COMETS. 



Comets, of the damned, but also the nature of their punishment. 

 S """Y"^*' Wheeled from the remotest limits of the system, the 

 chillino- regions of darkness and cold, the comet wafted 

 them into the very vicinity of the sun ; and thus alter- 

 nately hurried its wretched tenants to the terrifying ex- 

 tremes of chilling cold and devouring file. 



By other astronomers, comets were destined for more 

 scientific purposes. They were supposed to convey 

 back to the planets the electric fluid which is constant- 

 ly dissipating, or to supply the sun with the fuel which 

 it perpetually consumes. They have been regarded 

 also as the cause of the deluge ; and we must confess, 

 that if a natural cause is to be sought for that great 

 event, we can explain it only by the shock of some ce- 

 lestial body. The transient effect of a comet passing 

 near the earth, could scarcely amount to any great con- 

 vulsion ; but if the earth were actually to receive a di- 

 rect impulse from one of these bodies, the consequences 

 would be awful. A new direction would be given to 

 its rotatory motion, and the globe would revolve round 

 a new axis. The seas, forsaking their ancient beds, 

 would be hurried by their centrifugal force, to the new 

 equatorial regions ; islands and continents, the abodes 

 of men and animals, would be covered by the univer- 

 sal rush of the waters to the new equator, and every 

 vestige of human industry and genius at once destroy- 

 ed. The chances against such an event, however, are 

 so very numerous, that there is no dread of its occur- 

 rence. 

 Opinions Various opinions have been entertained by astrono- 



respecting mers respecting the tails of comets. They were sup- 

 the tails of p 0Sec l by Appian, Cardan, and Tycho Brahe, to be the 

 comets. light of the sun transmitted through the nucleus of the 

 comet, which they believed to be transparent like a 

 lens. Kepler thought, that the impulsion of the solar 

 rays drove away the denser parts of the comet's atmo- 

 sphere, and thus formed the tail. Descartes ascribes the 

 tail to the .refraction of light by the nucleus. Newton 

 maintained, that it is a thin vapour raised by the heat 

 of the sun from the comet. Euler asserts, that the tail 

 is occasioned by the impulsion of the solar rays driving 

 off the atmosphere of the comet ; and that the curva- 

 ture observed in the tail is the joint effect of this im- 

 pulsive force, and the gravitation of the atmospherical 

 particles to the solid nucleus. Mairan imagines that 

 comets tails are portions of the sun's atmosphere. Dr 

 Hamilton of Dublin supposes them to be streams of 

 electric matter ; and Biot supposes with Newton, that 

 the tails are vapours produced by the excessive heat of 

 the sun ; and also, that the comets are solid bodies be- 

 fore they reach their perihelion ; but that they are af- 

 terwards either partly or totally converted into vapour 

 by the intensity of the solar heat. 



Of all these theories, that of Euler seems to be most 

 philosophical. Since the comets are composed chiefly of 

 nebulous matter, and have very large atmospheres, the 

 external atmospheric strata must be drawn towards the 

 comet by very slight powers of attraction, and will 

 therefore yield to the smallest impulse. From the 

 great density of the planets, on the contrary, and the 

 small size of their atmospheres, the external strata are 

 attracted towards them with a very great force, and 

 therefore cannot yield, like those of the comets, to a 

 slight impulse. Hence we see the reason why the co- 

 mets have tails, while none of the planetary bodies ex- 

 hibit such a phenomenon. Whatever opinion may be 

 entertained of this explanation, it must, at least, be ad- 

 mitted, that if light is a material substance, the atmo- 

 spherical particles of a comet may have their gravity 



I 



diminished to such a degree, either by their distance Comets. 

 from its centre, or by the rarity of the nucleus, as to *"~~V"~*' 

 yield to the impulse of the solar rays, and be forced 

 behind the nucleus, in the same manner as smoke 

 yields to the impulse of the gentlest breeze. 



If this theory is well founded, may we not form an 

 opinion of the density of comets, by comparing their 

 magnitude with the length of their tails ; and may we 

 not suppose, that those comets which have no tails axe 

 more dense than others, and exercise over the particles 

 of their atmosphere an attractive force, which enables 

 them to resist the impulse of the solar rays. 



In the article Astronomy, p. 675, we have already Dr Brews- 

 noticed the peculiarities in the motion of the comet of ter '' theor y 

 1 770, which we attributed, along with other astrono- £ f J h * c0 " 

 mers, to the disturbances which it had experienced from met ot 

 the action of the planets. As we are convinced, how- 1770. 

 ever, that this explanation is too vague and general to. 

 be satisfactory, we shall now lay before our readers the 

 theory of the singular phenomena of the comet of 1770, 

 as recently proposed by Dr Brewster. This theory, 

 while it seems to give a rational explanation of these 

 phenomena, affords, at the same time, a plausible solu- 

 tion of some other difficulties in astronomy. 



A comet appeared in the year 1770, and was careful- 

 ly observed for nearly four months by M. Messier. 

 When Prosperin and Pingre applied themselves to cal- 

 culate the elements of its orbit, they found that a para- 

 bolic path would not represent the observations of Mes- 

 sier, and hence they suspected that its orbit might be 

 sensibly elliptical. M. Lexell of St Petersburgh com- 

 puted its elements in an elliptical orbit, and he found 

 that its period was five years and a half, and that its 

 greatest distance from the sun did not much exceed 

 that of Jupiter. This curious subject was investigated 

 rather unsuccessfully by Slop, Sejour, and Lambert; 

 and a few years ago it attracted the particular notice 

 of the National Institute of France. At the request of 

 that learned body, Dr Burckhardt repeated all the cal- 

 culations with the utmost care, and the result of his 

 investigations was a complete confirmation of Lexell's 

 conclusions. 



Here then is a most singular anomaly in the motion 

 of this comet. While all the other comets which have 

 been observed, move in orbits stretching far beyond 

 the limits of the solar system, and revolve in periods of 

 long duration, the comet of 1 770 never wanders beyond 

 the orbit of Saturn, and completes its revolution in the 

 short period of five years and a half. The return of this 

 body, therefore, was confidently expected by astrono- 

 mers ; but though it must now have completed nearly 

 eight revolutions round the sun, and though more obser- 

 vations have been made in the heavens during the last 

 40 years than perhaps during the two preceding centu- 

 ries, yet the comet of 1770 has never re-appeared. We 

 are consequently entitled to conclude, that the comet 

 of 1770 is lost, which could happen only from its uni- 

 ting with one of the planets, whose orbits it cros-r 

 sed. Now, if such an union took place, two conse- 

 quences would obviously flow from it. The planet . 

 would suffer a sensible derangement in its motions, and 

 its atmosphere would receive a vast accession of that ne- 

 bulous matter, of which the comets are often wholly 

 composed. Here, then, we have two distinct criteria 

 to enable us to ascertain the individual planet by which 

 the comet was attracted. The path of the comet inter- 

 sects the orbits only of Venus, the Earth, Mars, the 

 four new planets, and Jupiter, and therefore it must 

 have united with one of these bodies, or with their, sa? 



