1862.] on the Motion of Glaciers. 411 



means of which a glacier preserves both the continuity of its motion 

 and that of its mass in a degree with which the external conditions 

 under which it is placed might, at first sight, appear to be totally 

 inconsistent. Attempts had been made to confound the explanation of 

 glacial motion which rests on the property of regelation, with that 

 afforded by the property of the assumed viscosity of ice according to 

 the viscous theory. The speaker first endeavoured to remove this 

 vague and erroneous impression by a more careful definition of the 

 term viscous, as expressing a determinate property of the aggregates of 

 material particles which constitute bodies. lie remarked that any 

 body might, in a greater or less degree, be extended by tension, com- 

 pressed by pressure, or angularly distorted by forces acting in couples. 

 The same might be asserted of every indefinitely small element of the 

 body, the volume of which would be increased, and its density 

 diminished by tension, and the opposite effects would be produced by 

 pressure, while the forces acting as couples would distort or twist the 

 element without changing its volume or density. It might be said in 

 general terms that, if the force required to produce any of these 

 changes of volume or form in the elements composing the body were 

 comparatively large, the body was solid; if the required force were 

 much smaller, the body might be termed plastic, or if smaller still, 

 viscous or semifluid. Rigidity would be the limit of solidity, and 

 perfect fluidity , that of imperfect fluidity, two limits which of course 

 were iiever attained in nature, since in the first case an infinite force 

 would be required to produce any relative displacements of the con- 

 stituent particles ; and in the second, such displacement might be pro- 

 duced by a force which should be infinitely small. Such definitions, 

 however, would be far too vague and indeterminate for our immediate 

 purpose. If we would designate by any of these terms determinate 

 properties of a body, we must endeavour to give to the definitions of 

 them a degree of determinateness corresponding to that required in 

 the properties to be expressed. When a body, or each of its com- 

 ponent elementary portions, was affected by pressures or tensions as 

 above described, such pressures or tensions, whether acting externally 

 or internally, were called ybrcf* of displacement. If such forces were 

 of sufficient magnitude, they would of course dislocate the mass on 

 which they acted ; but in these definitions they were not supposed 

 sufficient to produce any kind of dislocation, but only a distortion 

 from the forms which the body or its component elements would have, 

 if acted on by no external forces whatever. When thus distorted, 

 every element of the mass would exert a certain force to regain its 

 undistorted form and position. This force was usually termed the 

 force of restitution ; in its greatest limit it would be equal to, but was in 

 fact always less than the force of displacement. The ratio (less than 

 unity) which it bore to the former force, measured what is termed the 

 elasticity of the mass. But it was to be observed that this property of 

 a body might not only be different at different points of it, if the body 

 were not homogeneous, but that it might be different at the same point 



