413 



case of glaciers, whose motion over their beds may, he thinks, be 

 accounted for in the same way, namely, by the alternate contraction 

 and expansion of the ice by diurnal changes of temperature, and he 

 then enters into certain calculations founded principally on data 

 contained in my ' Travels in the Alps of Savoy' in confirmation of 

 this view. 



Entertaining as I do the highest respect for Mr. Moseley's emi- 

 nent attainments as a theoretical mechanician, it is with extreme 

 regret that I find it necessary, in maintenance of the views regarding 

 glacier motion which I have elsewhere advanced, and in the interest 

 of scientific truth, to show (as I believe I can) that Mr. Moseley 

 has been led, apparently by a sudden inadvertency, to uphold an 

 opinion completely indefensible. 



I must first object to Mr. Moseiey's description or definition of 

 a glacier, as calculated to mislead the inquirer : he says (p. 339), 

 "glaciers are, on an increased scale" [compared to the sheet lead 

 covering of a roof], " sheets of ice placed upon the slopes of moun- 

 tains." There are certainly some inconsiderable glaciers of the 

 second order to which this description might possibly apply, with 

 the exception of the small thickness inferred by the word " sheet ;" 

 but the true glaciers, whose theory has been so often discussed 

 (which theory must evidently likewise include that of glaciers of the 

 second order), cannot fairly be called either sheets of ice nor be 

 accurately described as lying on the slopes of mountains. They are 

 vast icy accumulations whose depth bears a considerable proportion 

 to their breadth, and which fill mountain ravines or valleys. 



Glaciers are very generally hemmed in by precipitous rocks which 

 determine their contour or ground plan ; they have often to make 

 their way through contracted gorges where the ice occupies (as in 

 the case of the Mer de Glace of Chamouni), within a short distance, 

 a channel but half as wide as it did before. Yet the glacier, pre- 

 serving its continuity as a whole, expands or contracts in conformity 

 with the irregularities, not only of its lateral walls, but of its bed, 

 forcing itself over obstacles, or even occasionally allowing itself to 

 be cleft into two branches by them, and closing again into a united 

 mass after the insular obstruction has been past. To speak of such 

 resistances of the channel to the progress of the ice as mere friction, 

 or of a glacier considered as a solid body and in its whole extent 



