Hot and Thermal Springs, ^c. 253 



but the same decrease took place in every 618 feet, from Calj)i 

 to the highest point on the chain of the Chimborazo, or in the 

 last 8292 feet. But taking the whole column of air of 18,072 

 feet together, the decrease of 2o.25 took place in every 714 feet. 

 Between the surface of the sea and Qluito, Popnyan S. Fe de Bo- 

 gota, and Mexico, which lie on that extensive plateau, we find 

 from their respective mean temperatures a difference of eleva- 

 tion of 964 to 1090 feet* for every degree of temperature. 

 Idelerf shews, that the influence of plateaux and broad ridges 

 on the decrease of temperature in the Himalaya mountains is 

 still more considerable. 



Besides these general effects of the configuration of the earth's 

 surface which are independent of latitude, there are others 

 which are closely connected with the geographical situation of 

 the place. Thus, under the equator and in low latitudes, the 

 generation of heat must be greater on plains than on steep 

 mountains, because the rays of the sun fall almost or quite per- 

 pendicularly upon the former, while with the latter they make 

 an acute angle. In higher latitudes the contrary takes place on 

 the sides of mountains which lie opposite to the sun. In the 

 northern temperate zone, there may, therefore, be a slower 

 decrease of temperature on the southern declivity of a moun- 

 tain than on the northern or on gently sloping hills. 



Climate and local circumstances, which have the effect of 

 lowering the hmits of perpetual snow, must, of course, also 

 have some influence on the decrease of temperature. For 

 the nearer the limits of perennial snow are brought at any 

 place to the surface, the more rapid must be the decrease of 

 temperature in proportion to the height. In the middle of an 

 extensive chain of mountains, the decrease of temperature must 

 consequently be more rapid than on both sides where the warm- 

 ed air of the plains still operates. + 



" Von Humboldt, in Gilbert's Annal. xxir., 26-36 ; and xxxi, 369. 



t Ideler uebfr die Abnahnie des Warmestoffs mit der Hi3he, p. 103. 



t According to De Saussure (Voyages dans les Alpes, § 942), the limits of 

 perennial snow are lowered more than 600 feet, by accumulations of snow, 

 or by water resulting from the melting of snow. In like manner, Eamond as- 

 serts that the limits of perpetual snow, under the latitude of the Pyrenees, 

 form a curve which descends from both ends of the chain to the middle, and 

 which is therefore its lowest point. V. Buch, in Gilb. Ann., xlL p. 18. 



VOL. XXXV. NO. XLVIII. APBII. 1838. » 



