144 Remarks on Saturn. 



the rings (accepting Bond's estimate of their thickness) is less 

 than •g-J-oth P ar ^ °^ Saturn's volume. Hence, the mean density 

 of the system is more than double Saturn's mean density — or 

 exceeds, by more than one-half, the mean density of water. 

 Add to this the general permanence of the rings' form, and 

 their power of intercepting and reflecting light (for the planet's 

 shadow on the rings proves they are not self-luminous), and 

 the idea that the rings are vaporous may safely be dismissed. 



" The ' circumstance that the inner edge of the bright ring 

 is' generally ' clearly distinguished from its darker neighbour,' 

 is accounted for at p. 126 of Saturn. The objection drawn 

 from the appearance of the dusky ring where it crosses the 

 ball of the planet appears, at first sight, more formidable. It 

 would, no doubt, require a powerful telescope to detect f a 

 thinly-scattered stream of light-reflecting satellites in front of 

 the ball, supposing the latter uniformly illuminated. But we 

 must not forget that each satellite, thus placed, casts upon 

 Saturn a black shadow. Now, when the rings are open, the 

 inner edge (even) of the dusky ring is more than 10,000 miles 

 from the part of Saturn apparently behind that edge. Thus 

 it may easily be shown, that if the line of sight from the 

 observer to a satellite is inclined 10" to the line from the sun to 

 the satellite, the apparent place of the satellite on Saturn's 

 disc would be removed half a mile from the shadow of the 

 satellite. Hence, remembering that the satellites composing 

 the ring must be very small, that the equatorial horizontal 

 parallax of Saturn in opposition is 1", that Saturn's disc in 

 opposition subtends an angle of 19", that when Saturn is in 

 opposition lines from Saturn to the earth and sun may be in- 

 clined to each other in an angle as great as 2° 45', while when 

 Saturn is in other aspects the corresponding angle may be as 

 great as 6° 23', it is clear that nearly all the satellites are at 

 all times shifted from coincidence with their respective shadows, 

 and thus the black-spotted background of the disc is revealed. 

 Hence, as the shadows (like the satellites) are not separately 

 visible, the stripe of the disc crossed by the ring presents the 

 observed appearance — dusky, but not nearly so dark as the 

 other part of the ring, because the background is not black, 

 but merely strewn with black spots. 



"The fact is, that although, no doubt, f it is extremely 

 difficult to form any idea of the real nature of the rings from 

 observation,' yet, in the present state of science, the system is 

 far from 'offering a wide field for the admission of dissimilar 

 theories.' The rings must be solid, fluid, or vaporous, and 

 either continuous or discontinuous. I have shown that the 

 idea that they are vaporous must be rejected, and absolutely 

 insuperable dynamical reasons compel us to reject the idea that 



