150 VISCOUS THEORY OF GLACIER MOTION. [1846. 



singular complexity, and one which is not warranted by a more 

 accurate analysis. Yet for a time rival theories seemed poised 

 on the inappropriate question, " Are glaciers in a state of 

 internal tension or compression '?" Even if the glacier moved 

 as a mass of fragments, therefore without tension, the cohesion 

 must first have been broken before it could be reduced into 

 fragments. I have been inconsiderately censured for quoting, 

 with approbation,* the observation of M. Elie de Beaumont, that 

 a glacier appears to be rather in a state of distension than com- 

 pression, whilst I adopted a hydrostatic pressure, acting from 

 the origin as the source of motion. A careful examination of 

 the passages in question will show that my assent to the view 

 of M. Elie de Beaumont was limited to portions of the glacier, 

 and especially to those portions most crevassed, the parts, namely, 

 which connect the sides and centre, and which serve to drag the 

 more sluggish, because retarded, lateral portions after the freer 

 central part on which the vis a tergo acts with most advantage ; 

 and in a direction generally parallel to the blue bands, so far as 

 they are due to inequalities of motion in the horizontal plane.f 

 My earliest attempts to obtain clear views of the internal forces 

 acting on a semi-rigid body, impelled by self-contained hydros- 

 tatic forces, convinced me how little could be founded on the 

 completeness of any mathematical investigation of them, which 

 in our present state of knowledge may well be considered as 

 hopeless ; and reserving to myself the not so difficult task of 

 extricating at a future time the more important practical laws of 

 these strains and thrusts, I very carefully avoided, in my first 

 publication, any allusion to what might be considered as their 

 actual distribution ; a distribution varying not only from point 

 to point of the glacier surface, but throughout its thickness, and 

 most undoubtedly varying also for the same point at different 

 seasons of the year, or even changing its sign, so that a tension 

 at one season may become a thrust at another. 



I had no reason to repent of this caution, from which I only 



* Travels, 1st edit. pp. 178, 370 ; 2d edit. p. 370 and Note. 

 fSee Philosophical Magazine, May 1845, p. 408. 



