A hygrometer is made up of a dry bulb thermometer and a wet bulb thermometer. The dry bulb thermometer is an ordinary liquid-in-glass thermometer which measures the actual temperature of the air. The wet bulb thermometer is the same as the dry bulb thermometer, except that it is wrapped with a piece of muslin cloth which is kept moist by a piece of wick dipped in water.
When the air is not saturated with water, moisture in the muslin cloth evaporates, producing a cooling effect. This bring down the temperature of the bulb, thus the wet bulb thermometer will read a lower temperature than the dry bulb thermometer. This difference in temperature readings is called wet bulb depression. The drier the air, the more water will evaporate from the cloth, so the wet bulb depression will be larger. The relative humidity is obtained by refering to a relative humidity table.