${\pi }^{-}-p$ interactions have been observed in the hydrogen filling of diffusion cloud chambers exposed to a ${\pi }^{-}$ beam of average energy 1.37 Bev at the Brookhaven Cosmotron. If 9 interactions leading to heavy unstable particles are omitted, there remain 147 interactions observed with magnetic field and 323 interactions without field. The total cross section is estimated to be 34.6±2.7 millibarns. The elastic scattering cross section is 10.0±0.8 millibarns. The elastic scattering angles are mostly less than 60° in the center-of-mass system and are accordingly interpreted as mainly due to diffraction scattering. The observations are consistent with diffraction by a sphere with radius (1.18±0.10)×${10}^{-13\mathrm{}}$ cm and transparency 0.61±0.10. However, the 20 percent of elastic scattering observed through angles >60° may indicate a region of very strong interaction with radius ∼0.5×${10}^{-13\mathrm{}}$ cm surrounded by a region of much weaker interaction. Of the inelastic events only the 95 observed in the magnet cloud chamber can be analyzed further. Of these 71 to 81 are considered to involve the production of one secondary pion and 14 to 24 two pions, depending on the assignment of unidentified inelastic cases. This indicates a slightly greater multiplicity than predicted by Fermi's statistical theory. Angle and momentum distributions of emitted pions are discussed in terms of possible pion-pion interactions and excited nucleon states, but conclusions on these questions are uncertain.

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