It has been generally agreed that directions of cosmic rays are essentially isotropic, with some reservations according to some indications of anisotropy suggested by a few investigations. 1 The implication of the isotropy of primary cosmic rays seems to be that the trajectories of cosmic rays are so well disturbed by galactic magnetic fields that the memory of the original direction is lost. It is, then, natural to suppose that, if we consider the cosmic rays of extremely high energy, they are less affected by magnetic fields and hence the original anisotropy, if there is any, would show up. Attempts have been made to look for the anisotropy of extensive air showers. 2 The isotropy of extensive air showers of size ~10 5 particles has been well established. 1 However, above a size of 10 8 particles essentially no investigation had been made until the groups of M.LT. and Cornell obtained data on the isotropy of extensive air showers of this size and suggested some possible indication of an anisotropy, although it is still uncertain. 3 ' 4 We wish to inquire here if there is any particular component in primary cosmic rays which is anisotropic but is buried in the majority of isotropic components. As one of the possibilities, we may consider heavy nuclei of extremely high energy. The present report describes experimental results which may concern this problem, obtained by two experimental groups. The statistics of the results are by no means satisfactory; but, since preliminary results appear to be very suggestive from the astrophysical point of view, we thought it worth while to publish them. Part of the results was obtained by the air shower project of the Institute for Nuclear Study (INS). 5 The apparatus is essentially composed of an air shower array above the ground, with which the size, AT, the location of the center of the shower, and the arrival direction are determined for each extensive air shower; and an underground detector of eight square meters, with which we can measure the density of JLL mesons whose energy is greater than 5 Bev at the ground surface. Another part of the results comes from the project of the Osaka City University (OCU). 6 This is also a combination of an air shower array above ground and a detector array underground. Both groups used similar distribution functions for the lateral spread of particles in estimating the size N of
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