420 HISTORY OP SCIENCE. 



exceeds two hundred. These planets have been discerned by many 

 different observers in various parts of the world, in England, Italy, 

 France, the United States, Germany, and Denmark ; but the elements 

 have in most cases been calculated by Germans. It will of course be 

 unnecessary to give here particulars of the several members of this 

 numerous group, but the following remarks may have some interest. 

 Their mean distances from the sun are from 200,000,000 to more than 

 300,000,000 of miles, and their orbits have various degrees of inclina- 

 tions up to that of Pallas, which is inclined to the elliptic nearly 35, 

 while No. 33 (Polyhymnia) has an orbit the longer axis of which is to 

 the shorter as 100 is to 94. The largest of the group is Pallas, with 

 a diameter of 670 miles, while others have been calculated to be less 

 than 20 miles in diameter. Vesta may be seen by the naked eye as 

 a star of the sixth magnitude, and Ceres has also been seen under 

 favourable circumstances without optical aid. The hypothesis of 

 Olbers is considered to give a satisfactory explanation of the relation 

 between these minor planets, whose orbits are so entangled, that if 

 they were represented by material rings, it would be possible to lift 

 them all up by taking hold of any one of them. 



At first the members of the asteroid group discovered after Ceres, 

 Pallas, Juno, and Vesta, received only names taken as usual from 

 classical mythology, such as Astraea, Hebe, Iris, Flora, etc. ; but when 

 discoveries began rapidly to succeed each other, it was found desirable 

 to distinguish each new planet of the group by a number as well as a 

 name. The cause of the many discoveries of these small planets in 

 recent times is not so much improvements in telescopes as the increased 

 skill and systematic diligence of observers, and especially the greater 

 detail of the more recent star-maps, in which stars of very low magni- 

 tudes are set down. By the aid of these maps a moving point is readily 

 distinguished from the neighbouring fixed stars, and its path followed. 

 It is probable that these miniature planets exist in indefinite numbers, 

 and that many more await discovery. At present hardly a month 

 passes in which the astronomical journals have not to record at least 

 one addition to the asteroid group. 



Encke, on attempting to determine the orbit of a comet which had 

 been observed by Pons at Marseilles at the end of the year 1818, found 

 that the observations could not be represented by any other than an 

 elliptical orbit, in which the comet had a period of about 3^ years. 

 It was soon noticed that this comet was similar to one which had 

 appeared in 1805, with regard to which Bessel had remarked that a 

 parabolic orbit could not represent the observations, and it was found 

 on investigation that a series of observations extending over thirty-three 

 years showed ten returns of the same comet. Encke correctly pre- 

 dicted the return of the comet in 1822, when it was observed at Para- 

 matta. Encke observed that the period of this comet appeared to be 

 diminishing as it seemed to return to the point of its orbit nearest the 



