COSMIC BAYS — MILLIKAN AND CAMEEOlSr 217 



These experiments consisted in sinking sealed electroscopes in 

 deep, high-altitude, snow-fed lakes, and thus finding, to take a par- 

 ticular case, that the ionization in Muir Lake (altitude 11,800 feet 

 or 3,590 meters) decreased steadily with depth from 13.3 ions per 

 cubic centimeter per second at the surface to 3.6 ions at GO feet (18 

 meters) below the surface, below which point there was no further 

 measureable decrease with instruments of such sensibility as were 

 being used. This toas tJie frst time the zero of an electroscope — the 

 reading with all external radiations^ hoth local and cosmic, com- 

 pletely cut out — had been definitely determined, and the results 

 accordingly hegan to show that it was possible to make with cer- 

 tainty determinations of the absolute amount of the penetrating 

 radiation. 



Up to the point to which we have thus far described the experi- 

 ment, it proved merely either the existence at the surface of the 

 lake of a penetrating radiation so hard as to be able to penetrate 60 

 feet (18 meters) of water before becoming completely absorbed, or 

 else a very strange distribution of radioactivity in the water of the 

 lake. We therefore tested the radioactivity of the water and found 

 it immeasurably small, not one-hundredth part of the activity of 

 ordinary tap water in Pasadena. 



Next, by taking similar readings in another deep, snow-fed lake, 

 300 miles farther south and having an altitude 6,700 feet (2,060 

 meters) lower, we found a similar curve, but with each reading dis- 

 placed just 6 feet upward. But 6 feet of water was exactly the 

 equivalent in absorbing pow^ er, where the mass absorption law holds, 

 of the layer of atmosphere lying between the altitudes 11,800 feet 

 (3,590 meters) and 5,100 feet (1,530 meters). 



These experiments, supplemented by later similar findings in other 

 lakes, therefore proved definitely three things: 



First, that the effects in Muir Lake had not been due to any 

 radioactivity which happened to be distributed in the water in 

 a particular way. 



Second, that the source of the rays w^as not at all in the layer of 

 atmosphere between the two altitudes, for this layer acted in every 

 particular like an absorbing blanket, having precisely the absorption 

 that it should have if the 7'ays came in tvholly from above. 



Third, that in different localities 300 miles apart, north and south, 

 the rays were exactly alike at the same altitudes. 



These facts, combined with the further observation made both 

 before by Hess (loc. cit.) and others and at this time by ourselves, 

 that wathin the limits of our observational error the rays 

 came in equally from all directions of the sky, and supple- 

 mented finally by the facts that the observed absorption coeffi- 

 cient and total cosmic ra}^ ionization at the altitude of Muir Lake 

 predict satisfactorily the results obtained in the 15.5-kilonieter bal- 



