ATTRACTION IN THE CASE OF THE ENGLISH ARC. 
51 
Postscript. 
Since the above was written, I have had the opportunity of seeing a notice of the 
communication of the Astronomer Royal on the density of table-lands supposed to 
be supported by a dense fluid or semifluid mass, and the use he makes of his sugges- 
tions to remove the discrepancy, pointed out in my first communication, between the 
values of the deflection of the plumb-line in India, as determined by calculating the 
attraction of the Himalayas, and as indicated by the results of the Great Trigono- 
metrical Survey. The following difficulties occur to me in the way of this highly 
ingenious and philosophical method of removing the discrepancy; — 
1. It assumes that the hard crust of the earth is sensibly lighter than the fluid or 
semifluid 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 consolidating would expand and become lighter. One would rather expect 
it to become denser, by loss of heat and mutual approximation 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. Bab- 
bage 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 thick- 
ness of the crust be correct, viz. at least 1000 miles, these laws of change in the sur- 
face 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 principles of hydro- 
statics would occasion. Indeed, it seems to me highly probable, that the elevation 
of the Himalayas and the vast regions beyond may have taken place altogether from 
a slow upheaving force arising from this cause. 
I am inclined to think that the only explanation of tlie discrepancy between my 
calculation and the results of the Indian Survey is to be found in the greater curva- 
ture of the Indian arc. 
London, June 2, 1855. 
Note added after the reading of the Paper. 
A further difficulty arises from the peculiar law which the thickness of the crust 
must follow, if the present form of the surface arises altogether from hydrostatic 
principles. For if an excess of matter at the surface, in table-lands and mountains, 
implies a deficiency of matter below and therefore a protrusion of the light crust 
