342 



COMETS. 



L&bbock, lias also investigated the subject, am! tli 

 Nautical Almanack, for 1835, gives the results ac 

 cording to all the tliree. The comet of Halley has 

 a period of about seventy-six years. 



Besides the comet of 1759, of which there hnvi 

 been four authenticated returns, there are two 

 others, of which something like a return has been 

 traced at long intervals. One of these JI;I>M( 

 its perihelion at about eight o'clock on the morn 

 in- of the Cth of July, 1264, reckoning mean 

 time at Greenwich ; and again, at a little past eight 

 o'clock, on the evening of the 21st of April, 1556. 

 Thus its period is about 292 years, and it may be ex- 

 pected in 1848. The perihelion distance, however, 

 of this comet, which was more than half tliat of the 

 earth, in 1264, had diminished an eighth part by 

 1 556 ; and, as this must have caused a great elonga- 

 tion of its orbit, and as, from the length of its period, 

 it must go far into the regions of space, there is no 

 knowing how both the time of its revolution, and the 

 form and position of its orbit, may have been altered. 

 The other comet, in the elements of whose orbit 

 there is a similarity, from which its identity might be 

 with probability inferred, appeared in 1532, ana again 

 in 1661, having thus a period of about 129 years. 

 The return of that comet should, therefore, have been 

 about 1790. In that year, three comets made their 

 appearance ; but neither of them resembled the one 

 of 1661. Two of them moved in the opposite direc- 

 tion ; and the remaining one was more than twice 

 the distance from the sun in its perihelion, and its 

 orbit at nearly double the angle with that of the 

 earth. 



The comet denominated Encke s comet, which has 

 engrossed the public mind generally, and the scien- 

 tific world in particular, has justly claimed and re- 

 ceived the careful attention of astronomers, since its 

 appearance in 1818 engaged professor Encke to con- 

 sider the elements of its orbit. He was enabled to 

 identify it with a comet described by Messrs Me- 

 chain and Messier in 1789, in the constellation 

 Aquarius; also with a comet discovered in 1795, by 

 Miss Herschel, in the constellation Cycnus; and 

 with the comet in 1805. The investigation of this 

 diligent professor enabled him, from his observations 

 on its appearance in 1818, to foretell its re-appear- 

 ance in 1822, and to state the probability of its not 

 being observable in our climate. This anticipation 

 was realized by its discovery in New South Wales, 

 in the observatory of the governor, Sir Thomas Bris- 

 bane, June 2, 1822 ; and the accurate observations 

 of 'Mr Hunker, who discovered it, afforded Encke the 

 means of reconsidering the true elements of its orbit, 

 and with additional confidence computing its return 

 for 1825. This occurred as was expected. The 

 fresh data afforded by that return were carefully col- 

 lated by the professor. It was observed again on 

 October 30, 1828. This comet affords particular 

 interest to the mind of the astronomer, though it does 

 not offer a splendid object to his eye. Its orbit is an 

 ellipse of comparatively small dimensions, wholly 

 within the orbit of Jupiter : its period is 1260 days, 

 or about three years and three tenths a much shorter 

 period than has hitherto appeared to comprise the 

 revolution of any other comet, with the exception of 

 one seen in 1770, which did not satisfy, as for as ob- 

 servation has been able to show, the prediction of the 

 period of five years and a half, which was attributed 

 to it. In the opinion of Encke and other astrono- 

 mers, this comet may afford an opportunity of prov- 

 ing that the heavens oppose a resisting medium to 

 the motion of bodies. The subject has been dis- 

 cussed in the Transactions of the Astronomical So- 

 ciety of London by the able mathematician Massotti ; 

 and that gentleman offers reasons for considering 



comets capable of affording a demonstration of a re- 

 sisting medium in the heavens, though planets may 

 give no indication of it. 



Another comet, which encourages the anticipation 

 of much astronomical gratification, is one which 

 Biela discovered, Feb. 27, 1826, ami which was af- 

 terwards seen by Gambart and others. It seems to 

 possess claims to the attention of astronomers similar 

 to that of Encke, it being conceived to revolve about 

 the siui in about six years and seven-tenths, and t<> 

 be the same as the comet which appeared in 177, 

 and that which appeared in 1806. Encke's comet 

 was in its perihelion, by computation, Jan. 10. 1829. 

 Two other comets remain yet to be noticed, the 

 elements of which have been calculated, but which 

 have not as yet been confirmed by returns. The 

 orbit of the first was calculated by Newton, from its 

 appearance in 1<<80, and he estimated the periodic 

 time to be 575 years. It may therefore be expected 

 in 2255. The second appeared in 1556, is supposed 

 to have a period of 292 years, and will therefore re- 

 turn in 1848. 



The comet of 1770, to which allusion has been 

 made, would lead us to conclude that we are still ig- 

 norant of many of the causes by which the form ot 

 the orbits of comets, and the times of their revolution 

 and return, may be disturbed. That comet moved 

 almost in the plane of the earth's orbit, having an 

 inclination of only about a degree and a half; ithad 

 been observed with great care ; and the result of the 

 observations was, that it should return about every 

 five years and a half. Instead of going out of the 

 system, as may be presumed to be the case with 

 those comets that have long periods and eccentric 

 orbits, its greatest distance could not be much greater 

 than that of Jupiter, while its mean distance from 

 the sun was not much more than three times the 

 perihelion distance of the earth. No comet, at all 

 answering to that one, has, however, been again dis- 

 covered ; and therefore the conclusion is, that there 

 are, within the system itself, causes which can com- 

 pletely alter the motions of these bodies ; but what 

 those causes are, other than the attraction of the 

 planets, has not yet been ascertained. 



One remarkable difference between the comets 

 and the planets is in the angles which their orbits 

 make with that of the earth. Leaving out the small 

 planets that have recently been discovered, all the 

 others are contained within a zone extending only 

 7 on each side of the earth's orbit ; and, with the 

 exception of Mercury (by far the smallest of the old 

 Dlanets), they are within half that space. But the 

 orbits of the comets are at all possible angles ; and 

 the number increases with the angle, so that they ap- 

 jroximate to an equal distribution, hi all directions, 

 ound the sun as a centre. The numbers that have 

 >een observed are as follows : Under 10 of incli- 

 nation, 8 ; under 20, 19 ; under 30, 26 ; under 40, 

 37 ; under 50, 47 ; under 60, 63 ; under 70, 79 ; 

 under 80, 88 ; and under 90, about 100. Thus by 

 ar the greater number of the comets have their 

 )aths out of the direction of those of the planets ; 

 md hence, though they be bodies of such consistency 

 as that their collision with the planets would produce 

 serious consequences, there is but little chance that 

 uch collision can take place. The comets that have 

 >een observed have made their passages through 

 very different parts of the solar system: 24 have 

 >assed within the orbit of Mercury ; 47 within that 

 of Venus; 58 within that of the Earth; 73 within 

 hat of Mars ; and the whole within that of Jupiter. 

 Of a bundled, or thereabouts, mentioned by Lalande, 

 about one-half have moved from west to east, in the 

 same direction as the planets, and the other lialf in 

 he opposite direction. The direct and retrograde 



