lass of instrument
has sprung. If the wind 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 m