380 ANNUAL OF SCIENTIFIC DISCOVERY. 



their common centers of gravity ; perpetual collisions ere this would have re- 

 duced them to powder. "We assume now that the rings are fluid. Then they 

 may vary in form. It was first shown that they had varied by Otto Struve. 

 The diameter of the outer part of the ring is not known to have changed. 

 The inner edge is contracting, as it seems to me. Huygens, in 1657, made it 

 (allowing for the irradiation of his telescope) 6.5". Huygens and Cassini, in 

 1695, made it 6''; Bradley, in 1719, 5.4"; Herschel, in 1799, 5"; Struve, 

 senior, in 1826, 4.36" ; Encke and Galle, in 1838, 4.04" ; and Otto Struve, in 

 1851, 3.67". Does it decrease uniformly? I think it is decreasing more rap- 

 idly, and the present rate will bring the ring to an end, in certain parts of 

 it, in about 80 years from now. I will show in the meeting in section that 

 the planet does nothing either to maintain or destroy the equilibrium of the 

 ring. The satellites tend to maintain it in place. The ring is not gas ; its 

 density is nearly that of water. If the zodiacal light be a gaseous ring of the 

 earth it would need some solid parts to give body to it. May that gas be 

 the atmosphere around an infinity of small masses revolving about each other 

 and about the earth ? May there not be collisions among these revolving 

 masses that throw down parts of them to the earth ? Is not that as good a 

 reservoir of meteors as the moon ? Their melted state seems to lead us to 

 a lunar volcanic origin ; but unless some lunar volcano pointed expressly at 

 the earth were put in furious operation, such a bombardment could not hit 

 the earth with one in ten thousand of its projectiles. 



The action of Saturn would tend to bring a solid inflexible ring against 

 itself. But if almost the whole mass of the ring were gathered into a moon, 

 leaving the rest a ring not worth the name, then it might revolve as a satellite, 

 with the same side always toward the primary. The ring at all events must 

 rotate around the planet in about ten hours, so that were it solid any inequality 

 seen on one side would be on the other in about five hours. But it is often 

 seen to have an inequality remain on the same side for a number of days. 

 Now if the ring be liquid, it would revolve with a different velocity for dif- 

 ferent parts of the ring, so that a radius vector would pass over equal areas in 

 equal times. Those parts in perigee, so to speak, would therefore be smaller 

 and thinner in proportion to the velocity, and equal quantities of liquid would 

 pass any point at all times. In this state the planet does nothing to sustain 

 or destfo}^ the equilibrium of the ring, but the attraction of the exterior sat- 

 ellites would tend to maintain its equilibrium. 



The Adams's prize is from a fund established at Cambridge University, 

 England, in honor of the English discoverer of the planet Neptune. The 

 prize is to be given once in two years, for the best essay on some subject of 

 pure mathematics, astronomy, or other branch of natural philosophy, the com- 

 petition being open to all persons who have at any time been admitted to a 

 degree in this university. The fund having been accepted by the university, 

 the examiners announce the conditions of competition, and the method in 

 which the subject is expected to be treated. The successful candidate receives 

 about 130. 



