{\it p}--air Inelastic Cross-Section Measurement at $10^{18.5}$ eV Using Cosmic Ray Data

Cosmic ray data can be used to measure hadronic cross-section at the energies unreachable by modern accelerators. Using high quality cosmic ray data provided by the High Resolution Fly's Eye stereo fluorescence detector we find the p-air inelastic cross-section value to be $456\pm17(stat)+39(sys)-11(sys)$ mb at $10^{18.5}$ eV. We discuss the result, and the statistical and systematic errors.