158 VISCOUS THEORY OF GLACIER MOTION. [1846. 



order to allow for an equal ablation the succeeding summer ; 

 and at first I was disposed to admit so much of the dilatation 

 theory to be true as would account for the swelling of the sur- 

 face in a vertical direction by the freezing during winter of the 

 infiltrated water.* Further reflection convinced me, however, 

 that this explanation was insufficient and also not required, and 

 I accordingly concluded, " that the main cause of the restora- 

 tion of the surface is the diminished fluidity of the glacier in 

 cold weather, which retards (as we know) the motion of all its 

 parts, but especially of those parts which move most rapidly in 

 summer. The disproportion of velocity throughout the length 

 and breadth of the glacier is therefore less ; the ice more 

 pressed together and less drawn asunder ; the crevasses are con- 

 solidated, while the increased friction and viscosity causes the 

 whole to swell, and especially the inferior parts, which are most 

 wasted." t I have nothing to add to this explanation, except 

 that the observation of the motion throughout the whole 

 year confirms it in every particular. The more elevated por- 

 tions of the glacier, which during a large portion of the year 

 are exposed to a mean temperature under 32, move in a 

 manner comparatively uniform, the lower extremities undergo 

 great oscillations in their speed (in the ratio of four or five to 

 one, see page 129) ; hence the attenuation during the summer 

 regime, which is owing to the drag taking place downwards in an 

 excessive degree ; but the winter's cold, equalizing in some 

 measure the velocity everywhere, brings the plasticity into full 

 action, fills the crevasses, and swells the surface to its old level. 

 As it is universally admitted that the glacier proper does 

 not grow in thickness by snowy accumulations, the important 

 variations in its level in different years J cannot be ascribed to 

 the severity of certain seasons increasing the mass of snow 



* Fourth Letter. [Page 34 above.] 



f- Travels, p. 386, 2d edit. [See also the very striking analogous case of lava 

 streams mentioned in 2 of this paper, page 91.] 



\ For instance, it has been seen from Balmat's narrative (p. 134 above), that 

 in 1845 the glacier attained a much higher level at the Angle than it had done for 

 three previous years at least, since all the marks of measurements which were cut 



