296 EFFECT OF ASPECT. [SECT. xxvi. 



rigorous climate than the Asiatic, because it does not ex- 

 tend to so high a latitude. 



The interposition of the atmosphere modifies all the ef- 

 fects of the sun's heat. The earth communicates its tem- 

 perature 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 irregularities. 

 Nevertheless the mean annual temperature becomes gradu- 

 ally lower from the equator to the poles. But the diminu- 

 tion of mean heat is most rapid between the 40th and 45th 

 degrees of latitude both in Europe and America, which ac- 

 cords 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 towards 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 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 Fahren- 

 heit 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 59^; and 

 St. George's Bay, Newfoundland, latitude 48: and lastly, 

 the line of 32, the freezing point of water, passes between 



