386 NOTES OF A NATURALIST. 



on the western slope of the Cordillera the rate of decrease of 

 temperature in this region is much below the ordinary average. 

 Estimating the mean temperature of Mollendo at 22° at the 

 beginning of February, we find between Mollendo and Arequipa 

 a difference of 5'8'^ C, or a fall in summer of 1° for an ascent of 

 401 metres ; while in mid-winter we obtain a difference of 77°, 

 showing that an ascent of 364 metres is necessary to cause 

 a fall of 1°. This abnormal condition is, no doubt, mainly due 

 to the exceptionally low temperature of the coast-zone. Between 

 Arequipa and Vincocaya we may reckon the fall of temperature 

 on the 1st of March at I4"2° for an ascent of 2031 metres, 

 giving the proportion of 1° to 143 metres ; but in winter the 

 decrease is less rapid, as we have at the end of June a difference 

 of about 11*5° for an ascent of 2077 metres, or about 181 metres 

 for a fall of 1°. 



A remarkable contrast is shown when we compare the 

 temperature at Vincocaya with that of places on the plateau 

 surrounding the great lake of Titicaca. From Mr. Copeland's 

 observations we may estimate the mean annual temperature of 

 Vincocaya at 1° C, that of Puno at 8-5°, and that of La Paz at 

 8"8°. These figures would give a mean difference of 7*5° for a 

 difference in height of 537 metres between Vincocaya and Puno, 

 or a decrease of i^ for 72 metres. Between Vincocaya and La 

 Paz we have a difference of 7*8° for a difference in height of 

 732 metres, or a fall of 1° for 94 metres. The mean of the two. 

 comparisons gives a fall of 1° for 83 metres, or about twice as 

 rapid a change as the average of the comparison between 

 Arequipa and Vincocaya. I am not disposed to attribute this 

 remarkable difference of atmospheric conditions" exclusively to 

 the influence of plateaux in raising the mean temperature. 



In my own slight experience in the Peruvian Andes, in 

 ascending from Chicla, at about 3700 metres, to Casapalta, at 

 about 4200 metres, I observed so complete and rapid a change 

 in the character and aspect of the vegetation as to satisfy me 

 that the difference in the annual mean temperature must be 

 even greater than that observed by Mr. Copeland for a some- 

 what greater difference of height between Vincocaya and 

 Puno. It may be that, in this comparatively dry region of the 

 Andes, the higher stations receive more frequent, though not 

 copious, falls of rain or snow, the evaporation of which main- 



