230 Professor Forbes on the Leading Phenomena of Glaciers. 



After the detailed though scattered deductions which have 

 been made in the course of this work, from observations on 

 the Movement and Structure of Glaciers, as to the cause of 

 these phenomena, little remains to be done but to gather to- 

 gether the fragments of a theory for which I have endeavoured 

 gradually to prepare the reader, and by stating it in a some- 

 what more connected and precise form, whilst I shall no doubt 

 make its incompleteness more apparent, I may also hope that 

 the candid reader will find a general consistency in the whole, 

 which, if it does not command his unhesitating assent to the 

 theory proposed, may induce him to consider it as not unwor- 

 thy of being farther entertained. 



My theory of glacier Motion then is this : — A glacier is 



AN IMPERFECT FLUID, OR A VISCOUS BODY, WHICH IS URGED 

 DOWN SLOPES OF A CERTAIN INCLINATION BY THE MUTUAL 

 PRESSURE OF ITS PARTS. 



The sort of consistency to which we refer may be illustra- 

 ted by that of a moderately thick mortar, or of the contents of 

 a tar-barrel, poured into a sloping channel. Either of these 

 substances, without actually assuming a level surface, will tend 

 to do so. They will descend with different degrees of velocity, 

 depending on the pressure to which they are respectively sub- 

 jected, — the friction occasioned by the nature of the channel 

 or surface over which they move, — and the viscosity^ or mu- 

 tual adhesiveness, of the particles of the semifluid, which pre- 

 vents each from taking its own course, but subjects all to a 

 mutual constraint. To determine completely the motion of 

 such a semifluid is a most arduous, or rather, in our present 

 state of knowledge, an impracticable investigation. Instead, 

 therefore, of aiming at a cumbrous mathematical precision, 

 where the first data required for calculation are themselves un- 

 known with any kind of numerical exactness. I shall endeavour to 

 keep generally in view such plain mechanical principles as are, 

 for the most part, sufficient to enable us to judge of the com- 

 parability of the facts of Glacier motion with the conditions of 

 viscous or semifluid substances. 



That Glaciers are semifluids is not an absurdity. 

 The quantity of viscidity, or imperfect mobility in the par- 



