1885.] 



and on the Motion of Glaciers. 



93 



conversant with glaciers. The fatal objection taken by Forbes was 

 that in a glacier, considered as a whole, there is no snch variation of 

 temperatnre, and consequently no snch alternate expansion and con- 

 traction of the ice as Moseley's theory presupposes. The question of 

 the temperature of the interior of a glacier will be considered further 

 on. It will be sufficient to state here that theory and experiment 

 lead alike to the conclusion that the variations of temperature due to 

 the alternations of day and night, and even of summer and winter, 

 are local and superficial only, the great working mass of the glacier, 

 as it may be termed, being at a sensibly constant temperature. This 

 fact had been brought out in the controversy which arose out of 

 Charpen tier's " dilatation " theory of glacier motion. 



The late Mr. W. R. Browne, who recently came forward as a 

 defender of Moseley's hypothesis (" Proc. Roy. Soc," vol. 34, pp. 208 

 — 217), appeared to admit this fact, but argued that the upper layers 

 of ice in expanding and contracting may drag with them the lower 

 layers, or at least cause the upper layers to shear over the lower ones. 

 Without discussing the possibility of such action in the general case, 

 it may be sufficient to make two remarks. 



(1.) As the motion of a glacier is continuous during the year, it 

 must on Canon Moseley's hypothesis be caused by the diurnal as dis- 

 tinguished from the annual variations of temperature. Mr. Browne 

 quotes Dr. Rae as saying, " We know that ice 2 or 3 feet or more 

 thick contracts very considerably in a few hours by a sudden fall of 

 15 or 20 degrees of temperature." Now the cold of a summer's night 

 in the Alps is far less intense than that of a " cold snap " in the 

 American north-west, while it is very difficult to believe that even a 

 very considerable expansion or contraction of the upper 2 or 3 feet 

 of a glacier can affect the motion of a mass several hundred feet 

 thick. 



(2.) In many of the glaciers which move most rapidly the upper 

 layers of ice are intersected by a system of transverse crevasses at 

 short intervals. These extend so far as effectually to prevent any 

 interaction between portions of the surface layer at any considerable 

 distance from one another. 



Moseley did not make any satisfactory answer to the objections 

 brought against his theory, but in 1869 and the following years he 

 put forward a somewhat formidable objection to the current gravi- 

 tation theories of glacier motion. His paper was read to the 

 Royal Society on January 7, 1869 ("Proc. Roy. Soc," vol. 17, 

 p. 202 ; " Phil. Mag.," vol. xxxvii, p. 229), and was followed up by a 

 series of communications to the " Philosophical Magazine," of which 

 the most important are to be found in vols, xxxvii, pp. 363 — 370, 

 and xxxix, pp. 1 — 8. The gist of the objection is that the resist- 

 ance of ice to shearing is many times greater than the shearing force 



