148 HISTORY OF ASTEONOMY. 



whose longer axis the sun exercises a directive power, as 

 it would on a magnet, if it were itself magnetic ; or rather 

 as a positively electrified body would on a non-conduct- 

 ing body of elongated form having one end positively, 

 and the other negatively excited. 



As a comet approaches the sun, a portion of its matter 

 appears to be converted into vapor. In this vaporization, 

 the two electricities might be separated, the nucleus and 

 tail being in opposite electrical states. If now we sup- 

 pose the sun to be in a permanently excited electrical state, 

 we have an explanation of the repulsive force which has 

 been ascribed to the sun. If the repulsive force of the 

 sun upon the particles of the tail should overcome the 

 attraction of the nucleus, they must be driven off irre- 

 coverably. Such a separation could hardly be accom- 

 plished without carrying off some portion of the gravi- 

 tating matter ; and thus a new comet would be formed, 

 as in the case of Biela. 



Sir John Herschel mentions another mode in which 

 the division of a comet might be effected. The oscilla- 

 tions of a fluid covering a central body may, under cer- 

 tain conditions as to the coercive power of that central 

 mass, cease to continue of small extent, and may increase 

 in magnitude beyond any limit which analysis is capable 

 of assigning, even to the extent of destroying the con- 

 tinuity of the fluid, and separating it into distinct masses. 

 If such an extreme case could ever occur, it must be in a 

 comet like Biela's, consisting of a mass of vapor with 



