Longitudinal development of cosmic-ray showers

We report on new measurements of atmospheric Cherenkov radiation in cosmic-ray showers of primary energy 2×${10}^{15}$ to ${10}^{18}$ eV. The measurements are interpreted with the aid of detailed computer simulations to give the dependence of the depth of shower maximum upon primary energy. The change of depth of maximum between 5×${10}^{16}$ and 2×${10}^{17}$ eV is found to be greater than expected from plausible models with constant primary cosmic-ray composition.