Here rn = 54,t m 32 8 100 11 22 — = 0.404 c, and c X – — 54 :33 100 0.404 x 8.03 = 3.24 = d, and m — 54—.3.24 = 50.75: here we see that the temperature of the air 803feet above the surface of the earth is 50° 75".
Estimating the diminution from this method, which corresponds with obser vation, we find that heat lessens in an arithmetical progression ; and from the same premises it may be concluded, that the warmth of the air at some distance from the earth is not to be attributed to the rising of heated strata of air from the earth's surface, but to the conducting power of the air.
The upper strata of the atmosphere are frequently warmer in winter than the lower, and the preceding rule is appli cable to the temperature of the air dur ing the summer months only. According to the Philosophical Transactions for 1717, a thermometer placed on the sum mit of Arthur's Seat, the thirty-first of January, the year before, stood six degrees higher than a second at Hawkhill, situat ed 684 feet below it : this superior heat is considered by Mr. Kirwan to be pro duced by a current of heated air Bowing from the equator towards the north pole during our winter. A general idea has now been given of the method by which the mean annual temperature may be found throughout the known regions of the globe ; but there are some excep tions to the universality of the rules: for instance, the Pacific Ocean, between lati tude 52° and 66° north, and at the nor then extremity, is only forty-two miles in breadth, and at its southern is one thou sand three hundred miles ; it is therefore but reasonable to suppose, that the tem perature must be greatly affected by the land surrounding it, which rises into chains of mountains, with summits bear ing snow great part of the year, exclusive of the islands consisting of high lands scattered within it. Mr. Kirwan con cludes, in consequence, that its tempera ture is four or five degrees below the standard. This supposition cannot, how ever, be brought to any degree of cer tainty, from a deficiency of observations. It has been a generally received opi nion, that the southern hemisphere, beyond the fortieth degree of latitude, is much colder than corresponding parts of the northern : this our philosopher has proved to be true, with respect to the summer of the former; but that the !win ter in the same latitude is milder than in the latter.
Inconsiderable seas, in temperate and cold climates, are cold!? in winter and warmer in summer than the standard ocean, as they are necessarily under the influence of natural operations from the land, and its temperature, particularly the Gulph of Bothnia, which is generally frozen in the winter, but the water is sometimes heated in the summer to 70°, a state the opposite part of the Atlantic never acquires; the German Sea is five degrees warmer in summer than the At. lantic, and more than three colder in win ter; the Mediterranean is almost through out warmer, both in winter and summer, which therefore causes the Atlantic to Bow into it ; and the Black Sea. being colder than the Mediterranean, flows in to the latter.
It appears from meteorological tables, that the eastern part of North America has a much colder air than the opposite European coast, and falls short of the standard by about ten or twelve degrees. There are several causes which produce this considerable difference. The great est ales ati on in North America is between the 40th and 50th degree of north lati tude, and the 100th and 110th of longi tude west from London, and there the most considerable rivers have their origin. The height alone is sufficient to make this tract colder than it would otherwise be but there are other causes, and those are most extensive forests, and large swamps and morasses, each of which exclude heat from the earth, and consequently pre vent it from ameliorating the rigour of winter. Many extensive lakes lie to the east, and Hudson's Bay more to the north ; a chain of mountains extends on the south of the latter, and those equally prevent the accumulation of heat besides, this bay is bounded on the east by the moun tainous counties of Labrador, and has ma ny islands , from all which circumstances arise lowness of the temperature, and the piercing cold of the north-west winds. The annual decrease of the forests for the purpose of clearing the ground, and the consumption for building and fuel, is supposed to have occasioned a consider able decrease of cold in the winter; and if this should be the result, much will yet be done towards bringing the tempera ture of the European and American con tinents to something like a level.