ASTRONOMY. 137 



observed at Princeton in March and April, 1885, gave a period of 9 

 55™ 11^.14. "It is noteworthy that although this spot was in a higher 

 latitude (about 50° south) than the red spot, it yet rotates more rapidly." 

 Professor Young remarked the apparent overlapping of the southern 

 belt and the red spot which took place towards the end of March and 

 the beginning of April, and which was seen by many English observers 

 (Observatory, May, 1886, vol. 9: p. 188); but whilst admitting that it 

 was impossible to say which was uppermost, he was inclined, in oppo- 

 sition to Mr. Denning's view, to believe the red spot to be the lower. 

 Mr. Denning has pointed out that the apparent partial coalescence of 

 the two markings was simplj- due to an arm of the southern belt over- 

 taking the red spot, the former having a rotation period shorter by 

 about 19* than the latter. 



Mr. Denning finds evidence of regular recurrence in many of the 

 prominent markings on this planet. 



Saturn : The satellites of Saturn. — Professor Hall has finished a very 

 important discussion of the six inner satellites of Saturn, and his work 

 has been published as Appendix i to the Washington Observations for 

 1883. The observations of Professor Newcomb in 1871, and Professor 

 Hall's own observations from 1875 to 1884, are given in detail ; these are 

 followed by the formation of equations of condition and their solution, 

 and the "work concludes with useful tables of the satellites' motions. 



A remarkable result of the discussion is that the Washington obser- 

 vations of the five inner satellites can be satisfied within the limits of 

 their probable errors by circular orbits. It was hoped that the observa- 

 tions would determine the positions of the lines of apsides with such 

 accuracy that the motions of these lines would be known, and that thus 

 we might obtain data for a new determination of the mass of the ring 

 and of the figure of the jjlant. But the resulting circular orbits for the 

 inner satellites make the position of a line of apsides indeterminate, and 

 for the present the mass of the ring remains unknown. 



The mass of Saturn has been computed from the elements found for 

 Titan, Rhea, Dione, and Tethys with the separate results for the re- 

 ciprocal of the mass, — 



From Titan 3480. 07± 1. 138 



Rhea 3450. 43± 6. 202 



Dione . 3463. 68± 8. 379 



Tethys 3463. 41 i 10. 629 



or, the mean result from the four satellites, is 



Mass of Saturn = 3478 t-li 10 



the mass of the sun being unity. 



In a paper in the Astrouomische Nachrichten (No. 2743) entitled 

 " Comparison of the five inner satellites of Saturn made at Toulouse in 



