INTERNAL CONDITION OF THE EARTH. 317 



to resist. The crust would then be subject to up- 

 heavals and depressions taking place in time with 

 the revolutions of the sun and moon. If the crust 

 yielded perfectly, there would be no tides of the sea, 

 no rising and falling relatively to the land, at all. 

 The water would go up and down with the land, 

 and there would be no relative movement ; and in 

 proportion as the crust is less or more rigid the 

 tides would be more or less diminished in magni- 

 tude. Now we cannot consider the earth to be 

 absolutely rigid and unyielding. No material that 

 we know of is so. But I find from calculation * that 

 were the earth as a lukole not more rigid than a 

 similar globe of steel the relative rise and fall of the 

 water in the tides would be only H 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 -r- of 

 that on a pefectly 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 



3 Thomson and Tait, Natural Philosophy, Vol. I., 842. 



