This revolves rapidly, and soon
acquires a uniform rate, which it cannot greatly exceed, because
any addition to its velocity produces a much greater addition to
the resistance it meets with from the air. The interval between
the strokes on the bell of a clock is regulated in this way, and
the fly is so contrived, that the interval may be altered by
presenting the arms of it more or less obliquely to the direction
in which they move. This kind of fly, or vane, is generally used
in the smaller kinds of mechanism, and, unlike the heavy fly, it
is a destroyer instead of a preserver of force. It is the
regulator used in musical boxes, and in almost all mechanical
toys.
31. The action of a fly, or vane, suggests the principle of
an instrument for measuring the altitude of mountains, which
perhaps deserves a trial, since, if it succeed only tolerably, it
will form a much more portable instrument than the barometer. It
is well known that the barometer indicates the weight of a column
of the atmosphere above it, whose base is equal to the bore of
the tube. It is also known that the density of the air adjacent
to the instrument will depend both on the weight of air above it,
and on the heat of the air at that place.
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