The construction and operation of a torsion manometer for the measurement of the force exerted by a molecular ray is described. The manometer was used to measure the vapor pressures of KCl and CsI crystals in the range of temperature 800–950°K. The results obtained suggest that it is possible to develop this method to a degree where it will yield data at least as accurate as that obtained by other methods. Some applications of the torsion manometer are discussed.