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 rner- 

 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. 



63. Cog psychrometer. This instrument, commonly called the "egg- 

 beater" psychrometer, has been devised to obviate certain disadvantages of 



Fig. 6. Cog psychrometer. 



