48 PROF. O. REYNOLDS ON THE TAILS OF 



As to the possibility, or even the probability, of such a 

 discharge, I think it may be established on very good 

 grounds. 



The tails of comets may or may not be one with their 

 heads ; but whichever is the case, it is certain that the 

 difference in the appearance of comets and of planets in- 

 dicates some essential difference either in the materials of 

 which these bodies are respectively composed, or else in 

 the conditions under which their materials exist. Now, 

 ^rom the motion of comets, we know that their heads fol- 

 low the same laws of motion and gravitation as all other 

 matter ; and therefore we have good evidence, so far as it 

 goes, that comets and planets are similarly constituted as 

 regards materials. And since the appearance of a comet 

 changes very much as it passes round the sun, any assump- 

 tions with regard to the material of comets in order to 

 account for their difference from planets would not ac- 

 count for the variety of appearance the same comet pre- 

 sents at different times. On the other hand the conditions 

 of comets and planets must necessarily be very different, 

 from the extreme difference in the shapes of the orbits 

 they describe. Each planet remains nearly at a constant 

 distance from the sun (whatever that distance may be), 

 so that the heat or any physical effect the sun may have 

 upon it will also be constant ; on the comets its action 

 must change rapidly from time to time, particularly when 

 the comet is in certain parts of its orbit. Hence we may 

 say that the temperature and general physical condition of 

 planets is nearly constant, and that of comets, for the most 

 part, continually varying. 



There is, too, a very remarkable connexion between the 

 appearance of the comet and the rate at which the sun's 

 action on it changes. Herschel says : — " Sometimes they 

 first make their appearance as faint and slow-moving ob- 

 jects, with little or no tail, but by degrees accelerate, en- 



