258 Comets. 



possible, by an ephemeris), to all observers who have shewn 

 that they take an interest in the inquiry, so as to ensure the 

 full and complete observation of the new comet, so long as 

 it remains within the reach of our telescopes. 



It is by no means merely as a subject of antiquarian inte- 

 rest, or on account of the brilliant spectacle which comets oc- 

 casionally afford, that astronomers attach a high degree of 

 importance to all that regards them. Apart even from the 

 singularity and mystery which appertain to their physical 

 constitution, they have become, through the medium of exact 

 calculation, unexpected instruments of inquiry into points 

 connected with the planetary system itself, of no small im- 

 portance. We have seen that the movements of the comet 

 of Encke, thus minutely and perseveringly ti'aced by the emi- 

 nent astronomer whose name is used to distinguish it, has 

 afforded ground for believing in the presence of a resisting 

 medium filling the whole of our system. Similar inquiries 

 prosecuted in the cases of other periodical comets, will 

 extend, confirm, or modify our conclusions on this head. 

 The perturbations, too, which comets experience in passing 

 near any of the planets, may aflFord, and have affoi'ded, 

 information as to the magnitude of the disturbing masses, 

 which could not well be otherwise obtained. Thus the ap- 

 proach of this comet to the planet Mercury in 1838 af- 

 forded an estimation of the mass of that planet the more 

 precious, by reason of the great uncertainty under which all 

 previous determinations of that element laboured. Its ap- 

 proach to the same planet in the present year (1848) will be 

 still nearer. On the 22d of November their mutual distance 

 will be only fifteen times the moon's distance from the earth. 

 It is, however, in a physical point of view that these bodies 

 offer the greatest stimulus to our curiosity. There is, beyond 

 question, some profound secret and mystery of nature con- 

 cerned in the plienomenon of their tails. Perhaps it is not 

 too much to hope that future observation, borrowing every 

 aid from rational speculation, grounded on the progress of 

 physical science generally (especially those branches of it 

 which relate to the etherial or imponderable element), may 

 ere long enable us to penetrate this mystery, and to declare 



