An experiment has been performed in the upper atmosphere to study charged cosmic-ray particles that penetrate 20 cm of lead. These particles were divided into two groups depending on whether or not they interacted on traversing the lead. The experimental equipment, which consisted of the lead absorber, G—M tubes in coincidence, and radio telemetering devices, was carried to high altitudes by an array of 36 balloons. The rate of occurrence of particles in the two groups was determined as a function of atmospheric depth from 16 g-${\mathrm{cm}}^{-2\mathrm{}}$ to 400 g-${\mathrm{cm}}^{-2\mathrm{}}$. The experimental results can be interpreted in terms of high energy protons and $\mu$-mesons, with corrections for low energy protons and primary alpha-particles. The results are consistent with a primary proton intensity equal to 0.19 ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sterad}}^{-1\mathrm{}}$, an absorption length in air of 110 g-${\mathrm{cm}}^{-2\mathrm{}}$ for high energy protons, and the $\mu$-meson production spectrum given by Sands.

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