found the ground, at a depth of two feet, to be colder by 6°44' than at Conche; but the corresponding density of the air and its reciprocal were 872 and 1:147; consequently, 2° x 275 = 6°87'.

While the barometer at Conche stood at 28:500 inches, the mercurial column was only 19:836 inches on the summit of Mont Cervin, a still loftier mountain. The density of the air at this elevation was therefore 696, which being taken from its reciprocal 1:437, leaves 741 to be multiplied by 25°, including 18°52' as the diminution of temperature. The actual medium difference ascertained from corresponding thermometrical observations, made at depths in the ground from one to three feet, on the top of Mont Cervin and at Conche, was 18°25', almost exactly the same.

Such is the nice agreement on the whole, between theory and observation, with regard to the decrease of the mean temperature in the higher regions of the atmosphere. This gradation of cold varies, however, to a certain extent with the seasons. Since the heat derived from the sun is chiefly accumulated at the surface of the earth, the changes of temperature which take place through the year in the elevated strata of our atmosphere must evidently be less than what are experienced below. The lofty tracts of air, remote from the primary scene of action, preserve nearly and to that an equable temperature, and scarcely feel the extreme heat of summer or winter's frost. In ascending the atmosphere, the decrease of warmth is hence more rapid in the fine season, and more slow in the darkened period of the year. In many places, it will not be far from the truth, perhaps, to assume 30° for the multiplier during the summer months, and only 20° during those of winter.

Thus General Roy, a diligent and experienced observer, found, in the month of August, the air on the top of Snowdon was, in the course of a whole day, at an average, 7-2 centesimal degrees colder than on Caernarvon quay; but the difference between the density at that elevation and its reciprocal, or between 878 and 1:139, being only 261, would require nearly 28 for the multiplier.

In the early part of September, the same observer noticed the centesimal thermometer to stand 10 degrees lower on the top of Ben Lawers than at Weem, the relative density of the air at that height being 808. The difference from its reciprocal is 284, which would hence require to be multiplied by 35 to give the actual diminution of temperature.

Again, Saussure found, on his visit to Mont Blanc, the air on its summit to be 31 centesimal degrees colder than at Geneva. The relative density was 592, which being taken from its reciprocal 1:689, leaves 1:097; consequently the multiplier required is 28.

This ingenious philosopher passed several days encamped on the Col du Grand, where he found, from the mean of eighty-five observations, the temperature of the air to be only 4°54', or 20°3 colder than at Geneva; but the relative density of that elevated stratum and its reciprocal were 704 and 1:420; the difference of which, or 716, would require to be multiplied by 28½ to indicate the diminished temperature.

The observations made on the decreased temperature of the higher regions of the atmosphere by the ascension of balloons, appear generally to indicate rather a slow rate of diminution; but it should be recollected that those daring aerial flights have seldom been performed except in the fine season of the year. Besides, the car, the balloon, and its cordage, will not immediately acquire the temperature of the elevated strata, but continue for a considerable time to diffuse a sensible portion of heat. A memorable example, however, is entirely conformable to the general principle. Charles, the first who ascended the atmosphere by means of a balloon filled with hydrogen gas, found, on the 1st of December 1783, the thermometer depressed 11 centesimal degrees at the greatest elevation, the column of the barometer having sunk from 29:24 to 20:05 inches. This would require a multiplier less than 20.

The curve of perpetual congelation must evidently rise origin of higher during the tide of summer, and again descend in the glaciers, winter months, thus oscillating between certain limits of elevation. The intervening belt is narrow under the equator; but it enlarges to a very considerable extent in the higher latitudes. On the breadth of this zone, where frost holds a doubtful reign, depends the formation of glaciers along the flanks of the snowy mountains. The fields of snow which are alternately melted and congealed, become at last changed by this process into ice, often grouped and fashioned by such irregular action into the most fantastic shapes. In its native seat, this icy belt acquires continual additions to its height, till the accumulating pressure urges it downward, and at last precipitates its fragments to a lower level. In its new position, below the inferior boundary of congelation, the enormous pile suffers, on the whole, a very gradual thaw, which is sometimes protracted for several centuries. Meanwhile, in the higher magazine, another belt is again slowly collecting, which will in due time repeat the succession, and maintain the perpetual circle of production and decay.

