COMETS. 



comet of 1678. 44. Blainplan's comet of 1819. 45. Pons's comet of 

 1819. 46. Pigott's comet of 1783. 47. Peters's comet of 1846. 

 48. Tabular synopsis of the orbits of the comets which revolve within 

 Saturn's orbit. 49. Diagram of the orbits. 50. Planetary character 

 of their orbits. III. ELLIPTIC COMETS, WHOSE MEAN DISTANCES ARE 

 NEARLY EQUAL TO THAT OP URANUS : 51. Comets of long periods first 

 recognised as periodic. 52. Newton's conjectures as to the existence 

 of comets of long periods. 53. Halley's researches. 54. Halley 

 predicts its re-appearance in 1758-9. 



24. IT may be asked, If the existence of a resisting medium be 

 admitted, whether the same ultimate fate must not await the 

 planets ? To this inquiry it may be answered that, within the 

 limits of past astronomical record, the ethereal medium, if it 

 exist, has had no sensible effect on the motion of any planet. 

 That it might have a perceptible effect upon comets, and yet not 

 upon planets, will not be surprising, if the extreme lightness of 

 the comets compared with their bulk be considered. The effect 

 in the two cases may be compared to that of the atmosphere upon 

 a piece of swan's down and upon a leaden bullet moving through 

 it. It is certain that whatever may be the nature of this resisting^ 

 medium, it will not, for many hundred years to come, produce the 

 slightest perceptible effect upon the motions of the planets. 



25. The masses of comets in general are, as will be explained,, 

 incomparably smaller than those of the smallest of the planets ; 

 so much so, indeed, as to bear no appreciable ratio to them. A 

 consequence of this is, that while the effects of their attraction 

 upon the planets are altogether insensible, the disturbing effects 

 of the masses of the planets upon them are very considerable. 

 These disturbances, being proportional to the disturbing masses, 

 may then be used as measures of the latter, just as the movement 

 of the pith-ball in the balance of torsion supplies a measure of 

 the physical forces to which that instrument is applied. 



Encke's comet, near its perihelion, passes near the orbit of 

 Mercury ; and when that planet, at the epoch of its perihelion, 

 happens to be near the same point, a considerable and measurable 

 disturbance is manifested in the comet's motion, which being 

 observed, supplies a measure of the planet's mass. 



This combination of the motions of the planet and comet took 

 place under very favourable circumstances, on the occasion of the 

 perihelion passage of the comet in 1838, the result of which, 

 according to the calculations of Professor Encke" , was the discovery 

 of an error of large amount in the previous estimates of the mass 

 of the planet. After making every allowance for other planetary 

 attractions, and for the effects of the resisting medium, the 

 existence of which it appears necessary to admit, it was inferred 

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