Energy Distribution and Positive Excess of Mesotrons

Energy measurements of penetrating cosmic rays have been made near sea level with a vertical cloud chamber placed in a magnetic field. The chamber expansions were controlled by a threefold coincidence Geiger-Müller counter system. One of the counters was shielded by 10.2 cm of lead and set beneath the chamber. The soft component was eliminated by absorption by the lead. Our energy spectrum in the region from 0.2 Bev (billion electron volts) to 2 Bev differs slightly from that obtained by Blackett without lead filter, although at higher energies they coincide. The correspondence between the experimental results and Euler's theoretical curve for the hard component is discussed. Throughout the measured energy range (0.2 Bev to 10 Bev) we find approximately 29 percent more positive mesotrons than negatives for each energy.