PHYSICAL CONSTITUTION OF COMET8. 



483 



PHYSICAL CONSTITUTION OF COMETS. 



OF a.l the objects which attract attention in the heavens, none have excited 

 feelings of greater awe, or awakened sentiments of more intense curiosity, than 

 comets. What are these bodies ? or are they bodies at all ? What is their 

 character and constitution ? Whence do they derive their light ? Do they be- 

 long to our system ? Whence have they come, and whither do they go I Are 

 they, as was long believed, of the same class as the aurora borealis ? Although 

 much still remains to be discovered before full, clear, and definite answers can 

 be given to these and similar questions, yet much that is interesting has been 

 ascertained by the labors chiefly of contemporary astronomers. We shall, on 

 "the present occasion, present what is certainly known in as brief a space as 

 possible. 



ORBITUAL MOTIONS OF COMETS. 



Comets are attached to the solar system by the tie of gravitation, and in their 

 motions round the sun are governed by the same law of attraction, as that 

 which operates on the planets. Since they are susceptible of gravitation, they 

 must therefore be material. 



In their motions, however, they present circumstances strikingly different 

 from those which characterize the planets. The law of gravitation determines 

 nothing regarding the orbit of a body in moving round the sun, except that it 

 be one or other of those curves called conic sections, and that the place of the 

 sun shall be ihe focus of the curve. Subject to this restriction, the orbit of a 

 revolving body may be very various in magnitude, form, position, and direction. 

 The orbits of the planets are, nevertheless, all very nearly of the same form, 

 being all nearly circular, and all in the same position, being all very nearly in 

 the plane of the ecliptic; and they all move in the same direction, being that of 

 the annual motion of the earth. The comets observe none of these charac- 

 teristics in their orbitual motions. Their orbits vary indefinitely in form. None 



