66 EIGHTH LETTER ON GLACIERS. [1844. 



that this angle became continually, and without any exception, 

 more and more obtuse. During seventeen days, it revolved 

 through an angle of about a degree and a half. 



I reserve to another opportunity the publication of the 

 details of the measurements and the graphical ' projections, 

 which offer, when minutely examined, some interesting pecu- 

 liarities too long to specify. The main conclusion is, that even 

 the most compact parts of the ice yield to pressure, and that 

 where no fissures exist, there is a sliding of the parts of the ice 

 over one another, or else a plasticity of the whole mass. With 

 the abundance of blue bands before us in the direction in which 

 the differential motion must take place (in this case sensibly 

 parallel to the sides of the glacier), it is impossible to doubt that 

 these infiltrated crevices (for such they undoubtedly are) have 

 this origin, and are the main mechanism of the forward motion ; 

 but it occurred to me, on one occasion (the 23d August), to 

 obtain all but ocular evidence of the fact. Standing at the theo- 

 dolite with an assistant, we heard a dull noise in the ice within 

 a very few feet of us, attended (I think) with a slight tremor, 

 and followed by a rushing and hissing sound. As we were 

 very near the great crevasses of the moraine, it was, no doubt, 

 a subsidence of a portion of the glacier, and the rushing was 

 occasioned by the more rapid flow of the superficial streamlets 

 in the direction of increased inclination of the ice. I instantly 

 searched in all directions, but in vain, for the slightest evidence 

 of the fracture of the ice. All that I could see was, that where 

 the veined structure was best developed, innumerable air bubbles 

 escaped through the superficial water, which was slowly imbibed 

 in those parts where the strain had expanded the ice, and thus 

 enlarged the capillary fissures between the blue bands. 



Mr. Hopkins has done me the honour, in the memoirs before 

 alluded to, to mention with approbation my observations and 

 experiments on the subject of glaciers. He has been more 

 sparing either in praise or criticism of the theory which 1 have 

 founded upon them. Had Mr. Hopkins applied himself with 

 equal care to that as to other parts of my writings, he would 



