TERRESTRIAL HEAT. 



the same isothermal line, yet their climates and vegetable pro- 

 ductions are extremely different. 



18. One of the circumstances which produce the most marked 

 difference in the climates of places having the same mean tem- 

 perature is the difference between the extreme temperatures. In 

 this respect climates are classed as constant, variable, and extreme. 



Constant climates are those in which the maximum and minimum 

 monthly temperatures differ but little ; variable climates are those in 

 which the difference between these extremes is more considerable, 

 and extreme climates are those in which this difference is very 

 great. 



Constant climates are sometimes called insular, because the 

 effect of the ocean in equalising the temperature of the air is such 

 as to give this character to the climates of islands. 



19. Examples of the classification of climates. The following 

 examples will illustrate this classification of climates : 



Funchal offers the example of a constant or insular climate ; 

 London, Paris, and St. Malo, of a variable ; and New York and 

 Pekin of an extreme climate. 



20. The highest temperature of the air which has been observed 

 within the torrid zone is 130, which was observed by MM. Lyon 

 and Ritchie, in the Oasis of Mourzouk. This, however, is an 

 extreme and exceptional case, the temperature even in this zone, 

 rarely exceeding 120. 



The lowest temperatures observed by arctic voyagers in the polar 

 regions range from 40 to 60 below zero of Fahrenheit, which is 

 from 70 to 90 below the temperature of melting ice. Thus it 

 appears that the air at the surface of the earth ranges between 

 60 and + 120, the extremes differing by 180. 



21. Innumerable phenomena show that the temperature of the 

 air falls as the elevation increases. The presence of eternal snow 



70 



