1844.] UPPER STRATA OF ICE MOVE ALMOST UNIFORMLY. 55 



by M. Agassiz in the glacier of the Aar, is stated to have re- 

 mained vertical for a period of many weeks?" In the first 

 place, the fact of the verticality requires confirmation ; for it is 

 difficult to understand how, by means of a plummet, 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 constructed in the ordinary way, 

 should be either mathematically straight or vertical. I appre- 

 hend 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.* 



Bat, even setting aside this important consideration, the 

 principle of the variation of velocity being chiefly confined to 

 the neighbourhood of the sides and bottom, and the compara- 

 tively quiescent and passive state of the central and superficial 

 part, seem sufficient to explain the facts within the reasonable 

 limits of error. The depth of 140 feet appears, from M. 

 Agassiz' s own observations, not to exceed ONE SIXTH, at most, 

 of the depth of the glacier of the Aar in that part. Now, let 

 ABC, etc., represent points in the vertical section of the 

 glacier ; then, from all that we know of the superficial motion 

 of glaciers, or of the parallel case of rivers whose velocity has 

 been ascertained at different depths, the velocities will vary 

 in some such manner as A a, B b, C c, etc., the variation 

 being scarcely sensible at first, and very rapid at the bottom, 

 where the velocity may even be zero, if the curve be prolonged 



* Since this passage was written, I have had an opportunity of referring to the 

 description of the experiments of Agassiz in the JBibliotheque Universelle ; and I find 

 that there is no evidence whatever of the continued 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 

 t e xt. Aug. 1844. [See also page 30 of the present volume.] 



