? HELMHOLTfc 



tories to the attentive observer, histories which often stretch 

 far beyond the past of the human race into the obscurity of 

 the primeval world; a peaceful, uniform, and beneficent sway 

 of enormous natural forces, where at first sight only desert 

 wastes are seen, either extended indefinitely in cheerless, 

 desolate solitudes, or full of wild, threatening confusion 

 an arena of destructive forces- And thus I think I may 

 promise that the study of the connection of those phenomena 

 of which I can now only give you a very short outline, will 

 not only afford you some prosaic instruction, but will make 

 your pleasure in the magnificent scenes of the high moun- 

 tains more vivid, your interest deeper, and your admiration 

 more exalted. 



Let me first of all recall to your remembrance the chief 

 features of the external appearance of the snow-fields and 

 of the glaciers; and let me mention the accurate measure- 

 ments which have contributed to supplement observation, 

 before I pass to discuss the casual connection of those 

 processes. 



The higher we ascend the mountains the colder it becomes. 

 Our atmosphere is like a warm covering spread over the 

 earth; it is well-nigh entirely transparent for the luminous 

 darting rays of the sun, and allows them to pass almost 

 without appreciable change. But it is not equally penetrable 

 by obscure heat-rays, which, proceeding from heated terres- 

 trial bodies, struggle to diffuse themselves into space. These 

 are absorbed by atmospheric air, especially when it is moist; 

 the mass of air is itself heated thereby, and only radiates 

 slowly into space the heat which has been gained. The 

 expenditure of heat is thus retarded as compared with the 

 supply, and a certain store of heat is retained along the 

 whole surface of the earth. But on high mountains the 

 protective coating of the atmosphere is far thinner the 

 radiated heat of the ground can escape thence more freely 

 into space; there, accordingly, the accumulated store of heat 

 and the temperature are far smaller than at lower levels. 



To this must be added another property of air which acts 

 in the same direction. In a mass of air which expands, 

 part of its store of heat disappears, it becomes cooler, if it 

 cannot acquire fresh heat from without Conversely, by 



