those which do not require a wind vane or weathercock, (2) those
which do. The Robinson anemometer, invented (1846) by Dr. Thomas Romney
Robinson, of Armagh Observatory, is the best-known and most generally
used instrument, and belongs to the first of these. It consists
of four hemispherical cups, mounted one on each end of a pair of
horizontal arms, which lie at right angles to each other and form a
cross. A vertical axis round which the cups turn passes through the
centre of the cross; a train of wheel-work counts up the number of
turns which this axis makes, and from the number of turns made in any
given time the velocity of the wind during that time is calculated.
The cups are placed symmetrically on the end of the arms, and it is
easy to see that the wind always has the hollow of one cup presented
to it; the back of the cup on the opposite end of the cross also
faces the wind, but the pressure on it is naturally less, and hence
a continual rotation is produced; each cup in turn as it comes round
providing the necessary force. The two great merits of this anemometer
are its simplicity and the absence of a wind vane; on the other hand
it is not well adapted to leaving a record on paper of the actual
velocity at any definite instant, and hence it leaves a short but
violent gust unrecorded. Unfortunately, when Dr. Robinson first
designed his anemometer, he stated that no matter what the size of the
cups or the length of the arms, the cups always moved with one-third
of the velocity of the wind. This result was apparently confirmed by
some independent experiments, but it is very far from the truth, for
it is now known that the actual ratio, or factor as it is commonly
called, of the velocity of the wind to that of the cups depends very
largely on the dimensions of the cups and arms, and may have almost
any value between two and a little over three. The result has been
that wind velocities published in many official publications have
often been in error by nearly 50%.