lowers the temperature of the wet bulb ther-
by the velometer at the same time that wind
mometer. At any particular dry bulb tempera-
direction is observed from the compass. The
ture, the wet bulb depression increases as the
velometer is in an inclosed case with a vent
at one side and an aperture at the other side.
humidity increases, because moisture can be
evaporated from the wick at a greater rate
The vent may be set for either of two wind-
speed ranges. Between the vent and the aper-
when the air contains less moisture.
ture is a wind guide through which the air
b. When constructing the psychrometer, the
wet bulb is placed lower on the frame than
passes when a reading is to be taken. A wind
vane extends into an opening in the guide.
the dry bulb to saturate the wick more easily
This wind vane is attached to a pointer which
and to prevent the moisture from the wet
moves over a scale calibrated in knots (ML-
bulb from affecting the dry bulb.
4 3 3 A / P M ) , or mph (ML-497A/PM). As
the wind blows through the wind guide, the
wind vane is deflected in accordance with the
The timer is a mechanical timing device used
wind velocity, which is indicated by the posi-
in timing readings of the theodolite set set-
tion of the pointer on the scale. Deflection
tings during visual balloon observations. The
of the vane is controlled by a calibrated spring
clock is constructed so that the second hand
linked to the pointer.
will make a complete revolution in 1 minute,
while the minute hand will make a complete
circumference of the dial in 1 hour. The timer
is kept running by a mechanical spring which
a. The cooling effect due to the evapora-
must be wound at intervals.