1846.] ON THE DEPRESSION OF THE GLACIER SURFACE. 169 



XIV. ELEVENTH LETTER ON GLACIERS.* Addressed 

 to PROFESSOR JAMESON. 



Observations on the Depression of the Glacier Surface Ablation and Subsidence 

 distinguished and ascertained On the Kelative Velocity of the Surface and 

 Bottom of a Glacier. 



MY DEAR SIR In my Tenth Letter on Glaciers, which 

 you did me the favour to publish lately, a question was dis- 

 cussed respecting the apparent depression of the surface of a 

 glacier. I had already pointed out in the first edition of my 

 Travels, that several causes combine to produce this depression, 

 but that observations were wanting to distinguish them. The 

 causes then enumerated were (if I mistake not) these : 1. The 

 actual waste or melting of the ice at its surface. 2. The sub- 

 sidence of the glacier in its bed, owing to the melting of its 

 inferior surface, whether by the healj of the earth, or that due 

 to currents of water. 3. The effect of the drawing out of the 

 glacier where it is in a state of distension, which tends to reduce 

 the thickness of the mass of ice ; (when a glacier is violently 

 compressed the effect will be contrary, or an elevation will 

 result) ; to which may be added the influence of the slope of 

 the bed of the glacier, by which, as it moves forward, its abso- 

 lute elevation is diminished, or the contrary if it ascends. I 

 had also pointed out a method f by which the first of these 

 effects, or the absolute ablation of the ice (as it has been termed 

 by M. Agassiz), might be distinguished from the other two, 

 namely, by driving a horizontal hole into the wall of a crevasse, 

 and observing the diminution of the thickness of the stratum of 

 ice above it. The partial and total effects I have observed in 

 the following manner, during the present summer, on the Mer 



* Edinburgh New Philosophical Journal, October 1846. [The Tenth Letter of 

 the Series is not reprinted, being mainly controversial, and not containing new 

 original observations. It will be found in the same Journal for January 1846.] 



f Travels, 1st Edition (1843), p. 154. 



