An instrument for measuring the vapor pressure and the relative humidity of the air or the quantity of moisture in the air. It consists of a dry-bulb thermometer and a wet-bulb thermometer, the latter having its bulb covered with a layer of muslin kept moist with water. The rate of evaporation from the moist muslin depends upon the quantity of moisture in the air. The more rapid the evaporation, the greater the cooling, and hence the greater the difference in the temperature readings of the two thermometers.