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and many more, concur in disturbing the regular distribution of heat over the globe, and occasion numberless local irregularities: nevertheless the mean annual temperature becomes gradually lower from the equator to the poles; but the diminution of mean heat is most rapid between the 40° and 45° of latitude both in Europe and America, which accords perfectly with theory, whence it appears that the variation in the square of the cosine of the latitude which expresses the law of the change of temperature, is a maximum towards the 45° of latitude. The mean annual temperature under the line in Asia and America is about 814° of Fahrenheit; in Africa it is said to be nearly 83°. The difference probably arises from the winds of Siberia and Canada, whose chilly influence is sensibly felt in Asia and America, even within 18° of the equator.
The isothermal lines are parallel to the equator, till about the 22° of latitude on each side of it, where they begin to lose their parallelism, and continue to do so more and more as the lati
tude augments. With regard to the northern hemisphere, the isothermal line of 59° of Fahrenheit passes between Rome and Florence, in latitude 43°; and near Raleigh, in North Carolina, latitude 36°; that of 50° of equal annual temperature runs through the Netherlands, latitude 51°;
and near Boston, in the United States, latitude 421°; that of 41° passes near Stockholm, latitude 591°; and St. George's Bay, Newfoundland, latitude 48°; and lastly, the line of 32°, the freezing point of water, passes between Ulea, in Lapland, latitude 66°, and Table Bay, on the coast of Labradore, latitude 54°.
Thus it appears, that the isothermal lines which are parallel to the equator for nearly 22°, afterwards deviate more and more; and from the observations of Sir Charles Giesecke in Greenland, of Mr. Scoresby in the Arctic seas, and also from those of Sir Edward Parry and Sir John Franklin, it is found that the isothermal lines of Europe and America entirely separate in the high latitudes, and surround two poles of maximum cold, one in America and the other in the north of Asia, neither of which coincides with the pole of the earth's rotation. These poles are both situate in about the eightieth parallel of north latitude; the Trans-. atlantic pole is in the 100° of west longitude, about 5° to the north of Sir Graham Moore's Bay, in the Polar Seas, and the Asiatic pole is in the 95° of east longitude, a little to the north of the Bay of Taimura, near the North-East Cape. According to the estimation of Sir David Brewster, from the observations of M. de Humboldt and Captains Parry and Scoresby, the mean annual temperature
of the Asiatic pole is nearly 1° of Fahrenheit's thermometer, and that of the transatlantic pole about 3 below zero, whereas he supposes the mean annual temperature of the pole of rotation to be 4° or 5o. It is believed that two corresponding poles of maximum cold exist in the southern hemisphere, though observations are wanting to trace the course of the southern isothermal lines with the same accuracy as the northern.
The isothermal lines, or such as pass through places where the mean annual temperature of the air is the same, do not always coincide with the isogeothermal lines, which are those passing through places where the mean temperature of the ground is the same. The mean heat of the earth is determined from that of springs, and if the spring be on elevated ground, the temperature is reduced by computation to what it would be at the level of the sea, assuming that the heat of the soil varies according to the same law as the heat of the atmosphere, which is about a degree of Fahrenheit's thermometer for every 656 feet. From a comparison of the temperature of numerous springs with that of the air, Sir David Brewster concludes that there is a particular line passing nearly through Berlin, at which the temperature of springs and that of the atmosphere coincide; that in approaching the Arctic Circle the tempe
rature of springs is always higher than that of the air, while proceeding towards the equator it is lower. He likewise found that the isogeothermal lines are always parallel to the isothermal lines, consequently the same general formulæ will serve to determine both, since the difference is a constant quantity, obtained by observation, and depending upon the distance of the place from the neutral isothermal line. These results are confirmed by the observations of M. Kupffer, of Kasan, during his excursions to the north, which show that the European and the American portions of the isogeothermal line of 32° Fahrenheit actually separate, and go round the two poles of maximum cold. This traveller remarked also, that the temperature both of the air and of the soil decreases most rapidly towards the 45° of latitude. The temperature of the ground at the equator is lower on the coasts and islands than in the interior of the continents; the warmest part is in the interior of Africa, but the temperature is obviously affected by the nature of the soil, especially if it be volcanic.
It is evident that places may have the same mean annual temperature, and yet differ materially in climate. In one the winters may be mild and the summers cool: whereas another may experience the extremes of heat and cold. Lines passing through places having the same mean summer
or winter temperature, are neither parallel to the isothermal, the geothermal lines, nor to one another, and they differ still more from the parallels of latitude. In Europe, the latitude of two places which have the same annual heat never differs more than 8° or 9°; whereas the difference in the latitude of those having the same mean winter temperature is sometimes as much as 18° or 19°. At Kasan, in the interior of Russia, in latitude 55° 48, nearly the same with that of Edinburgh, the mean annual temperature is about 37° 6; at Edinburgh it is 47° 84. At Kasan, the mean summer temperature is 64°. 84, and that of winter 2012, whereas at Edinburgh the mean summer temperature is 58° 28, and that of winter 38°.66. Whence it appears that the difference of winter temperature is much greater than that of the summer. At Quebec, the summers are as warm as those in Paris, and grapes sometimes ripen in the open air; whereas the winters are as severe as in Petersburg; the snow lies five feet deep for several months, wheel-carriages cannot be used, the ice is too hard for skating, travelling is performed in sledges, and frequently on the ice of the river St. Lawrence. The cold at Melville Island, on the 15th of January, 1820, according to Sir Edward Parry, was 55° below the zero of Fahrenheit's thermometer, only 3° above the tem