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 pure alcohol in order to remove any traces of grease. Under no circumstances should gasoline be used. In adjusting it to the bulb, every trace of oil, grease, or other substance should be removed from the fingers. If the capillarity of the wick is sluggish it is better to try another. In an emergency, if the fabric about the bulb is dry it may be wet with a camel’s hair brush dipped in water. Water containing any sort of impurity is apt to impair the capillarity of the wick or even destroy it. In “hard water” localities distilled water would better be used. In catching rain water it is well to bear in mind that the water falling during the first part of a shower may be very dirty.

When the moisture is close to saturation two or three successive determinations may be necessary for a satisfactory result. When the humidity is very low the difference between the dry- and the wet-bulb reading of several determinations may be considerable. In this case, too, the observer must use his judgment. A mean of several determinations is a fairly safe record.

When the water in the cup of the Mason hygrometer is frozen, care should be used in making the reading. If the upper end of the wick is dry—sometimes this is the case—the determination may be regarded with suspicion. It is better to wet the part around the bulb by means of a camel’s hair brush and wait a few minutes until the thermometer has settled to a fixed temperature before reading. A sling psychrometer gives a more accurate result in freezing weather, and its use is more convenient. The wick, or covering, may be wet with water at ordinary temperature, but the whirling must be continued until no further reduction of the wet-bulb temperature occurs.

The Hair Hygrometer.—Human hair freed from its natural oil and from grease of every sort, is highly sensitive to moisture. It may be made chemically clean by a bath, first in water with a mild soap and, after drying, in ether. After the ether bath it should not come in contact with bare hands. If one end of a clean hair—or a strand of several hairs—be made fast to a binding post and the other wound around the axle of a dial needle and kept taut by a spring, the lengthening and shortening of the hair by changing moisture may be made to indicate humidity with a fair degree of accuracy.

Commercial hair hygroscopes and hair hygrometers of various