410 



R. M. Deeley — Viscous Floio of Glacier-Ice. 



When saturated with water many sands, clays, and loams do become 

 very plastic, and tiow almost like viscous fluids. Excellent instances 

 of such flow may be seen at the base of the Cromer cliifs. 



We must regard the solid crust of our earth as being hard, truly 

 plastic, or plastic under great pressure, not viscous. Except as the 

 result of denudation, or from the action of internal forces, which 

 occasionally bring the stresses above the limit of perfect elasticitj'', 

 our mountain ranges are stable and not tending to level up as they 

 would do if composed of even a much more viscous substance than 

 pitch. To quote Maxwell,^ "A tallow candle is much softer than 

 a stick of sealing-wax ; but if the candle and the stick of sealing- 

 wax are laid horizontally between two supports the sealing-wax 

 will in a few weeks in summer bend with its own weight, while 

 the candle remains straight. The candle is, therefore, a soft solid, 

 and the sealing-wax a very viscous liquid." 



The term viscosity, however, has been used to indicate a property 

 of solids of quite another kind. That it should have been used in 

 such a way is unfortunate, for the property known as solid viscosity 

 is quite different from that of a liquid. To make this quite clear 

 the phenomenon presented by a solid substance under the action of 

 a stress will be considered before treating in detail the conditions 

 of liquid flow. 



The space between two planes. Fig. 1, one of which YY is free 

 and the other X X fixed, we will regard as being occupied by a solid 

 substance. Under a given stress, acting along the plane F F in the 

 direction of the arrow, the solid undergoes an elastic shear equal, 

 we will say, to the displacement of Bi to Aj^. But, the stress 

 remaining constant, it will be found that with the lapse of time 

 this displacement slowly increases in amount, especially if the solid 

 be set in vibration, and only reaches a maximum after a very 

 considerable interval has elapsed. On removing the stress the plane 

 returns at once to almost its original position. But the last traces 

 of distortion only disappear after the lapse of many hours. Indeed, 

 in many instances the original configuration is never regained, the 

 behaviour of the material as it were depending upon its previous 

 history. 



We will now consider the conditions of flow of a thin stratum 

 of a viscous liquid placed between the planes X X and Y Y, the 



1 " Theory of Heat," 1894 edition, p. 303. 



