172 Prof. J. D. Forbes's Sixteenth Letter- on Glaciers. 



soften at a temperature of 2° centigrade below its thawing 

 point : that, consequently between 28-°4 and 32° of Fahrenheit, 

 ice is actually passing through various degrees of plasticity, 

 within narrower limits, but in the same manner that wax, for 

 example, softens before it melts. M. Person deduces this 

 fi'om the examination of the heat requisite to liquify ice at 

 different temperatures. The following sentences contain his 

 conclusions in his own words : — " II parait d'apres mes 

 experiences que le ramollissement qui precede la fusion, est 

 circonscrit dans une intervalle d'environ 2 degres. La 

 glace est done un des corps dont la fusion est la plus nette ; 

 mais cependant le passage de I'etat solide a I'etat liquide 

 s'y fait encore par degres, et non par un saut brusque."* 



Now it appears very clearly from M. Agassiz' thermometrical 

 experiments, and from my own observations, that from 28° to 

 32° Fahr. is the habitual temperature of the great mass of a 

 glacier ; that the most rigorous nights propagate an intense 

 cold to but a very small depth ; and I am perfectly convinced 

 that in the middle and lower regions of glaciers which are habi- 

 tually saturated with water in summer, the interior is little, if 

 at all, reduced below the freezing point, even by the prolonged 

 cold of winter ; it would be contrary to all just theories of 

 the propagation of heat if it were otherwise, when we recol- 

 lect the enormous mass of snow which such glaciers bear dur- 

 ing the coldest months of the year, is a covering sufficient to 

 prevent any profound congelation in common earth ; and 

 admitting that ice is probably a better conductor of heat 

 than the ground, it is quite incredible that a thickness of 

 many hundred feet of ice, saturated with fluid water, should 

 be reduced much below the freezing point, or should even 

 be frozen throughout. And that it is not, the striking testi- 

 mony of the continued stream of water issuing all winter from 

 under the ice can hardly fail to convince us ; still more, the cir- 

 cumstance mentioned in my Fourteenth Letter, that evenin the 

 month of February the source of the Arveron becomes whitish 

 and dirty:, as in summer^ before a change of weather, proving 

 (as I have there remarked) that *' in the middle of winter a 

 temporary rise of temperature over the higher glacier regions 

 (which is the precursor of bad weather) not only produces a 



* Coniptcs Kcrdup, 29th ^pril 1860. 



