SXOW LIXE. 



on the elevated parts of mountain ranges, in every part of the 

 globe, not excepting even the torrid zone, is a striking evidence of 

 this. 



It appears, from observations made upon the declivities of the 

 vast mountain ranges -which traverse the equatorial regions, that 

 the decrease of temperature is neither uniform nor regular. 



The observations made in temperate climates give results equally 

 irregular. Gay-Lussac found ascending in a balloon, that the 

 thermometric column fell one degree for an elevation of about 320 

 feet. On the Alps the height which produces a fall of one degree 

 is from 260 to 280 feet, and on the Pyrenees from 220 to 430 feet. 

 It may be assumed, that in the tropical regions, an elevation of 

 300 feet, and in our latitudes from 300 to 330 feet, corresponds to 

 a fall of one degree of temperature on an average, subject, however, 

 to considerable local variation. 



22. It might appear that in those elevations at which the tem- 

 perature falls to 32, water cannot exist in the liquid state, and 

 we might expect that above this limit we should find the surface 

 invested with perpetual snow. Observation nevertheless shows 

 such an inference to be erroneous. Humboldt in the equatorial 

 regions, and M. Leopold de Buch in Norway and Lapland, have 

 shown that the SXOW-LINE does not correspond with a mean 

 temperature of 32 for the superficial atmosphere, but that on the 

 contrary, within the tropics, it is marked by a mean temperature 

 of about 35, while in the northern regions, in latitudes of from 

 60 to 70, the mean temperature is 26|. 



It appears that the snow-line is determined not so much by the 

 mean annual temperature of the air as by the temperature of the 

 hottest month. The higher this temperature is, the more elevated 

 will be the limit of perpetual snow. But the temperature of the 

 hottest month depends on a great variety of local conditions, such 

 as the cloudy state of the atmosphere, the nature of the soil, the 

 inclination and aspect of the surface, the prevailing winds, &c. 



23. At a given place the surface of the ground undergoes a 

 periodical variation of temperature, attaining a certain maximum 

 in summer and a minimum in winter, and gradually, but not 

 regularly or uniformly, augmenting from the minimum to the 

 maximum, and decreasing from the maximum to the minimum. 



The question then arises as to whether this periodic variation 

 of temperature is propagated downwards through the crust of 

 the earth, and if so, whether in its descent it undergoes any and 

 what modifications ? 



To explain the phenomena which have been ascertained by 

 observation, let us express the mean temperature by M, and let the 

 maximum and minimum temperatures be T and #. 



71 



