LESSONS IN ASTRON 



between us and the star, it* light WM not so much enfeebled 

 M it would have been bj the faintest tog on the earth'* rarfaoe. 



During tho time that the Uil of thin comet WM being thrown 

 out, the nucleus WM watched, and presented tho appearance 

 of a Horios of coverings being thrown off and passing into the 

 t:ul. As many M seven of these envelopes were dutin 

 nerved. The toil appeared to be maoh brighter at the edge*, 

 dark baud passed down the middle, M if it consisted 

 of a hollow oane. Tho observations of this comet were 

 very numerous, and there is little donbt bnt that it revolve* 

 in an elliptic orbit, completing its journey in about 

 your-. 



The great comet of 1861 was also a very remarkable one, from 

 the great rapidity with which it burnt upon as. It WM din- 

 covered in the southern hemisphere about the middle of May. 

 < MI tho 29th of June it WM first seen in this country, only a 

 i of tho tail being then visible above the horizon. IU 

 uriK'litness WM very remarkable, being even greater than that 

 of the one noon in 1858. Its tail, when at its greatest length, 

 lod over nearly 80, 

 as perfectly straight 

 1 1 .\ .in, however, somewhat 

 i:in-liko iii shape. Fig. 31 

 ii presents it, and exhibits 

 tho appearance of the 

 i.urlnis when the tail was 

 being thrown off. Mr. 

 Him! states that it is 

 probable that on the 30th 

 of June the earth actually 

 -I'd through the tail of 

 this comet ; and it is a 

 remarkable fact, in con- 

 nection with this, that on 

 that day Mr. Lowe and 

 one or two other observers 

 noticed a peculiar phos- 

 phorescent glare in the 

 sky. 



In 1826 a comet was 

 discovered by M. Biela, 

 which has since been 

 known by his name, and 

 has become remarkable. 

 Observations made at dif- 

 ferent times soon showed 

 that it moved in an 

 elliptical orbit, and its 

 period was found to be 

 about 6J years. Its re- 

 turn in 1832 took place 

 within a few hours of the 



predicted time. On its next visit to the sun it was invisible, 

 owing to the position of the earth with respect to it and the 

 sun, which was such that the comet was lost in the solar rays. 

 When it again returned towards the close of 1845, a very 

 strange phenomenon was seen. The comet, which at first had 

 been almost circular, gradually became elongated, and at 

 length divided into two parts, which continued to travel sepa- 

 rately till lost sight of. Both parts re-appeared in 1852, the 

 distance between them having somewhat increased in the mean- 

 while. In 1859 it was again in an unfavourable position for 

 observation, so was not seen ; and in 1866, from some unknown 

 cause, it could not be found. In November, 1872, it suddenly 

 made its appearance ; but in 1879, no body answering to the 

 calculated position, or even to the supposed appearance, of 

 Biela' s comet was observed. 



There are several other comets that have a strange history ; 

 bnt we can only now refer to one Encke's, which revolves in 

 a period of about 3J years. More appearances of this comet 

 have been carefully observed than of any other, and, by com- 

 paring these different observations, Encke found that on each 

 return it accomplished its passage round the sun 2fc or 3 hours 

 earlier than he expected. Its period thus appears to be 

 diminishing by this amount in each revolution. He conjec- 

 tured that this might be accounted for by assuming that all 

 space was occupied by an extremely rare medium, but yet 

 one sufficiently dense to retard the comet to this extent, and 





thus to Oftuse it gradually to be falling fa 

 Other conjectures have hew started, but the 

 .1 Ml ; i M M*ajsJ MfcM MM 



'f the pby.ical oonetituMoti of these bodies 

 little is M yet knows. They appear to 

 of vajwur revolving round the ami 

 that in a few , 



on this point does not appear to he very strong, and tf one doe-- 

 exist, it evidently is very small. The general opinion is. how. 

 ever, against Ma erietemte at all, and the great majiaitj o. 

 oomete are known to be devoid of one. 



That the mass of these bodies is iisirmslj minute i* seen 

 by the way in which they are affected by any of the heavenly 

 bodies which they come near. LexeU's comet, for faataie*. fa 

 1770, when approaching the sun, pa sis d so near to the pmaet 

 Jupiter, that it WM en Angled for several mooths among ita satel- 

 lites. Ite orbit WM completely shsnged by this eoatect. hut no 

 effect whatever could be disosrned upon the -nttffHtir. haioas. 

 had the comet's mass bean at all appreciable, their times of 



revolution must have been 

 slightly modified. 80. 

 too. the comet* of 1M 

 and 1861. though they 

 both paered near the earth, 

 did not alter the length 

 of the year by a single 

 second. The fact that the 

 light of even faint star* 

 is scarcely at all dimi- 

 nished by passing through 

 the tail of a comet baa 

 ' iuently 



>N ki.''W i. w .: *. .it- v 



air would expand if re- 

 noved far from the earth 

 serf ace, and possibly the 

 matter of tho comet'. Uil 



mar* otmjeetuie; hot we 

 know that the whole matt 

 of a comet is so small that 

 van if one were to come 

 into full collision with tho 

 earth, no injurious 

 would result from 

 blow. 



Several large 

 have at 



clone to the sun M almost 

 to graae ite surface. The 

 heat, therefore, to which 



they were exposed must have been extremely great, but thry 

 appeared not to be at all affected by it. 



The number of oomete which are connected with our s/sttm 

 it is impossible to estimate. Some have imagined that it must 

 be very great indeed. This fact, at all events, is certain, that 

 there must be many which, from their position, are altogether 

 hidden from us. 



The following is a list of the principal oomete recognised M 

 belonging to the solar system. There are many others which 

 have been calculated, but some need confirmation. 



Yam*. 



Eocke'* 

 De Vico's 



WsauaaWa 



Bror*en' 

 Biela'* . 

 D'Amst's 



7-44 



Tt 



De Tire's 

 Brorsra's . 7V 



Mfehoin's 



8-tM rears. 



i 

 V54 



As some of these latter closely agree in period, it has been 

 conjectured that they may originally have formed part of one 

 huge comet which divided in a similar way to Biela's. 



