132 VISCOUS THEORY OF GLACIER MOTION. [1846. 



of glacier motion (more particularly on the Glacier des Bois) 

 affords a proof of the justness of the principle laid down by me 

 in 1842, that the motion of the ice " is more rapid in summer 

 than in winter, in hot than in cold weather, and especially more 

 rapid after rain,. and less rapid in sudden frosts ;"* the evidence 

 of the connection is plainer by mere inspection than any detail 

 could make it. But I request attention to the apparent anomalies 

 of the curves, as affording a stronger evidence of the fidelity with 

 which the measurements have 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 the wetness of the sur- 

 face, whether derived from mild rain, from thawing snow, or 

 from any other meteorological accident which the register of 

 the thermometer cannot of itself indicate.f Further, a thick coat- 

 ing of snow on the glacier must defend it from the excessive 

 cold of winter just as it defends the earth and plants, and con- 

 sequently the minimum of motion will not necessarily coincide 

 with the minimum of temperature. Now, to estimate these more 

 irregular causes is not so easy ; but some light is thrown upon 

 them by a register of the weather and state of the snow, volun- 

 tarily 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. 



* Fourth Letter on Glaciers, Edinburgh Philosophical Journal, Jan. 1843 ; and- 

 p. 35 of this volume. 



f " The proportion 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. 



