CLIMATOLOGY. 335 



general question what fraction of the mean annual or summer 

 temperature corresponds to the difference of one degree of 

 geographical latitude, taken in the same meridian? In each 

 system of isothermal lines of equal curvature there reigns a 

 close and necessary connection between three elements; 

 namelv, the decrease of heat in a vertical direction from 

 below upwards ; the difference of temperature for every one 

 degree of geographical latitude; and the uniformity in the 

 mean temperature of a mountain station, and the latitude 

 of a point situated at the level of the sea. 



In the system of eastern America the mean annual tempe- 

 rature, from the coast of Labrador to Boston, changes 1'6 

 for every degree of latitude ; from Boston to Charleston about 

 l-7; from Charleston to the tropic of Cancer, in Cuba, the 

 variation is less rapid, being only 1 0> 2. In the tropics this 

 diminution is so much greater, that from the Havana to 

 Cumana the variation is less than 0-4 for every degree of 

 latitude. 



The case is quite different in the isothermal system of cen- 

 tral Europe. Between the parallels of 38 and 71 I found 

 that the decrease of temperature was veiy regularly 0'9 for 

 every degree of latitude. But as, on the other hand, in Cen- 

 tral Europe the decrease of heat is 1'8 for about every 534 

 feet of vertical elevation, it follows that a difference of eleva- 

 tion of about 267 feet, corresponds to the difference of one 

 degree of latitude. The same mean annual temperature as 

 that occurring at the Convent of St. Bernard, at an elevation 

 of 8173 feet, in lat. 45 50', should, therefore, be met with 

 at the level of the sea in lat. 75 50'. 



In that part of the Cordilleras which falls within the tropics 

 the observations I made, at various heights, at an elevation of 

 upwards of 19,000 feet, gave a decrease of 1 for every 341 feet; 

 and my friend, Boussingault, found, thirty years afterwards, 

 as a mean result 319 feet. By a comparison of places in the 

 Cordilleras, lying at an equal elevation above the level of the 

 sea, either on the declivities of the mountains, or even on exten- 

 sive elevated plateaux, I observed that, in the latter, there was 

 an increase in the annual temperature, varying from 2 '7 to 

 4*1. This difference would be still greater if it were not for 

 the cooling effect of nocturnal radiation. As the different 

 climates are arranged in successive strata, the one above the 



