72 NINTH LETTER ON GLACIERS. [1845 



The fact established on the glacier of the Aar satisfactorily 

 refutes the notion that a predominant state of compression in a 

 glacier is incompatible with the existence of transverse crevasses. 



II. Continuity of motion. Until recently it was a question 

 entirely unresolved, whether the glaciers move by insensible 

 and nearly uniform degrees, or whether they start forward by 

 short jerks, as might be expected if the movement in their 

 irregular channels were effected by piecemeal fractures, local 

 subsidences, arid the justling of independent fragments. Accord- 

 ingly, when I determined, for the first time, in June 1842, the 

 absolute continuity of the motion down even to the interval of 

 an hour,* it seemed impossible to reconcile this to the only 

 modification of De Saussure's theory applicable to the case, and 

 the fact seemed to point, as a necessary consequence, to an 

 insensible yielding of parts throughout the whole mass, which 

 therefore moves as a whole, and not by jerks occasioned by 

 strains upon a nearly rigid mass when they attain the limit 

 consistent with the small play of flexibility of the particles, as 

 some authors would have us believe.f 



It is satisfactory to have an entire confirmation of these 

 particulars, from the observations made on the glacier of the 

 Aar in 1844. The observations were made " to the accuracy 

 of a millimetre on the movement of the glacier from hour to 

 hour"^ and " at the lower extremity of the glacier, as in the 

 upper part of its course, the glacier does not advance abruptly, 

 by jerks (saccades) as formerly supposed, but its march is 

 gradual and continuous "\\ 



III. The influence of warm and damp weather in accelerating 

 the continuous march of the glacier, and of cold weather in 

 checking it, 1 deduced in 1842 from a careful comparison of its 



* See first Letter on Glaciers, [p. 1 1 of this volume.] 



f Mr. Hopkins' experiment of a box full of ice descending an even plane, does 

 not apply to this case, because, though it moves as a whole, it does so without 

 change of figure, and without the resistance arising from the irregularity of the 

 channel of a glacier ; and hence the seeming analogy to a glacier entirely fails. 



I Comptes Kendus, p. 1302, line 12. 



Jadis. It is to be inferred that the writer meant previous to 1842- 



|| Comptes Rendus, p. 1303, line 9. 



