ASTRONOMY. 193 



SECT. XI. 



OF COMETS. 



201. A COMET is a luminous body, which ap- 

 pears in the heavens only for a limited time, 

 seldom exceeding a few months ; during which, 

 beside the diurnal motion, of which it partakes 

 in common with the other heavenly bodies, it 

 has always a motion peculiar to itself, by which 

 it changes its place among the fixed stars. Its 

 appearance is usually that of a collection of va- 

 pour, in the centre of which is a nucleus, for the 

 most part, but indistinctly defined. 



In some Comets, the peculiar motion is progressive, 

 in others retrograde. In the same comet, the 

 motion is all nearly in one plane ; but in different 

 Comets, these planes make all different angles with 

 the ecliptic. At the beginning and end of the ap- 

 pearance of a Cometj it deviates from the plane in 

 which the middle part of its course lies. NEWTON 

 de Systemate Mundi, 59. The Comets have no 

 parallax, and are therefore certainly beyond the 

 limits of our atmosphere. 



VOL. II. N 202. A 



