148 Professor Forbes' s Fifteenth Letter on Glaciers. 



more viscid than common water. But, I concluded that, 

 when the frontal resistance (due to friction and cohesion) 

 becomes very great, the planes of least resistance may assume 

 an inclination of 60" or more, a notion which has been treated 

 as practically untenable by a mathematical critic of my theory, 

 whilst he admits that it is theoretically possible. The iron 

 shavings in question demonstrate the truth and feasibility of 

 my anticipation. There is no difficulty in determining the 

 exact line of pressure, for it is obviously that in which the 

 tool is made to act, or it is mathematically parallel to the 

 flat side of the shaving itself, if we suppose it straightened. 

 (Fig. 7.) In one of the specimens now before me, the planes 

 i>f detrusion or frontal dip, make an angle, as nearly as can 

 be estimated, of seventy degrees, with the base or line of pres- 

 sure. From the fibrous appearance of the whole mass, I have 

 little doubt that it is traversed by numberless fissures or 

 flaws parallel to the planes of actual sliding, flaws which 

 might probably be made evident by immersing the whole in 

 dilute acid. 



Time does not allow me to add more. Some may consider 

 these approximations and analogies trifling, but I persuade 

 myself that you will not do so, being well aware how much 

 has resulted in the progress of science from the patient study 

 of minute facts not obviously related to one another. It is 

 some pleasure to me to persuade myself that my speculations 

 upon the cause of the motion of glaciers have had some slight 

 influence in drawing attention to the loose manner in which 

 bodies have hitherto been classified as solid and fluid, rigid, 

 flexible, or plastic. On the one hand, attention is directed to 

 the way in which stress or strain is exerted upon masses, and 

 modified by their internal constitution in a way which no 

 theory not embracing an expression of that constitution 

 founded on experience, can possibly represent. On the other 

 hand, the imperfect views which practical men have enter- 

 tained as to the manner in which intense strains affect ma- 

 terials of certain kinds, and in certain forms, are apparently 

 about to undergo a considerable revolution. I remain, my 

 dear Sir, yours very truly, 



James D. Forbes. 



Rev. Dr Whewell. 



