370 NOTES OF A NATURALIST. 



below the normal amount the rate of decrease of temperature, 

 is the comparative absence of strong winds, and the feebleness 

 of the sea-breezes which are usually so conspicuous in the 

 tropics. For reasons that will be further noticed, the fall in 

 temperature in ascending mountain ranges is largely due to 

 currents of air carried up from the lower region. In mountain 

 countries an air-current, encountering a range transverse to its 

 own direction, is mechanically forced to rise along the slopes, 

 and thus raises large masses of air to a higher level ; the same 

 effect in a less degree occurs with isolated peaks. But in the 

 Peruvian Andes, as well as in many other parts of the great 

 range, although storms arise from local causes on the plateau, 

 westerly winds from the ocean are infrequent and feeble ; and 

 the sea-breezes, due to the heating of the soil by day, much less 

 sensible than usual in warm countries. 



Making full allowance for the operation of the two causes 

 here specified, it yet appears that the difference of temperature 

 between the coast and the higher slopes of the Peruvian Andes 

 is exceptionally small. It is not merely due to the abnormal 

 cooling of the coast-zone, but to the exceptionally high tempera- 

 ture found in the zone ranging from 3500 to 4000 metres. I 

 should not have attached much importance to the few observa- 

 tions of the thermometer that I was able to make during a 

 hurried visit, if the conclusion which they suggest had not been 

 strongly confirmed by the character and aspect of the vegetation. 



When I found that the table given by Humboldt, which has 

 been copied and adopted by so many writers on physics, in 

 which the mean temperature at a height of 2000 toises, or 3898 

 metres, in the Andes of Ecuador, close to the equator, is set 

 down at 7°, while at Chicla, thirteen degrees of latitude south, 

 at a height less only by 174 metres, there is reason to believe 

 that we find a mean annual temperature of not less than 12°, I 

 was led to enter more fully into the subject. 



The result of somewhat careful study has been to convince 

 me that, while the physical principles involved in the attempt to 

 discover the vertical distribution of temperature in the atmo- 

 sphere prove the problem to be one of extreme complexity, the 

 results hitherto obtained from observation are altogether in- 

 sufficient to guide us to an approximate law of distribution. I 

 may remark that the problem has not merely a general interest 



