The whirling hygrometer also known as a sling psychrometer is made out of two thermometers mounted on a frame which rotates around a handle, so it can be whirled in the air by hand.
One thermometer is covered with a tight-fitting muslin sock which is kept wet by preferably with distilled water. This type of thermometer is known as the wet-bulb thermometer. The other is the dry-bulb thermometer and exposed to the atmosphere directly.
With air less than 100 per cent relative humidity, the wet bulb will display a lower temperature than the dry bulb and is known as wet-bulb depression. A special chart is used to convert the wet-bulb depression to relative humidity or the relative humidity can be determined by correlating the readings with those on a simple slide rule, which is supplied with each instrument.