218 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 19 2 8 



loon flight — all this constitutes frettif unamhiguous evidence that the 

 high-altitude rays do not originate in our atmosphere^ very certainly 

 not in the lower nine-tenths of it, and justifies tlie designation " cos- 

 iivic rays^'' the most descriptive and the most appropriate name yet 

 suggested for that portion of the penetrating rays which come in 

 from above. We shall discuss just how unambiguous the evidence 

 is at this moment after having presented our new results. 



These represent two groups of experiments, one carried out in 

 Bolivia in the high Andes at altitudes up to 15,400 feet (4,620 

 meters) in the fall of 1926, and the other in Arrowhead Lake and 

 Gem Lake, Calif., in the summer of 1927. 



PENETRATING RADIATION IN THE HIGH ANDES 



The ex])eriments in the high Andes had four prime objectives as 

 follows: (1) To see whether in lakes in the Southern Hemisphere 

 the altitude-ionization curve would coincide with that found in lakes 

 in the Northern Hemisphere. This curve was particularly sensitive 

 in the very high altitude la^lies obtainable in the high Andes, and 

 the spectral distribution found in 1925 could be more accurately 

 tested. If the Northern Hemisphere and the Southern Hemisphere 

 curves coincided, it would go a long way toward eliminating the 

 possibility that the rays are generated by the incidence of high-speed 

 beta rays on the very outer layers of our atmospheres — about the 

 only hypothesis which could put the source of these rays in the last 

 tenth of the air about the earth. For such beta rays would be ex- 

 pected to be influenced by the earth's magnetic field so as to generate 

 stronger radiation over the poles than over the Equator. In latitude 

 17° S. we should be completely screened from such pole effects, par- 

 ticularly if we could get into suitable high-altitude pockets in the 

 mountains. (2) To obtain further crucial tests of the C. T. R. 

 Wilson hypothesis that these rays may be due to the integration of 

 the effects of the impact in the earth's atmosphere of electrons en- 

 dowed with many millions of volts of energy acquired in thunder- 

 storms. Lakes in suitable pockets in the high Andes would be com- 

 pletely screened from such effects. Also, a comparison of the rays 

 found in thunderstorm areas with those found in large regions like 

 California which are comparatively free from thunderstorms might 

 furnish check observations upon this point. (3) By determining, as 

 outlined above, the zero readings of new electroscopes to obtain new 

 checks on our value of the ionization due to the cosmic rays at sea 

 level, a quantity upon which as yet there have been wide divergences 

 between the results of different experimenters. (4) To get into suit- 

 able pockets or valleys in very high mountains where the rays are 

 three or four times as intense as at sea level, and there to make more 

 trustworthy tests on directional effects in cosmic rays — in particular 



