1 06 CELESTIAL PHENOMENA. 



greater length on these phenomena as well as on comets, 

 and notice particular cases more than might appear suitable 

 in a general view of nature, it is not without a purpose. 

 The great diversity of character in different comets has been 

 already mentioned. The little knowledge which we yet pos- 

 sess of the physical qualities of these bodies, renders it 

 difficult to separate the essential from the accidental in phse- 

 nomena recurring at intervals, and which have been observed 

 with very unequal degrees of accuracy. It is only the mea- 

 suring and computing parts of the astronomy of comets, 

 which, in modern times, have made such admirable progress: 

 in the present imperfect state of our knowledge, therefore, a 

 scientific consideration must restrict itself to physiognomical 

 diversity in the nucleus and tail, to instances of remarkable 

 approximation to other cosmical bodies, and to extreme 

 cases either of dimensions of orbit, or of periods of revolution. 

 In these phenomena, as well as in' those which will next be 

 treated of, fidelity to nature can only be sought in the 

 careful description of individual instances, and by such an 

 animated and graphic mode of expression, as may serve to 

 bring the reality vividly before the mind. 



Shooting stars, fire balls, and meteoric stones, are with 

 great probability regarded as small masses moving with 

 planetary velocities in space, and revolving in conic sections 

 round the Sun, in accordance with the laws of universal 

 gravitation. These masses approach the Earth in their 

 path, are attracted by her mass, and enter our atmosphere, 

 becoming luminous at its limits ; when they frequently let 

 fall stony fragments, heated in a greater or less degree, and 

 covered with a shining black crust. A careful investigation 

 of what has been observed at the epochs when periodic 



