Relative humidity may be measured reasonably accurately with a wet and dry bulb hygrometer. This is an instrument equipped with two thermometers, side by side. The bulb of one of the thermometers is covered with a muslin wick dipping into water. The water is absorbed up the muslin and evaporates, cooling the bulb, which results in the column of liquid mercury retreating down the stem. The difference between the wet and dry bulb temperatures is called the wet bulb depression.
The extent to which the bulb is cooled depends on the amount of water already in the air and can be found from tables relating the relative humidityto the dry bulb temperaure and the wet bulb depression.