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%.

The other forms of velocity anemometer may be described as belonging
to the windmill type. In the Robinson anemometer the axis of rotation
is vertical, but with this subdivision the axis of rotation must
be parallel to the direction of the wind and therefore horizontal.
Furthermore, since the wind varies in direction and the axis has to
follow its changes, a wind vane or some other contrivance to fulfil
the same purpose must be employed. This type of instrument is very
little used in England, but seems to be more in favour in France. In
cases where the direction of the air motion is always the same, as
in the ventilating shafts of mines and buildings for instance, these
anemometers, known, however, as air meters, are employed, and give
most satisfactory results.

Anemometers which measure the pressure may be divided into the plate
and tube classes, but the former term must be taken as including a
good many miscellaneous forms. The simplest type of this form consists
of a flat plate, which is usually square or circular, while a wind
vane keeps this exposed normally to the wind, and the pressure of the
wind on its face is balanced by a spring. The distortion of the spring
determines the actual force which the wind is exerting on the plate,
and this is either read off on a suitabl