﻿396 Scientific Intelligence. 



If such external evidence be gone,* we must rely on the incom- 

 patibility of the known value of the precessional constant with an 

 ellipticity of internal strata of equal density greater than that 

 appropriate to the actual ellipticity of the surface. Might there 

 not be a considerable excess of internal ellipticity without our 

 being cognizant of the fact astronomically? 



And, further, have we any right to feel so confident of the in- 

 ternal structure of the earth as to be able to allege that the earth 

 would not through its whole mass adjust itself almost completely 

 to the equilibrium figure? 



Tresca has shown in his admirable memoirs on the flow of 

 solids that when the stresses rise above a certain value the solid 

 becomes plastic, and is brought into what he calls the state of 

 fluidity. I do not know, however, that he determined at what 

 stage the flow ceases when the stresses are gradually diminished. 

 It seems probable, at least, that flow will continue with smaller 

 stresses than were initially necessary to start it. But if this is 

 so, then, when the earth has come to depart both internally and 

 externally from the equilibrium condition, a flow of solid will set 

 in, and will continue until a near approach to the equilibrium 

 condition is attained. 



When we consider the abundant geological evidence of the 

 plasticity of rock, and of the repeated elevation and subsidence 

 of large areas on the earth's surface, this view appears to me more 

 probable than Sir William Thomson's. 



On the whole, then, I can neither feel the cogency of the argu- 

 ment from tidal friction itself, nor, accepting it, can I place any 

 reliance on the limits which it assigns to geological history. 



The second argument concerning geological time is derived 

 from the peculiar cooling of the earth. 



We know in round numbers the rate of increase of temperature, 

 or temperature gradient, in borings and mines, and the conduc- 

 tivity of rock. These data enable us to compute how long ago 

 the surface must have had the temperature of melting rock, and 

 when it must have been too hot for vegetable and animal life. 



Sir William Thomson, in his celebrated essay on this subject 

 (republished in Thomson and Tait's " Natural Philosophy," 

 Appendix D), concludes from this argument that "for the last 

 96,000,000 million years the rate of increase of temperature under- 

 ground has gradually diminished from about- -^th to about -g^th 

 of a degree Fahrenheit per foot. ... Is not this, on the whole, 

 in harmony with geological evidence, rightly interpreted ? Do 

 not the vast masses of basalt, the general appearances of moun- 

 tain-ranges, the violent distortions and fracture of strata, the great 



* I find by a rough calculation that Mjths of the land in the northern hemis- 

 phere are in the equatorial half of that hemisphere, viz., between 0° and 30° N. 

 lat. ; and that f-fths of the land in the southern hemisphere are in the equatorial 

 half of that hemisphere, viz., between 0° and 30° S. lat. Thus for the whole 

 earth, |§£ths of the land lie in the equatorial half of its surface, between 30° N. 

 and S. lat. In this computation the Mediterranean, Caspian and Black Seas are 

 treated as land. 



