Encyclopaedia Britannica, 11th Edition | Page 5

blows into the mouth of a tube it causes an increase
of pressure inside and also of course an equal increase in all closed

vessels with which the mouth is in airtight communication. If it blows
horizontally over the open end of a vertical tube it causes a decrease of
pressure, but this fact is not of any practical use in anemometry,
because the magnitude of the decrease depends on the wind striking the
tube exactly at right angles to its axis, the most trifling departure from
the true direction causing great variations in the magnitude. The
pressure tube anemometer (fig. 1) utilizes the increased pressure in the
open mouth of a straight tube facing the wind, and the decrease of
pressure caused inside when the wind blows over a ring of small holes
drilled through the metal of a vertical tube which is closed at the upper
end. The pressure differences on which the action depends are very
small, and special means are required to register them, but in the
ordinary form of recording anemometer (fig. 2), any wind capable of
turning the vane which keeps the mouth of the tube facing the wind is
capable of registration.
[v.02 p.0003]
The great advantage of the tube anemometer lies in the fact that the
exposed part can be mounted on a high pole, and requires no oiling or
attention for years; and the registering part can be placed in any
convenient position, no matter how far from the external part. Two
connecting tubes are required. It might appear at first sight as though
one connexion would serve, but the differences in pressure on which
these instruments depend are so minute, that the pressure of the air in
the room where the recording part is placed has to be considered. Thus
if the instrument depends on the pressure or suction effect alone, and
this pressure or suction is measured against the air pressure in an
ordinary room, in which the doors and windows are carefully closed
and a newspaper is then burnt up the chimney, an effect may be
produced equal to a wind of 10 m. an hour; and the opening of a
window in rough weather, or the opening of a door, may entirely alter
the registration.
[Illustration: FIG. 1 & FIG. 2 Anemometers.]
The connexion between the velocity and the pressure of the wind is one
that is not yet known with absolute certainty. Many text-books on
engineering give the relation P=.005 _v_^2 when P is the pressure in lb
per sq. ft. and v the velocity in miles per hour. The history of this
untrue relation is curious. It was given about the end of the 18th

century as based on some experiments, but with a footnote stating that
little reliance could be placed on it. The statement without the
qualifying note was copied from book to book, and at last received
general acceptance. There is no doubt that under average conditions of
atmospheric density, the .005 should be replaced by .003, for many
independent authorities using different methods have found values very
close to this last figure. It is probable that the wind pressure is not
strictly proportional to the extent of the surface exposed. Pressure
plates are generally of moderate size, from a half or quarter of a sq. ft.
up to two or three sq. ft., are round or square, and for these sizes, and
shapes, and of course for a flat surface, the relation P=.003 _v_^2 is
fairly correct.
In the tube anemometer also it is really the pressure that is measured,
although the scale is usually graduated as a velocity scale. In cases
where the density of the air is not of average value, as on a high
mountain, or with an exceptionally low barometer for example, an
allowance must be made. Approximately 1-1/2% should be added to
the velocity recorded by a tube anemometer for each 1000 ft. that it
stands above sea-level.
(W.H. Di.)

ANEMONE, or WIND-FLOWER (from the Gr. [Greek: anemos],
wind), a genus of the buttercup order (Ranunculaceae), containing
about ninety species in the north and south temperate zones. _Anemone
nemorosa_, wood anemone, and _A. Pulsatilla_, Pasque-flower, occur
in Britain; the latter is found on chalk downs and limestone pastures in
some of the more southern and eastern counties. The plants are
perennial herbs with an underground rootstock, and radical, more or
less deeply cut, leaves. The elongated flower stem bears one or several,
white, red, blue or rarely yellow, flowers; there is an involucre of three
leaflets below each flower. The fruits often bear long hairy styles which
aid their distribution by the wind. Many of the species are favourite
garden plants; among the best known is _Anemone coronaria_, often
called the poppy anemone, a tuberous-rooted plant, with