Comets, 259 



whether it is really matter in the ordinary acceptation of the 

 term which is projected from their heads with such extrava- 

 gant velocity, and if not impelled, at least directed in its com'se 

 by a reference to the sun, as its point of avoidance. In no 

 respect is the question as to the materiality of the tail moi'e 

 forcibly pressed on us for consideration, than in that of the 

 enormous sweep which it makes round the sun in perihelio, in 

 the manner of a straight and rigid rod, in defiance of the law of 

 gravitation, nay, even of the received laws of motion, extending 

 (as we have seen in the comets of 1C80 and 1843) from near 

 the sun's surface to the earth's orbit, yet whirled round un- 

 broken ; in the latter case through an angle of 180° in little 

 more than two hours. It seems utterly incredible that in 

 such a case, it is one and the same material object which is 

 thus brandished. If there could be conceived such a thing 

 as a negative shadow, a momentary impression made upon the 

 luminiferous ether behind the comet, this would represent, in 

 some degree the conception such a phenomenon irresistibly 

 calls up. But this is not all. Even such an extraordinary 

 excitement of the ether, conceive it as we will, will afford 

 no account of the projection of lateral streamers ; of the effu- 

 sion of light from the nucleus of a comet towards the sun ; 

 and its subsequent rejection ; of the iri'egular and capricious 

 mode in which that effusion has been seen to take place ; 

 none of the clear indications of alternate evaporation and 

 condensation going on in the immense regions of space occu- 

 pied by the tail and coma, — none, in short, of innumerable 

 other facts which link themselves with almost equally irre- 

 sistible cogency to our ordinary notions of matter and force. 

 The great number of comets which appear to move in para- 

 bolic orbits, or orbits at least undistinguishable from para- 

 bolas during their description of that comparatively small 

 part within the range of their visibility to us, has given rise 

 to an impression that they are bodies extraneous to our sys- 

 tem, wandering through space, and merely yielding a local 

 and temporary obedience to its laws during their sojourn. 

 What truth there may be in this view, we may never have 

 satisfactory grounds for deciding. On such an hypothesis, 

 our elliptic comets owe their permanent denizcnship within 



