Royal Astronomical Society. 309 
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a globe uniformly consisting of that substance might take the place 
of our earth in the planetary system, retaining its size, the answer 
would be that nothing under the ores of silver or the lighter ores of 
lead weuld serve the purpose. The increase of density, then, from 
the surface towards the centre, is fully confirmed by this experi- 
ment: a highly probable result of the theory of gravitation is made 
as sure as any result can be. But while we thus admire the man- 
ner in which one inquiry is made to confirm the consequences of 
another, we must not forget that it is no small part of our province 
to apply the conclusions of sound knowledge to the destruction of 
those remains of the age of speculation which yet linger about the 
porch of inductive philosophy ; and at which those who have safely 
gained the inner court will feel little temptation to laugh, when 
they remember how many may be, and probably are, led finally 
away by the delusions which lie in wait at the entrance. Among 
these the assumption that the earth is a mere hollow shell has held 
a conspicuous place: let us take this assumption, and grant that 
the depth of the solid matter is even one-eighth of the whole radius, 
which is more than many speculators would admit. ‘To make such 
a supposition consistent with the result of the experiment before us, 
it must be inferred that the actual matter of the shell considerably 
exceeds mercury in mean density, and is all but equal to hammered 
gold. Let such a result be established, if it be possible ; but in the 
meantime, and until presumption can be shown in its favour, let it 
be the office of the experiment before us to check the wildness 
of mere hypothesis. 
The French academicians, in measuring their South American 
degree, were the first who found sufficient local attraction in a 
mountain adjacent to their observations (Chimborago) to give any 
hope of making that phenomenon useful in future inquiry. Mas- 
kelyne, in 1772, suggested the employment of astronomical obser- 
vation in the neighbourhood of a mountain, for the determination 
of the earth’s mean density. Schehallien was chosen for the purpose 
by a committee of the Royal Society, and the result of this cele- 
brated experiment was announced in 1775. The description of this 
purely statical experiment is easier than in that of Cavendish: the 
position of a plumb-line, in a state of deviation from the vertical of 
the place, on account of the attraction of a mountain, is first to be 
accurately determined by making that plumb-line the regulator of 
an instrument for measuring zenith distances, and comparing the 
zenith distances thus obtained with those determined in other 
places of known differences of latitude. ‘The weight of the plumb- 
line is then acted on by three forces of known direction, one of 
them being of known magnitude. The remaining forces (one of 
which is the attraction of the mountain) can then be determined : 
so that if the distance of the mountain be known, its mass can be 
compared with that of the whole earth. The next step, and the 
most difficult one, is to compare the mean density of the mountain 
with that of water, which requires an accurate knowledge of its 
