Encyclopaedia Britannica, 11th Edition | Page 4

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 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 the
ordinary pressure plate is admirably adapted for measuring the force of
a steady and uniform wind, it is entirely unsuitable for following the
rapid fluctuations of the natural wind. To make matters worse, the pen
which records the motion of the plate is often connected with it by an
extensive system of chains and levers. A violent gust strikes the plate,
which is driven back and carried by its own momentum far past the
position in which a steady wind of the same force would place it; by the
time the motion has reached the pen it has been greatly exaggerated by
the springiness of the connexion, and not only is the plate itself driven
too far back, but also its position is wrongly recorded by the pen; the
combined errors act the same way, and more than double the real
maximum pressure may be indicated on the chart.
A modification of the ordinary pressure-plate has recently been
designed. In this arrangement a catch is provided so that the plate being
once driven back by the wind cannot return until released by hand; but
the catch does not prevent the plate being driven back farther by a gust
stronger than the last one that moved it. Examples of these plates are
erected on the west coast of England, where in the winter fierce gales
often occur; a pressure of 30 lb per sq. ft. has not been shown by them,
and instances exceeding 20 lb are extremely rare.
Many other modifications have been used and suggested. Probably a
sphere would prove most useful for a pressure anemometer, since
owing to its symmetrical shape it would not require a weathercock. A
small light sphere hanging from the end of 30 or 40 ft. of fine sewing
cotton has been employed to measure the wind velocity passing over a
kite, the tension of the cotton being recorded, and this plan has given
satisfactory results.
Lind's anemometer, which consists simply of a U tube containing liquid
with one end bent into a horizontal direction to face the wind, is
perhaps the original form from which the tube class of instrument has
sprung. If the wind