PORTION OF THE COSMOS. THE PLANETS. 375 



The possibility of determining by calculation,, even ap- 

 proximately, the probable epoch of such a cosmical event, 

 which should also be that of the origin of the small planets, 

 is more than doubtful, seeing the complications arising 

 from the great number of the supposed " fragments" with 

 which we are already acquainted, the secular retrogressions 

 of the apsides, and the movement of the nodal lines ( S1 ). 

 Olbers marked the region of the nodal line of the orbits of 

 Ceres and Pallas as corresponding to the northern wing of 

 Yirgo and to the constellation of Cetus. In the last-named 

 constellation, scarcely two years later, Juno was discovered 

 by Harding accidentally, however in the course of the 

 construction of a star catalogue : in the former, after a long 

 five years' search directed by the hypothesis, Olbers himself 

 discovered Yesta. This is not the place for deciding whether 

 these two results, standing by themselves, are sufficient to 

 support the hypothesis. The cometary mists or nebulosities, 

 in which the small planets were at first imagined to be enve- 

 loped, have disappeared before the examinations made with 

 more perfect instruments. Olbers attributed the conside- 

 rable alterations of light to which these planets were supposed 

 to be subject, to their irregular figure as "fragments of a 

 disrupted planet" ( 611 ). 



Jupiter. 



Jupiter's mean distance from the Sun is 5*202767 in 

 parts of the Earth's mean solar distance. The true mean 

 diameter of this largest of all the planets is 19294 German, 

 or 77176 English geographical miles, equal, therefore, to 

 11 '25 5 diameters of the Earth, and about 'th more than 



