xv.] FORBES* GLACIER DISCOVERIES. 515 



Trans, and Phil. Magazine) as irrelevant mathematical 

 exercitations. His later writings I have not studied, as 

 I perceived that he had made no enlargement of his 

 physical basis of reasoning. A proper theory of glaciers 

 must include a constant, or constants, expressive of the 

 state of the internal friction, according to the magnitude 

 of which, in relation to the forces acting to set the body 

 in motion, it will break up as a solid or flow like a 

 liquid. I never felt myself equal to attempting this 

 very general problem, but I recollect well that in 1851, 

 AY lu -n Air. Stokes* paper on the Friction of Fluids 

 appeared, I urged him to consider the motion of glaciers 

 from the same point of view. I am glad to think from 

 what you tell me, that there is now a prospect of our 

 having a real solution of this interesting problem. 



'Tresca's paper on the quasi-fluid motion of solids 

 under intense pressure (Comptes Rendus, Nov. 7, 1864) 

 seems to have excited some notice through a paragraph 

 in some English journal, to which two of my correspond- 

 ents refer 



' Yours very sincerely, 



* JAMES D. FORBES.' 



We find the Thomsons' explanation of at least a great 

 part of the puzzling phenomena of glaciers given in a 

 ( -harming popular form by Helmholtz in his Populdre 

 Wissenschaftliche Vortrtige, from which we have para- 

 phrased the following passages, with which this chapter 

 may fitly terminate. 



' One of the fixed points of our thermometric scale, 



which we call zero or the freezing point, is determined 



ly placing the thermometer in a mixture of ice and pure 



water. Water, so long as it is in contact with ice, 



not be cooled below the freezing point without 



ming ice; ice cannot be heated above this point 



without mc-ltin^. A mix tine of ice and water must 



therefore remain stationary at the temperature zero. 1 1 



one tries to warm such a mixture, the in- im-lts but the 



temperature of the mixture docs imt rifle ab .\ e /.-TO SO 



I. !. i! 



