The Humidity Meter

The amount of moisture present or the huruidity of the air is determined by a comparison of dry and wet bulb thermometers. They are both ordiriary thermometers, but the bulb of the latter is covered with muslin that is wet. In the latest form of instrument the thermometers are motmted on arms carried by a shaft that is rotated by a crank which is geared to the shaft. The motion of the shaft rotates the thermometers in vertical planes and causes the water in the muslin to evapórate more or less rapidly, according to the amount of moisture in the air. This evaporation lowers the ternperature of the thermometer, and from tables constructed after long experiments the degree of moisture can be determined by the difference in temperature between the two