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In 4949 photographs of cosmic-ray phenomena, not associated with dense air showers, occurring in a counter controlled cloud chamber surrounded by lead, the numbers of nuclear interactions in which secondaries were produced and observed to occur in and above the chamber were 182 and 223, respectively. The mean free path for nuclear interaction of the penetrating particles produced in these nuclear interactions is 316=fc70 g/cm 2 of lead, while for nuclear scattering (large angle scattering without the production of secondaries) it is at least 4 or 5 times this value. The projected zenith angular distributions of the secondaries from these interactions are given. The lightly ionizing secondaries going in the forward direction have an angular distribution ^cos 3 0, while those in the backward direction from nuclear interactions occurring in the chamber have a uniform distribution.

The penetrating particles in extensive air showers have been observed in a cloud chamber under 6 inches of lead at an elevation of 3260 meters. If a group of air showers is selected in such a way that their cores mostly land between 4 and 24 meters from the chamber, the penetrating particles are seen to consist mainly of ^-mesons which have been produced in the air above the apparatus. If a group of air showers is selected so that their cores are mostly <6 meters from the chamber there are again many ^-mesons and in addition there is evidence for local production of penetrating particles in mixed showers and high energy nuclear disintegrations produced by both neutral and ionizing penetrating particles. Thus we have evidence that high energy neutral particles and highly interacting ionizing particles make up an appreciable portion of the penetrating particles in the cores of air showers. There are more cascades of energy > 10 8 ev under the lead shield than can be accounted for by the radiation and knock-on processes of the observed number of mesons.

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