Within the tropics, the zone of undecided frost is so very narrow, that scarcely any trace of a glacier has been ever observed. But as that zone enlarges in the higher latitudes, the appearance of vast glaciers constitutes a very striking feature in the aspect of the lofty mountains. They occur frequently along the sides of the Pyrenees, but they are still more conspicuous in the recesses of the central chain of the Swiss Alps. Glaciers are likewise seen as far north as the verge of the arctic circle. Along the western shore of Norway and the coast of Lapland, stretching onwards to the promontory of the North Cape, huge masses of glacier ice descend from the cliffs, or against the precipitous sides of the mountains, almost to the surface of the ocean.

Climate is affected by the intensity of solar light, or what Climate has been termed insolation. This influence was not much remarked until of late years. The effect of light on the colour of plants, indeed, attracted the attention of philosophers in the last century, especially of Ingenhouz and Sennebier; and practically, the effect of etiolation, or blanching of vegetables growing in darkness, was known to gardeners. But the effects of light on animals attracted little attention. Some had remarked the comparative rarity of bodily defects in the natives of hot climates where the light is strong; but the experiments of Dr W. F. Edwards on tadpoles proved that light exerts considerable influence on the animal frame. He showed, that if one portion of the same brood be exposed to light, while another portion was suffered to live in darkness, while every other circumstance was the same in each, that the change to the perfect frog took place far sooner in those exposed to light than in those kept in darkness, the latter arriving almost to the size of their parents before the change took place. Instruments, therefore, for measuring the degree of solar light form now part of a meteorological apparatus. Two instruments of this sort are chiefly employed, the actinometer and the photometer of Professor Leslie. The former consists of a thermometer with a large oblong bulb, and filled with a dark coloured fluid. As it has no fixed point, it is reduced for each observation to a zero by means of a screw acting on the elastic extremity of the bulb, formed of a slip of caoutchouc; the instrument, in a case covered by a glass, is exposed to the direct rays of the sun, and its rise then noted. It affords comparative indications of the intensity of solar light.

For measuring the intensity, or at least the calorific action of light, no instrument is so finely adapted, by its peculiar delicacy, as the Photometer, which consists of a Differential Thermometer inclosed in a thin pellicular case, and having one ball made of black and the other of clear glass. It will besides admit of some variety in its form and construction, and may be rendered on the whole very commodious and portable. Yet, owing to a combination of circumstances, this elegant instrument has only been partially and reluctantly admitted; and the philosophic world has still to discharge an act of justice, by receiving it into the favour and distinction which it so well deserves. Some, indeed, affecting to display superior sagacity, have taken the trouble to remark that it was only a species of thermometer, and not strictly a photometer, since it measures heat and not light. But what does the thermometer itself indicate, except expansion? As heat is measured by the expansion it occasions, so light is determined by the intensity of the heat which, in every supposition, invariably accompanies it. What other mode, after all, could be imagined for detecting the presence of light? How can an unknown quantity be expounded, but in terms of one already known?

The photometer is adapted for a variety of important meteorological researches. If such instruments, in the hands of skilful observers, had been dispersed to the remote regions of the globe, we should ere now have obtained a body of precise facts, highly instructive in themselves, and calculated to illustrate the nature of different climates. Meanwhile, we shall endeavour to state the general consequences which may be drawn from even a scanty range of photometrical observations.

