the Earth's Crust. 



351 



otherwise this state of things could not exist. If the advocates 

 of the theory that the crust has expanded in becoming solid say 

 that I have considered the weight of m r in my first diagram too 

 great, what will they say of the density of m s in my second dia- 

 gram ? In order to nullify the buoyancy, the density ought to 

 be greater than that of the crust under the continent. But by 

 what process could it become denser, if it became at first lighter 

 in becoming: solid ? 



13. Another means of determining the inferior limit of the 

 thickness of the crust, seems to be suggested by a consideration 

 of the varying pressure the attraction of the moon and sun must 

 cause the fluid in the interior^ of the earth to exert against the 

 crust. If the crust were very thin, this varying pressure might 

 cause it to crack. In the following calculation I show that, to 

 resist the force caused by the moon alone, the crust should be at 

 least 100 miles thick. The force of the sun is to that of the 

 moon as 2 : 5. Hence to counteract their combined efi'ect at 

 new moon, the thickness should be at least 140 miles. 



14. In the following diagram, let E be the centre of the earth 



Fig. 3. 



^3j 



supposed to be a sphere, consisting of a crust of uniform thick- 

 ness and with fluid within. E A is the radius directed towards 

 the moon M. I propose to find the increase of pressure of the 



