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 occa. sionally 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 with. out 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, 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. Forme.ly 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 weather; 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 bere referred to bears the date of 1676.

EDIT.

and yet less frequent at north or north-east, insomuch that many here do not scruple to affirm, that for at least 2 of the year the wind is westerly; and we have sometimes known passengers wait at Chester and Holyhead no less than three months for a fair wind to come hither.

The Honourable Daines Barrington is well known to have entertained a similar opinion, and to have carried to a much greater extent. He communicated it chiefly in an article in a much later volume of the valuable journal we have just referred to*; and its substance we shall lay before our readers in the admirable summary of it which is given in the twelfth volume of the recent abridgement of this work.

Mr. Barrington, observes the Editor, had long entertained a notion that the seasons are become much milder in the northern latitudes than they were 16 or 17 centuries past; and from this it has happened, that many passages in the classical writers decriptive of the severity of the climates, had struck him more perhaps than they would a common reader.

If this same question should be agitated 2000 years hence, it might receive an absolute demonstration; as a journal of the changes in a well-constructed thermometer would show the tem. perature which prevailed in any particular place, during the present century. No such accuracy can be expected from any passages in the classical writers; but in order to state the alteration which may have happened in so long a course of years, the most proper method seems to be to compare their accounts with those of more modern travellers, who have equally wanted the assist. ance of a thermometer for their observations.

Mr. B. chiefly relies on many of Ovid's letters from Pontus (though he was not only a poet, but a writer of most glowing fancy and imagination), in which he describes the effects of cold at Tomas, probably the modern Temisware, during his seven years residence there, and afterwards contrast this description with that of later travellers. Ovid was born at Sulmo in Italy, about 90 Roman miles S. W. from the capital. He afterwards resided chiefly at Rome, and was there at the time he received the emperor's orders for his immediate banishment: Mr. B. therefore con. siders him as then leaving the 42d degree of northern latitude, the

Phil. Trans. Vol. Ivii. year 1768.

elimate in which he was born, and continued to live. He was thence removed. to Tomos, which Dr. Wells, in his maps of ancient geography, places only in the 44th degree of northern lati. tude: the change was therefore only of 2 degrees, and yet Ovid immediately describes it as the winter of Hudson's Bay, with the Euxine sea frozen over, with people and cattle walking on it; as well as other instances of extreme cold.

Besides the quotations from Ovid, Mr. B. gives several others from the ancients, as Virgil, Strabo, Pliny, &c. descriptive of the excessive cold of that latitude. He then contrasts these with the accounts of modern travellers in that country, who have not noticed any such severities of climate there.

Mr. B. now leaving Tomos, compares the accounts of the wea ther in Italy, with those of the present times: it being first pre mised, that the country was better cultivated in the Augustan age than it is now, which should consequently have made the tempera. ture of the air more warm than it is now experienced to be. He begins with some passages from Virgil's Georgics. This most excellent husbandman is constantly advising precautions against snow and ice in the management of cattle; and he may be generally supposed to give these directions for the neighbourhood of Naples, or Mantua his native country, where he does not evidently from the context mean some other parts of Italy. Speaking afterwards of Calabria, the most southern part of Italy, he expresses himself, with regard to the rivers being frozen, as what was commonly to be expected. Pliny too in a chapter, De natura cæli ad arbores, and speaking of Italian trees, says, Alioqui arborum frugumque communia sunt, nives diutinas sedere. But perhaps the strongest proof of that very remarkable fact, the Italian rivers being constantly frozen over, is to be collected from a chapter in Ælian, which consists entirely of instructions how to catch eels while the water is covered with ice. Now, if we may believe the concurrent accounts of modern travellers, it would be almost as ridiculous to advise a method of catching fish in the rivers of Italy, which de. pended entirely on their commonly being frozen over, as it would be to give such directions to the inhabitants of Jamaica. Mr. B. cannot find that the precautions, which Virgil gives in his Georgies, against the damages which sheep and goats might receive from the snow and frost, are now necessary; and both these animals are

known to stand the severest winters of the Highlands of Scotland, conceived to be in Virgil's time almost the ultima Thule. On the whole Mr. B. infers, that there appears to have been a general melioration of temperature in the air and the seasons, in many, perhaps most parts of the earth.

[Phil. Trans. 1768.

CHAP. XXXVI.

NATURE, PROPERTIES, AND VARIATIONS OF HEAT.

THE

SECTION 1.

Sources and Effects of Heat.

HE sources ofheat are various, but its effects uniform whatever. the cause that produces it; or rather, perhaps, the essence or ma. terial of which it consists is the same in every instance. The most powerful and extensive source of heat with which we are ac. quainted is the sun, from which perhaps this invisible matter (if matter at all) is emitted, in consequence of the chemical processes that continually take place at its surface. Friction is another powerful source of heat; percussion is a third; and there are a few others which have not yet been sufficiently traced out and explained.

The recent opinions upon this curious subject are given with so much perspicuity, and at the same time such convenient succinct. ness by Sir Humphry Davy, that through the remainder of this chapter we shall take leave to borrow his words.

WHEN a body which occasions the sensation of heat on our or. gans, is brought into contact with another body which has no such effect, the result of their mutual action is, that the hot body contracts, and loses to a certain extent its power of communicating heat, and the other body expands, and in a degree acquires this power.

This law may be exemplified with respect to every form of pon