But even a pendulum is not without its uncertainties.
1. The difficulty of ascertaining, in practice, its centre of
oscillation, as depending on the form of the bob, and its distance
from the point of suspension; the effect of the weight of the
suspending wire towards displacing the centre of oscillation; that
centre being seated within the body of the bob, and therefore
inaccessible to the measure, are sources of considerable uncertainty.
2. Both theory and experience prove that, to preserve its
isochronism, it must be shorter towards the equator, and longer
towards the poles.
3. The height of the situation above the common level, as being
an increment to the radius of the earth, diminishes the length of the
pendulum.
4. The pendulum being made of metal, as is best, it varies its
length with the variations in the temperature of the atmosphere.
5. To continue small and equal vibrations, through a sufficient
length of time, and to count these vibrations, machinery and a power
are necessary, which may exert a small but constant effort to renew
the waste of motion; and the difficulty is so to apply these, as that
they shall neither retard or accelerate the vibrations.
1.
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