443 



culties occur to me in the way of this highly ingenious and philoso- 

 phical method of removing the discrepancy : 



" 1. It assumes that the hard crust of the earth is sensibly lighter 

 than the fluid or semi-fluid mass, imagined to be a few miles below 

 the surface. But I know of no law, except the unique law of water 

 and ice, which would lead us to suppose that the fluid mass in con- 

 solidating would expand and become lighter. One would rather 

 expect it to become denser, by loss of heat and mutual approxima- 

 tion of its particles. 



"2. There is, moreover, every reason to suppose, that the crust 

 of the earth has long been so thick, that the position of its parts 

 relatively to a mean level cannot be any longer subject to the laws 

 of floatation. If the elevations and depressions of the earth's surface 

 have always remained exactly what they were at the time when the 

 laws of floatation ceased to have an uncontrolled effect, then the 

 same reasoning would no doubt apply in our case, as if they still had 

 their full sway. But geology shows that other laws are in constant 

 operation (arising most probably, as Mr. Babbage has suggested, 

 from the expansion and contraction of the solid materials of the 

 crust), which change the relative levels of the various parts of the 

 earth's surface, quite irrespectively of the laws of floatation. If 

 Mr. Hopkins's estimate of the thickness of the crust be correct, viz. 

 at least 1000 miles, these laws of change in the surface must have 

 been in operation for such an enormous interval of time, as quite to 

 obliterate any traces of the form of surface which the simple prin- 

 ciples of hydrostatics would occasion. Indeed, it seems to me highly 

 probable that the elevation of the Himalayas and the vast regions 

 beyond may have arisen altogether from the slow upheaving force 

 arising from this cause. 



" I am inclined to think that the only explanation of the discre- 

 pancy between my calculations and the results of the Indian Survey, 

 is to be found in the greater curvature of the Indian Arc." 



VOL. VII. 2 T 



