Dr. Harold Jeffreys — Causes of Mountain- Building. 217 



by means of the widespread movements that one associates with 

 such readjustment. This hypothesis, while doubtless an important 

 secondary agency, can therefore scarcel)^ be regarded as giving the 

 primary cause of the formation of the great mountain chains. 



A digression in the meanings of the terms rigidity, plasticity, 

 viscosity, and^uidity may not be out of place here. When a body is 

 exposed to a tangential or shearing stress (such a stress, for instance, 

 as would exist if a rectangular block of Avood had one face rigidly 

 fixed and a tension were applied in the plane of the opposite one) it 

 ordinarily changes its shape to some extent. The ideal type of sub- 

 stance described as perfectly elastic will spring back completely and 

 immediately to its original shape when the stress is removed (the 

 complication due to inertia being ignored). In ordinary substances, 

 however, the recovery may be incomplete, gradual, or absent. If it 

 occurs at all, the substance possesses elasticity, and the coefficient of 

 rigidity is measured by the ratio of the stress to the reduction of strain 

 when the stress is released. If it is absent, so that the body retains 

 the shape it had just before the release, the substance is a fluid. 

 Thus a fluid is a substance with no elasticity. One of its properties 

 is that when a constant tangential stress is applied for some time it 

 goes on changing its shape or flowing at a constant rate ; the viscosity 

 is measured by the shearing stress needed to cause flow at a certain 

 definite rate. When the viscosity is zero, the substance yields 

 completely and instantly to any shearing stress, however small ; such 

 a substance is o. perfect fluid. 



Substances possessing any elasticity are called solids. Imperfection 

 of elasticity in a solid may be shown by the recovery after stress 

 being partial or gradual. If it is partial, we see that the body has 

 undergone a permanent change of shape, called permanent set ; such 

 a substance is called plastic. Suppose now that a certain stress i^ 

 applied for a certain time T causes a permanent set of amount s, 

 where s may be small. If the body is then stressed again by the 

 same amount and for the same time and released, it will have 

 acquired a further permanent set s, making 25 in all. Repeating 

 the process n times will give a set ns. Now a constant stress F 

 Avould naturally be expected to produce at least as great an effect in 

 the same time as an intermittent one ; hence a stress F applied 

 for a time equal to the sum of nT, and the times when there was no 

 stress would produce a permanent set not less than ns. This can be 

 made as great as we like by making n great enough ;' hence a plastic 

 solid can be made to change its shape as much as we like by applying 

 a constant stress for a long enough time. In this respect it therefore 

 resembles a viscous fluid.' 



' Mr. Deeley suggests that the repetition of the stress would not give the 

 same set each time, but a smaller one, leading to a convergent series which 

 could never exceed a certain small amount. This implies that when a body 

 has been strained beyond the limits of recovery it is stronger and more difficult 

 so to strain again. The contrary seems more probable, and agrees with 

 the phenomena of malleability and ductility, which are inconsistent with 

 Mr. Deeley's hypothesis. Clay can undergo an indefinite amount of 

 permanent set, and if the stress needed to give it a definite increase of set 



