IV. — SIRONA AND CERES. 



Sirona. Ceres. 



Distance 2. 7G61 2. 7G73 



Period 1^80^.3 lC81*i.4 



Lougitude of perihelion . 152^ 53' 149^ 38' 



Longitude of ascending node 64° 26' 80° 47' 



Eccentricity 0.1439 0.0705 



Inclination 11° 35' 10^37' 



V. — URDA AND GERDA. 



In the American Journal of Science for February, 1877, Dr. Peters, 

 tbe discoverer of these asteroids, calls attention to several striking 

 coincidences between the elements of their orbits. The mean distance, 

 inclination, and ascending node of the former are very nearly identical 

 with those of the latter. "The fact of two planets moving iu the same 

 plane, with the same time of revolution, having also the line of apsides 

 iu common,* but with a widely different eccentricity," as Dr. Peters 

 justly remarks, "is worthy of note." The results of further observa- 

 tions will be looked for with interest. 



CONCLUSION. 



13. When the nebular hypothesis wa^ proposed by Laplace, but four 

 minor planets had been discovered. According to that hypothesis those 

 new members of the system had been formed by the breaking up of a 

 nebulous ring into fragments so small that no one had sufiBcient attrac- 

 tive force to unite the others about its center as a single spheroidal 

 mass. After the number of asteroids had been largely increased the 

 celebrated Plana regarded their phenomena as confiruiiug, if not dem- 

 onstrating, Laplace's theory. "The double fact," he remarks, "of the 

 multitude of those bodies and of their circulation in the same direction 

 around the sun, is now too imposing to admit an explanation of their 

 origin and formation different from that developed by Laplace, iu his 

 Systeme-du Monde."t 



But the form and extent of the zone of the minor planets, as well as 

 the known facts in regard to Saturn's rings, seem to require a moditica- 

 tion of Laplace's theory. Throughout the greater part of the interval 

 between Mars and Jupiter an almost continuous succession either of 

 very narrow rings or of extremely small planetary masses appears to 

 have been abandoned at the solar equator. The entire cluster, distrib- 

 uted throughout a ring whose outer radius exceeds the inner by IGO 



* Tbe longitudes of their perihelia differ by nearly 180°. 



t Note sur la Formation probable de la Multitude des Asteroides qui, entre Mars et 

 Jupiter, circulent autour du Soleil. Par Jean Plana. Presentee le 2 Mars, 185G, a 

 I'Acaddmie Koyale des Sciences de Turin. 