The direct and absolute action of the sun's rays on the photometer, at the elevation of 30°, may be reckoned in this climate at 120 millesimal degrees. The effect is produced by the incidence of a pencil of light, which has for its base a circle of the same diameter as the black ball, but modified and regulated in its amount by the subsequent dispersion of the accumulated heat from the whole surface of the sphere, which is four times greater than that of the generating circle. If a thin disc were, therefore, substituted instead of the ball, and presented to the perpendicular rays of the sun, the impression would be doubled, or raised to 240°; or, if the emission of heat from the posterior surface of the disc were prevented, the calorific effect would amount by this accumulation to 480°, or 86° on Fahrenheit's scale. Such is the rise of temperature which a dark surface of dry mould, sloping at an angle of 30°, yet exactly facing the sun, might acquire under a diaphanous shell of glass, if scarcely any portion of the heat were supposed to be conducted downwards into the mass of the earth. But since the rays of light which traverse the atmosphere under an obliquity of 30° have, in comparison with perpendicular beams, their force diminished in the ratio of 750 to 563; the action of a vertical sun, through a thin capsule of glass, might heat up a dark horizontal surface 113° by Fahrenheit's scale. On removing the glass cover, this effect, in a calm still air, would be reduced about two-thirds, or to 75°.

It is obvious that the accumulated effect of the incident calorific rays much increase in proportion as the conducting power action of the medium is diminished. Hence, at an elevation of the rays in three miles and a half, where the density of the atmosphere different is reduced to one-half, the heat communicated would, on circumstances this account alone, augment from 75° to 83 degrees. But the effect would be further increased, from the smaller absorption of heat in its passage to the surface. Under the equator, the whole accumulated action would, therefore, amount to 96 degrees. All travellers, accordingly, complain of the scorching rays which the sun darts from a dark azure sky on the summits of lofty mountains. Yet the contrast is more striking in the higher latitudes. Thus, in the middle parallel of 45°, the action of the sun at the summer solstice would excite a heat of 69° at the level of the sea, and of 90° at an elevation of three miles and a half; but at the winter solstice it would communicate only 17° below and 46° at the altitude assumed. Saussure was accordingly very much struck with the force and brilliancy of the sun-beams on the top of Mont Blanc.

It might easily be computed that, on the supposition of a perfect calm, the surface of the earth, under the equator, will, at the medium of a year, have its temperature raised 12°; and that, in the latitude of 45°, the mean annual impression would be only 5°. But, of the whole of the light received, the calorific action on a black mould, whether emitted from the sun or shed indirectly by the sky, may be deduced from the indication of the photometer. It is only required to diminish the power of the sun's rays in the ratio of the sine of their obliquity, and to reduce the action of the light reflected from the canopy of clouds to one-half, or what is due to the medium inclination of 30°, then to multiply the sum of these quantities by eight, and divide by three, or to take two-thirds of the quadrupled effect. Thus, suppose, while the sun's altitude is 40°, that the photometer marks 155°, which it very seldom ever reaches in this climate, and that it indicates only 20° if merely screened from the direct action of the sun. Now, 155° multiplied by the sine of 40° makes 87°-8, which is augmented to 97°-8 by the addition of the half of 20°; and this number again being increased in the ratio of 3 to 8, gives finally 261 millesimal degrees, or 47° on Fahrenheit's scale. When the sun is obscured in clouds, the reflected light, from a dappled sky, will sometimes in summer affect the photometer to the extent of 50°. This corresponds to a heat of 16° of Fahrenheit communicated to the ground. During the fine season the photometer seldom in cloudy weather indicates less than 15°, which is equivalent to an impression of 6° on the embrowned surface of the earth. While the sun is enveloped in clouds, if the rest of the sky assumes a fine azure hue, the photometer will only mark 10°. But in the gloomy days of winter, the minute portion of light which pierces through the congregated mass of clouds will scarcely affect the photometer 5°, or excite a heat of 2° by Fahrenheit on the ground. These augmentations of temperature are communicated to the ground unimpaired, only in the case, however, of a perfect calm. The agitation of the atmosphere will scatter the heat before it has accumulated. When the wind creeps along the surface of the earth at the rate of eight miles in the hour, it diminishes the calorific action of the light from the sun and from the sky one-half; but if it sweeps with a velocity of 16, 24, or 32 miles in the hour, it will reduce the whole effect successively to the third, the fourth, or the fifth of its standard. The impression made on the ground seldom, therefore, exceeds the third part of the computed measure, and often will not amount to one-fifth.

