Variation of Motion at different Seasons. 337 



been made, and to the truth of the plastic theory, than perhaps even 

 the general coincidence just referred to. 



If the velocity of the glacier depend upon the completeness of its 

 infiltration with water, rendering the whole an imbibed porous mass 

 like a sponge, it cannot depend solely on the mean temperature of 

 any period, but also upon tlie wetness of the surface, whether 

 derived from mild rain, from thawing snow, or from any other 

 meteorological accident which the register of the thermometer can- 

 not of itself indicate.* Further, a thick coating of snow on the gla- 

 cier must defend it from the excessive cold of winter, just as it de- 

 fends the earth and plants, and consequently the minimum of mo- 

 tion will not necessarily coincide with the minimum of temperature. 

 Now, to estimate these more irregular causes is not easy ; but some 

 light is thrown upon them by a register of the weather and state of 

 the snow, voluntarily kept for me at Chamouni by Auguste Balmat ; 

 which forms a valuable supplement to the thermometrical register 

 of Geneva and St Bernard. Although the daily details would take 

 up too much space, I will endeavour to give a faithful abstract of 

 them so far as to give a general idea of the climate of Chamouni 

 from October 1844 to November 1845. This diary includes (at my 

 request) occasional notes on the state of the source of the Arveiron, 

 which are of considerable interest. 



Weather at Chamouni. 



1844, October. — A good deal of rain during the month, which 

 on the 10th and 16th fell as snow on the hills (nine inches at Mon- 

 tanvert), and subsequently to the latter day the glacier at the Mon- 

 tanvert was not clear of snow during the winter. 14th, Source of 

 the Arveiron diminished to one-fourth (of the summer volume). 

 Ice- vault more than half closed. 



November. — Till 14th much rain and snow ; fine with frost after, 

 20(A, Source of the Arveiron very low ; has not shifted its usual 

 position. 



December. — Weather generally fine throughout ; cold most severe 

 from 7th to 12th. 



1845,VanMary. — The weather continued splendid till the 20th; 

 greatest cold from —2° to —5° Reaumur. 19th, The vault has 

 disappeared at the source of the Arveiron. 20th, The first snow 

 fell which lay at Chamouni, and continued from this day, attaining 

 a depth of 1^ foot in February. Up to this time all the secondary 



* '• The pro2)ortion of velocity does not follow the proportion of heat, because 

 any cause, such as the melting of a coating of snow by a sudden thaw, as in the 

 end of September 1842, produces the same effect as a great heat would do." — 

 Travels, 2d Edit., p. 372. 



