Solar System. 49 



present imagine them to be. Upon the 

 whole, the astronomy of comets is very 

 imperfect ; for but little can be known 

 with certainty where but little can be 

 seen. Comets afford few observations, 

 on which to ground conjecture, and are 

 for the greatest part of their course be- 

 yond the reach of human vision. 



Like the planets, they are observed to 

 be opaque bodies, shining only by the in- 

 fluence of the sun, and like them are car- 

 ried along in their orbits, by the combi- 

 nation of the centripetal and centrifugal 

 forces ; sometimes seeming to go for- 

 wards, sometimes backwards, and some 

 times to be stationary. The great eccen- 

 tricity of their orbits make them liable to 

 suffer considerable alterations, from the 

 attraction of the planets, and of each oth- 

 er. 



They are called comets, from their 

 having a long tail, somewhat resembling 

 the appearance of hair. This, however, 

 is not always the case ; for some comets 

 have appeared, which were as well defi- 

 ned, and as round as planets ; but in gen- 

 eral, they have luminous matter diffused 

 around them, or projecting out from 

 them. 



