OUTLINES OF NATURAL PHILOSOPHY. 



. A Comet, when it first appears, is usual- 

 ly surrounded by a faintly luminous vapour, to 

 which the name of Coma has been given. As 

 the Comet approaches the Sun, the coma be- 

 comes more bright, and at length shoots out 

 into a long train of luminous transparent va- 

 pour, very much resembling a streamer, and in 

 a direction opposite to the Sun. This forms the 

 Tail of the Comet. 



As the Comet retires from the Sun, the tail grows 

 less, and resumes nearly its first appearance. 

 Those Comets which never come very near the 

 Sun, have nothing but a coma or nebulosity round 

 them during the whole time of their continuance. 



The tail is always transparent, so that the stars are 

 distinctly seen through it, as they are even said to 

 have been in some instances through the central 

 part, or what was supposed the nucleus of the Co- 

 met. 



The length and form of the tail are very various. 

 Sometimes it is only a few degrees, ait others it is 

 more than a quadrant. In the great Comet which 

 appeared in 1680, the tail subtended an angle of 

 70 ; in that of 1618, an angle of 104. 



The tail sometimes consists of diverging streams of 

 light; that of 1744 consisted of six, all proceeding 

 from the head, and all a little bent in the same di- 

 rection. 



The 



