A simple grating spectrophotometer is used in conjunction with a tungsten filament lamp and projector to measure directly the precipitable water in the atmosphere. Utilization is made of the 1.380 μ water vapor absorption band and the 1.250 μ region for a point of reference. Vacuum thermocouples and high sensitivity galvanometer are employed for energy measurement. Included is a discussion on the calibration and field tests made at the Laboratory of the Institute of the Aeronautical Sciences, Sands Point, Long Island, New York, August and September, 1942. Calibration tests were conducted over optical paths of from 1.5 meters to 52 meters; precipitable water paths were from 0.0023 centimeter to 0.083 centimeter.