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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 no. ticed 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 ex. cellent 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 oeighbourhood 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 TOL, IV.
Beyond this height, which has been called the lower term of con. gelation, 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.
09 | 28000 26°2290) 48° 122+5 70° | 4413
11253 72 4295
57 5439 79 3911 14 | 26268 36 18577
58 5345 80 3861 15 | 26061 37 17985 59 5251 81 3815 16 25781 38 17393 | 60 5148 82 3769 17 25501 39 | 16801
3723 18 25221 40 16207 4989 84 3877 19 24941 41 15712 63 4910 85 3631 20 21661 42 | 15217 61 4831 86 3592 21 | 24 104 43 | 14722 65 4752 87 3553 22 24147 44 | 14227
3514 23 23890 45 13730 67
4616 89 3475 24 23633 46 13235 68 4548
3432 25 23423 47 12740 69 4180
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 duriog 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, 520 and 66° 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 observa. tions 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 t.
Small seas surrounded with laod, 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 I. The German sea is above three degrees colder in winter, and five degrees warmer in summer than the Atlantic g, 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 ll.
The eastern parts of North America are much colder than the opposite coast of Europe, and fall short of the standard by about 10° or 12°, as appears from American Meteorological Tables The causes of this remarkable difference are many. The highest part of North America lies between the 40th and 50th degree of north latitude, and the 100th and 10th degree of longitude west from London ; for there the greatest rivers originate. The very height, therefore, makes this spot colder than it otherwise would be. It is covered with immense forests, and abounds with large swamps and morasses, which render it incapable of receiving any great degree of heat; so that the rigour of winter is much less tempered by the heat of the earth than in the old continent. To the east lie a number of very large lakes; and farther north, Hudson's Bay; about 50 miles on the south of which there is a range of mountains, which prevent its receiving any heat from that quarter. This bay is bounded on the east by the mountainous country of Labrador and by a number of islands. Hence the coldness of the north.west winds and the lowness of the tempera: ture. But as the cultivated parts of North America are now much warmer than formerly, there is reason to expect that the climate will become still milder when the country is better cleared of woods, though perhaps it will never equal the temperature of the old continent.
* Irish Trans. viii. p. 375. + Irish Trans. viii. p.417. Mem. Stock, 1776. Kirwan's Temperature of Lat. p. 53.
Islands are warmer than continents in the same degree of lati. tude ; and countries lying to the windward of extensive mountains or forests are warmer than those lying to the leeward. Stones or sand have a less capacity for heat than earth has, which is always
• For the following statement of the extremes of heat and cold at Montreal and Three Rivers in Canada, I am indebted to an ingenious officer, who kept a register for eight years, from the year 1776 to 1784, inclusive.
“ In the warmest summer the thermometer was not observed to rise higher than 91°, though it has been said to have risen so high as 96° and eveo 99o at Quebec ; but where these observations were made, the thermometer was generally from 80° to 84° in the warmest summers, and the average of the ordinary summers was about 70°.
“ In the severe frosts the thermometer sunk to 45° below 0. This happened three times within this period, viz. on the 23 and 25th February 1782, and 08 the 10th February 1784.
“ In the ordinary winters, at Three Rivers, the mercury stood at from 10° to 25" below 0, and in thr severe winters from 250 to 35° below 0
« The summer observations were taken at about nine o'clock in the morning, and three in the afternoon. The winter, before sun-sise, and about nine, and sometimes ten o'clock at night.