Mr. Hopkins on the Mecha7iism of Glacial Motion. 247 



7j -\ — - — lz)lxly will be the forces acting respectively 



on the two sides of the element parallel to the plane of xy. 

 Also let y denote the tangential force on the element in a di- 

 rection parallel to the axis of z ; then will 2/ {I xlz + '^yZ z) 

 be the whole tangential force on the element parallel to z. 

 Hence we must have 



or -^|=2/(8.r + 8j/). 



Consequently, Ix and ly being indefinitely small, -^ — must 



be indefinitely less than f, whence it will also follow that the 

 same must hold with respect to Z. Hence, if the surfaces of 

 the contiguous elementary parallelopipeds be capable of ex- 

 erting a finite tangential force {/) on each other, it would 

 require an indefinitely great force (Z) to make contiguous 

 elements move in opposite directions. Consequently, the 

 relative motion of contiguous particles here considered will 

 be impossible, not only when the mass has any considerable 

 cohesion, but also when there exist planes of no cohesion in 

 the direction of extension, provided those planes be very nu- 

 merous, and the friction along them be not very small *. I 

 maintain, therefore, that separation plafies cannot be produced 

 in glaciers in this manner. 



7. In comparing the slidiiig and viscous theories, with refer- 

 ence to the causes assigned by them respectively for the mo- 

 tion of glaciers, we may observe, that they agree in assigning 

 gravity as the primary cause of that motion ; but in the one 

 theory it is contended that the efficiency of gravity is princi- 

 pally due to the state of disintegration of the lower surface of 

 the glacier, while in the other it is maintained that this effi- 

 ciency is due to the plasticity of the general mass. At the 

 same time it will not, I presume, be denied by the advocates 

 of the viscous theory that some sliding does take place over 

 the bed of the glacier, of which, in fact, the polished and 

 striated rocks associated with glaciers affijrd the most indu- 

 bitable proof; and on the other hand it will be admitted, that 



* Most persons will be aware from experience of the difficulty of draw- 

 ing out one bundle of rods from another, when intermingled and arranged 

 as supposed above. I have endeavoured to test the conclusion of the text 

 by stretching elastic substances, as for example, a piece of sealing-wax, but 

 have never detected the slightest indication of the relative motion there 

 spoken of. 



