1892.] liquid condition of the Earth's interior. 339 



The above appears to be a sufficient answer to the objection 

 brought against the theory of internal liquidity that in such a 

 case there could be no measurable ocean tides. 



Prof. Darwin appears when he wrote to have held the view 

 that the earth must be very rigid probably in consequence of his 

 investigation by which he had proved that on a solid globe 

 nothing short of a high degree of rigidity could sustain the 

 weight of continents and mountains. This necessity is of course 

 entirely removed by Airy's hypothesis that the crust is supported 

 in a state of approximate hydrostatic equilibrium on a yielding 

 nucleus*. 



Assuming therefore the necessity of a high degree of rigidity, 

 Darwin finds a certain coefficient of viscosity, which according to 

 his calculations would cause the obliquity of the ecliptic to 

 increase most rapidly at the present time (p. 526), and uses this 

 particular value in his numerical calculations. Thus, when he 

 estimates the length of time, since the moon may have been 

 detached from the earth, at about 57 million years -J*, the estimate 

 depends upon that particular value of the viscosity. So also do 

 his estimates of the amounts of heat generated by tidal friction 

 within the earth during certain intervals of time dating from the 

 same epoch. And in short all the numerical results in Table IV. 

 at p. 494, depend upon the particular assumed high degree of 

 viscosity. It cannot therefore be too carefully borne in mind by 

 Geologists that none of those numerical estimates, which relate 

 to time, are applicable to the case of a liquid interior. 



With respect to the obliquity of the ecliptic, it seems probable 

 that it may have originated when the moon broke away from the 

 earth, however much the amount of it may have since changed ; 

 for the rupture must have occurred at what was then the equator ; 

 but the alteration in the principal axes of the earth owing to its 

 removal must have caused the axis of rotation to shift its place 

 within the mass, so that the plane of the moon's orbit would 

 represent that of the original equator, while the plane of the new 

 equator would have become oblique to it. 



Although, as just mentioned, the amounts of heat generated 

 in the earth during certain intervals of time depend upon a 

 particular assumed value for the viscosity, not so the whole 

 amount since the rupture. Darwin says "According to the 

 present hypothesis [of the generation of the moon] looking for- 

 ward in time [from that epoch], the moon-earth system is from 

 a dynamical point of view continually losing energy from the 



* Phil. Trans. Roy. Soc, vol. 14.5, p. 101. See also a lecture by Sir G. B. Airy 

 "On the interior of the Earth." Nature, vol. 18, p. 41, 1878. 



t See "Precession of a viscous spheroid and remote history of the earth." 

 Phil. Trans. Pt. n. p. 531, 1879. 



