226 SIXTEENTH LETTER ON GLACIERS. [i860. 



be frozen throughout. And that it is not [so frozen], the 

 striking testimony of the continued stream of water issuing all 

 winter from under the ice can hardly fail to convince us ; still 

 more, the circumstance mentioned in my Fourteenth Letter, 

 that even in the month of February the source of the Arveiron 

 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 thaw there, but finds the usual channels 

 still open for transmitting the accumulated snow-water."* 



It thus appears quite certain that ice, under the circum- 

 stances in which we find it in the great bulk of glaciers, is in 

 a state more or less softened even in winter ; and that, during 

 nearly the whole summer, whilst surrounded by air above 32, 

 and itself at that temperature, it has acquired a still greater 

 degree of plasticity, due to the latent heat which it has then 

 absorbed.f 



I have mentioned that the observations of this and some 

 previous summers have enabled me to extend the survey of 

 the valley of Chamouni beyond the limits to which my Map 

 was originally confined. I have also obtained a great number 

 of approximate altitudes of all the highest summits of the chain 

 of Mont Blanc, from the extended base which the distance from 

 the Mont Breven to the Croix de Flegere (above 15,400 feet) 

 has afforded me. But the results are as yet only partially 

 calculated. I have also made some additions to our knowledge 

 of the geography of the eastern part of the chain of Mont 

 Blanc, by examining the Glacier of La Tour in its whole 

 extent, which proved the configuration of the mountains to be 

 different from what has been represented on all the maps and 

 models which I have seen. The Glaciers of Argentiere and 



* [See p. 133, note, of this volume.] 



f [See note to Philosophical Transactions, 1846, p. 209 (p. 166 of this volume), 

 and also the paper following this one.] 



