202 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 193 5 



Because of the small number of entries in the table and because of 

 the recognizable differences in the way rays of different charge and 

 mass act upon matter, it is usually easy, except in the case of ex- 

 tremely high energy rays of equal charge, to identify the ray by its 

 track. Practically all of the rays listed have been found associated 

 with the cosmic radiation. 



We know, however, that many of the rays found in the cloud 

 chamber are secondaries produced from surrounding matter and 

 there is difficulty in distinguishing these from the primary cosmic 

 rays. Occasionally clusters of rays or " showers ", such as repre- 

 sented in plate 1, figure 2, are fomid, apparently emanating from one 

 or two points in surrounding material, as though they had been 

 generated there by the impact of some very energetic primary ray. 

 Even if the point of origin of a shower should chance t<) occur within 

 the gas of the chamber and the ray generating it could be photo- 

 graphed, there would still be uncertainty whether it were not just an- 

 other secondary from some previous shower. The presence of matter 

 between us and the source of the radiation confuses the problem, and 

 if we are to know what the primary cosmic rays are, we must have 

 an apparatus which operates in the space beyond the atmosphere. 



The proposal seems fantastic, but actually the earth's magnetic 

 field constitutes such an apparatus. The confusion of secondaries 

 begins at the top of the atmosphere, only a few miles above the sur- 

 face, but the bending force of the magnetic field begins to curve the 

 primary rays at distances of thousands of miles. In determining the 

 amount of such curvature, for calculation of rigidities, it is impossi- 

 ble to trace out the paths of single rays as was done in the cloud 

 chamber analysis, for observations are limited to those which can 

 be made on the earth's surface and within the atmosphere. But it 

 will appear that this is no handicap, for the variations of intensity 

 with changes of direction and position on the earth's surface give the 

 equivalent information for the determination of the rigidity, and 

 absorption in the atmosphere contributes the supplementary evidence, 

 analosfous to the track densities, for further identification of the 

 type of ray. The two methods are equivalent, except that the earth- 

 magnetic analysis concerns the primary rays alone. 



6. SIMPLIFIED ANALOGY OF THE EARTH-MAGNETIC ANALYSIS 



The relations between the rigidities of the primary rays and the 

 measured intensities are complex and mathematical, but without 

 going into the rigorous theory all of the essential points can be made 

 clear by a simple analogy. The complexity of the real problem is 

 due entirely to the peculiar form of the earth's field and if we con- 

 sider an imaginary field of uniform extent and intensity the prob- 

 lem is simple indeed. 



