The Mason hygrometer, nearly two hundred years old in principle, has been a standard in all countries for many years. It is made in various forms, but the essential features do not 
Mason Hygrometer. Weather Bureau pattern. vary. The instrument consists of two thermometers mounted on the same base. One measures the temperature of the free air; the bulb of the other is covered with a single thickness of thin bolting silk or muslin, the lower end of which is in a small vessel of water attached to the baseboard. Capillary attraction keeps the fabric wet and evaporation is almost always taking place.
The evaporation of the water chills the bulb and the wet-bulb thermometer therefore registers a lower temperature. The more rapidly the evaporation takes place, the less is the moisture content of the air; and the percentage may be determined by the difference of the readings of the two thermometers. Tabulated determinations accompany the hygrometer, and the percentage of moisture already calculated is found from tables contained in the Weather Bureau Circular of Instruction.
The hygro-autometer is a very convenient form of the Mason hygrometer. The tables are carried on a roll attached to the hygrometer. Thumbscrews turn the rolls until the difference between air temperature and wet-bulb temperature appears in the space at the top; the per cent of humidity is opposite the air temperature reading. The hygro-autometer is a most excellent hygrometer for auditoriums and for household use.
