Archdeacon Pratt on the Curvature of the Indian Arc. 343 



that the table-land is supported by the buoyancy of a mass of 

 crust projecting downwards into the heavy fluid. This repre- 

 senting the state of things, a mass ad rising above the surface 

 and attracting a distant station A, necessarily implies a corre- 

 sponding deficiency of mass in the locality be, causing an almost 

 equal negative attraction at A counteracting the former positive 

 attraction. 



9. There are some difficulties in the way of this ingenious 

 hypothesis which I wish now to state. 



1st. On what physical principle can it be assumed that the 

 crust of the earth is lighter than the fluid immediately below it ? 

 It might rather be imagined, that by the process of consolidation 

 in using heat it would become contracted and therefore heavier. 



2nd. If the result at which Mr. Hopkins has arrived in his 

 calculations on precession be correct, that the solid part of the 

 crust is at least 1000 miles thick, an enormous intei"val of time 

 must have elapsed since the crust was sufficiently thin for the 

 form of its surface to be affected by the fluid below. Duiing 

 this immense interval, there has been abundance of time for the 

 geological law, of which I have spoken at the beginning of this 

 paper, to have altered the form of the surface, originally given 

 by hydrostatic principles, to such a degree as to have obliterated 

 all traces of it ; indeed, the whole table-land and all the moun- 

 tain range may have been heaved up dui'ing this time. 



10. But I have another, a third difficulty. If the crust be so 

 thin, or was so when the earth assumed its present contour, what 

 must be the state of the crust beneath deep and wide oceans ? 



Fi^. 3. 



■Xhicrii3l-';'/Ay-r - . :' ^' ^'{;^'-^"~l\^r'^5 ''- - v.'' '"^-.' '-'I'i-'f;;-;- 



The state of things in fig. 3 could no more exist than that of 

 fig 1. If we take the width of the ocean to be only 100 miles, 

 and average depth 2 miles, and density half that of the crust, the 

 cohesive force necessary to prevent the upward pressure of the 

 iaternal fluid from breaking up the crust and letting in the ocean, 

 is a force equal to the weight of about 14 miles of rock. This 

 state of things cannot exist, and we must substitute fig. 4, in 



