200 SEE— TEMPERATURE, SECULAR COOLING [April 20, 



the globe behaves throughout as a soHd ; and it must be held there- 

 fore that the matter in confinement acts as an seolotropic elastic solid, 

 though if released from pressure it would instantly expand as vapor, 

 owing to the enormously high temperature. 



In the paper on the " Cause of Earthquakes," however, it is 

 shown that notwithstanding this general law for the globe as a 

 whole, there is a thin layer just beneath the crust which in seismic 

 disturbances behaves as a fluid; and the disastrous shaking of the 

 earth is due mainly to the enforced movement of currents in this 

 layer. Yet this substratum of fluid is under such great pressure 

 beneath the confining crust that the compressed lava transmits earth- 

 quake waves almost as if the globe were solid from the surface to 

 the centre. Thus even this viscous substratum, under the least 

 pressure of any of the heated matter within the globe, is sufficiently 

 rigid to transmit to us faithfully the waves of compression and dis- 

 tortion which have been communicated to it. It is held that the 

 actual rigidity may be greater than that calculated from the pressure, 

 but it can not be less. We are therefore safe in following the rule 

 that the rigidity is everywhere proportional to the pressure, and this 

 gives us a definite view of the condition of the matter in the differ- 

 ent layers. 



Mr. R. D. Oldham, F.R.S., has shown that, so far as the propa- 

 gation of earthquake waves enables one to judge {Quarterly Journal 

 of the Geological Society, August, 1906), the matter of the earth's 

 interior is essentially homogeneous down to within 0.4 of the radius 

 from the centre, where some change appears to take place. This 

 probably indicates that the nucleus is a magma of all the elements, 

 with the density and rigidity increasing towards the center, owing 

 to the augmentation of pressure; but the cause of the discontinuity 

 about 0.4 of the radius from the center is not yet understood. 



On the whole the most remarkable feature of the earth's consti- 

 tution is the great increase of pressure towards the center. This 

 gives our encrusted planet enormous rigidity and effective strength, 

 or tenacity, to withstand any disrupting force. The average pressure 

 or theoretical rigidity of all the layers composing the earth's mass 

 is (cf. A. N., 4104) : 