The simplest and most accurate method of examining the temperature acquired during the day on the surface of the earth, is to employ a differential thermometer of the pendant kind, about one or two feet in length, and having its lower ball surmounted by a small cylindrical cavity supporting the coloured liquor. This instrument being suspended or held in a vertical position, the lower ball resting on the ground, will evidently mark, by its movable column, the difference between the temperature of the surface and that of the ambient air. In this way, we have found, during the summer months in this climate, that the ground was, by Fahrenheit's scale, generally two or three degrees warmer than the air near it in cloudy weather, and perhaps ten or fifteen degrees warmer when the sun shone powerfully upon it. But, under similar circumstances, the effect varies very considerably, according to the nature of the surface. While fresh ploughed land, for instance, indicates an increased temperature of perhaps 8°, a grass plot close beside it will scarcely show a difference of 3°. Nor is this distinction owing to any greater absorption of light by the black mould; the reflection from the surface, in both cases, being extremely small. A thin layer of hay, whether spread on the naked soil or on the green turf, will betray the same diminished effect. The fibres of the grass exposing a multiplied surface to the contact of the air, the greater portion of the heat is hence dissipated before accumulation. A corresponding effect has been remarked with respect to the impressions of cold. Thus, in the neighbourhood of Edinburgh, after a long tract 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 water-pipes. The greater density and solidity of the pavement had no doubt conducted the frigorific impressions more copiously downwards, while the loose and spongy blades of grass had mostly scattered and wasted those impressions in the open field. This consideration, it is obvious, might lead to very important practical results.

The unequal action of light at the surface of the earth, whether produced by the various obliquity of the sun's rays, the different inclination of the horizon, or the alternating succession of day and night, is attested, we have seen, by the actual flow of the heated portions of the atmosphere. Between the poles and the equator, a perpetual circulation of air is maintained, which confines the accumulating effects of heat within narrow limits. The prevalence, on the whole, of northerly winds in this hemisphere during summer, and of southerly winds in winter, tends likewise to mitigate the extreme impressions of hot and cold. But a current of warm air excited at first by the presence of the sun, continues to rise from the ground, and occasions the descent, therefore, of an opposite current of cold air, which, as the equilibrium of temperature is not soon restored, may be protracted through a great part of the night. The combined influence of these currents is hence continually exerted in cooling down the surface of the earth; but their activity being the greatest while the solar beams fall most copiously the accumulation of heat is checked in little more than an hour after mid-day, while its further dissipation is prolonged through the whole of the night, sun-rise being generally the moment when the ground is coldest.

Such a concatenated system of aerial currents might hence appear sufficient to explain the gradation and general balance of temperature which prevails on the surface of our globe. An horizontal stream of air must evidently cause the flow of an opposite one, since the action must be the same on every part of the same parallel of latitude. The difference of the temperature of the surface from that of the ambient air will maintain the constant play of an ascending and a descending current. In clear and calm weather, this interchange between the higher and lower strata of the atmosphere will be the most vigorous, owing then to the concentrated impression of the sun-beams. The perpetual commerce maintained in our atmosphere by the medium of these combined horizontal and vertical currents, forms no doubt an essential part of the system which attempts the constitution of this globe. But it is not the only mode by which nature seeks to preserve the harmony of her productions; and recent discovery has detected the existence of another auxiliary principle, extremely active, of most rapid and extensive influence, and continually at work, though subject to various modifications. To understand rightly, however, the operation of this principle, it will be necessary to recall the chief facts which have been disclosed relative to the propagation of heat.

It is well known that, though partial causes may disturb the equilibrium of temperature among bodies, there is a constant tendency to restore it again. Yet heat still remains in the state of combination, without ever assuming a distinct form. Its balance is, therefore, maintained by a very different process from that which establishes the equilibrium between the several communicating parts of a liquid. The substratum of heat is not passive, nor do the calorific particles themselves merely flow from their redundancy towards another situation where they happen to be deficient. But since the presence of heat is invariably accompanied by corporeal distention, that portion of the substance which loses it must successively contract, while the portion which gains it will in the same degree expand. The actual transfer of heat through any mass will hence give occasion to a connected series of minute internal contractions and expansions. To consider the subject more fully, we shall suppose the conducting substance to be, 1. a solid; 2. a liquid; and, 3. a gaseous fluid.

