s8 



KNOWLEDGE & SCIENTIFIC NEWS. 



[March, 1907. 



tinualiy varied from the 3 1 1 1 to the (2nd of October. At 

 one time two or three emanations were seen to issue in 

 different directions, these having forms sometimes like 

 thai "I .1 gas flame coming from a Rattened opening, at 

 other times only slightly divergent, and again occasion- 

 ally only one jet was seen. When more than one such 

 jet or emanation was visible, the principal jet ol light 

 oscillated in direction tn and fro on either side of the 

 line directed towards the Sun. " like a compass needle 

 thrown into vibration and oscillating about a mean 

 position." Sir J. 1 lerschel concluded, from his own ob- 

 servations and those of others, that the matter of the 

 nucleus is largely converted into vapour by the Sun's 

 heat, and escapes in jets and streams from the parts 

 turned towards the Sun. This matter is, however, pro- 

 vented from proceeding in this direction by some force 

 directed from the Sun, much more powerful than gravi- 

 tation (and repulsive). Being thus repelled from the Sun 

 with considerable velocity, it must leave the nucleus 

 altogether, and, consequently, at each approach the 

 comet must lose a portion of its substance, for the 

 feeble attractive power of the nucleus will prevent this 

 matter being retained within the comet's sphere of at- 

 traction, and it will be too far away to be re-absorbed 

 afterwards. ITius it is probable that at each ap- 

 parition the comet will be less conspicuous. After 

 passing its perihelion, the comet was not seen again till 

 near the end of Janur.ry, when it had no longer a tail, 

 but was seen as a small, round disc, surrounded by a 

 " coma " or nebulous envelope. As the comet gradually 

 receded from the Sun this coma disappeared as though 

 absorbed into the disc, and this latter increased greatly 

 in size, so that during one week (from January 25 to 

 February 1) it increased in volume 40 times. This in- 

 crease of size continued, so> that mainly from this cause 

 it became invisible, its illumination iDecoming fainter 

 and fainter as its size increased. The shape of the 

 disc changed gradually from a nearly' circular form 

 to that of a paraboloid. The nucleus meanwhile re- 

 mained nearly unchanged, but the ray or jet pro- 

 ceeding from it increased in length and brightness, its 

 direction being along the axis of the paraboloid. " If," 

 says Hcrschel, " the office of the jets was to feed the tail, 

 the office of the ray would seem to have been to conduct 

 back its successively condensing matter to the nucleus." 



The comet's envelope and ray gradually faded, and as 

 last seen it had the same form as in the previous 

 August, viz., that of a small, round nebula, with a 

 bright point near the centre. In all, it was visible from 

 the 5th of August, 1835, to the 5th of May, 1836, a 

 period of 9 months. 



The period of revolution of this comet is given in 

 Herschel's " Outlines of Astronomy," as 27,865.74 days, 

 so that, since it passed its perihelion on the 15th of 

 November, 1835, it should again return to this position 

 on March 2, 1912; but on account of the considerable 

 disturbing action of the planets Jupiter, Saturn, and 

 Uranus, the actual date may differ considerably from 

 this. So eccentric is the position of the Sun in its orbit, 

 that whilst at perihelion the comet's distance from the 

 Sun is about 0.586 of the earth's distance, or about 55 

 millions of miles, it recedes to a distance of 35.4 times 

 that of our earth, or about 3,300 millions of miles, con- 

 siderably greater than that of Neptune. Whilst the 

 planets all move in orbits lying nearly in the same plane, 

 the comet's orbit makes an angle of 17 with the 

 Ecliptic (plane of the earth's orbit), and its motion 

 therein is in a direction contrary to that of the planets 

 (and of most of the other short period comets), or is 



retrograde. Whilst the planets move in a direction op- 

 posite to the hands of a clock, as seen from our northern 

 latitudes, the comet of Halley moves in the "clock- 

 wise " direction (as shown in the accompanying dia- 

 gram'). This comet, as also five others, viz., Pons' 



Comet, seen in 1812 and [884; Olbers' Comet, seen 



Perihcl 



Aphelion ., 35-4 



Longitude of Perihelion 305 in-i 



, ,, ascending node 55°(&) 



in 1 81 5 and 1887; De Vico's Comet of 1846; Brorsen's 

 Comet of 1847, and Westphal's Comet of 1852, passes 

 near Neptune's orbit at its aphelion, and these comets 

 are sometimes known as Neptune's family of comets. 

 If at any time a comet enters our system from an infinite 

 distance, moving in a parabola under the Sun's attrac- 

 tion, it will have its motion either accelerated or 

 retarded when it comes near any of the planets. The 

 smallest increase of velocity will change the parabolic 

 orbit into a hyperbolic one, the smallest decrease will 

 convett it into an ellipse. In the latter case the comet 

 will become a permanent member of our system. This 

 it is possible is what has actually happened, and the 

 converse case of a loss seems also to have occurred. A 

 comet was discovered in 1770, and was shown by 

 Lexell to move in an elliptic orbit, with a short period 

 of about 5^ years. It was, however, never seen again, 

 nor were any former records of its appearance to be 

 found. Lexell, however, showed that in 1767, when at 

 its aphelion, or furthest from the Sun, the comet must 

 have been fifty-eight times nearer to the planet Jupiter 

 than to the Sun, and that then the planet's attraction on 

 it was three times that of the Sun; that in all probability 

 it had been moving in a parabola, which orbit was con- 

 verted into an ellipse bv the planet's action. He 

 further showed that since the aphelion was close to 

 Jupiter's orbit and the comet's period 5 1 years, that of 

 Jupiter being n years, at the end of two revolutions of 

 the comet and one of the planet, they would again be 

 close together, 500 times less than their distance from 

 the Sun, so that in all probability the comet's orbit 

 would become again parabolic or hyperbolic. Thus he 

 anticipated its eventual disappearance, and, in fact, it 

 was never again seen. Laplace and Leverrier later 

 showed, however, that Lexell 's results were liable to 

 considerable uncertainty. 



