1869.] of the Descent of Glaciers by their Weight only. 203 



Bishop of Annecy, has since been confirmed by the measurements of 

 Agassiz, Forbes, and Tyndall. There is a constant displacement of the par- 

 ticles of the ice over one another, and alongside one another, to which is 

 opposed that force of resistance which is known in mechanics as shearing 

 force. 



By the property of ice called* regelation, when any surface of ice so 

 sheared is brought into contact with another similar surface, it unites with 

 it, so as to form of the two, one continuous mass. Thus a slow displace- 

 ment of shearing, by which different similar surfaces were continually 

 being brought into the presence and contact of one another, would exhibit 

 all the phenomena of the motion of glacier ice. 



Between this resistance to shearing and the force, whatever it may be, 

 which tends to bring the glacier down, there must be a mechanical rela- 

 tion, so that if the shearing resistance were greater the force would be in- 

 sufficient to cause the descent. The shearing force of cast iron, for in- 

 stance, is so great that, although its weight is also very great, it is highly 

 improbable a mass of cast iron would descend if it were made to fill the 

 channel of the Mer de Glace, as the glacier does, because its weight would 

 be found insufficient to overcome its resistance to shearing, and thus to 

 supply the work necessary to those internal displacements, of which a 

 glacier is the subject, or even to shear over the irregularities of the rocky 

 channel. The same is probably true of any other metal. 



I can find no discussion which has for its object to determine this me- 

 chanical relation between what is assumed to be the cause of the descent 

 of a glacier, and the effect produced,— to show that the work of its weight 

 (supposing that alone to cause it to descend) is equal to the works of the 

 several resistances, internal and external, which are actually overcome in its 

 descent. It is my object to establish such a relation. 



The forces which oppose themselves to the descent of a glacier are, 

 — 1st, the resistance to the sliding motion of one part of a piece of solid 

 ice on the surface of another, which is taking place continually throughout 

 the mass of the glacier, by reason of the different velocities with which 

 its different parts move. This kind of resistance will be called in this 

 paper (for shortness) shear, the unit of shear being the pressure in lbs. 

 necessary to overcome the resistance to shearing of one square inch, which 

 may be presumed to be constant throughout the mass of the glacier. 



2ndty. The friction of the superimposed laminae of the glacier (which 

 move with different velocities) on one another, which is greater in the 

 lower ones than the upper. 



3rdly. The resistance to abrasion, or shearing of the ice, at the bottom 

 of the glacier, and on the sides of its channel, caused by the roughnesses 



sack." " This," says Mr. Cowell, from whose paper read before the Alpine Club in 

 April 1864 the above quotation is made, " is not an isolated example of the scattering 

 that takes place in or on a glacier, for I myself saw on the Theodule Glacier the remain! 

 of the Syndic of Val Tournanche scattered over a space of several acres." 



Q 2 



