252 TRANSACTIONS OF THE [-MAY 17, 



and this with a probable error hardly exceeding a single second, 

 while the old-fashioned method gave results doubtful by not less 

 than a quarter of a minute. Cornu and Obrecht have indepen- 

 dently introduced the same method at Paris. When we have a 

 complete twelve years' series of such observations, they will give 

 an exceedingly precise determination of the time required by 

 light to traverse the earth's orbit, and so, indirectly, of the solar 

 parallax. 



As regards Saturn, there is nothing to report so startling as 

 Jupiter's red spot. A white spot, which appeared in 1877, en- 

 abled Hall to make a new determination of tlie rotation period, 

 which came out 10 h. 14 m. 14 s, This is in substantial accord 

 with an earlier determintition of W. Herschel's (10 h, 16 m. 

 07 s.), but involves a serious correction of the value 10 h. 

 29 m. 17 s. given in most of the text-books. The error prob- 

 ably came from a servile copying of a slip of pen made by some 

 book compiler, fifty years ago or more, in accidentally writing 

 Herschel's value of the rotation of the inner ring, instead of 

 that of the planet. 



Much time has been spent in observations of the rings, and 

 Trouvelot has reported a number of remarkable phenomena, 

 most of which, however, he alone has seen as yet. The most 

 recent micrometric measures have failed to confirm Struve's 

 suspicion that the rings are contracting on the planet. Exten- 

 sive series of observations have been made ui)on the satellites by 

 H. Struve, Meyer, and others in Europe, and by Hall in this 

 country. Hall's observations are especially valuable, and the 

 series is now so nearly completed that we may soon hope to have 

 most accurate tables. In the case of Hyperion, there is found a 

 singular instance of a retrograde motion of the line of apsides of 

 the orbit, produced by the action of an outside body, the effect 

 being due to the near commensurability of the periods of Hy- 

 perion and Titan. This most peculiar and paradoxical disturb- 

 ance first showed itself as an observed fact in Hall's observations; 

 and, soon after, Newcomb gave the mathematical explanation 

 and development. He finds the mass of Titan to be about y^^u-jy 

 that of Saturn. It may be noted, too, that Hall's observations 

 of the motions of Mimas and Enceladus indicate for the rings a 

 mass less than -^ that deduced by Bessel: instead of being y^^ 

 as large as the planet, they cannot be more than y^yVoj '-^^^^ '^^'^ 

 probably less than yolou- 



The satellites of Uranus have also been assiduously observed 

 at Washington, so that at present the Uranian system is prob- 

 ably as accurately determined as the Jovian, perhaps more so. 

 The form of the planet has been shown to be decidedly elliptical 

 (about -jV) by observations of Schiaparelli and at Princeton ; 



