Professor Forbes on the Leading Phenomena of Glaciers, 233 



Now, it is by observations on the glacier itself that we can 

 best make experiments on great masses of ice, as here sug- 

 gested. 



The Motion of a Glacier resembles that of a Viscous Fluid. 



All experimental philosophers are agreed as to the facts, that 

 a fluid like water, heavy and slightly viscid, moves down an 

 inclined plane or canal, with a velocity which varies according 

 to the slope, and which varies also from point to point of the 

 section of the stream. The part of the stream which moves 

 fastest is the surface, and especially the central part of the 

 surface. The velocity of motion diminishes on the surface 

 from the centre to the sides, and/rom the surface towards the 

 bottom. 



The cause of these variations is admitted to be the friction 

 of the sides and bottom of the canal or bed, which retards the 

 fluid particles immediately in contact with them, and the ad- 

 hesion of these particles to their neighbours, that is, their vis- 

 cosily^ communicates this retardation by certain gradations, 

 which are not correctly known, to the interior mass of the 

 fluid. Hence, — 



I. The centre and top of the stream move faster than the 

 sides and bottom, especially if the friction of the fluid particles 

 over one another he less than their friction against the sides of 

 the canal. If this be not the case — if the friction of the con- 

 tained mass against the containing or supporting walls be less 

 than the friction which exists amongst its own particles, the 

 mass will slide out of its bed^ and will so far act as a solid body. 

 If it have a certain mobility amongst its own particles, it will, 

 whilst sliding over its bed, alter, at the same time, the relative 

 position of its own particles ; — it will move partly as a solid, 

 partly as a fluid. We may then fairly call it a semifluid or a 

 semisolid. 



II. From this it also evidently appears, that the greater the 

 viscosity of the fluid, the farther will the lateral and funda- 



greater part of the present volume was written, before I succeeded in obtaining 

 access to M. Rendu's work, in the 10th volume of the Memoirs of the Academy of 

 Ckamberif, which I owe at length to the kindness of the right reverend author. 



