APPENDIX. 387 



tains a constant low temperature in the surface and the sur- 

 rounding air. 



In comparing observations in Peru, Bolivia, or Chili with 

 those made in the Andes of Ecuador, it must not be forgotten 

 that the climatal conditions are essentially different. Owing to 

 the fact that in the latter the range of the Andes is much narrower, 

 and on one side the main valleys descend in a nearly due 

 easterly direction, the hot, vapour-laden, easterly winds reach 

 the plateaux still charged with moisture, and at all seasons 

 rain is frequent and abundant. Farther south, the winds from 

 the Atlantic have deposited the greater part of their moisture 

 before they arrive at the western side of the main range, and the 

 annual rainfall must be comparatively trifling. 



I have sought in vain in the records of mountain observations 

 in other parts of the world for materials from which any pro- 

 bable inference may be drawn as to a law regulating the ratio 

 of decrease of temperature with increasing height above the 

 sea-level. There is reason to admit that isolated peaks of no 

 great height show a more rapid decrease as compared with the 

 plain than do considerable mountain masses. Of mountains 

 exceeding the height of 3000 metres in the tropics, the most 

 rapid rate of decrease is that recorded for Pangerango in Java, 

 being 1° for I78'5 metres. 



The greater mountain masses in or near the tropics show 

 nearly the same rate of decrement, by comparison with the 

 sea-level, that I have been led to infer from the observations 

 in Ecuador. The average rate for the Himalayas is about 1° 

 for 194 metres of ascent, and for the less lofty peaks of Mexico 

 Humboldt's observations show a decrease of 1° for 188 metres. 

 The great irregularities due to local conditions make it impos- 

 sible to derive any positive conclusions as to the comparative 

 rate of decrease in successive zones of elevation. 



In Europe and North America comparisons between the 

 temperatures at niountain summits and the sea-level give rates 

 of decrease varying between 1° for 160 metres, and 1° for 170 

 metres ; but it must be remarked that the averages are mainly 

 founded on observations made in summer, and it is certain 

 that the rate of decrease is much slower in winter. Where the 

 difference of height is not very great, it not uncommonly 

 happens that in winter the phenomenon is reversed, and that 



