Mass of Cosmic-Ray Mesons at Mountain Altitudes

Mass measurements of cosmic-ray particles have been carried out by cloud-chamber observations at 3240-meters' elevation to determine the relative rate of occurrence of mesons of different types, in particular of $\mu$-mesons of the hard component and of $\pi$-mesons produced locally above the apparatus. From momentum and range observations, the mass of individual particles could be obtained with a probable error of 5 percent, thus allowing accurate resolution between $\mu$- and $\pi$-mesons.