ASTRONOMY. 413 



sary conditions. The rings must move about the planet without there 

 being revoUition i)roperly so called of the rings ; they must be solid 

 without being rigid. There seems to be no way of satisfying these con- 

 ditions and at the same time easily explaining all the observed plienom 

 ena other than the hypothesis conceived by Cassini II and recently de- 

 veloped by Proctor and Clerk-Maxwell, which supposes the rings to be 

 formed of disconnected material of thousands of little independent bod- 

 ies, asteroids, circling around the planet in independent and concentric 

 orbits not exactly in the same plane, and completing their revolutions 

 in different times. It is evident that all the combinations necessary for 

 explaining the observed i)henomena would be possible in such a system; 

 these bodies would explain certain phenomena in proportion as they are 

 more or less numerous, more or less separated by space, more or less 

 rare, &c. If we suppose (what is a very probable thing) that these 

 bodies possess different reflective powers, we can conceive of all sorts 

 of combinations of light, from the brilliant rings to the dark-gray bands. 

 In short this hypothesis appears to be entirely sufficient to explain the 

 rings of Saturn and their variability." (Trouvelot in Btill. Astron., 

 January, 1885.) 



The orbit of Tethys. — "Herr Karl Bohlin has recently communicated 

 to the Swedish Academy of Sciences an interesting discussion of the 

 elements of the orbit of Tethys. The observations discussed are those 

 of Sir William Herschel, 1789, reduced by Lament; Lamont, 1836; Sir 

 J. Herschel, 1835-'37 ; the Bonds, 1848-'52 ; Secchi,18o6; Captain Jacob, 

 1857-'58; Newcomb and Holden, 1874-'75, and Meyer, 1880-'81. The 

 elements are calculated for each period of observation, without taking 

 account of perturbations. Herr Bohlin, then specially treating the 

 mean longitude of the epoch, and adopting 190O'69812 as the value of 

 the mean motion, draws up tables of the differences between observa- 

 tion and calculation, and attemps to represent them by an empirical 

 formula. The corrected value of the mean motion is 190°.698169, almost 

 identical with that found previously by M. Baillaud. Herr Bohlin finds 

 that the annual motion of the peri-saturnium amounts to 33°. M. Bail- 

 laud's results and M. Tisserand's investigations had given the value as 

 70°. The eccentricity is found as 0-00803 i 0-00077." {Nature, January 

 28, 1886.) 



The orbit of lapetus.— An elaborate paper on the orbit of lapetus, the 

 outermost satellite of Saturn, by Prof. Asaph Hall, forms Ai)i)endix i, a 

 quarto pamphlet of 82 pages, recently published, to the Washington 

 Observations for 1882. lapetus was discovered on the 25th of October, 

 1671, by Cassini with a telescope of 17 feet focal length. Titan, the 

 brightest of the satellites of Saturn, was discovered by Huyghens on the 

 25th of March, 1655, and Huyghens was deterred, apparently, from fur- 

 ther search by his belief that the solar system was now complete, this 

 discovery making the number of satellites equal to the unmber of planets 



