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ence of matter that it is a universal law that heat and ex- 
pansion, cold and contraction are correlative terms. 
Having laid down an abstract axiom, let us now apply it 
to a concrete example, namely, the earth. 
The old notions about the stability of the earth when 
compared with the mobility of the sea have been long since 
exploded. We now know that there is no such thing as 
absolute terra firma. We know that the earth in a less 
degree is mobile like the water, rising here and sinking 
there in apparently restless pulsations, waves, swellings, and 
subsidings. This being so, it becomes an interesting problem 
to determine whether these risings and sinkings compensate 
one another; that is, whether a rising in one neighbourhood 
corresponds to a sinking elsewhere, or whether the whole 
periphery of the earth is undergoing enlargement or dimi- 
nution, is stretching or shrinking. To decide this by direct 
experiment is not easy, for since water is our gauge the 
same relative effect will be produced either by the sinking 
of an ocean-bed or the rising of an adjacent continent. But 
we have a considerable amount of evidence notwithstanding 
which points, so far as I know, in one direction and one 
only. There is first the a priori evidence. 
There are two sciences which deal with the inner consti- 
tution of the earth : astronomy, which deals with it as a 
part of the great macrocosm, the universe ; and geology, 
which deals with its inner life as a universe of its own. The 
question of the alteration in the earth’s bulk has naturally 
been discussed by both astronomers and geologists, and dis- 
cussed too from very different points of view, but both are 
agreed in one conclusion, namely, that the earth is shrinking. 
Since the days of La Place the Nebular hypothesis has 
been generally received by astronomers as the one which 
best meets observed facts. This hypothesis predicates the 
existence of gravitation everywhere, and shows how by its 
influence the various heavenly bodies have become con- 
