VARIATIONS OF APPEARANCE. 



unknown force by which it is repulsed in the opposite direction, 

 and so forms the tail. * 



(4.) That this acts unequally on the cometary matter, which is 

 not all vaporised, and of that which is, a considerable portion is 

 retained, so as to form the head and coma. 



(5.) That this force cannot be solar gravitation, being contrary 

 to that in its direction, and very much greater in its intensity, as 

 is manifest by the enormous velocity with which the matter of the 

 tail is driven from the sun. 



(6.) That the matter thus repelled to a distance so great from 

 a body whose mass is so small must, to a great extent, escape 

 from the feeble influence of the gravitation of the mass composing 

 the head and coma, and, unless there be some more active agency 

 in operation, a large portion of such vaporised matter must be 

 lost in space, never to reunite with the comet. This would 

 lead to the consequence, that at every passage through its 

 perihelion the comet would lose more and more of its vaporisable 

 constituents, on which the production of the coma and tail 

 depends, so that, at each successive return, the dimensions of 

 these appendages would be less and less, as they have in fact 

 been found to be. 



99. On receding from the sun after its perihelion, the comet 

 was observed under very favourable circumstances at the Cape 

 by Sir J. Herschel and Mr. Maclear. It first reappeared there 

 on the 24th of January, under an aspect altogether different 

 from that under which it was seen before its perihelion. It 

 had evidently, as Sir J. Herschel thinks, undergone some great 

 physical change, which had operated an entire transformation 

 upon it. 



" Nothing could be more surprising than the total change 

 which had taken place in it since October. ... A new and 

 unexpected phenomenon had developed itself, quite unique in the 

 history of comets. Within the well-defined head, somewhat 

 eccentrically placed, was a vivid nucleus resembling a miniature 

 comet, with a bead and tail of its own, perfectly distinct from 

 and considerably exceeding in intensity the nebulous disc or 

 envelope which I have above called the ' head.' A minute bright 

 point, like a small star, was distinctly perceived within it, but 

 which was never quite so well defined as to give the positive 

 assurance of the existence of a solid sphere, much less could any 

 phase be discerned." * 



100. The phenomena and changes which the comet presented 

 from its reappearance on the 24th of January, until its final 



* "Cape Observations," p. 397. 



205 



