468 THE POPULAR SCIENCE MONTHLY. 



described by the innumerable disconnected masses of which the curious 

 appendage is now generally believed to be composed. The opening 

 and closing of its chasms, and other observed signs of its restless char- 

 acter, are calculated to give the impression that the extent to which it 

 is altered during many years must be considerable. But, considering 

 the conditions necessary for the phenomena it exhibits, it would seem 

 that the rapidity with which its changes proceed might be approxi- 

 mately determined from the amount of heat which it radiates into space, 

 and which must have been, for the most part, produced at the expense 

 of its motion. Now, though the rings may be principally composed of 

 water, and have a superficial temperature near the freezing-point, yet, 

 their surfaces being over a hundred times as extensive as that of the 

 earth, they may be reasonably supposed to lose by radiation about 

 thirty times as much heat as our planet receives from the sun and 

 allows to escape into space. Taking Bessel's estimate for the mass of 

 the double girdle, it will be found that such an amount of heat might 

 be generated hy the conflicting movements of its parts without reducing 

 their orbits more than one per cent, in ten thousand years. There is, 

 indeed, reason to believe that, in this case, Bessel's results are un- 

 reliable, in consequence of the uncertain and defective character of the 

 data with which they were obtained. If we assign to each ring the 

 probable amount of matter in the neighboring moons of the gigantic 

 planet, it would seem that their permanent change of size may be so con- 

 siderable that it might be detected by the observations of a few centuries. 

 It is likely that, in the inner ring, especially at the zone nearest to 

 the primary, the temperature is much higher than that which I have 

 supposed, and that alterations in its condition might proceed at a rate 

 sufficiently rapid to be discoverable by the telescope. More than twenty 

 years ago Otto Struve, having carefully compared observations since 

 the time of Huygens, announced as the result of his labors that the 

 inner ring is changing its dimensions so rapidly that before two hun- 

 dred years it will be united to the planet. Other astronomers have 

 expressed a belief of the recent origin and of the mutable nature of the 

 obscure or vapor ring which lies closer to Saturn. The conclusions of 

 Struve, however, have been disputed ; and indeed it is probable that 

 they give an exaggerated picture of the transitory state of things in 

 the Saturnian dominions ; nor can the conflict of opinions on this point 

 be settled by observation alone. But, though taking place too slowly 

 to be at once detected in this way, the changes in question are still 

 inevitable ; and they give safe groimd for tracing the history of past 

 events in this part of the celestial regions. It is evident that the mat- 

 ter composing Saturn's wonderful appendage must have once moved in 

 a wider zone, where it could exist only in the form of two secondary 

 planets. I have shown in a previous article that a dismemberment and 

 a conversion into a ring must be the general fate of every planetary 

 body which, by a slow contraction of its orbit, revolves at last too close 



