A sensitive instrument for measuring changes in atmospheric pressure and adapted for use in elevation surveying is described. Differences in atmospheric pressure are measured by measuring the changes in volume of an isothermal gas chamber that are required to maintain the chamber at atmospheric pressure. The instrument is sensitive to a change in atmospheric pressure equivalent to an elevation difference of 0.1 foot. Some of the theoretical aspects relating barometric pressure to elevation are reviewed, and the practical limitations imposed by the erratic nature of the atmosphere are discussed. Two field techniques are described, and a typical survey around an eight mile loop of thirteen stations is presented in which the closure is 1.2 feet, and the average error is 0.8 foot.