Jacques Wardlaw Redway

“ Evaporimeters.—The rate of evaporation depends on the amount of moisture in the air. If the humidity is low, evaporation is more rapid than when it is high. With the humidity above 95 per cent a piece of wet muslin in the open air may require more than an hour to dry; with the humidity at 100 per cent it does not dry at all. A high temperature also favors evaporation—chiefly from the fact that, with rising temperature, the relative humidity decreases without any change in absolute humidity. ”

“ 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. ”

“ When the air—or rather, the water vapor itself—is near the point of saturation it is moist to the senses. Hygroscopic substances, such as sugar, salt and many other substances, absorb moisture; sized paper and starched fabrics swell and become limp. ”