Under conditions that demand more exact humidity records than are obtainable with hygrometer, the psychrometer furnishes means of making more accurate observation. The psychrometer shown in Fig. 159 is of the form used by the U. S. Weather Department. Like the hygrometer, it is composed of a wet-bulb and a dry-bulb thermometer but no water cup is attached to the instrument for moistening the wick of the wet bulb. When ready for use the wick is wet with water before each observation. The greater accuracy to be attained by the use of this instrument is on account of the maximum evaporation which is obtained from the wet bulb for any atmospheric condition. The evaporation which takes place from the wet-bulb thermometer in quiet air is not so great as that which occurs if the same air is in motion. In moving air, however, there is a certain maximum rate beyond which no further evaporation will take place. The motion of the air may be produced either by blowing on the bulb with a fan or air blast, or by whirling the thermometer. With the psychrometer the latter method is used. This instrument is provided with a handle which is pivoted to the frame and about which it is swung to produce a maximum evaporation from the wick. When a motion of the air is attained sufficient to produce a saturated atmosphere about the bulb, the temperature will remain constant. A velocity of air or the motion of the wet-bulb thermometer 10 feet per second is that usually taken as the rate for observation and the swinging is kept up 3 or 4 minutes or until the temperature of the wet-bulb thermometer remains stationary. [Illustration: FIG. 160.--Dial hygrometer.] Then the temperature of each thermometer is read and the humidity found in the table. Relative humidity determinations may be made at temperatures below the freezing point if sufficient precaution is taken in the observations. When the instrument is not in use, it is kept in the metallic case shown in the picture, to protect it from injury. =Dial Hygrometers.=--Various forms of hygrometers are in use, in which a pointer is intended to indicate on a dial the percentage of atmospheric humidity. That shown in Fig. 160 is one of the common forms. Instruments of this kind depend for their action on the absorptive property of catgut or other materials that are sensitive to the moisture changes of the air. These instruments give fairly accurate readings in a small range for a limited time, but they are apt to go out of adjustment from causes that cannot be controlled. Unless they are occasionally compared with a standard humidity determination, their readings cannot be relied upon for definite amounts of atmospheric moisture.