The momentum spectra of cosmic ray protons and pions up to an energy of I50 GeV have been measured at Durham, 198 feet above sea-level. Assuming a particular model for the interaction of high energy nucleons, a new estimate has been made of the primary cosmic ray nucleon spectrum. This may be represented in the energy range 10(^10) to 3 x 10(^13) eV nucleon by the expression:-I(>E) = 0.87(^+0.52) (_-0.30) E(^-1.58) cm(^-2) sec(^-1) Sterad(^-1) where E is in units of GeV. It has been shown that a model in which the mean value of the elasticity of nucleon=air nucleus collisions is not a function of ' incident energy describes the experimental results accurately. The value found for the fraction of this energy retained by the incident nucleon is 53%. The effect of large fluctuations in elasticity from collision to collision has been investigated and found to be negligible. The result of taking fluctuations into account is to give a mean value of elasticity ~3% lower than the value obtained assuming the elasticity not to fluctuate
