[171] 

 XVII. On Glacier Theories. By Dr. Wheweli.. 



To Richard Taylor, Esq. 

 Dear Sir, 



I SHALL be obliged by your allowing me to make a remark 

 or two in your pages on the subject of glaciers; which 

 subject has been recently discussed both there and elsewhere. 

 In the course of these discussions, expressions have been let 

 fall, as if there were some particular care requisite to avoid 

 confusion with regard to notions generally considered very ele^ 

 mentary, tis rigidity, solidity, Jlexibility,Jluidity. I will there- 

 fore, at the risk of provoking a smile Worn your mathematical 

 readers, begin with distinguishing these. Rigid bodies are 

 absolutely incapable of any motion of their parts among each 

 other. To these are opposed ^exihle bodies, in which the 

 parts move without changing their attachments ; antl ^uid 

 bodies, in which the parts have no attachment to each other, 

 and moveamongst each other freely. Flexible bodies are elastic, 

 when they tend to resume their shape. Lines and developable 

 surfaces may be flexible without being elastic ; but a flexible 

 solid must be elastic, for when it is bent, some of its dimensions 

 must be lengthened, or shortened, or both. Fluids are imper- 

 fectly fluid when their parts have a slight attachment which 

 requires some force to overcome it. In this case, when a par- 

 ticle is detached from its original companions, it attaches itself, 

 with a like force, to a new set among whom it is drawn. Such 

 bodies are also called viscous or plastic, these terms implying 

 various degrees of force in the attachment of the particles. 

 And when this force is so great as not to be easily overcome, 

 the body may be termed solid, as opposed to fluid ; but in this 

 sense solidity is merely a comparative term, and cannot be 

 properly opposed to rigidity, which is an absolute one. Flex- 

 ible and viscous are very clearly distinguished, for in viscous 

 bodies the particles slide past each other, though not freely ; 

 in flexible bodies they move round, but not past each other. 

 A block of caoutchouc is flexible ; if melted, it is viscous; and 

 this, however little it be melted. 



This being premised, we shall have no difficulty in com* 

 paring the results of supposing the ice of glaciers flexible and 

 viscous. Let an alpine sloping valley be filled with a vast 

 mass of solid caoutchouc, the slope and friction being such that 

 the mass slides slowly downwards. What will be the condition 

 of the mass? Plainly the sides and bottom will be held back 

 by friction ; the middle and upper parts will drag forwards. 

 The straight lines originally transverse will become curves, 

 with the convexity downwards. The wholemass will be in a 



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