474 Prof. P. Lowell on 
of argument in each case is independent o£ the other. For 
in the one case, in the shadow, we are reasoning on what we 
note from a transverse viewing of the tores ; in the other, 
the rings themselves, from a longitudinal aspect of them 
in the bright agglomerations. As the two deductions 
lead to the same result, each gains corroboration from the 
other. 
So much for the facts. They conduct us to a conclusion 
of interest from the point of view of celestial mechanics. To 
see this we will briefly recapitulate what has previously been 
shown of the stability of the rings. Laplace first showed 
that the rings could not be, as they appear, wide solid rings, 
inasmuch as the strains due to the differing attraction of 
Saturn for the several parts must disrupt them. Peirce then 
proved that even a series of very narrow solid rings could 
not subsist, and that the rings must be fluid. Finally, 
Clerk-Maxwell demonstrated that even this was not enough, 
and that the rings to be stable must be made up of discrete 
particles, a swarm of meteorites, in fact. But, if my memory 
serves me right, Clerk-Maxwell himself pointed out that even 
such a system could not eternally endure, but was bound 
eventually to be forced both out and in, a part falling upon 
the surface of the planet, a part going to form a satellite 
farther away. 
Even before Clerk-Maxwell's time, Edward Eoche in 1848 
had shown that the rings must be composed of discrete 
particles, — mere dust and ashes. He drew this conclusion 
from his investigations on the minimum distance at which a 
fluid satellite could revolve around its primary without being- 
disrupted by tidal strains. 
The dissolution which Clerk-Maxwell foresaw can easily 
be proved to be inevitable if the particles composing the 
swarm are not at considerable distances from one another ; 
and that they are not at such distances apart is certainly the 
case with rings A and B, as is witnessed by the light those 
rings send us, even allowing for the comminuted form of 
their constituents. Now a swarm of particles thus revolving 
round a primary are in stable equilibrium only in tlie absence 
of collisions. But in a crowded company collisions, due 
either to the mutual pulls of the particles or to the pertur- 
bations of the satellites, must occur. At each collision, 
although the moment of momentum of the two particles 
remains the same, energy is lost unless the bodies be per- 
fectly elastic, a condition not found in nature, the lost energy 
being converted into heat. In consequence, some particles 
will be forced in toward the planet while others are driven 
