138 



RECORD OF SCIENCE FOR 1886. 



1876 and 1877," Professor Hall discusses the 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 present 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 with the Ginch 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 Ehea are assumed to 

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



Elements of the satvllites of Satur^i, 1880. 



Satellite. 



Mimas . .. 

 Enceladus 

 Tethys . . - 



Dione 



Rhea 



Titan .... 

 Hyperion 

 lapetus . . . 



Mean daily motion. 



381.99078572 

 262. 73177276 

 190. 69838434 



131. 53.500629 

 79. 69010973 



22. 57700000 

 16. 919883 

 4. 53794773 



Time of revolu- 

 tion. 



d 



0. 9424311 



1.3702 1 H75 

 1. 88779785 



2.7369140 

 4.5174991 



15.9454245 

 21. 276742 

 79.3310152 



Mean distance from 

 Saturn. 



26.80 

 34.40 

 42.734 



54.734 ±0.0442 

 76.537 it 0.0459 



176.915 ±0.0193 



213 98 



515! 5195 + 0. 02645 



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- 

 l^al effect to transform this motion into motion in a Keplerian 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 

 jS'ewcomb, and in which there is one of the nearest approaches to com- 

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



