Comets. 253 



appear to indicate that the structure of a comet, must be 

 that of a hollow envelope, of a parabolic form, enclosing 

 near its voi-tex the nucleus and head. This would ac- 

 count for the apparent division of the tail into two princi- 

 pal lateral branches, the envelope being oblique to the line 

 of sight at its borders, and therefore a greater depth of illu- 

 minated matter being there exposed to the eye. In all pro- 

 bability, however, they admit great A'arieties of structure, and 

 among them may very possibly be bodies of widely different 

 physical constitution, and there is no doubt that one and the 

 same comet at different epochs undergoes great changes, 

 both in the disposition of its materials and in their physical 

 state. 



We come now to speak of the motions of comets. These 

 are apparently most irregular and capricious. Sometimes 

 they remain in sight only for a few days, at others for many 

 months ; some move with extreme slowness, others with ex- 

 traordinary velocity; while not unfrequently the two ex- 

 tremes of apparent speed are exhibited by the same comet 

 in different parts of its course. The comet of 1472 described 

 an arc of the heavens of 40° of a great circle* in a single day. 

 Some pursue a direct, some retrograde, and others a tortuous 

 and very irregular course ; nor do they confine themselves, 

 lilve the planets, within any cei-tain region of the heavens, 

 but traverse indifferently every part. Their variations in 

 apparent size, during the time they continue visible, are no 

 less remarkable than those of their velocity ; sometimes they 

 make their first appearance as faint and slow-moving objects, 

 with little or no tail ; but by degrees accelerate, enlarge, and 

 throw out from them this appendage, which increases in 

 length and brightness till (as always happens in such cases) 

 they approach the sun, and are lost in his beams. After a 

 time they again emerge, on the other side, receding from the 

 sun with a velocity at first rapid, but gradually decaying. 

 It is for the most part after thus passing the sun, that they 

 shine forth in all their splendour, and that their tails acquire 



* 120° in extent in the former editions; but tliis was the arc described in 

 longitude, and the comet at the time referred to had great north latitude. 

 VOL. XLVII. NO. XCIV. — OCTOBER 1849. S 



