Professor Forbes's Sixth Letter on Glaciers. 243 



would not repay the labour which it would involve, which 

 would be of the most serious kind ; — for we must not expect 

 to find the difference of velocity apparent in the superficial 

 strata, even to a considerable depth, since we know that the 

 retardation is a maximum near the sides and bottom, and that, 

 for the same reason, the motion of all the central part of a 

 glacier is nearly uniform, so will the motion of all the part of 

 the ice near the surface be nearly uniform. 



These considerations suggest the explanation of a difficulty, 

 kindly suggested to me by a most competent judge, who ex- 

 pressed himself at the same time persuaded of the truth of 

 the viscous theory of glaciers. '• How comes it, that, if the 

 motion of the diff'erent parts of a glacier diminishes from the 

 surface to the bottom, the ' trou de sonde' or bore^ 140 feet 

 deep, made by M. Agassiz in the glacier of the Aar, is stated 

 to have remained vertical for a period of many weeks ?" In 

 the first place, the fact of the verticality requires confirma- 

 tion ; for it is difficult to understand how, by means of a plum- 

 met, a hole 140 feet deep, and only 3 or 4 inches in diameter, 

 could have its verticality tested. Such bores, so far as I 

 have seen them, are more or less twisted, owing to the softness 

 of the material, and the method of working ; and it seems 

 beyond all probability, that a hole of such a depth construct- 

 ed in the ordinary way, should be either mathematically 

 straight or vertical. I apprehend that the verticality alluded 

 to by M. Agassiz, or his coadjutors, is merely that of popular 

 language, indicated by the boring rods standing vertically 

 outwards when plunged into the hole, which, on account of 

 their flexibility, would not be an indication of the verticality 

 of more than the upper twenty or thirty feet of the bore at the 

 most.* 



* Since this passage was written, I have had an opportunity of refer- 

 ring to the description of the experiments of Agassiz in the Biblioihique 

 Univ^rselle j and I find that there is no evidence whatever of the con- 

 tinued verticality of the bore of 140 feet, which existed (to that depth), 

 I believe, but a few days ; the observations of continued verticality, 

 such as they are, applied to small bores only, not exceeding 25 or 30 feet, 

 which, of course, greatly increases the force of the reasoning^ in the text. 

 Aug. 1844. 



