COMETS. 



405 



different distances. This notion is of course 

 wholly inapplicable to those cases where the lumi- 

 nous train does not lie in the direction of the sun's 

 rays. It is, moreover, less entitled to considera- 

 tion, now that light itself, instead of being an 

 emanation of particles, is beginning to be more 

 generally considered as depending upon the undula- 

 tion of a medium, which medium may be that very 

 ether indicated by Encke's comet, as already men- 

 tioned. 



It seems to be taken for granted, that this train 

 is a material substance, and that this substance is 

 of the same nature with the nebulous atmosphere of 

 the comet ; and that it once made a part of that 

 atmosphere. But how is it separated and borne 

 along to such enormous distances? What power 

 retains it, and keeps it in its relative place, when, 

 as it should seem, the attraction exerted by such an 

 inconsiderable mass as that of a comet, is wholly 

 insufficient ? What should cause the extremity of 

 the tail to move ten or twenty times as fast as the 

 comet itself, when it is so many times farther from 

 the sun, as it must do in order to keep the same 

 position with respect to the central body? If it is 

 said in reply, that it is perpetually renewed by every 

 new emission of solar light, we beg leave to ask 

 what becomes of these long trains, when they have 

 served their momentary purpose, and why should 

 they not continue to reflect the light that falls upon 

 them, long after they have been generated ? thus 

 producing a broad luminous tract throughout the 

 space over which the tail has swept. 



It is indeed, generally admitted, that more or less 

 of the luminous train is left behind, and wholly de- 

 tached from all connection with the body whence 

 it proceeded. This is rendered probable, not only 

 for the reasons above suggested ; but also from the 

 fact that in several instances this appendage is 

 found to become shorter and shorter, and less and 

 less conspicuous, at each successive return. Not 

 only the train, but the body of the comet seems to 

 be wasting away, as in the case of Biela's, which 

 at its last return could scarcely be discerned with 

 a good ten feet telescope, in the most skilful 

 hands ; and was actually invisible to the inhabitants 

 of America; for the want of sufficient aid to the 

 natural sight. 



If comets are thus destined to part with the 

 matter of which they are composed, and the celes- 

 tial spaces are continually strewed with this highly 

 rarified, and all but immaterial substance, we have 

 arrived at a probable source of that medium, which 

 seems to have been detected by the motions of 

 Encke's comet. We may remark also, that the sub- 

 stance surrounding the sun, and lying in the direc- 

 tion of its equator, and extending many millions of 

 miles, giving rise tc what is called the zodiacal 

 light,' may have the same origin, that is, may be 

 composed of the tails of innumerable comets.f 

 Indeed, the sun itself may be said to have a tail or 

 train extending in opposite directions, borrowed, 

 perhaps, by little and little from each passing 

 comet , and this train may in some degree resemble 



* Thi? light is of a pyramidal or leaf-like shape, having the 

 fun for its basis and the ecliptic, or rathorthe solar equator, for 

 its axis, or line, of direction. It is faint like the t;iil of 11 comet, 

 or the milky way, and is for the most part confounded with tin- 

 twilight, except in low latitudes, where it is more frequently 

 seen. Hut in the month of April, in the evening, or in Octo- 

 ber, in the morning-, it makes so large an angle with the hori- 

 zon in the temperate' latitudes, and oxteinl-s RO high, that it 

 may be seen after the twilight has ceased in the evening-, and 

 before it begins in the morning. 



t Herschcl's Astronomy Art. G2S. 



that of a comet, and be a conspicuous object to a 

 spectator situated in such a manner as to admit of 

 its being seen unmixed and unobscured by the liglit 

 of his own atmosphere. 



It will be inferred, we are aware, from what we 

 have said, that comets may, by slow degrees, be 

 dispersed through the celestial spaces and absorbed 

 into the sun's atmosphere, and thus cease to be 

 comets any longer. There is also another way in 

 which they may terminate their existence as comets 

 This very medium, this solar atmosphere, strewed 

 with the wreck of we know not how many of these 

 ill fated bodies, seems fitted to prove destructive to 

 many more. Encke's comet, by its short period 

 and frequent returns, affords the best opportunity 

 hitherto presented, of judging of the consequences 

 that must result from such a state of things. This 

 comet is evidently approaching nearer and nearer to 

 the sun, each revolution. There is no cause known 

 to astronomers that can save it from its impending 

 fate ; and hereafter, when we shall have become 

 familiar with the return of comets, and are well 

 acquainted with their diminishing periods, it may 

 become an object of emulation to astronomers, to 

 calculate and foretell the time when a comet shall 

 terminate its career by falling into the sun. The 

 comet of 1680 has already approached so near as to 

 be only one sixth of the sun's diameter distant 

 when in perihelion. It appeared in Newton's time, 

 and no doubt seems to have existed in the mind of 

 that great man, that it would, at some future epoch, 

 cease to revolve. This opinion he retained to the 

 latest period of his life, and he seems to have made 

 some estimate of the time that might be expected 

 to elapse before such a catastrophe would take 

 place ; " possibly," he says, " after five or six re- 

 volutions." 



This idea of the destiny of comets, that they 

 were designed ultimately to fall into the sun, and 

 be consumed, was the more readily received for- 

 merly, when the sun was regarded as a mass of ig- 

 nited matter, that required replenishing like our 

 domestic fires. Light itself, moreover, being sup- 

 posed to be an emission of particles, it seemed to 

 be necessary to find some means of supplying this 

 continual exhaustion of the sun's substance. Other 

 and more remote phenomena were referred to as 

 strikingly analogous to the case in question. Se- 

 veral of the fixed stars had, at different times, sud- 

 denly burst forth with great splendour, so as to sur- 

 pass even Jupiter and Venus, and in some instances 

 to be visible in the day time. These stars, it was 

 supposed, owed their increased brilliancy to a fresh 

 supply of fuel in the form of a precipitated comet. 

 Such are the conjectures and speculations of some 

 of the greatest men the world has produced. 



But among astronomers of the present time, who 

 allow this tendency in cometary bodies toward the 

 sun, without being able to point out any sufficient 

 check, some maintain that matter so exceedingly 

 rare, and indeed matter of any kind and of any 

 form with which we are acquainted, must in all 

 probability be completely volatilized by the solar 

 heat, and absorbed into the solar atmosphere, long 

 before it could reach the body of the sun. It is 

 believed that some comets already give indications 

 of a rapid approach to this sort of dissolution. 



Another fruitful subject of speculation relates to 

 the possibility or probability of a comet's coming 

 so near the earth, as to occasion a great revolution 

 in the physical condition of our globe. The nearest 

 approach of one of these bodies that has hitherto 



