i9o8.] THE PHYSICS OF THE EARTH. 183 



two-thirds of that which it would be were the rigidity perfect; while, if the 

 rigidity were no greater than that of a globe of glass, the relative rise and 

 fall would be only two-fifth of that on a perfectly rigid globe. 



" Imperfect as the comparison between theory and observation as to the 

 actual height of the tides has been hitherto, it is scarcely possible to be- 

 lieve that the height is only two fifths of what it would be if, as has been 

 universally assumed in tidal theories, the earth was perfectly rigid. It seems, 

 therefore, nearly certain, with no other evidence than this afforded by the 

 tides, that the tidal effective rigidity of the earth must be greater than that 

 of glass. This is the result taking the earth as a globe uniformly rigid 

 throughout. That a crust fifty or a hundred miles thick could possess such 

 preternatural rigidity, as to give to the mass, part solid and part liquid, a 

 rigidity as a whole, equal to that of glass or steel is incredible; and we 

 are forced to the conclusion that the earth is not a mere thin shell filled 

 with fluid, but is on the whole or in great part solid." (Paper read to 

 Geological Society of Glasgow, February 14, 1878; Kelvin's "Popular Lec- 

 tures and Addresses," Vol. II, pp. 317-318.) 



In his presidential address to the iklathematical and Physical section of 

 the British Association at Glasgow, September 7, 1876, Lord Kelvin remarked 

 of the earth's crust that " were it of continuous steel and 500 kilometers 

 thick, it would yield very nearly as much as if it were india rubber to the 

 deforming influences of centrifugal force and of the sun's and moon's at- 

 tractions." " The solid crust would yield so freely to the deforming influence 

 of sun and moon that it would simply carry the waters of the ocean up and 

 down with it, and there would be no sensible rise and fall of water relatively 

 to the land." (" Popular Lectures," Vol. II., pp. 251-2.) 



Lord Kelvin's final conclusion was that " the earth as a whole is 

 certainly more rigid than glass, but perhaps not quite so rigid as 

 steel." 



§ 15. Darzi'iji's Researches on the Tidal Method of Evaluating 

 the Earth's Rigidity. — As the natural successor of Lord Kelvin in 

 the researches on the physics of the earth. Professor Sir G. H. 

 Darwin took up the problem of the earth's internal physical con- 

 dition and confirmed and extended these conclusions by several 

 important lines of inquiry. Darwin's researches on the bodily tides 

 of viscous and semi-elastic spheroids and on the oceanic tides upon 

 a yielding nucleus tended to strengthen the argument for a high 

 effective rigidity so decidedly that he concluded that " no very 

 considerable portion of the interior of the earth can even distantly 

 approach the fluid condition." 



But whilst Darwin's researches confirmed Kelvin's conclusions 

 as to the great effective rigidity of the earth, yet a more critical 



