A hygrometer measures the moisture content of a gas. One kind is called a sling psychrometer, and consists of two thermometers mounted on a board that pivots from a handle. The user swings the board around, getting a lot of air to pass over the two themometers, one of which has a bulb that is kept wet with a water-soaked wick. This bulb will be cooler than the other one because of evaporation of the water. A chart is then consulted to translate the two temperatures into relative humidity.