380 Professor Forbes's Eighth Letter on Glaciers, 



measured, from time to time, the angle between this mark 

 and the several marks transverse to the glacier, and I found 

 that this angle became continually, and without any excep- 

 tion, more and more obtuse. During seventeen days, it re- 

 volved 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 specif}^ 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 evi- 

 dence of the fact. Standing at the theodolite with an assist- 

 ant, 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 frac- 

 ture 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 im- 

 bibed 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 be- 

 fore 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 

 I have founded upon them. Had Mr Hopkins applied him- 



