Dec. 6, 1888J 



NA rURE 



14: 



Both head and tail are so transparent that all but the faintest 

 stars are easily seen throngh them. The star Arcturus was seen 

 throii2h the tail of Donati's comet in 1858 at a place where it 

 was 90,000 miles in diameter. 



As a comet approaches the sun, its velocity, like that of the 

 planets, increases, and it j;radually gets hotter and gives out 

 more light. 



When the comet gets sufficiently hot, aigrettes or Jets make 

 their appeaiance ; these are socalh d because the/ seem to shoot 



perihelion passage, while travelling at the rate of 1,200, coo miles 

 an hour, in two days shot out a tail 6o,cco,oco miles in length. 



We must now enter somewhat more into details with regard 

 to some of these cometary characteristics. 



First of all, it must be pointed out that the meteoritic swarms 



I iG. 16. — Donati's comst (general view). 



out from the nucleus like sparks shoot out from a squib. The 

 jets rapidly change their positions and directions, and the tail is 

 formed, apparently at the expense of the mailer of which the 

 head was in the first instance built up. The tail is always turned 

 from the sun, whether the comet be approaching or receding. 



Drawings of a comet, as seen at diffeient times, show how the 

 jets vary in appearance and direction. Instead of jets, some 

 comets present phenomena of a very different character, called 

 envelopes, which are thrown off concentrically from the nucleus. 



Fig. 17. — Comet with single nucleus (Crui's come*, 1882, RiccO). 



These are among the chief physical peculiarities about the 

 heads of comets ; and we see at once that we have something 

 perfectly distinct from the planets, and that some comets are 

 at first sight different from others. The envelopes have been 

 observed to rise from the nucleus with perfect and exquisite 

 regularity in exactly the same way thatlhe jets swing backwards 

 and forwards. 



Fig. iS.— Nucleus surrounded by ellipsoidal he.id (Comet 1882 October 25, 

 in Washington refractor). 



are not always single, for in some comets the nuclei are double 

 or triple. 

 i In the case of single nuclei the nucleus may be the origin, 

 and lie in the brighter region the extension of which forms the 



Fig. 19.— Compound nucleus (same comet November 5). 



tail. But this is not invariable : the nucleus may be caught 

 forming part of an elliptic head (Fig. l8) before any very great 

 extension of the tail begins to take place, owing to reasons 

 which will be stated further on. 



-Co.nmander S.^mpson's sketch of the great comet, 

 October lo. 



In the case of double or multiple nuclei we have a clear 

 indication of the existence of more than one chief mete)iitic 



The enormous effect produced by a near approach to the sun j swarm, whether they be enveloped in the same atmosphere or 

 may be gathered from thi fact that the comet of 1680, at its give rise to the same tail (Fig. 19). But it would seem th.\t, in 



