THE STORY OF COSMIC RAYS — SWANN 



257 



There was a time when a strong belief existed that the primary 

 particles were photons and not charged particles at all. J. C. Clay's 

 discovery of the variation of cosmic-ray intensity with latitude, a 

 discovery confirmed by a series of worldwide measurements by A. H. 

 Compton and his associates, first led to the conclusion that there were 

 at least some charged particles in the primary radiation. Later, as the 

 matter was more deeply studied in connection with intensities from 

 different directions, it appeared that there was no room for anything 

 else but charged particles and, further, that the particles were posi- 

 tively charged. 



Of course, the latitude effect is greatly complicated by the fact that 

 our observations are made in the atmosphere itself. Suppose that ob- 

 servations could be made at an altitude so high that a negligible 

 amount of air existed above. Then we might expect that the intensity 

 of the radiation would mount continually as we passed from the mag- 

 netic equator to the magnetic pole and included in our measurements 

 more and more of the less energetic radiation that can reach the atmos- 

 phere at higher latitudes. But if we should observe at a depth in 

 the atmosphere, and even if we could be sure that we were measuring 

 only the primary cosmic rays, we should expect that the increase of 

 intensity with latitude would continue only until a latitude was reached 

 at which the low-energy rays, deflected away by the magnetic field 

 below that latitude, would still be unable to reach us because they 

 were stopped by the atmosphere. 



From this viewpoint the variation with latitude of, let us say, the 

 vertical intensity of the primary cosmic rays might take the form 

 shown in the accompanying simple diagram (fig. 5) . The curve ABC 

 would hold at very high altitudes ; ABD would apply at a place where 

 there is an appreciable amount of the atmosphere above ; AEF, where 



Figure 5. — The effect of absorption by the atmosphere on the intensity of cosmic rays at 



various latitudes. 



