320 KELVIN 



weighed and found wanting in conformity to the truth. On 

 a great many different essential points it has been 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 (Fie. 134, p. 314) represents a solid shell 

 with water outside, you can understand if the solid shell 

 yields with sufficiently great freedom, there will be exceed- 

 ingly 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 

 increased 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 crust at fifty miles thick, or two hun- 

 dred and fifty thousand feet, and with these proportions 

 make the calculation, we find that something scores of times 

 more rigid than steel would be required to keep the shape so 

 well as to leave any appreciable degree of difference from 

 the shape of hydrostatic equilibrium, and allow the water 

 to indicate, by relative displacement, its tendency to take 

 the figure of equilibrium; that is to say, give to us the 

 phenomena of tides. The geological inference from this con- 

 clusion is, that not only must we deny the fluidity of the 

 earth and the assertion that it is encased by a thin shell, 

 but we must say that the earth has, on the whole, a rigidity 

 greater than that of a solid globe of glass of the same 



