The integral intensity of the cosmic ray muon flux (> 0.28 GeV/c) near sea level has been measured in the zenith angle range 75–90°. Very few other experimental measurements have been made in this range. The present results agree closely with the predictions of a current theoretical model except at very large zenith angles, where the measured intensities are somewhat higher than predicted.
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Flint et al. (1972) studied this question.