June, 1834. glaciers. 279 



land, is concealed by one dense forest, dripping with moisture, 

 and abounding with ferns and other plants that love a humid 

 atmosphere : while the soil of central Chile, where not irri- 

 gated, is arid and nearly desert. These two countries, so 

 remarkably opposed to each other in every character, blend 

 together rather suddenly near Concepcion, in lat. 37°. I do 

 not doubt, the plain of perpetual snow undergoes an extra- 

 ordinary flexure in the district where the forest ceases ; for 

 trees indicate a rainy climate, and hence a clouded state of 

 atmosphere.* 



From central Chile to Bolivia, a space of 16°, the rise of 

 the snow-line is only 2000 feet. If Bolivia possessed an 

 atmosphere as clear as that of Chile, the limit in all proba- 

 bility would be even higher than the present 1 7,000. The 

 cause why the limit in the equatorial regions should be lower 

 than in a latitude seventeen degrees to the southward, I 

 leave to those to explain, who have more means of informa- 

 tion respecting the dryness and clouded state of the atmo- 

 sphere in the respective regions. 



The presence of glaciers depends on the accumulation of a 

 large mass of snow, subject to some variations of tempera- 

 ture, sufficient partially to thaw, and then reconsolidate the 



* The average degree of atmospheric transparency seems to be a most 

 important element in determining the climate of any place. Dr. Richard- 

 son (Report to Brit. Assoc, for 1836, p. 131) has remarked that Professor 

 Leslie, from experimenting on the effects of radiation only in an insular 

 climate, deduced theoretical inferences respecting the mean temperature 

 of the year, extremely different from the results obtained under the clear 

 atmosphere of the polar regions. I apprehend central Chile will bear 

 comparison with any part of the world for the clearness of its sky, and 

 Chiloe, for one of an opposite condition : therefore we should not feel 

 surprised, if the effects of two such opposite climates at first appear and 

 malous. The remarkable difference in the height of the snow-line, on the 

 opposite sides of the Himmalaya, has been explained by Humboldt and 

 Jacquemont, on the same principle : and in a like manner, the difference 

 between the heights on the Pyrenees and on Caucasus, the latter moun- 

 tains being characterized by a climate more excessive, than that of the 

 former. 



