844 Professor Forbef*' j^ccount of his recent 



opposite the Montanvert, amounts to 27.1 inches. (7.) I have 

 ascertained the velocity of motion much nearer the origin of 

 the glacier than when I last wrote. Tliis, which would appear 

 to be nearly, if not quite an expcrimcntimi cruets between the 

 sliding and dilatation theories, does not yield a result so favour- 

 able to the latter as I had at first supposed ; for though it is 

 undoubtedly true, as stated in my last, that the head of the 

 glacier moves slower than the foot, the middle part moves 

 rather slower than either, owing probably to the greater width 

 and thickening of the ice there. This source of error from 

 the varying section of the glacier I had fully anticipated ; but 

 still, when we push the experiment to a limit, and take tlie 

 velocity very near the origin, the velocity ought to diminish, 

 on the theory of Charpentier, with a rapidity not to be mis- 

 taken. Yet very near the head of the Glacier de Lechaud, 

 the diurnal velocity is considerably more than a foot per day. 

 I am far, however, from thinking that I am yet in a position 

 to judge finally of the merits of any theory ; my belief is, 

 that both of those cited will as yet require great modification. 

 By insisting upon the treatment of the problem as one of 

 pure mechanics, I am far from denying that the kind of in- 

 vestigations to which the glacier theorists have hitherto al- 

 most exclusively referred, are also of great value, such as 

 those on the temperature and structure of the ice. The latter, 

 in particular, is a sort of standing evidence of its mechanism, 

 and, rightly understood, must lead to the most important con- 

 firmation of any mechanical theory. This you may believe 

 I have made an object of very particular attention. I have 

 now examined so many glaciers as to have a very clear idea 

 of the empirical laws which that structure follows. Lately, I 

 begin to perceive a connection between tliat structure and the 

 facts of motion already cited. If these two classes of facts 

 can be well brought into harmony with one another, we should 

 have a very good chance of consolidating them into something 

 like a theory. In my next letter, I will give you some ac- 

 count, at all events, of my observations on the subject, whicli 

 are sufficiently definite, and probably also (without consider- 

 ing it as proved), of what seems likely enough to be its true 

 explanation. I go to-morrow to the Great St Bernard, to 

 meet M. Studer. — Believe me, very sincerely yours, 



James Forbes. 



