SECT. XXV. ISOTHERMAL LINES. 245 



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 8U 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 Ame- 

 rica, 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 : that of 41 

 passes near Stockholm, latitude 59s ; 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 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 



