A large cloud‐chamber‐ion‐chamber stack has been designed and operated to measure the properties of the interactions in light materials of cosmic‐ray protons and pions in the energy range greater than 140 GeV. The usable flux of these particles at sea level is limited to the measured value of (1.3±0.1)×10−4m−2ster−1sec−1by the requirement that each particle be unaccompanied by time‐coincident particles in the aperture of the apparatus of 0.22 m2‐ ster. With the use of this flux and the apparatus described, an interaction cross section for protons in carbon can be measured for energies up to 300 GeV in about a year. In the use of the ionization spectrometer, large fluctuations in the absorption curve have been observed that preclude the use of the shape of the curve alone to measure the energy of individual events.