386

THE PLANET SATURN.

It is especially worthy of notice, in the above table, that theperiod of Mimas is almost exactly one-half of that of Tethys ;and that of Enceladus almost exactly one-half of that ofDione. Professor D. Kirkwood has also pointed out (see TheObservatory , vol. i., p. 199) that the sum of five times themean motion of the 1st (or innermost) satellite, added to thatof the 3rd, and to four times that of the 4th, appears to beexactly equal to ten times that of the 2nd,—a relation which mayhe compared with that discovered by Laplace in the case of thethree innermost of Jupiter’s Moons (see Lecture XIII., p. 334).

It may also be interesting to notice that Mimas, the nearestof all the eight satellites, revolves at a distance of only about34,000 miles from the outer boundary of the rings; and thatthe above table shows, that the distances of the first four, orfive, satellites are apparently arranged upon a different systemfrom that of the others. There is a certain regularity in theincrease of distance for the first four, but a much larger intervalbetween the 4th and 5th; while the gap between Hyperionand Iapetus is again exceedingly wide in comparison with thatbetween Hyperion and Titan. Certainly no such regularity isapparent in the distances of these satellites as in those of thefour moons of Jupiter , which it will be remembered are nearlyas the numbers 6, 9, 15, 27, in which series each successiveincrease is about double of the preceding.*

* The following are the only attempts with which we are acquainted tofind anyseries at all resembling Bode’s so-called Law of PlanetaryDistances,in the case of the distances of the Saturnian satellites. The former isquoted from Mr. Proctor’s “ Saturn, ” p. G3 ; the latter from the EmjlishMechanic , vol. xxiii., p. 198.

The distances of the satellites from the centre of the planet are approxi-mately as the numbers—

4 543 G'3G 844 1P37 2G'3G 3P88 TG'GO

Now if we take 44444444and add 0 1 2 4 8 1G 32 G4

we obtain 4 5 G 8 12 20 3G G8

most of which are in very fair accordance with the real values.

Or we may take 1 2 345G78

add 1 2 4 8 1G 32 G4 128

add G G G G G G G G

and we obtain K 10 13 18 27 44 t't 142

the halves of which 4 5 04 9 134 22 384 71

also resemble the true ratios.