THE TIDES. (A PP. A.) 199 



found at variance with the truth. One of these 

 points is, that unless the material of this supposed 

 shell were preternaturally rigid, were scores of 

 times more rigid than steel, the shell would yield so 

 freely to the tide-generating forces that it would 

 take the figure of equilibrium, and there would be 

 no rise and fall of the water, relatively to the solid 

 land, left to show us the phenomena of the tides. 



Imagine that this (Fig. 30, p. 190) represents a 

 solid shell with water outside, you can understand if 

 the solid shell yields with sufficiently great freedom, 

 there will be exceedingly little tidal yielding left 

 for the water to show. It may seem strange when 

 I say that hard steel would yield so freely. But 

 consider the great hardness of steel and the smaller 

 hardness of india-rubber. Consider the greatness 

 of the earth, and think of a little hollow india- 

 rubber ball, how freely it yields to the pressure of 

 the hand, or even to its own weight when laid on a 

 table. Now, take a great body like the earth : the 

 greater the mass the more it is disposed to yield to 

 the attraction of distorting forces when these forces 

 increase with the whole mass. I cannot just 

 now fully demonstrate to you this conclusion ; 

 but I say that a careful calculation of the 

 forces shows that in virtue of the greatness of 

 the mass it would require an enormously in- 

 creased rigidity in order to keep in shape. So 

 that if we take the actual dimensions of the 

 earth at forty-two million feet diameter, and the 



