3. By making the experiment in the level of the ocean, the
difference will be avoided, which a higher position might occasion.
4. The expansion and contraction of the rod with the change of
temperature, is the fourth source of uncertainty before mentioned.
According to the high authority so often quoted, an iron rod, of
given length, may vary, between summer and winter, in temperate
latitudes, and in the common exposure of house clocks, from 1/1728 to
1/2592 of its whole length, which, in a rod of 58.7 inches, will be
from about two to three hundredths of an inch. This may be avoided
by adjusting and preserving the standard in a cellar, or other place,
the temperature of which never varies. Iron is named for this
purpose, because the least expansible of the metals.
5. The practical difficulty resulting from the effect of the
machinery and moving power is very inconsiderable in the present
state of the arts; and, in their progress towards perfection, will
become less and less. To estimate and obviate this, will be the
artist's province. It is as nothing when compared with the sources
of inaccuracy hitherto attending measures.
Before quitting the subject of the inconveniences, some of
which attend the pendulum alone, others both the pendulum and rod, it
must be added that the rod would have an accidental but very precious
advantage over the pendulum in this country, in the event of our
fixing the foot at the nearest aliquot part of either; for the
difference between the common foot, and those so to be deduced, would
be three times greater in the case of the pendulum than in that of
the rod.
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