140 Canon Moseley on the Mechanical Impossibility of 



the case, since in the imaginary glacier the weight is found to 

 be insufficient to cause it to descend, much more must it be so 

 in the actual glacier. This is my argument ; and if it cannot be 

 overthrown, it follows from it that no theory is to be received 

 which accounts for the descent of glaciers by their weight only. 

 The data I have used in my computation are Professor TyndalFs 

 observations on the velocity of the surface-ice of the Mer de 

 Glace at Les Ponts, and those on the velocity of the side-ice on 

 the Glacier du Geant near the Tacul. These last observations 

 were made under circumstances of great difficulty and some 

 danger*. No conceivable error in the data can, however, ac- 

 count for the enormous disproportion which is shown to exist 

 between the work of the force of gravity which is supposed to 

 cause the glacier to descend and that of the resistances it has to 

 overcome. Among these resistances I have reckoned those of 

 the sides and bottom of the channel to be as great as though the 

 ice were frozen to them ; and considering what are the obstacles 

 in the actual channel from projecting rocks, bends in its direction, 

 and frequent contractions, the assumption of a resistance at least 

 equal to that which would result m the imaginary form of gla- 

 cier from the ice being frozen to its bottom and sides, is not 

 perhaps unreasonable. My result is not, however, practically 

 affected by throwing the resistance of the channel wholly out of 

 the question. If its bottom and sides be conceived perfectly 

 smooth, and if the ice of the glacier be supposed not to adhere 

 to them at all, its differential motion remaining nevertheless un- 

 altered, it still follows t that the work of the resistances through 

 any distance of the descent is forty-seven times as great as the 

 work of the weight of the glacier through that distance will supply. 

 The differential motion is, in point of fact, by far the greater 

 part of the motion of the glacier — thirteen fourteenths of it on 

 the Aar Glacier, according to Professor Forbes J; so that the 

 resistance to the differential motion, measured by its work, is by 

 far the greatest resistance. Nor will any possible error in my 

 assumed value of the unit of shear in ice affect the general result 

 at which I have arrived. My experiments on it were made in a 

 high temperature of the air. I have repeated them when it was 

 below freezing, and found it greatly increased. As the tempe- 

 rature of the ice of a glacier is assumed not to be above freezing, 



* It is desirable they should be repeated at more leisure and with greater 

 precautions than Professor Tyndall could command, and that the velocity 

 of the surface-ice should be determined on the same section as that where 

 the velocity of the side-ice is observed. 



f Phil. Mag. May 1869. In equation (9), U 2 and U 3 are to be assumed 

 to vanish ; whence the above result will follow. 



% Occasional Papers, p. 74. 



