The circumstances which have been enumerated, 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 40th and 45th degrees 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 (N. 127), which expresses the law of the change of temperature, is a maximum towards the 45th degree of latitude. The mean annual temperature under the equator in America is about 81-1/2° 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 nearly parallel to the equator, till about the 22nd degree of latitude on each side of it, where they begin to lose their parallelism, and continue to do so more and more as the latitude 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 42-1/2°: that of 41° passes near Stockholm, latitude 59-1/2°; 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 Labrador, latitude 54°.
Thus it appears that the isothermal lines, which are nearly parallel to the equator for about 22°, afterwards deviate more and more. From observations made during the numerous voyages in the Arctic Seas, 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 79° N. lat. and 120° E. long., has a mean temperature of 2° Fahrenheit; and the other, whose temperature was determined by Sir David Brewster to be 3-1/2° Fahrenheit, from the observations of Sir Edward Parry is near Melville Island. The pole of the earth’s rotation, whose mean temperature is probably not below 15° Fahrenheit, is nearly midway between the two; and the line which joins these points of maximum cold is almost coincident with that diameter of the polar basin which bisects it, and passes through its two great outlets into the Pacific and Atlantic Oceans, a most remarkable feature, and strongly indicative of the absence of land, and of the prevalence of a materially milder climate in the polar Ocean, probably not under 15° Fahrenheit.[12] 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.