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 suitable gauge, or leaves a record in
the ordinary way by means of a pen writing on a sheet of paper moved
by clockwork. Instruments of this kind have been in use for a long
series of years, and have recorded pressures up to and even exceeding
60 lb per sq. ft., but it is now fairly certain that these high values
are erroneous, and due, not to the wind, but to faulty design of the
anemometer.

The fact is that the wind is continually varying in force, and while