228 B, ROSSI AND D. B, HALL mesotron, but detected also mesotron coming removed from the beam by scattering, and this in directions inclined up to an angle of almost may have reduced the magnitude of the ab- 45. The inclined mesotrons travel a longer sorption anomaly due to decay. We may add distance and have on that account a smaller that the results recently obtained by neher and probability of survival than the vertical ones. Stever with an ionization chamber, concerning The increase in the path length, however, is mesotron coming in all directic partially compensated by an increase in the with our present results and with those of effective momentum. Thus the correction is not Hilberry and Hoag and of Nielsen, Ry large and can be disregarded at the present state Nordheim and Morgan than with the results of of the experimental accuracy. Ageno, Bernardini, Cacciapuoti, Ferretti and Wick COMPARISON WITH PREVIOUS RESULTS CONCLUSION Table II summarizes the data on the mesotron The experiments described have shown, in decay which can be deduced from the measure- agreement with previous results, that the number for vertical mesotron. L is calculated, according by a layer of air than by a dense absorber 4), from the probability of survival. The results of Nielsen, ionization losses.The indication from earlier groups with pa from 1.8X10 to 3.5X10 ev/cand experiments that the difference in stopping from35×105to59×10sev/ c are not ower between air and condensed materials enough for a quantitative comparison with our decreased has been definitely established. This to 4.5X108 ev/c. No other measurements on se. result verifies a theoretical prediction based upon lected groups of mesotrons are available. all the the disintegration hypothesis, thus confirming remaining data in Table Ii refer to mesotron for which only the lower limit of the momentum is by spontaneous decay of mesotron in the defined. A comparison between these data is not atmosphere. A value of the proper lifetime straightforward because L depends not only on To=2.4X10-6 sec is deduced from measurements the minimum effective momentum Pmin of the a fairly monokinetic group of mesotron. mesotron recorded, but also on the shape of the We are greatly indebted to Professor A. H Compton for discussions of the problem and for momentum spectrum, which is probably different the encouragement given to this work. We also at different altitudes. One may expect, however an approximate correlation to exist between the express our appreciation to Professor J. C values of pmin and l in the various experiments. collaboration, and to Professor N. Hilberry and Table II shows that this is actually the case if we Mrs. Jane E. Hamilton for their valuable assist- exclude the measurements of Ageno, bernardini Cacciapuoti, Ferretti and Wick, who found a Company for making available for two months a value of L much larger than that determined by suitable truck, to the Denver City Parks for other authors for nearly the same value of Pmin. their helpful cooperation and to the Carnegie The reason for the disagreement is not com- Institution of Washington for financial support, pletely clear. It may be noted that Ageno and we wish to express our sincere gratitude. One of collaborators used a lead absorber placed bet veen us(B R. acknowledges with thanks the fi the counters to compensate for the difference in support granted to him by the Committee in aid atmospheric depth between the higher and the of Displaced Foreign Scholar lower station. With this arrang preciable number of mesotron may have been 57, 3(1940) ference 2. Also M. A. Pomerantz, Phys. Rev