ASTRO-PHYSICS 277 



binary with a period of four days, which revolves 

 in a period of some twenty years round a third 

 star invisible to us. 



Passing from these questions to the problems 

 of our own planetary system, we find the same 

 principle applied to the examination of Saturn's 

 rings. These remarkable structures, which in 

 the telescope look like rings of continuous matter 

 encircling the planet, were long a puzzle to the 

 astronomer. Theory indicates that such rotating 

 rings of continuous matter, whether solid or 

 liquid, would be unstable, and would break up 

 under the forces which must necessarily exist. 

 The alternative hypothesis, that the rings consist 

 of a swarm of tiny meteorites, each revolving 

 round the planet in its own separate orbit, was 

 elaborated mathematically by Clerk Maxwell, 

 but no confirmation of this view was obtained 

 till Keeler examined the rings with the spectro- 

 scope. He found that the inner parts of the 

 rings revolved faster than the outer parts, in 

 accordance with the requirements of the meteoritic 

 hypothesis. If, on the other hand, the ring 

 were solid, the outermost parts would possess 

 the highest velocity, on the same principle by 

 which the circumference of a fiy-wheel moves 

 faster than its inward parts. 



While the knowledge of sun and stars 

 delivered to us by spectrum analysis has been 

 both extensive and striking, the interpretation 

 of spectral phenomena has proved a much more 

 complicated problem than was anticipated when 

 Bunsen and Kirchhoff's great discovery first 



