COSMIC RADIATION — BLACKETT 



181 



did the early experiments of Hess, that the radiation cannot come 

 from the sun — at least, if it travels in straight lines — for if it came 

 from the sun it would be very much more intense by day than by 

 night. One can, in fact, conclude from the constancy of the radiation 

 with time that the rays, 

 as they reach the earth, 

 must be isotropic; that 

 is, they must be com- 

 ing from all directions 

 equally; for if they were 

 coming from any one di- 

 rection predominantly, 

 then since the earth is 

 rotating, any part of the 

 earth's surface would 

 receive more radiation 

 when it was facing the 

 direction from which 

 more rays were coming. 

 Thus, the constancy in 

 time of the rays implies 

 their isotropy in space. 

 Now this is one of the 

 most difficult things to 

 explain about the cos- 

 mic rays, for it is very 

 difficult to find plausible 

 sources for the rays 

 which are uniformly dis- 

 tributed with regard to 

 the earth. As has been 

 mentioned already, the 

 sun is obviously ex- 

 cluded as a possible 

 origin, but so also are 

 the stars of our galactic 

 system, for these are far 

 from being uniformly 

 distributed around the 

 earth. If one looks at the night sky one sees a great concentration 

 of stars which we call the Milky Way. There are many times more 

 stars in this direction than in a direction at right-angles, so if the 

 rays came from the stars of the Milky Way there would be a greater 

 intensity of the rays when the Milky Way is overhead. This means 

 that the rays would show a variation with sidereal time, but careful 



62 



\s—\-~\-4~~\ 



12 



Solar time 



18 24 hrs. 



(Messer schmidt) 



Figure 4 — Absorbor. 



1. None. 4. Absorber 3 cm Pb. 



2. Absorber 0.5 cm Pb. 5. Absorber 10 cm Pb. 



3. Absorber 1.5 cm Pb. 6. Absorber 20 cm Pb. 



