COMETS. 



403 



longer a meteor, a transient fire in our atmosphere. 

 It is no longer a messenger of evil. It is no longer 

 to be dreaded as a mysterious and unwelcome in- 

 truder among the celestial luminaries. It is to be 

 ranked among the great works of nature. It is 

 not only a curious object to behold, but it affords 

 an interesting subject of contemplation. We 

 delight to trace it through its long journey of three- 

 fourths of a century; and when the eye can no 

 longer distinugish it, to follow it in imagination, as 

 it pursues its upward flight, leaving planet after 

 planet far behind, penetrating the unexplored depths 

 of space, with an almost inexhaustible force, and 

 not ceasing its ascent, till it has doubled the dis- 

 tance of the most remote of the planetary bodies. 



Other comets are at length found, as might be 

 expected, to possess the character of revolving 

 bodies. A comet appeared in 1818, which" was re- 

 cognized as one which had been observed several 

 times before. Its orbit was of so small an extent, 

 compared with that of most comets, as to admit 

 of its period, or time of a complete revolution, 

 being computed. This was ascertained by Encke, 

 to be about three years and a third, and its return 

 was predicted to take place in June 1822. Astro- 

 nomers were apprized at the same time, that its 

 situation in the heavens, would be such as to render 

 it invisible in Europe and America ; but that it 

 would be seen by the inhabitants of New South 

 Wales. It actually returned according to the pre- 

 diction ; and after sufficient time had elapsed for 

 intelligence to arrive from that quarter of the globe 

 we were greeted with a regular set of observations, 

 made by Mr Rumker, an experienced astronomer, 

 at Paraimta, completely justifying the anticipations 

 of the sagacious Encke. This name has therefore 

 already become familiar to us, like that of Halley, 

 as a convenient means of designating acomet, with 

 which we are continually becoming more and more 

 acquainted, and through it, with the whole class to 

 which it belongs. As it completes a revolution in 

 about three years and a third, it has returned seve- 

 ral times since 1822, namely, in 1825, 1828, 1832, 

 1835 and 1838. These frequent returns have been 

 applied to a valuable purpose. They have served 

 to teach us that the celestial spaces are in all pro- 

 bability not absolutely void, as we have been ac- 

 customed to regard them. There is the strongest 

 reason for believing that this comet is resisted by 

 an ether or some extremely rare medium. At each 

 successive return, the evidence has accumulated, 

 till at last the position assumed seems scarcely to 

 admit of a doubt. The effect of the supposed 

 medium has been, by checking the motion of the 

 comet, in the direction of a tangent to its orbit, to 

 cause the attraction toward the sun to predominate, 

 whereby the path described is continually contract- 

 ed into a sort of spiral, the time of a revolution 

 at the same time being continually diminished. 

 The amount of this diminution is found to be about 

 two days each revolution, after making allowance, 

 with all possible precision, for the disturbing influ- 

 ence of the planets. No other effect has been ob- 

 served to take place, after the most careful scru- 

 tiny, except that which would necessarily arise 

 from the alleged cause. 



Another comet is now supposed to afford addi- 

 tional evidence on this all important point, which 

 gives an extraordinary interest to each successive 

 return of those comets whose periods are known. 

 " It cannot be doubted," says Sir John Herschel, 

 "that many more will be discovered, and by their 



resistance, questions will come to be decided, such 

 as the following: What is the law of density of 

 the resisting medium which surrounds the sun? Is 

 it at rest or in motion ? If the latter, in what 

 direction does it move ? Circularly round the sun, 

 or traversing space. If circularly, in what plane ? 

 It is obvious that a circular or vorticose motion of 

 the ether would accelerate some comets and retard 

 others, according as their revolution was, relative 

 to such motion, direct or retrograde. Supposing 

 the neighbourhood of the sun to be filled with a 

 material fluid, it is not conceivable that the circu- 

 lation of the planets in it for ages, should not have 

 impressed upon it some degree of rotation, in their 

 own direction, and thus may preserve them from 

 the extreme effects of accumulated resistance." 



Another comet, known as Biela's, appeared in 

 1826. It is recognised as being the same which 

 was observed in 1805 and in 1772. It was found 

 to have a period of six years and three quarters. 

 Its return was accordingly predicted to take place 

 in October, 1832 ; and it actually made its appear- 

 ance at the time and place assigned. It is remark- 

 able for its near approach, not to the earth itself, 

 but to the earth's orbit, the least distance being 

 less than the sum of the semidiameters of the 

 comet and the earth, so that if the earth had hap- 

 pened to be in that point, at the same time with 

 the comet, it would certainly have been involved, 

 to a greater or a less degree, in the nebulous mat- 

 ter of the comet. The earth, in fact, reached this 

 point a month later than the comet, and did not, 

 at any time, come within about fifty millions of 

 miles, or half the distance of the earth from the 

 sun. This comet, at its greatest distance from the 

 sun, departs about six times the earth's distance, or 

 somewhat beyond the orbit of Jupiter. Its last 

 appearance was in 1838. 



These are the only comets of whose periods we 

 have any certain knowledge. There are, however, 

 two others whose periods have been stated ; but 

 the times are not yet arrived for testing the accu- 

 racy of the predictions. One of these comets ap- 

 peared in 1264, and again in 1556 an interval of 

 292 years ; and it is expected again in 1848. The 

 other comet was observed by Newton in 1680, 

 and he predicted its return in 575 years ; a long 

 period must therefore elapse before the truth 

 of this calculation, or conjecture, can be verified. 

 The tail of this comet, which is several times 

 noticed in history, was of such vast extent, that 

 when the head of the comet was in the horizon, 

 the extremity of its tail was in the zenith. 



It is thus abundantly proved that comets are re- 

 volving bodies like the planets, and that they obey 

 the same great laws of attraction. There seems, not- 

 withstanding, to be a marked difference in their 

 physical constitution. Comets appear to be com- 

 posed of a substance extremely light compared with 

 that of the planets. Thus Halley's comet which, 

 as we have seen, was detained a year and eight 

 months by the attraction of Jupiter and Saturn, 

 stems to have had little or no reciprocal influence 

 upon either of these planets. A still more strik- 

 ing instance to the same purpose, occurs in acomet 

 which appeared in 1770, which, according to the 

 highest authority, had been revolving in an orbit 

 that did not admit of its being seen from the earth, 

 completing a revolution in about fifty years; but 

 passing in 1767 within a short distance of Jupiter, 

 its course was entirely changed, so as to bring it 

 near to the earth in 1770, and cause it to complete 

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