=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 accumulate in the cloth. Tap-water and the water of streams may be used without appreciable error, if the cloth is changed somewhat more frequently. The temperature of the water is practically negligible under ordinary conditions. Readings 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 mercury 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 mercury begins to rise. For this reason it is desirable to shade the psychrometer 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.