Accurate observations on cosmic-ray intensities as measured by Neher electroscopes were made in the equatorial belt (Madras, India, mag. lat. 3 N) up to more than 98 percent of the way to the top of the atmosphere with the following results: 1. Cosmic rays whatever their nature are so rapidly absorbed in the outer layers of the atmosphere that even in the equatorial belt they get into equilibrium with their secondaries and produce their maximum ionization before they have penetrated through the first tenth of the atmosphere. 2. From that point on they fall off exceedingly rapidly in intensity following an exponential equation, their law of absorption being like that of x-rays and not that of particles that exhibit range phenomena such as low energy $\beta$-rays, proton rays or $\alpha$ rays. 3. The differences between the present curve, the San Antonio curve (mag. lat. 38.5 N) and the curve obtained on the Settle-Fordney flight (mag. lat. 53 N) fix for the first time the complete curves of ionization produced in the atmosphere by incoming charged particles having energies in the 2.5-6.7×${10}^3$ Mev and 6.7-17×${10}^3$ Mev ranges. These curves are reasonably well in accord with the Bethe Heitler theory as extended by Carlson and Oppenheimer. 4. The exceedingly rapid absorption of this latitude sensitive radiation with a coefficient nearly constant and independent of energy qualitatively justifies the shower theory of the foregoing authors. 5. The latitude sensitive part of the cosmic-ray ionization found in the lower portion of the atmosphere is practically all due to the secondary effects of varied nature resulting from the absorption of the incoming electrons in the upper tenth of the atmosphere. 6. The apparent absorption coefficient of the whole progeny of secondary influences resulting, down to sea level from the absorption of incoming electrons in the very top layers of the atmosphere is approximately the same as that found by Johnson and by Neher for the east-west effect thus proving that the particles causing the latitude and the east-west effects are of the same type. Both absorption coefficients are such as to suggest that these particles are electrons (predominantly positive) and not protons.

• Received 8 May 1937

DOI:https://doi.org/10.1103/PhysRev.52.80