Measurement OF atmospheric moisture — The quantity of moisture mixed with the air under different conditions of tempera ture and degree of saturation may be measured in several dis tinctly different ways. Many of these, however, are not practicable methods for daily observations, or are not sufficiently accurate. Probably the most convenient of all methods and the one most generally employed is to observe the temperature of evaporation that is, the difference between the temperatures indicated by wet and dry bulb thermometers. The most reliable instrument for this purpose is the sling, or whirled psychrometer. In special cases rotary fans, or other means, may be employed to move the air rapidly over the thermometer bulbs. In any case satisfactory results can not be obtained from observations in relatively stag nant air. A strong ventilation is absolutely necessary to accuracy. Sling psychrometer.-this instrument consists of a pair of thermometers, provided with a handle as shown in fig. 1, which permits the thermometers to be whirled rapidly, the bulbs being thereby strongly affected by the temperature of and moisture in the air. The bulb of the lower of the two thermometers is covered with thin muslin, which is wet at the time an observation is made.