ON COMETS. £11 



' If these vapours then rise to a considerable height above a 

 comet, before they are phosphorescentiv decomposed ; not 

 having, like tho^e of the Sun, a fixed limit of ascent; when 

 the comet approaches its perihelion these vapours will be- 

 come luminous, and the comet may appear as a lucid nebu- Comets with- 

 losity merely, with an irregular boundary, and without our outanuc l eu « 

 being able to distinguish any determinate disk : and when 

 such a disk is observed, it is probably owing, as in the case and with. 

 of the Sun, to the regularity of the lucid vapour, without 

 the body of the comet, which is surrounded by vapours, 

 being at all visible. 



If such be the cause of the light emitted by comets ; May not their 

 that is, if it be owing to luciferous vapours, which are de- nSe^feadTo" 

 composed; and if the rays of the Sun produce or accele- errours in cal- 

 rate this decomposition, when they approach their perihe- cu at,on * 

 lion ; their light may be at first faint, then increase more 

 than in proportion to their approximation, and decrease 

 more rapidly than in proportion to their, recession. If then 

 their apparent magnitude enter into the calculation of their 

 orbit, may not this be a cause of errour? 



Farther, as the light of comets appears to be certainly a comet may 



phosphoric, and not reflected; as phosphoric effects ungues- not be know n 



i i i i i • i • ,. „ a * its return 



tionably depend . on chemical operations proceeding from ft m the difFe- 



the composition of bodies: and as these operations in co- renceof itsa P" 

 mets do not appear to follow as regular a course as in the ' 



Sun; may not this be the reason, that we are so little capa- 

 ble of recogniziug comets on their return, though their ele- 

 ments have been accurately determined, and we have rea- 

 son to expect their return at stated periods? for they may 

 actually return at these periods without being known, on 

 account of their having a very different appearance; or even 

 without being visible, as the circumstances, which caused 

 them to emit light at the time when they were observed, 

 may not exist. This would render unnecessary the suppo*- «° that a per- 

 sition, which some astronomers have adopted, that in tHeir. ^bi^is^not ne* 

 very eccentric pfaths they meet with bodies, the vicinity of cessary to ac- 

 which obliges them to alter their course, ; count for it. 



One of the most remarkable phenomena, of ^comets, , the Tails of co- 

 fa*/ which they commonly have, excites us strongly to in- mets * 

 quire into its ca»s? : .but not to give up the rein*, too much 

 am P 2 to 



