These yet, by their presumed
and apparent equality, serve as well to reckon time by (though not
to measure the parts of duration exactly) as if they could be proved
to be exactly equal. We must, therefore, carefully distinguish betwixt
duration itself, and the measures we make use of to judge of its
length. Duration, in itself, is to be considered as going on in one
constant, equal, uniform course: but none of the measures of it
which we make use of can be known to do so, nor can we be assured that
their assigned parts or periods are equal in duration one to
another; for two successive lengths of duration, however measured, can
never be demonstrated to be equal. The motion of the sun, which the
world used so long and so confidently for an exact measure of
duration, has, as I said, been found in its several parts unequal. And
though men have, of late, made use of a pendulum, as a more steady and
regular motion than that of the sun, or, (to speak more truly), of the
earth;- yet if any one should be asked how he certainly knows that the
two successive swings of a pendulum are equal, it would be very hard
to satisfy him that they are infallibly so; since we cannot be sure
that the cause of that motion, which is unknown to us, shall always
operate equally; and we are sure that the medium in which the pendulum
moves is not constantly the same: either of which varying, may alter
the equality of such periods, and thereby destroy the certainty and
exactness of the measure by motion, as well as any other periods of
other appearances; the notion of duration still remaining clear,
though our measures of it cannot (any of them) be demonstrated to be
exact.
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