200 POPULAR LECTURES AND ADDRESSES. 



crust at fifty miles thick, or two hundred 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 appreci- 

 able degree of difference from the shape of hydro- 

 static equilibrium, and allow the water to indicate, 

 by relative displacement, its tendency to take the 

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

 the phenomena of tides. The geological inference 

 from this conclusion 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 dimensions ; and perhaps greater than that 

 of a globe of steel of the same dimensions. But 

 that it cannot be less rigid than a globe of glass, 

 we are assured. It is not to be denied that there 

 may be a very large space occupied by liquid. We 

 know there are large spaces occupied by lava ; but 

 we do not know how large they may be, although 

 we can certainly say that there are no such spaces, 

 as can in volume be compared with the supposed 

 hollow shell, occupied by liquid constituting the 

 interior of the earth. The earth as a whole 

 must be rigid, and perhaps exceedingly rigid, 

 probably rendered more rigid than it is at the 

 surface strata by the greater pressure in the 

 greater depths. 