1. When the surplus heat is conducted through a solid substance, a sort of alternating vermicular motion is excited along the whole train of communication. If heat were left to the energy of its own repulsion, it would, like light, dart with a speed almost instantaneous. But the time consumed by those interior oscillatory movements retards immensely the rate of transmission. The quickness of the oscillations themselves depends on the elasticity of the conducting substance; but their energy and extent are proportioned to the extreme difference of temperature, and the shortness of the tract, modified essentially by the peculiar nature of the conducting substance. In equal circumstances, glass transmits heat faster than wood, and metal faster than glass. But, even in the same class of conductors, the effects are very different; thus, box delivers the impressions of heat more quickly than cork, and silver conveys them with greater rapidity than platinum.

2. When the conducting substance is a liquid. The ordinary transmission of heat through a solid is now greatly augmented, from the diffusion occasioned by the mobility of the affected portions of the medium. Below the freezing point, ice will conduct heat through its substance; but after it has melted into water, a new and powerful agency Climate. is brought into play. The liquid particles, as they become successively warmer, acquire a corresponding expansion, and, therefore, rise upwards and spread through the mass, carrying with them and dispersing the heat which they have received. This diffuse buoyancy will depend evidently on the dilatable quality of the liquid. It is greater in alcohol than in water, and in water than in mercury; it is even more active in hot than in cold water. Near the point of congelation, indeed, the joint conducting power of water is scarcely superior to that of mere ice. The actual flow of a liquid, by whatever cause it is produced, must evidently accelerate the dissipation of heat.

3. When the medium of transmission is a gaseous substance, the heat is partly still conducted through the substance of the communicating mass, as if this were solid, and partly transferred by the streaming of the corpuscles, which come to be successively affected. But a new principle seems here to combine its influence, and the rate of dispersion in aeriform media is found to depend chiefly on the nature of the mere heated surface. From a metallic surface the heat is feebly emitted; but from a surface of glass, or still better, from one of paper, it is discharged with profusion. If two equal hollow balls of thin bright silver, one of them entirely uncovered, and the other closely enveloped in a coat of cambric, be filled with water slightly warmed, and then suspended in a close room, the former will only lose 11 parts of heat in the same time that the latter will dissipate 20 parts. Of this expenditure 10 parts from each of the balls is communicated in the ordinary way, by the slow recession of the proximate particles of air as they come to be successively heated. The rest of the heat, consisting of 1 part from the naked metallic surface, and of 10 from the cased surface, is propagated through the same medium, but with a certain diffusive rapidity, which in a moment shoots its influence to a distance, after a mode altogether peculiar to the gaseous fluids.

But those effects are modified by the different proximity of the air to the metallic surface. If the silver ball be covered with the thinnest film of gold-beater's skin, which exceeds not the 3000th part of an inch in thickness, the power of dispersion will be augmented from 1 to 7; if another pellicle be added, there will be a farther increase of this power from 7 to 9; and so repeatedly growing, till after the application of five coats, when the repellent energy will reach its extreme limit, or the measure of 10.

The approximation of the metallic substratum thus evidently diminishes the power of the external pellicle in darting heat. No absolute contact exists in nature; but air must approach to a boundary of pellicle, or cambric, much nearer than to a surface of metal, from which it is always divided by an interval of more than the 500th part of an inch. A vitreous surface has very nearly the same property as one of cambric or paper: from its closer proximity to the recipient medium, it imparts its heat more copiously and energetically than a surface of metal in the same condition.

