COSMIC RADIATION — BLACKETT 



183 



It is of interest to note that if the earth had been completely covered 

 with cloud so that no stars could ever have been seen, it would still 

 have been possible to show that the earth was moving very rapidly 

 toward a certain direction in the heavens, or, more accurately, moving 

 relative to something, the only property of the something being the 

 power of producing cosmic rays. We will take it, therefore, that it is 

 experimentally demonstrated that the cosmic rays are of extragalactic 

 origin. 



We must, therefore, seek for some extragalactic origin for the rays 

 which is uniformly distributed all round the earth. Now there are 

 such bodies — in fact, the whole universe is filled with nebulae — and 

 these nebulae are, in fact, nearly uniformly distributed around us. 



12 16 



Sidereal Time 



Figure 5.— Percentage variation in intensity of the cosmic rays with sidereal time. Curve, piedicted 

 effect due to galactic rotation. Data, Hess and Steinmaurer; open circles, half-hour means; solid circles, 

 3-hour means (Compton). 



Each nebula may be considered to be another galaxy similar in prin- 

 ciple to our own, but in general of rather smaller size. But if we 

 imagine that these nebulae are the origin of the cosmic rays, we meet 

 with a great difficulty, for it has been shown that the rays do not 

 come from our own galaxy, so why should they come from any other 

 galaxy? These nebulae are of widely different types, but it seems 

 probable that these different forms represent mainly different stages 

 in very similar life histories. The youngest nebulae seem to consist 

 of a great mass of diffuse gas which gradually condenses into stars, 

 and develops finally, in many cases, into a spiral form such as is 

 shown by the great nebula of Andromeda. Our galaxy is probably 

 a nebula of this last type. Thus, it is, of course, possible that the 



