SCIENCE- GOSSIP. 



.183 



the inner. It is usuallj' believed the rings are 

 really composed of an immense multitude of tiny 

 satellites, each travelling in its orbit around its 

 primary. It really seems, ton, that the rings are 

 not quite so flat as some ha\e supposed. The 

 shadow of the planet on the rings has often ap- 

 peared to be notched, as will be noticed on Mr. 

 Bolton's drawing. This gentleman has given what 

 seems to be a plausible e.'iplanation. He considers 

 that " B " .slightly thickens toward its outer edge, 

 whilst the inner edge of "A" is nearly twice the 

 thickness of the outer edge of " B," and tlien 

 rapidly thins towards its outer edge. This may 

 explain the peculiar distortion of the shadow. 



The dimensions of this ring system are such 

 that a train travelling fifty miles an hour would 

 take a year and ten weeks to travel round tlie 

 outer border of '■ A." Professor T. J. J. See, from 

 measures witii the 2(3-inch Washington achromatic 

 in 1900, makes the greatest diameter of " A " 

 173,115 miles, Avith a breadth of 1 1,S4() miles ; the 

 greate-st diameter of "B" li5,828 miles, and its 

 width 17,181 miles. The "crape veil" has a width 

 of 11,533 miles, whilst its inner edge is only sepa- 

 rated by 6,730 miles from the surface of the planet. 

 A comparison of the measures taken from time to 

 time makes it e\ident that the dimensions of the 

 system are slowly but surely changing. 



Huyghens so early as i()55 discovered Titan, 

 Saturn's largest satellite, and later Cassini added 

 four other memljers to tlie system, named Japetus, 

 Rhea, Tethys, and Dione. All these, except Tethys, 

 may at times be seen with a 3-inch achromatic ; 

 but to reach Jlimas and Enceladus, discovered by 

 Sir W. Herschel in 178il, an aperture of from (1 to 

 8 inches at least is required, and Hyperion, which 

 was discovered simultaneously by W, C. Bond and 

 Lassell in September 1848. needs even greater 

 instrumental power. Titan is easily seen witli a 



their revolution round tire planet, always pre- 

 senting the same face to their primary. With 

 the exception of Japetus they all revolve in the 

 same plane as the rings, but tliat satellite's orbit is 

 inclined about 10°. 



From time to time, as already mentioned. Titan 

 may be seen in transit the shadow may also be 



8.\'lL'RN .\s SKES' r:v Mr. Si_rivi:n Uolton', July aril, l'J(U. 

 ( /ill permission from the '• Cainhriaii Natural Observer") 



seen with even a 3-inch aperture nearly as easily 

 as the shadow transit of Jupiter's first satellite. A 

 similar instrument is also sufficient to show the 

 eclii^se of Titan. 



The distance, period, diameter, and apparent 

 star magnitudes of the satellites are : — 





Miles 



d. h. m. 



Miles 



Mag, 



Mimas . . 



115,000 . 



. 0.22.37 . 



. 1,000 



.. 17 



Enceladus 



147,000 . 



. 1. 8.63 . 



? 



.. 15 



Tethys . . 



183.000 . 



. 1.21.18 . 



500 



.. 13 



Dione 



234,000 . 



. 2.17.41 . 



600 



.. 12 



Rhea 



327,000 . 



. 4.12.25 . 



. 1,200 



.. 10 



Titan 



758,000 . 



. 16.22.41 . 



. 2,092 



8 



Hyperion 



010,000 . 



. 21. 7. 7 . 



•J 



.. 17 



.T.ipetus .. 



2,221,000 . 



. 79. 7.63 . 



'. 1,800 



9 



P[..\XEl' Satcun's Rixi:s. (Orairit,bii E. AiitoiiiaiU.) 



1-incli telescope, but Japetus is variable, being 

 much brighter near the western elongation than in 

 any other part of its orbit. The other satellites 

 were thought by Schriiter to be brighter in the 

 eastern sections of tlieir orbits. These variations 

 make it probable that Saturn's moons, like our 

 ■ own. revolve on their axes once in the course of 



Sir John Herschel jpointed out a singular rela- 

 tionship between the periods of the four inner 

 sateUites. If the periods of Mimas and Enceladus 

 arc doubled you get almost exactly the periods of 

 Tethys and Dione. The orbit of Mimas is less than 

 30,000 miles from the outer edge of the rings. 

 (To ie c&nthnted.) 



