1851 



HOPKINS ON CHANGES OF CLIMATE. 



n 



for our purpose. It is necessary to make a distinction between those 

 observations which have been made in balloons or on the sides of 

 comparatively steep mountains, and those which have been made on 

 extensive elevated table lands ; the results in the two cases being very 

 diiferent. Of the first class, Humboldt has given us* the results of 

 nine cases, in which the observations were made at different heights, 

 varying from about 5000 to 18,000 feet. The mean of the results 

 gives 191*4 metres, or about 638 feet for 1° Cent., which is equiva- 

 lent to 355 feet for 1° Fahr. In Gay-Lussac's balloon ascent, the 

 observations gave 193 metres for 1° C, up to the height of about 

 12,000 feet. At greater heights the decrease of temperature was 

 somewhat more rapid, and was at the rate of 1° for 187 metres for the 

 whole height ascended, which amounted to upwards of 23,000 feet. 

 We may adopt, without risk of material error, 190 metres for 1° C, 

 or 350 feet for 1° F. In some particular cases, however, 320 feet 

 would probably be nearer the truth. 



Humboldt has also made some valuable observations, which show 

 the effect of extensive high table-lands in raising the temperature 

 above that which would be given by calculations founded on the re- 

 sult just enunciated. In ascending from one table-land to another, 

 the decrease of temperature is much slower than if we should ascend 

 in a balloon, or up the side of a steep mountain. The following table 

 exhibits the results of Humboldt's observations on four of the highest 

 table-lands on the new continent : — 



Place of Observation. 



Latitude. 



Height. 



Mean 

 Temperature. 



Increase of 

 Height for 1°C. 



Quito 



13 s. 

 2 26 N. 

 4 35 N. 



19 26 N. 



metres. 

 2907 

 1796 

 2660 



2277 



150 C. 

 20-6 

 16-5 

 16-9 



metres, 

 244-4 

 288-1 

 256-0 

 249-3 



Popavan .... 



San. Fe di Bogota 

 Mexico 





The mean of the numbers in the last column is 258*4 metres. 

 This is equivalent to 4/8 feet of height for 1° F., instead of 350 feet, 

 as in the former case. The high general temperature of the plains 

 of Tibet (as indicated by the nature of their produce), in proportion 

 to their enormous elevation, is doubtless due to the same cause. 



From these results it appears, that we may take a decrease of 

 1° F. as corresponding to an elevation varying, according to circum- 

 stances, from 320 to 500 feet ; the smaller number being applicable 

 to small, or, if high, very steep mountains, and the latter to large 

 massive ranges presenting extensive table-lands along their sides. 

 For intermediate cases, intermediate numbers must be adop1:ed. 



21. Before the publication of the admirable observations of Hum- 

 boldt, it was usually assumed, in speculations respecting the line of 

 perpetual snow, that it coincided with that of 32°. Later observa- 

 tions have shown the error of this hypothesis, especially in the 

 higher latitudes. Humboldt has given the following results, deduced 



* Recueils Astronomiques of his Travels in S. America, vol. i. p. 129. 



