176 Professor C. Piazzi Smyth's 



He then considers the possibility of their being streams of fluid 

 somewhat denser than water, and flowing around the planet. The 

 extraordinary thinness of the ring allowing the fluid to be supposed 

 to be of uniform density ; then with the mass, width, diamet(3r, and 

 velocity of rotation as observed, a proportion for the thickness and 

 width of each ring, may be computed by the respective lengths in 

 each direction, which will give equality of fluid pressure in the 

 centre. Hence the whole number of component rings to make up 

 the grand ring as observed, may be computed, and the first approxi- 

 mation has given 20, which Prof. Pierce considers the maximum 

 limit. The definite width of the elementary rings, is, he says, to a 

 certain extent, analogous to the definite size of the drops of water 

 from a vial ; and as those drops may be observed to break off from 

 the fluid above with quite an abrupt line of separation, so will the 

 dividing of the rings of Saturn, as the fluid approaches that state of 

 rest which is necessary for the phenomenon, occur with a swift and 

 abrupt sinking in at the proper distances. 



Investigating then into the stability of such a fluid ring or rings, 

 Prof. Pierce was surprised to find that after all, the motion of the 

 centre of gravity was not controlled by the primary ; that it retains 

 the individual particles from flying apart, and keeps them in the 

 form of a ring, but it has no influence on their motion as one body. 

 The ring might therefore, so far, move to such an extent in its plane 

 as to be brought into contact with the ball of the planet and destroyed. 

 He found, however, that the sustaining power exists in the satellites 

 exterior to the ring, as was indeed supposed by Sir John Herschol, 

 in his Outlines of Astronomy. 



Thence Prof. Pierce concludes, that no planet can have a ring, un- 

 less it is surrounded with a sufficient number of satellites properly 

 arranged ; and that Saturn is the only one so furnished. He shews 

 moreover, that the Sun has not its satellites, i.e. the planets, properly 

 arranged for supporting a ring, or that the only possible part of the 

 system for such a phenomenon would be just within the powerful 

 masses of Jupiter and Saturn, and that supposing the Sun ever had 

 been surrounded by some ring of matter comparable to the zodiacal 

 light, then defects in the maintaining power of the planets outside 

 might cause the centre of gravity of the ring to begin to move until 

 is was broken by being brought into contact with the Sun or some 

 of the interior planets, when either asteroidal planets or comets 

 might result under different circumstances. 



Pressure of business has prevented Prof. Pierce at present from 

 giving the whole of his mathematical investigations, but meanwhile 

 his results will be received with great faith, after the unusual 

 abihty and the scrupulous regard to absolute truth of hypotheses 

 which he shewed in his difficult researches into the mutual pertur- 

 bations of Uranus and Neptune. 



New Planet Eunomia. — As Messier complained that when grieving 



