OE THE PLUIMB-LINE IN INDIA. 747 
untenable ioi- three reasons :—(l) It supposes the thickness of the earth’s solid crust 
to be considerably smaller than that assigned by the only satisfactory physical calcula- 
tions made on the subject— those by Mr. Hopmxs of Cambridge. He considers th<. 
thickness to be about 800 or 1000 miles at least. (2) It assumes that this thin crust 
IS lighter than the fluid on which it is supposed to rest. But we should expect that in 
becoming solid from the fluid state, it ivould contract by loss of beat and become 
leaner, (o) The same reasoning by which Mi-. Airy makes it appear that every pro- 
tu erance oihside this thin crust must be accompanied by a protuberance inside, down 
into the fluid mass, ivould equally prove that wherever there was a hollow-, as in deep 
seas, in the outward surface, there must be one also in the inner surface of the crust 
corresponding to it,- thus leading to a law of varying thickness which no process of 
cooling could have produced. 
§ 2. HjjiMtliesis of Deficieney of Matter adopted in this Paper. 
.3 It is nevei-theless to this source— I mean a Deficiency of Matter below— that we 
ook, I feel fully assured, for a compensating cause, if any is to be found. My 
piesent object is to propose another hypothesis regarding deficiency of matter below 
tue mountaiii-niass, as first sus’s'estcd hv AIt 4ipv* onri i/-. -i.rvi i -> • 
test of calculation. liypothes.s to the 
I wdl here observe, that all the more laborious numerical calculations in this Papei- 
lave been perfonned for me-as I could not possibly find leisure for the work— 
by a pmctised Computer of the Great Trigonometrical Survey Office in Calcutta 
obligingly recommended to me by the Deputy Surveyor-General. In the course of the 
pre.,ent paper the attraction of the Mountain Mass, in the direction of the meridian at 
the three pi-inc.pal stations of the Great Arc-Kaliana, Kalianpur, and Damargidi- 
™ again calculated, and a test so far afforded of the general accuracy of my Lmer 
the'til '"V'T, l^ceeds. At 
perfect 1 r been a 
pe.fect splieioid, with no mountains and valleys nor ocean-hollow-s. As the crust formed 
oritfrt“"‘”’“f expansions may have taken place in an; 
of Its paits, so as to depress and elevate the corresponding portions of the surface If 
he.se changes took place chiefly in a vertical direction, then at any epoch a verM lii 
drawn dowm to a sufficient depth from any place in the surface will pL thi-ough 1 mZ 
of matter which has remained the same in amount all through the changes. By the 
die lel h?“T" ‘f' "P’ «‘us raised above 
is most likdv^' cori-esponding attenuatim, of matter below. This attenuation 
cause wfill P^bably e.xists through a great depth. Whether this 
rmitlin iT ! “f compensation can be determined only by 
submitting it to calculation, which I proceed to do. ^ ^ 
5 F 2 
