ve their proper place here, and those which
should be left to the historian of geology. There are some, however, of
which the cosmical connections are so close that it is impossible to
overlook them. Among these is the ascertainment of the solidity of the
globe. At first sight it seems difficult to conceive what the apparent
positions of the stars can have to do with subterranean conditions; yet
it was from star measurements alone that Hopkins, in 1839, concluded the
earth to be solid to a depth of at least 800 or 1,000 miles.[884] His
argument was, that if it were a mere shell filled with liquid,
precession and nutation would be much larger than they are observed to
be. For the shell alone would follow the pull of the sun and moon on its
equatorial girdle, leaving the liquid behind; and being thus so much the
lighter, would move the more readily. There is, it is true, grave reason
to doubt whether this reasoning corresponds with the actual facts of the
case;[885] but the conclusion to which it led has been otherwise
affirmed and extended.

Indications of an identical purport have been derived from another kind
of external disturbance, affecting our globe through the same agencies.
Lord Kelvin (then Sir William Thomson) pointed out in 1862[886] that
tidal influences are brought to bear on land as well as on water,
although obedience to them is perceptible only in the mobile element.
Some bodily distortion of the earth's figure _must_, however, take
place, unless we suppose it of absolute or "preternatural" rigidity, and
the amount of such distortion can be determined from its effect in
diminishing oceanic tides below their calculated value. For if the earth
were perfectly plastic to the stresses of solar and lunar gravity,
tides--in the ordinary sense--would not exist. Continents and oceans
would swell and subside together. It is to the _difference_ in the
behaviour of solid and liquid terrestrial constituents that the ebb and
flow of the waters are due.

Six years later, the distinguished Glasgow professor suggested that this
criterion might, by the aid of a prolonged series of exact tidal
observations, be practically applied to test the interior condition of
our planet.[887] In 1882, accordingly, suitable data extending over
thirty-three years having at length become available, Mr. G. H. Darwin
performed the laborious task of their analysis, with the general result
that the "effective rigidity" of the earth's mass mu