138 



RECOKD OF SCIENCE FOR 1886. 



187G and 1877," Professor Hall discusses tlie old method of observing 

 these difficult objects by noting their conjunctions with the ends of the 

 ring, or with some other marked feature of the Saturnian system, but 

 concludes that the filar micrometer measures are at i^resent among the 

 best we have. He is inclined to think that the heliometer, if it can be 

 made large enough, must be one of the best instruments for dealing 

 with measurements of such objects as Saturn and Jupiter. This sug- 

 gestion is being carried out by Mr. Asaph Hall, jr., in a series of obser- 

 vations of Titan Avith the Cinch heliometer of the Yale College Obser- 

 vatory. 



The following table represents the results of Professor Hall's inves- 

 tigations upon these satellites. The elements of Titan, however, and 

 the values of the node and inclination of the ring aie adopted from 

 Bessel. Mimas, Enceladus, Tethys, Dione, and Rhea are assumed to 

 move in the plane of the ring, and Hyperion in the plane of Titan. 



Elements of the satdlites of Saturn, 1880. 



Satellite. 



Lonpritudeof 

 Pen-Saturnium. 



Mimaa 



Enceladna 

 Tethya .... 



Dione 



Kbea 



Titan 



Hyperion . 

 lapetus . . . 



Circular .. 



... do 



...do 



do 

 do 



268 37 56. 



83 37 55. 2 



353 14 56. 5 



Eccentric- 

 . ity. 



Zero . 

 ..do .. 

 -do .. 



-do 

 .do 



0. 02841836 



0. 1 



0. 027795 



Inclination to 

 ecliptic. 



28 10 10.7 

 28 10 10.7 

 28 10 16.7 



28 10 16. 7 

 28 10 16.7 



27 33 56. 7 

 27 33 56. 7 

 18 33 39. 5 



Lonarftude of 

 node. 



167 55 5. 

 167 55 5. 

 167 55 5. 



167 55 

 167 55 



5.9 



168 10 34. f 

 168 10 34. i 

 142 26 41. '. 



The motion of Hyperion. — Tisserand in investigating the case of two 

 satellites moving around their primary in orbits but little inclined to 

 each other has shown that if the mean motions are very nearly com- 

 mensurable, and if the motion of one was originally circular and uni- 

 form, the perturbations caused by the other would have for their princi- 

 pal effect to transform this motion into motion in a Kepleriau ellipse 

 with a uniform rotation of the major axis. Applying this to the case of 

 Hyperion perturbed by Titan, which has been investigated by Hall and 

 Newcomb, and in which there is one of the nearest approaches to com- 

 mensurability of mean motions to be found in the solar system, M. 



