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also still more affected by the sun's vertical rays, because their passage through the atmosphere is shorter than that of the oblique rays. As far as the sun's mean altitude only is concerned, it appears from Simson's calculations, that the heat received at the equator in the whole year, is nearly twice and a half as great as at the poles; this proportion being nearly the same as that of the meridan heat of a vertical sun, to the heat derived, at the altitude 2340, in the middle of the long annual day at the poles. But the difference is rendered still greater, by the effect of the atmo sphere, which interrupts a greater portion of the heat at the poles than elsewhere. Bouguer has calculated, upon the supposition of the similarity of the affections of heat and light, that in latitude 45°, 80 parts out of 100 are transmitted at noon in July, and 55 only in December. The heat intercepted by the atmosphere is perhaps not wholly, but very nearly, lost with respect to the climate of the neighbouring places. It is obvious that, at any individual place, the climate in summer must approach in some degree to the equatorial climate, the sun's altitude being greater, and in winter to the climate of the polar regions.
While the earth is becoming warmer at any particular spot, the heat thrown off by radiation into the atmosphere, and thence into the empty space beyond it, together with that which is transmitted to the internal parts of the earth, must be less than the heat received from the sun; and when the earth is growing colder, more heat must pass off than is received: but whenever the heat of the surface is stationary, neither increasing nor diminishing, as at the times of the greatest and least heat, it is obvious that the heat re ceived from the sun must be precisely equal to the heat which is thrown off. Now this quantity may be estimated by the degree of refrigeration in the night; and hence Mr. Prévost has very inge niously deduced the proportion of the sun's heat arriving at the surface of the earth in the latitude of Geneva, in July, and in December; which he finds to be as 7 or 8 to 1; and this result agrees very well with a calculation deduced from the length of the day, the sun's altitude, and the interception of his rays by the atmosphere.
In London the temperature generally varies, in the course of the day and night, somewhat more than 5o, and less than 20°. In January, the mean diurnal variation of temperature is 6o, in
July 10°, and in September, 18°. Hence, says Mr. Kirwan, we may understand the reason of the great frequency of colds in spring and in autumn.
Some philosophers have supposed the earth to become progressively warmer in the course of ages, while others have imagined that its heat is exhausted. Both these opinions appear in general improbable. The greater heat the earth receives by day, the more it throws off, both by day and by night; so that in the course of a few ages the heat must probably have attained its maximum. Local changes may indeed arise from local circumstances; thus, the climate of America is said to have become considerably warmer, since a large part of its surface has been cleared from its dense forests by human labour; and to judge from the descriptions of the ancients, it appears that even in Europe the winters were formerly much colder than they are at present. If, however, Dr. Herschel's opinion of the variation of the heat of the sun be confirmed, it will introduce a great uncertainty into all theories upon the subject: since in these calculations the original heat of the sun has always been supposed unalterable.
The sea is less heated than the land, partly because a greater quantity of water evaporates from it, and partly because the sun's rays penetrate to a considerable depth, and have less effect on the surface, while the water is also mixed, by the agitation of its waves and currents, with the colder water below. It is also more slowly cooled than the land, since, when the temperature of the superficial particles is depressed, they become heavier, and sink to the bottom. For similar reasons, the sea is colder than the land in hot climates, by day, and warmer in cold climates, by night. These circumstances, however, nearly balance each other, so that the mean temperatures of both are equal, that of the sea being only less variable. Although the process of evaporation must cool the sea, yet when the vapours are condensed without reaching the land, their condensation must compensate for this effect by an equal extrication of heat.
. There is another cause which perhaps contributes in some degree, in temperate climates, to the production of cold; that is, the alternation of freezing and thawing. Mr. Prévost observes that congelation takes place much more suddenly than the oppo. site process of liquefaction; and that of course the same quantity
of heat must be more rapidly extricated in freezing than is absorbed in thawing; that the heat, thus extricated, being disposed to fly off in all directions, and little of it being retained by the neighbouring bodies, more heat is lost than is gained by the alternation: so that where ice has once been formed, its production is in this manner redoubled. This circumstance must occur wherever it freezes, that is, on shore, in latitudes above 35°; and it appears that from about 30° to the pole, the land is somewhat colder than the sea, and the more as it is further distant from it; and nearer the equator the land is warmer than the sea: but the pro cess of congelation cannot by any means be the principal cause of the difference, and it is probable that the different capacity of earth and water for heat is materially concerned in it.
