372 SCIENCE PROGRESS 



in it, and the mass may ultimately break up into several parts. 

 If the mass is incompressible, the cataclysm does not occur so 

 readily ; with a homogeneous incompressible gas, it will not 

 occur at all. Mr. Jeans divides such encounters of two masses 

 into two classes, transitory and lasting, the former being those 

 in which the tidal forces come and go before the major axis of the 

 spheroid increases by more than 10 per cent. Lasting en- 

 counters are extremely rare ; for the stellar universe as a whole, 

 and excluding star clusters, they occur on the average about 

 once in 4X io 13 years. It follows that the large proportion 

 of binary stars in the sky cannot be explained by lasting 

 encounters. On the other hand, a transitory encounter will 

 not produce a binary system, unless the initial density was 

 below about io -19 . This value is so improbably small that it 

 seems that in general binaries cannot have been formed by 

 tidal action. 



Similar conclusions hold for nebulae, and in addition it is 

 found that tidal action does not in any case give rise to the 

 well-known spiral form, but rather to a curve of a boomerang 

 shape. As regards the solar system, although it is possible to 

 conceive it as having been produced by intense tidal action, 

 the odds are very heavy against this being so. If formed in this 

 way, our system must be regarded as a sort of freak in the sky ; 

 as far as we are aware, it is so, and since its existence cannot be 

 explained on the rotational hypothesis, it does not seem un- 

 reasonable to suppose it to have been formed in this way. The 

 most probable conclusion to be drawn from Mr. Jeans 's in- 

 vestigations is, therefore, that binary systems and spiral nebulae, 

 in general, have evolved from a mass of rotating gas, and that 

 our solar system has probably been evolved from a gaseous 

 mass by the intense tidal actions of another mass. 



The Aurora Borealis. — The corpuscular theory of the aurora 

 first advanced by Goldstein and subsequently developed by 

 Paulsen and Birkeland, supposes that the sun is continually 

 emitting into space charged particles, somewhat of the nature 

 of cathode rays. Such of these as enter the magnetic field of 

 the earth are converged by the action of the latter towards the 

 earth and, entering the upper layers of the atmosphere either 

 directly, or indirectly through the production of secondary 

 cathode rays, produce the luminescence observed as the aurora. 

 The theory has the observational support that the curve showing 



