120 M. Ch. Martins' Bemarks on M. Durochefs Memoir 



at its lower surface, slides over a slope of 40'.* In making 

 this objection, M. Durocher supposes, along with Mr Forbes, 

 that the movement of glaciers is nothing else than that of 

 heavy bodies.^ If he had studied this movement, he would 

 have found that such is not the case. The glacier of the Aar 

 moves on a very unequal bed, sloping 1° 30', an inclination 

 amounting almost to nothing, and on which a solid body 

 would remain stationary ; for, according to M. Morin, the 

 two bodies which slide most easily on one another are cop- 

 per and greased iron ; but the former does not get into motion 

 till the inclination reach 4° 35'. In a heavy solid body, vis- 

 cous or liquid, the lower extremity advances with a degree 

 of quickness sensibly equal to that of the upper parts. In a 

 glacier, the movement becomes slower towards its lower ex- 

 tremity. Thus in the present year, whilst at the middle por- 

 tion of its length, the glacier of the Aar had traversed a 

 space of 75 metres in 396 days, it had advanced only 41 

 metres near its extremity. The influence of the slope is even 

 so slight on the progress of glaciers that we cannot compare 

 it to the sliding of heavy bodies. M. Desor has already 

 shewn this ; J M. Dollfus and myself have confirmed it. In 

 fact, while the glacier of Griinberg, one of the tributaries of 

 the Aar, had advanced 2''22 in 17 days, the latter had ad- 

 vanced 2'-94 ;^and yet the surface of the first mentioned had 

 an inclination of 30°, that of the second only 3°.§ 



The second difficulty which M. Durocher advances against 

 the ancient extension of glaciers in Scandinavia, is, that they 

 cannot traverse planes inclined to opposite slopes. In order 

 to be convinced that this objection is not real, we have only 

 to go to Switzerland and examine the profile of some gla- 

 ciers, where we see the rock on which they rest. We will 

 perceive that all the glaciers pass over planes inclined to 

 opposite declivities. I shall content myself by citing the 



* On the Motion of Glaciers. Philosoph. Mag., vol. xxvi., p. 4, 1845. 

 t Comptes llendus de I'Acadoraie des Sciences, t. xxiii., p. 209, 27th July 

 1846. 



+ Ibid., t. xix., p. 1303, 9th Dec. 1844. 

 § Ibid., t. xxiii., p. 823, 26th October 1846. 



