and yet less frequent at north or north-east, insomuch that many here do not scruple to affirm, that for at least 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 temperature 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 considers him as then leaving the 42d degree of northern latitude, the * Phil. Trans. Vol. lvii. year 1768. climate in which he was born, and continued to live. He was thence removed to Tomos, which Dr. Wells, in his maps of an. cient 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 premised, that the country was better cultivated in the Augustan age than it is now, which should consequently have made the temperature 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 Beyond this height, which has been called the lower term of congelation, and which must vary with the season and other circum. stances, Mr. Bouguer has distinguished another, which he called the upper term of congelation; that is, the point above which no visible vapour ascends. Mr. Kirwan considers this line as much less liable to vary during the summer months than the lower term of congelation, and therefore has made choice of it to determine the rate of the diminution of heat, as we ascend in the atmosphere. Bouguer determined the height of this term in a single case, and Kirwan has calculated the following Table of its height for every degree of latitude in the northern hemisphere. TABLE of the Height of the Upper Line of Congelation in the different Latitudes of the Northern Hemisphere. From this method of estimating the diminution of temperature, which agrees remarkably well with observation, we see that the heat diminishes in an arithmetical progression. Hence it follows, that the heat of the air at a distance from the earth, is not owing to the ascent of hot strata of air from the surface of the earth, but to the conducting power of the air. 3. This rule, however, applies only to the temperature of the air during the summer months of the year. In winter the upper strata of the atmosphere are often warmer than the lower. Thus on the 31st of January 1776, the thermometer on the summit of Arthur's Seat stood six degrees higher than a thermometer at Hawkhill, which is 684 feet lower *. Mr. Kirwan considers this * Roy, Phil. Trans. 1777, p. 777. superior heat, almost uniformly observed during winter, as owing to a current of warm air from the equator, which rolls towards the north pole during our winter *. 4. Such, then, in general, is the method of finding the mean annual temperature over the globe. There are, however, several exceptions to these general rules, which come now to be mentioned. That part of the Pacific Ocean which lies between north lat. 52° and 66o is no broader at its northern extremity than 42 miles, and at its southern extremity than 1300 miles: it is reasonable to suppose, therefore, that its temperature will be considerably influenced by the surrounding land, which consists of ranges of mountains covered a great part of the year with snow; and there are besides a great many high, and consequently cold, islands scattered through it. For these reasons Mr. Kirwan concludes, that its temperature is at least four or five degrees below the standard. But we are not yet furnished with a sufficient number of observations to determine this with accuracy. It is the general opinion that the southern hemisphere, beyond the 40th degree of latitude, is considerably colder than the corre sponding parts of the northern hemisphere. Mr. Kirwan has shown that this holds with respect to the summer of the southern hemisphere, but that the winter in the same latitudes is milder than in the northern hemisphere +. Small seas surrounded with land, at least in temperate and cold climates, are generally warmer in summer and colder in winter than the standard ocean, because they are a good deal influenced by the temperature of the land. The Gulph of Bothnia, for instance, is for the most part frozen in winter; but in summer it is sometimes heated to 70°; a degree of heat never to be found in the opposite waters of the Atlantic ‡. The German sea is above three degrees colder in winter, and five degrees warmer in summer than the Atlantic. The Mediterranean sea is, for the greater part of its extent, warmer both in summer and winter than the Atlantic, which therefore flows into the former. The Black Sea is colder than the Mediterranean, and flows into it . The eastern parts of North America are much colder than the opposite coast of Europe, and fall short of the standard by about Irish Trans. viii. p. 375. + Irish Trans. viii. p.417. Mem. Stock. 1776. Ibid. |