Hot and Thermal Springs. 351 



If wc trace a glacier from its lower to its upper end, we find 

 that the crystals of ice become continually harder, till, at a certain 

 height, it consists of nothing but a loose granular mass of snow, 

 whicii, in Sxcitzerland, is called Firn* The height at which this 

 Fh'n first appears, and which Hugi proposed to call the Firn-line 

 ( Firnlinie ) is, according to his observations in the Alps, a fixed 

 line, which undergoes neither elevation nor depression, whether 

 on a northern or southern dcchvity, or from any other cause. 

 He found the Firn-line, from observations on a great number 

 of glaciers, to lie between 7614 and 7698 feet above the level 

 of the sea. In the Pennine Alps, the Firn-line seems to lie 

 somewhat higher, at least the observations on the Gries, and on 

 the ridges of the Binnenthal, give it at near 7800 feet. Gla- 

 ciers properly so called do not reach this Firn-line; for, at 

 the height of '600 feet, where the mean temperature, according 

 to Chap. XVIII., is about S6°.5, the glaciers of the Alps are 

 quickly converted into Firn.-f- ,: 



All these circumstances may be satisfactorily explained, if we 

 consider that, where the mean temperature of the soil becomes 

 32°.0, the glacier no longer melts away from underneath, but 

 only on its upper surface, during the summer. The greater 

 number of the glaciers in the Alps extend far below the linntof 

 6165 leet, and consequently rest upon a soil, the mean tempera- 

 ture of which, if not covered with ice and snow, would be 

 above 32°. It is this excess of heat above 8!<i° which may 

 possibly be employed in promoting a continual melting of the 

 glaciers from underneath. Theoretical considerations bring us 

 to the following conclusions: — The covering of an alpine val- 

 ley with a glacier, causes a disturbance in the original law of 

 the progressive decrease of heat from the centre of the earth to- 

 wards the surface, which cannot be repaired so long as the co- 

 vering remains ; but the law is not entirely destroyed A dis- 

 tinction must therefore be made between the actual temperature 



• Hugi in Berghaus Annal, iii., 295, Some of the assertions of this dili- 

 gent explorer of the Alps, may perhaps be found to require some corrections, 

 when subjected to a closer investigation. It certainly is not true that the 

 masses which are converted into glacier ice lie deeper the higher tlie glacier 

 lies above the Fim. I. 1. p. 294. 



t I. 1. P. 289. 



