128 ON COMETS. 



tember, had become unmistakable, and continued to 

 increase in amount as the latter extended in apparent 

 dimension, till it assumed at length that superb aigrette- 

 like form, like a tall plume wafted by the breeze, which 

 has never probably formed so conspicuous a feature in 

 any previous comet. To a certain extent, it is a common 

 enough feature in the tails of comets, and is usually re- 

 garded as conveying the idea of their moving in a 

 resisting medium ; in a space, that is to say, not quite 

 empty, as smoke is left behind a moving torch. But 

 this is a very gross and inadequate conception of the 

 peculiarity in question. The resistance of the " ether," 

 such as the phenomena of Encke's comet already 

 noticed, may be supposed to indicate, is far too infin- 

 itesmally small to be competent to produce any per- 

 ceptible deviation from straightness. Nor is it at all 

 necessary to resort to any such explanation of the fact. 

 Such an appearance would naturally arise from a combi- 

 nation of the motion the matter of the tail had (in 

 participation with that of the nucleus) with the impulse 

 given it by the sun each particle of it describing, from 

 the moment of quitting the head, an orbit quite different 

 from that of the latter; being necessarily, under the 

 influence of the repulsive force directed from the sun, a 

 curve of the form called by geometers an hyperbola, 

 nearly approaching to a straight line, and having its con- 

 vexity turned towards the sun : the visible form of the 

 tail (be it observed) being, not the perspective view of 

 such an orbit, but that of the portion of space contain- 

 ing, for the time being, all those particles, each describ- 



