THE OUTER PLANETS. Ill 



for the past two hundred years, he reached the 

 conclusion that the inner diameter of the ring 

 was decreasing at the rate of sixty miles a-year, 

 and that the bodies composing the rings were 

 being drawn closer to the planet. Accordingly, 

 Struve calculated that only three centuries would 

 be required to bring about the precipitation of 

 the ring-system on to the globe of Saturn. In 

 1881 and 1882 Struve, expecting a further 

 decrease, made another series of measures, but 

 these did not confirm his theory, which was 

 accordingly abandoned. 



The study of the globe of Saturn has made 

 less progress than that of the rings. The surface 

 of the planet had been known since before the 

 time of Herschel to be covered with belts, but 

 as spots seldom appear on Saturn, only one deter- 

 mination of the rotation period had been made, 

 that by Herschel. Much interest was aroused, 

 therefore, by the discovery, by Hall, at Washing- 

 ton, on December 7, 1876, of a bright equatorial 

 spot. Hall studied this spot during sixty rota- 

 tions of the planet, determining the period as 

 10 hours 14 minutes 24 seconds. This was con- 

 firmed by Denning in 1891, and by Stanley 

 Williams, an English observer, in the same year. 

 On June 16, 1903, Barnard, at the Yerkes 

 Observatory, discovered a bright spot, from 



