SECT. XXVI. ISOTHERMAL LINES. 259 



open to the western ocean, the superficial temperature 

 of which, as far north as the 45th and 50th degrees of 

 latitude, does not fall below 48 or 51 of Fahrenheit, 

 even in the middle of winter. On the contrary, the 

 cold of Russia arises from its exposure to the northern 

 and eastern winds. But the European part of that em- 

 pire has a less rigorous climate than the Asiatic, because 

 it does not extend to so high a latitude. 



The interposition of the atmosphere modifies all the 

 effects of the sun's heat. The earth communicates its 

 temperature so slowly that M. Arago has occasionally 

 found as much as from 14 to 18 of difference between 

 the heat of the soil and that of the air two or three 

 inches above it. 



The circumstances which have been enumerated, and 

 many more, concur in disturbing the regular distribution 

 of heat over the globe, and occasion numberless local ir- 

 regularities. 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. 123), which expresses the 

 law of the change of temperature, is a maximum to- 

 ward the 45th degree of latitude. The mean annual 

 temperature under the line in America is about 81^ 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 22d 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 isother- 

 mal 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 tem- 

 perature runs through the Netherlands, latitude 51; 

 and near Boston in the United States, latitude 42 : 

 that of 41 passes near Stockholm, latitude 59| ; and 



