. Research methods in ecology. Plant ecology. 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
. Research methods in ecology. Plant ecology. 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 djmger 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 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. 63. Cog psychrometer. This instrument, commonly called the "egg- beater" psychrometer, has been devised to obviate certain disadvantages of. Fig. 6. Cog Please note that these images are extracted from scanned
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