MARS ASTEROIDS JUPITER SATURN URANUS. 99 



but a little to the west, a position which is found to contribute to the stability of the 

 Saturnian system. The surface of the globe exhibits belts, bright and dark, which 

 point to the same cause as similar phenomena in the case of Jupiter, whatever that cause 

 may be. But, unlike his neighbour, the axis of Saturn is inclined 29 to the plane of his 

 orbit. He has days and nights therefore of unequal length, a diversity of seasons, and 

 alternately for years continued winter and darkness reign at his poles. 



The theory of the satellites of Saturn is far less perfect than that of the moons of 

 Jupiter. With reference to their elements in general only approximate results have 

 been obtained. Their respective distances from their primary vary from about half the 

 distance of our own moon to upwards of two millions of miles. Their periodic revolu- 

 tions also vary from twenty- two hours to seventy days. Nothing is known as to their 

 precise magnitude, but the most distant is evidently the largest, and is supposed to be 

 nearly equal to Mars in size. The first discovered, or the sixth as to distance, called in 

 honour of the discoverer the Huygenian satellite, is the brightest. These two may be 

 discerned with ordinary optical aid. The rest are more difficult objects, and it requires 

 the mightiest telescopes to reach the two which just skirt the rings discovered by 

 Herschel. The dimensions of the rings, as given from the micrometrical measurements 

 of Struve, made at Dorpat in the year 1828, with Frauenhofer's large refractor, are as 

 follows : 



Exterior diameter of exterior ring - 176,418 miles 



Interior diameter of exterior ring - 155,272 



Exterior diameter of interior ring - - 151,690 . 



Interior diameter of interior ring - - 117,339 



Interval between the inner ring and Saturn - - 19,090 . 



Interval between the rings - 1,791 



Thickness of the rings not more than - - - 100 . 



The above measurements give to the exterior ring a width of 10,573 miles, and to the 

 interior of 17,175 miles, making the entire width of the double ring, including the interval, 

 29,539 miles. That these appendages are opaque substances is shown by the fact that 

 they throw shadows on those parts of the planet that are on a line with them and the 

 sun, and on the other hand they receive the shadow of the planet. Both the rings have 

 a motion of rotation, accomplished in 10 h 29 m 17 s , a condition plainly essential to their 

 stability, the centrifugal force engendered by the rotatory motion balancing the attraction 

 of the planet, and preventing their precipitation to its surface. But notwithstanding the 

 centrifugal force, it may be demonstrated to be a condition necessary to the stable equi- 

 librium of the rings, that they should not be of uniform thickness or density, otherwise 

 the system would be in a state of unstable equilibrium, like that of a needle balanced on 

 its point, and the slightest disturbance, such as must arise from the action of the satel- 

 lites, would bring on a catastrophe. Accordingly, evidence appears that the rings are 

 not alike in all their parts. The edge is not flat but spheroidal, and according to 

 Messier the surface is diversified with inequalities, an irregularity of form which satisfies 

 the demands of theory, and guarantees the preservation of this remarkable appendage. 

 The system of Saturn, the most complicated with which we are acquainted, is an 

 astonishing instance of artistic skill, of nicely adjusted mechanism a ball launched in 

 space, surrounded by two ponderous zones, each independent of the other, all acted upon 

 by the attraction of the sun and of the planets, and acted upon by one another and by 

 the satellites of the planet yet no confusion has arisen, but the original order is main- 

 tained, after the lapse of ages of constant antagonism ! 



The planet is here shown in different parts of its vast circuit round the sun, with the 



