196 SEE— FURTHER RESEARCHES ON [April 24, 



In the "Nat. Phil.," Vol. I, part II, §832, Lord Kelvin and 

 Professor Tait remark that 



" The precise circumstances under which elastic bodies break have not 

 hitherto been adequately investigated by experiment. It seems certain that 

 rupture cannot take place without difference of stress in different directions. 

 One essential element therefore is the difference between the greatest and 

 least of the three principal stresses. How much the tendency to break is 

 influenced by the amount of the intermediate principal stress is quite un- 

 known. The difference between the greatest and least stresses may however 

 be taken as the most important datum for estimating the tendency to break. 

 This difference has been called by JNIr. G. H. Darwin (to whom the investi- 

 gation of which we speak is due) the 'stress-difference.'" 



Stress-difference is a term which when applied to matter within 

 the earth denotes the tendency to flow. For rupture is not possible 

 when the matter is in confinement under such pressure and at high 

 temperature. Now if the earth were homogeneous, as assumed in 

 Darwin's inquiry, the inequalities of surface due to the mountains, 

 plateaus, and continents would give rise to a stress-difference in 

 the underlying layers ; and Darwin showed that the stress-difference 

 would increase with the depth, being at the center, for inequalities 

 of the type represented by harmonics of the second order, eight 

 times what it is at the surface. 



If the earth were not eft'ectively solid throughout, a flow ought 

 to take place either near the surface or at greater depth ; and thus 

 the inequalities of surface would disappear. But the plateaus and 

 mountains do not sink in, and this fact proves that the globe is not 

 fluid, and even that the plastic or viscous layer just beneath the crust 

 is quite stiff. As we have seen that the rigidity increases very 

 rapidly towards the center, we easily see why movement should 

 not occur at great depth, since the rigidity there exceeds that of 

 any known substance, and at the centre is about three times that 

 nickel steel used in armor plate. 



In the paper on the " Temperature of the Earth " we have shown 

 from the evidence of stability aft'orded by geological pinnacles 

 millions of years old, that no movements of deep seated character 

 occur within the earth. This evidence supports the view that the 

 earth is effectively solid, and has behaved as such since the con- 

 solidation of the crust. 



