  Measuring the angles of muons and electrons in air showers is proposed as a
method for studying the primary cosmic-ray mass composition near the knee of
the cosmic-ray energy spectrum at a few $10^{15}$ eV. Conventional tracking
detectors at existing air shower arrays could serve this purpose, like the CRT
detectors at the HEGRA array. When the average radial muon angles are examined
as a function of shower core distance, the experimental resolution can be very
well calibrated from the tangential angle distribution. The method is
particularly promising for measuring changes in the average mass number of the
primary cosmic rays with energy. The method is described and experimental and
theoretical constraints are discussed.
