344 Archdeacon Pratt on the Curvature of the Indian Arc. 



Fig. 4. 



7 



which there is a hollow in the under part of the crust, allowing 

 the heavy fluid to rise to such a height above its mean level that 

 the deficiency of weight of the ocean may exactly equal the excess 

 of weight of the heavy fluid below. Any departure from this 

 form would endanger a catastrophe. By introducing, then, the 

 principles of floatation to explain the cause of the exterior form 

 of the earth's surface, we come to the following conclusion re- 

 garding the inner surface of the crust. 



'(^i^c' 





/, ' / 'i •' ' 



?ftv 



Wherever the exterior surface of the crust rises into moun- 

 tains or sinks into ocean-beds, immediately beneath this the 

 inner surface takes a precisely similar but inverse form, so as 

 nearly to double the amount of increase or decrease in the thick- 

 ness of the crust ; and any decided departure from this law would 

 endanger a catastrophe. JN^ow the singularity of this curious 

 inverse law militates against its reality. But more especially 

 does it seem to contradict all notions of the process of solidifi- 

 cation by coohng down. We can conceive no reason why the 

 crust should have thickened so much less at ef than at ab and cd, 

 but quite the contrary. It might have been expected that at/ 

 the crust would have thickened downwards faster than at b and 

 d, till a pretty uniform thickness had been attained and kept up. 



11. I feel disposed, therefore, to hesitate about the existence 

 of a deficiency of matter below mountains, and rather to suppose 

 that the variations of the present external form of the earth have 



