HUMIDITY 39 
62. Readings. All observations should be made facing the wind, and the 
observer should move one or two steps during the reading to prevent the 
possibility of error. The cloth of the wet bulb is moistened with water by 
means of a brush, or, much better, it is dipped directly into a bottle of water. 
Distilled water is preferable, as it contains no dissolved material to accumu- 
late in the cloth. Tap-water and the water of streams may be used with- 
out appreciable error, if the cloth is changed somewhat more frequently. 
The temperature of the water 
is practically negligible under 
ordinary conditions. Read- 
ings can be made more 
quickly, however, when the 
temperature is not too far 
from that of the air. The 
psychrometer is held 
firmly and swung rapidly 
through the air when the 
space is not too confined. 
Where there is danger of 
breakage, it is swung back 
and forth through a short arc, 
pendulum-fashion. As_ the 
reading must be made when 
the mercury of the wet bulb 
reaches the lowest point, the 
instrument is stopped from 
time to time and the position 
of the column noted. The 
lowest point is often indicated 
by the tendency of the mer- 
cury to remain stationary; 
as a rule it can be noted with certainty when the next glance shows a rise 
in the column. In following the movement, and especially in noting the 
final reading, great care must be taken to make the latter before the mer- 
cury begins to rise. For this reason it is desirable to shade the psychro- 
meter with the body when looking at it, and to take pains not to breathe 
upon the bulbs nor to bring them too near the body. At the moment when 
the wet bulb registers the lowest point, the dry bulb should be read and the 
results recorded. 
Fig. 6. Cog psychrometer. 
63. Cog psychrometer. This instrument, commonly called the “egg- 
beater” psychrometer, has been devised to obviate certain disadvantages of 
