228 Professor Forbes on the Leading Phenomena of Glaciers. 



3. The inclination of the bed is seldom such as to render 

 the overcoming of such obstacles as the elbows and prominen- 

 ces, contractions and irregularities of the bed of glaciers, even 

 conceivable, being, on an average of the entire Mer de Glace, 

 only 9°, a slope practicable for loaded carts ; but the greater 

 part of the surface inclines less than 5°, which is below the 

 steepest slope on the great highway of the Simplon, an artil- 

 lery road. 



4. It has been convincingly proved by me, that the mo- 

 tion of the glacier varies not only from one season to an- 

 other, but that it has definite (though continuous) changes 

 of motion, simultaneous throughout the whole, or a great part 

 of its extent, and therefore due to some general external 

 change. This change has been shewn to be principally or sole- 

 ly the effect of the temperature of the air, and the condition 

 of wetness or dryness of the ice. In order to reconcile this 

 to the sliding theory, it should be shewn, that the disengage- 

 ment of the glacier from its bed, depends on the kind of weather 

 which affects its surface and temperature. In no part of the 

 summer is the glacier actually frozen to its lateral walls ; the 

 difference, then, must be due to the action of the earth^s heat, 

 in gradually melting away the irregularities of the interior 

 surface of the ice, in contact with the rocky bed on which it 

 reposes. T have already said, that I consider such an influ- 

 ence of the proper heat of the earth to be distinctly included 

 in De Saussure''s theory, as it has been stated by himself, and 

 understood by his successors.* It was, however, suggested to 

 me very distinctly by M. Studer last summer, as not incon- 

 sistent with a motion by gravity without acceleration ; and I 

 admit the ingenuity of the thought, which, as it will be seen 

 in the sequel, I am disposed to allow, may be one wai/ of gla- 

 cier motion, though not exactly the cause of it. The same 

 thought was afterwards suggested to me by Sir John Her- 

 schel, and more lately I have seen it stated, that Mr Hopkins 



* Any one who carefully reads De Saussure's § 535 in connection with § 533, 

 will be convinced that he gives all due weight (we should be inclined to say 

 more than due weight) to the effects of subterranean heat in detaching the ice 

 from its bed, lubricating it on its bed, and even elevating it over obstacles by the 

 Jiydrostatic pressure of confined water. 



