190 POPULAR LECTURES AND ADDRESSES. 



bottom. The earth's crust would yield to and 

 from the moon, and the water would not move at 

 all relatively to the crust. If the earth were even 

 as rigid as glass all through, calculation shows that 

 the solid would yield so much that the tides could 

 only be about one third of what they would be if 

 the earth were perfectly rigid. Again, if the earth 

 were two or three times as rigid as glass, about as 

 rigid as a solid globe of steel, it would still, con- 



FIG. 30. Elastic Tides. 



sidering its great dimensions, yield two or three feet 

 to that great force, which elastic yielding would 

 be enough to make the tides only twc thirds of 

 what they would be if the earth were perfectly 

 rigid. Mr. G. Darwin has made the investigation 

 by means of the lunar fortnightly tides, and the 

 general conclusion, subject to verification, is that the 

 earth does seem to yield somewhat, and may have 

 something like the rigidity of a solid globe of steel. 



