406 



COMETS. 



taken place, to far as our knowledge extends, wa- 

 that of the comet of 1770, as already mentioned. 

 The least distance, in tins case, was no less than 

 1 ,456,840 miles. No effect upon the earth or moon 

 was perceived, although an exceedingly slight one, 

 if it had existed, must have been detected. 



A much nearer approach than this is liable to 

 take place, from the known position of the orbits 

 of other comets whose elements have been calculat- 

 ed. The great comet of 1680, so remarkable in 

 other respects, is also distinguished as coming near 

 to the earth's path. Its least distance is about 

 twice that of the moon, or 440,000 miles. Its at- 

 tractive force, therefore, would be only one-fourth 

 of that of the moon, on the supposition that it con- 

 tained as much matter. Even this force would be 

 exerted only for a short time, not long enough, 

 probably, to produce any sensible disturbance in 

 the waters of the ocean. The effect of the moon 

 in causing the tides, is the accumulated result of a 

 long continued effort, with comparatively little 

 change in the position of the attracting body, or 

 the direction of its action. With all the force 

 that the moon actually exerts, it would have very 

 little, if any, sensible influence in disturbing the 

 ocean, if it were to dart by us with the rapidity of 

 the comet under consideration. 



Biela's comet has already been described as 

 coming so near the earth's track, as to be liable 

 actually to involve a part of the earth at least, in 

 its nebulous atmosphere. Let us now see what 

 chance there is of the earth's being, for instance, 

 within half a million miles either way, of the par- 

 ticular point of danger. There being in the cir- 

 cumference of the earth's orbit six hundred millions 

 of miles, in round numbers; on the supposition that 

 the comet passed the point in question, once a 

 year, there would be an equal chance for each por- 

 tion of a million of miles, that the earth would be 

 there at that juncture ; of course, according to the 

 doctrine of chances, the earth may be expected to 

 be within half a million of miles of the point 

 under consideration, once in six hundred years, it 

 being supposed that the comet passes this point 

 once a year. But as it actually passes it only once 

 in six years and three quarters, the above period 

 must be increased in the same proportion ; that is, 

 the near approach of the eartb and comet, above 

 referred to, cannot be supposed to occur more than 

 once in about forty centuries. 



Allowing the possibility of the event in ques- 

 tion, what would be the probable consequences of 

 a near approximation, or even a rencontre ? Not, 

 so far as we can judge, a violent shock or concus- 

 sion, like that occasioned by one vessel running 

 against another, not a sudden and overwhelming 

 rush of the waters of the ocean over the land. 

 This comet of Biela, according to the best infor- 

 mation we possess, seems to be merely a collection 

 of the thinnest possible vapour. Let us imagine a 

 person in a balloon approaching a cloud. The ob- 

 ject might seem a formidable one ; but the aeronaut 

 is plunged in the vaporous mass before he is aware 

 of it. He is sensible of no material change ; he 

 has felt no jar. He only perceives a little obscu- 

 rity in the objects around him, and wonders what 

 has become of the cloud that seemed so menacing 

 in the distance. So it may be with regard to 

 comets. We may have already encountered one of 

 these bodies without ever dreaming of it. We 

 may have been enclosed in its vast envelope with- 

 out perceiving any change, except, perhaps, some 



peculiar hue in the atmosphere, or some tincui 

 tint iu the objects about ti-. We are passing fre- 

 quently through parts of space that have, in all 

 probability, been visited by the enormous train of 

 some comet, and which, we have reason to believe, 

 still retain traces of this substance. It is hardly 

 possible to conceive that the earth should have 

 escaped all contact with an clement, so widely dil- 

 fused, or that it should (ail to attract and appro. 

 priate to itself sonic portion of this matter. We 

 see the zodaical light, supposed to be identical with 

 the substance in question, embracing the orbits oi 

 Mercury and Venus, and of course enveloping thc.-e 

 planets, and mixing with their atmospheres, and 

 subjecting them to all the consequences that we 

 seem to dread from the introduction of matter so 

 foreign. There is little reason to believe that 

 the earth is exempt from the same lot, and little 

 ground to apprehend evil from a condition that 

 seems the necessary result of the original constitu- 

 tion of the system to which we belong. What the 

 tail of a comet is, it were vain to conjecture. 

 Some future chemist may perhaps tell us. But 

 whatever it be, whether watery vapour or any 

 other vapour, or one of the permanent gases, or 

 something different from all these, and wholly un- 

 like any of our aeriform substances, there is little 

 reason to suppose that it would, on the whole, be 

 injurious. Even if it were deleterious in itself, if 

 it were the most virulent poison, still, according to 

 all that we know of it, it is so exceedingly rare, and 

 would constitute so very small a proportion as an 

 ingredient of the air we breathe, that no evil could 

 be apprehended from it. The fixed air and other 

 noxious gases, that are disengaged by volcanoes and 

 discharged into our atmosphere, may often be found 

 to exist in much greater quantity, compared with 

 the vital principle, especially in the vicinity of the 

 volcano, than we are liable to be exposed to, from 

 the largest influx from a comet's train. Such an ac- 

 cession of matter to our globe, although it is be- 

 lieved to have actually taken place, perhaps more 

 than once, has not as yet been distinctly and satis- 

 factorily recognised. There are several instances 

 on record, of a peculiar state of the atmosphere, 

 that has been ascribed to the presence of matter 

 from this source. The following are Mr Arago's 

 statements and reasoning upon this subject. 



" The fog of 1783 began nearly on the same day 

 (the 18th of June) in places very distant from each 

 other, as Paris, Avignon, Turin, Padua. It ex- 

 tended from the northern coast of Africa to 

 Sweden; it was also observed in a great part of 

 North America ; it lasted more than a month. 

 The air, at least that of the lower regions, did not 

 appear to be its vehicle, because in some places it 

 came on with a north wind, and in others with a 

 south or east wind ; travellers found it on the 

 highest summits of the Alps. The abundant rains 

 which fell in June and July, and the highest winds, 

 did not disperse it. In Languedoc, its density was 

 occasionally so great that the sun did 'not become 

 visible, in the morning, till it was 12 above the 

 horizon ; it was very red the rest of the day, and 

 might be looked at with the naked eye. This fog 

 or smoke, as some meteorologists have called it, 

 had a disagreeable odour. The property by which 

 it was particularly distinguished from common fogs, 

 was its being, by all accounts, very dry, whereas 

 most fogs are moist. At Geneva, Senebier found 

 that the hair hygrometer of Saussure, which in real 

 fogs stands at 100, ranged in the midst of th 



