BOOK OF NATURE LAID OPEN. 205 



directions. These comets, in one part of their orbit, 

 called the perihelion, approach very near the sun, 

 and appear all in a blaze: this appearance, however, 

 they gradually lose, as they fly off, with inconceivable 

 rapidity, to the opposite part of their orbit, called, by 

 astronomers, their aphelion ; there they are totally di- 

 vested of their luminous tail, and, being at such an 

 immense distance from the sun, it is concluded they 

 must experience a degree of cold, of which we can 

 have no adequate conception. There are a variety 

 of opinions as to the number of comets belonging to 

 our system: Riccioli enumerated 154; others assert 

 that 450 had been seen previous to 1771 : the tables 

 of Berlin estimate them at 700, and some have even 

 supposed that there are millions. And, as these bo- 

 dies appear but seldom, and their stay is so short, it 

 is not surprising that so much ignorance should pre- 

 vail respecting them. It is, however, now sufficient- 

 ly ascertained, that these, like the planets, shine not 

 by their own, but by a reflected light; and, were it 

 not for the sun, these bodies, instead of alarming our 

 fears by their glaring magnificence, would move un- 

 seen and unheeded by us; for, however naturalists 

 may have differed respecting the composition of their 

 tails, it seems now to be pretty well understood that 

 they possess nothing fiery or combustible in them, 

 and may, with more propriety, be likened to the mild 

 radiance that proceeds across the street from the il- 

 luminated glass in an apothecary's window in a dark 

 night, than to any thing of a baneful or pestilential 

 nature. 



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