The basic design of a wet bulb thermometer is to insert a thermometer into a reservoir of pure, distilled water. Wrapped around the thermometer is a piece of cloth (usually muslin) that acts as a wick, drawing water up through the reservoir via capillary action. As the moisture in the cloth above the surface of the water evaporates, it draws heat from the thermometer, thus lowering the temperature until the surrounding air reaches a point of saturation. A dry bulb thermometer is simply a regular thermometer that is not dipped into the water reservoir or wrapped in cloth.