By what process the several portions of heat thus delivered to the atmosphere shoot through the fluid mass, it seems more difficult to conceive. They are not transported by the streaming of the heated air, for they suffer no derangement from the most violent agitation of their medium. The air must, therefore, without changing its place, disseminate the impressions it receives of heat by a sort of undulatory commotion, or a series of alternating pulsations, like those by which it transmits the impulse of sound. The portion of air next the hot surface, suddenly acquiring heat from its vicinity, expands proportionally, and begins the chain of pulsations. In again contracting, this aerial shell surrenders its surplus heat to the one immediately before it, now in the act of expansion; and thus the tide of heat rolls onwards, and spreads itself on all sides.

But these pulsations are not propagated with equal intensity in all directions. They are most powerful in the perpendicular to the projecting surface, and diminish as they deviate from that axis in the ratio of the sine of the angle of obliquity.

Nor are the vibratory impressions strictly darted in radiating lines, but each successive pulse, as in the case of sound, presses to gain an equal diffusion. Different obstructions may hence cause the undulations of heat to deflect considerably from their course. Thus, if a cornucopia, formed of pasteboard, present its wide mouth to a fire, a strong heat will, in spite of the gradual inflection of the tube, be concentrated at its narrow end; in the same way, probably, as waves flowing from an open bay into a narrow harbour, now contracted and bent aside, yet without being reflected, rise into furious billows.

But the same pulsatory system will enable the atmosphere to transmit likewise the impressions of cold. The shell of air adjacent to a frigid surface, becoming suddenly chilled, suffers a corresponding contraction, which must excite a concatenated train of pulsations. This contraction is followed by an immediate expansion, which withdraws a portion of heat from the next succeeding shell, itself now in the act of contracting; and the tide of apparent cold, or rather of deficient heat, shoots forwards with diffusive sweep.

That quality which enables a surface to propel the hot or cold pulses, likewise fits it under circumstances to receive their impressions. If a vitreous or varnished surface emits heat most copiously, it will also, when opposed to the tide, arrest with entire efficacy the affluent wave; and if, on the other hand, a surface of metal sparingly parts with its own heat, it detains only a small share of each warm appulse, and reflects all the rest.

Hence the construction of the Pyroscope, a delicate and Pyroscope, valuable instrument, adapted to measure the warm pulses of air, or the intensity of the heat that darts continually from a fire into a room, which has been vaguely and inaccurately termed radiant heat. It is in fact only a modification of the differential thermometer, one of the balls being completely gilt with a thick gold or silver leaf. The pyroscope being placed at some distance from the fire, the hot pulses are mostly thrown back from the bright metallic surface; but on the naked glass ball they produce their full impression; and the same instrument will serve equally to indicate the pulsations excited from a cold surface. Thus, in a warm apartment, the pyroscope placed before a mass of snow, a block of ice, or even a pitcher of water from the fountain, will quickly intimate the chilling impressions propagated through the ambient medium. Nor has the brightness of the fire or the glare of the snow any sensible influence to affect the result; for, of the small portion of light transmitted, what falls on the diaphanous ball passes almost without obstruction, and what strikes the gilt ball, especially if this be covered with silver leaf, is nearly all reflected.

But the pyroscope, in its simple form, is scarcely calculated for making nice observations, when exposed out of doors to the agitation of winds and the effulgence of light. A greater share of this light will generally be detained by the gilt surface than what is absorbed in its passage through the diaphanous ball. On the other hand, all the effects on the instrument will be diminished by the rapidity of the circulation of the air.

The requisite adaptation is attained by adapting the py- Athrioscopescope to the cavity of a polished metallic cup of rather an ellipse oblong spheroidal shape, the axis, having a vertical position, being occupied by the sentient ball, while the section of a horizontal plane passing through the upper forms the orifice. The cup may be made of thin brass or silver, either hammered or cast, and then turned and polished on a lathe; the diameter being from two to four inches, and the ec- centricity of the elliptical figure varied within certain limits according to circumstances. The most convenient proportion, however, is to have this eccentricity equal to half the transverse axis, and, consequently, to place the focus at the third part of the whole height of the cavity, the diameter of the sentient ball being likewise nearly the third part of that of the orifice of the cup.

The athrioscopé might be reduced to a compact form, having the lower ball ensconced by a hollow sphere of brass, composed of two pieces which screw together, while the upper ball occupies the focus of the cup, which needs scarcely be more than two inches wide.

