The globe of the earth itself, indeed, might be considered as
invariable in all its dimensions, and that its circumference would
furnish an invariable measure; but no one of its circles, great or
small, is accessible to admeasurement through all its parts, and the
various trials to measure definite portions of them, have been of
such various result as to show there is no dependence on that
operation for certainty.
Matter, then, by its mere extension, furnishing nothing
invariable, its motion is the only remaining resource.
The motion of the earth round its axis, though not absolutely
uniform and invariable, may be considered as such for every human
purpose. It is measured obviously, but unequally, by the departure
of a given meridian from the sun, and its return to it, constituting
a solar day. Throwing together the inequalities of solar days, a
mean interval, or day, has been found, and divided, by very general
consent, into 86,400 equal parts.
A pendulum, vibrating freely, in small and equal arcs, may be
so adjusted in its length, as, by its vibrations, to make this
division of the earth's motion into 86,400 equal parts, called
seconds of mean time.
Such a pendulum, then, becomes itself a measure of determinate
length, to which all others may be referred to as to a standard.
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