492 Mr. J. Prescott on the 



Then 5 



ra = C — k D 



ft + 5 



=4-5 + 



20-8 

 on substituting for n and D. 



Now the densest materials we know in the earth's crust 

 nave specific gravities only about 22 or 23. It is highly 

 probable then that the density at the earth's centre is not 

 more than 30. The fact that we find in the earth's crust matter 

 so dense as platinum and osmium mingled with much lighter 

 substances, rather indicates that the density at the centre is 

 not nearly so great as 30. For, if substances of such dif- 

 ferent densities are to, be found within a few feet of the 

 earth's surface, it is very likely that lighter matter will be 

 mingled with the denser in the interior. But at the same 

 time there must be a gradual increase of density from the 

 surface towards the centre. 



When we have determined m the value of B can be 

 calculated from the equation 



B -3I6 ? 4 R5 



= — R 5 . 

 316 



Substituting for B in equation (a), obtained from the 

 rotation of the earth, we find 





g 4-^ m E 5 1 

 6 Ra> 2 ~ 5 316 RV + 2 





3 1 m KE 1 1 

 "5 316* 5^ W *Ko) 2 + 2 





3 1 m g 1 



Now 



^=289. 



Hence 



3m , 1 1 





5x316x5-5 2 2«y* 