This instrument, exposed to the open air in clear weather, will at all times, both during the day and the night, indicate an impression of cold shot downwards from the higher regions. Yet the effect varies exceedingly. It is greatest while the sky has the pure azure hue; it diminishes fast as the atmosphere becomes loaded with spreading clouds; and it is almost extinguished when low fogs settle upon the surface. The name Athrioscopé (from Athros, serenus, sudus, frigidus) may, therefore, be justly appropriate to this new combination of the pyroscope. The sensibility of the instrument is very striking, for the liquor incessantly falls and rises in the stem with every passing cloud. But the cause of its variations does not always appear so obvious. Under a fine blue sky, the athrioscopé will sometimes indicate a cold of 50 millesimal degrees; yet on other days, when the air seems equally bright, the effect is hardly 30°. Particular winds at different altitudes seem to modify the result, and so perhaps may the transition from summer to winter. The pressure of hygrometric moisture in the air probably affects the indications of the instrument.

On replacing the metallic lid, the effect is entirely extinguished, and the fluid in the stem of the differential thermometer sinks to zero. A cover of pasteboard has at first precisely the same influence; but after it has itself become chilled by this exposure, it produces a small secondary action on the sentient ball, scarcely exceeding, however, the tenth part of the naked impression. A lid of glass or of mica intercepts the impressions like one of paper; for the admission of light has no deranging effect if the athrioscopé be rightly constructed and highly polished. The minute secondary action is almost extinguished, if screens of paper, glass, or mica, be held at some distance above the mouth of the instrument.

The climate of any region is greatly modified by the moisture or dryness of its atmosphere. This has an important influence on its vegetable productions. The modes and instruments employed in estimating this quality of the atmosphere are detailed under the articles Hygrometry and Physical Geography.

The quantity of rain which falls periodically in different countries has great influence on climate. As a general fact, the annual quantity of rain decreases from the equator towards the poles. It is greatest among mountains; but diminishes as we ascend lofty ridges, as the Andes or the Himalayas. For the instruments employed in determining this circumstance, and the inferences deduced from observation, see Physical Geography.

For the effect of climate on the geographic distribution of plants, see Botany, Part iii., where the subject is treated as fully as our limits admit.

(J.L.) (T.S.T.)

END OF VOLUME SIXTH.

NEILL AND CO., PRINTERS, EDINBURGH. ### Elements of Chinese Characters

#### Elements of One Stroke

| Stroke | Character | Meaning | |--------|-----------|---------| | 1 | 一 | one | | 2 | 二 | two | | 3 | 三 | three | | 4 | 四 | four | | 5 | 五 | five | | 6 | 六 | six | | 7 | 七 | seven | | 8 | 八 | eight | | 9 | 九 | nine | | 10 | 十 | ten |

#### Elements of Two Strokes

| Stroke | Character | Meaning | |--------|-----------|---------| | 11 | 一 | one | | 12 | 二 | two | | 13 | 三 | three | | 14 | 四 | four | | 15 | 五 | five | | 16 | 六 | six | | 17 | 七 | seven | | 18 | 八 | eight | | 19 | 九 | nine | | 20 | 十 | ten |

#### Elements of Three Strokes

| Stroke | Character | Meaning | |--------|-----------|---------| | 21 | 一 | one | | 22 | 二 | two | | 23 | 三 | three | | 24 | 四 | four | | 25 | 五 | five | | 26 | 六 | six | | 27 | 七 | seven | | 28 | 八 | eight | | 29 | 九 | nine | | 30 | 十 | ten |

#### Elements of Four Strokes

| Stroke | Character | Meaning | |--------|-----------|---------| | 31 | 一 | one | | 32 | 二 | two | | 33 | 三 | three | | 34 | 四 | four | | 35 | 五 | five | | 36 | 六 | six | | 37 | 七 | seven | | 38 | 八 | eight | | 39 | 九 | nine | | 40 | 十 | ten |

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Published by A & C Black, Edinburgh.