Since the atmosphere is very little heated by the passage of the sun's rays through it, it is naturally colder than the earth's surface; and for this reason, the most elevated tracts of land, which are the most prominent, and the most exposed to the effects of the atmosphere, are always colder than situations nearer the level of the sea. The northern hemisphere is somewhat warmer than the southern, perhaps because of the greater proportion of land that it contains, and also in some measure on account of the greater length of its summer than that of the southern; for although, as it was long ago observed by Simpson, the different distance of the sun compensates precisely for the different velocity of the earth in its orbit, with respect to the whole quantity of heat received on either side of the equinoctial points, yet Mr. Prévost has shown, that in all probability the same quantity of heat must produce a greatet effect when it is more slowly applied; because the portion lost by radiation from the heated body is greater, as the temperature is higher. Since, therefore, on account of the eccentricity of the earth's orbit, the north pole is turned towards the sun seven or eight days longer than the south pole, the north. ern winters must be milder than the southern: yet the southern summers, though shorter, ought to be somewhat warmer than the northern but in fact they are colder, partly perhaps from the much greater proportion of sea, which in some degree equalises the temperature, and partly for other reasons. The comparative intensity of the southern summer and winter is not exactly known;
but in the island of New Georgia the summer is said to be extremely cold.
The northern ice extends about 9° from the pole: the southern 18° or 20°; in some parts even 30'; and floating ice has occasionally been found in both hemispheres as far as 40° from the poles, and sometimes, as it has been said, even in latitude 41° or 42°. Between 54° and 60° south latitude, the snow lies on the ground, at the sea side, throughout the summer. The line of perpetual congela. tion is three miles above the surface at the equator, where the mean heat is 84°; at Teneriffe, in latitude 28°, two miles; in the latitude of London, a little more than a mile; and in latitude 80° north, only 1200 feet. At the pole, according to the analogy deduced by Mr. Kirwan, from a comparison of various observations, the mean temperature should be 31°. In London the mean temperature is 50°; at Rome and at Montpelier, a little more than 60°; in the island of Madeira, 70°; and in Jamaica, 80°.
There are frequently some local causes of heat and cold which are independent of the sun's immediate action. Thus, it has been observed, that when the weather has been clear, and a cloud passes over the place of observation, the thermometer frequently rises a degree or two almost instantaneously. This has been partly explained by considering the cloud as a vesture, preventing the escape of the heat which is always radiating from the earth, and reflecting it back to the surface: the cloud may also have been lately condensed, and may itself be of a higher temperature than the earth. Mr. Six has observed that in clear weather, the air is usually some degrees colder at night, and warmer by day, close to the ground, then a few feet above it; but that in cloudy wea ther there is less difference and it is possible that this circumstance may be derived from the difference of the quantity of evapo. ration from the earth's surface, which occasions a different degree of cold in different states of the atmosphere.
An idea has frequently been started, that the temperature of se veral, perhaps of all climates, has varied at different epochs, and is in truth perpetually varying; in some instances for the better, and in others apparently for the worse. And the more or less active cultivation of the soil, the clearing and draining of the ground, or the suffering it to lie barren and unproductive, co
vered with woods and morasses, are the causes which have chiefly been adverted to for the purpose of explaining these phænomena.
After all, however, the assertion, as relating to a general fact, requires to be more attentively examined than it appears to have been; and admitting its truth, the cultivation or neglect of the soil does not seem in every instance to constitute the actual cause of this difference in the temperature.
In America, observes an intelligent Irish writer in the Philoso phical Transactions, at least as far as the modern plantations are extended, an extraordinary alteration has been perceived in the temperature of the country since the Europeans began to settle there. This change, continues he, is generally attributed to the cutting down of vast woods, with the clearing and cultivating of the country. But that Ireland should also considerably alter without any such manifest cause, either invalidates that reason, or else evinces that quite different causes, may produce the same effect. For if it be true, as some compute, that this kingdom was better inhabited and cultivated before the late civil wars, then at present t, it should, according to the reasons alledged for the change of temperature in America, be rather grown more intemperate, viz. for want of cultivation: but the contrary is observa ble here, and almost every one begins to take notice, that this country becomes every year more and more temperate. Formerly it was not unusual to have frost and deep snows of a fortnight or three weeks continuance; and that twice or thrice, sometimes oftner, in a winter; nay we have had great rivers and lakes frozen all over; whereas of late, especially these two or three years last past, we have had scarcely any frost or snow at all. Neither can I impute this extraordinary alteration to any fortuitous concourse of ordinary circumstances requisite to the production of fair wea ther; because it is manifest, that if has proceeded gradually, every year becoming more temperate than the preceding. Though it be observed that frosty and snowy winters make early springs, and for as little as we have had of either this winter, yet there has not within the memory of any now living happened a forwarder. spring in Ireland; since this island could produce some store of ripe cherries in the midst of April. The wind keeps for the most part here between the north-west and the south, seldom at east,
• Vol. xi. year 1676.
+ The paper here referred to bears the date of 1676.