1844.] GREAT GLACIER OF ALETSCH. 61 



IX. EIGHTH LETTER on GLACIERS * addressed to PRO- 

 FESSOR JAMESON by PROFESSOR FORBES. 



Observations on the Motion of the Glacier of Aletsch Experiments on the Plas- 

 ticity of the Ice of the Mer de Glace, by Observing the Distortion of a Limited 

 Space of Ice devoid of Crevasses Movement of a Glacier of the Second Order 

 8000 Feet above the Sea. 



GENEVA, 30th August 1844. 



My Dear Sir The theory of glaciers has now reached that 

 point when it can only receive some material addition by the 

 multiplication of accurate measurements; and these measure- 

 ments must be conducted in the manner which will best discri- 

 minate between rival hypotheses, and, if possible, yield direct 

 instead of indirect proofs of each fundamental fact assumed. In 

 my former letters I have insisted sufficiently upon the import- 

 ance of the results which a system of nice measurement has 

 introduced into this branch of science, and their value to the 

 theorist who afterwards wishes to put numerical for unknown 

 quantities in his investigations ; I also showed that there is a 

 continuity and approximate constancy in the motions of glaciers, 

 which permits us to obtain, with certain precautions, in a few 

 days, better results than any one had previously acquired during 

 the lapse of months or years. I have now to announce to you 

 that I have pushed these measurements to a still greater degree 

 of minuteness, and with results which show that the methods I 

 have employed are trustworthy, and are able to afford the direct 

 solution of questions which at first appeared to admit of only 

 indirect or inductive proof. 



Of this class, by far the most important appeared to be the 

 manner in which the glacier alters its form in such a way, and 

 to such a degree, as to suffer its central portion to descend 

 towards the valley with double or treble the velocity of its 

 lateral parts. Such, for instance, I have found to be the case 

 in the middle region of the great glacier of Aletsch, where its 

 inclination is small (about 4), and where the continuity of the 



* Edinburgh New Philosophical Journal, October 1844. 



