tt 303, 304] Structure of the Sun : Comets 393 



prominences have been explained in a variety of different 

 ways as joint results of solar disturbances of various 

 kinds ; but no detailed theory that has been given explains 

 satisfactorily more than a fraction of the observed facts 

 or commands more than a very limited amount of assent 

 among astronomical experts. 



304. More than 200 comets have been seen during the 

 present century ; not only have the motions of most of them 

 been observed and their orbits computed ( 291 ), but in a large 

 number of cases the appearance and structure of the comet 

 have been carefully observed telescopically, while latterly 

 spectrum analysis and photography have also been employed. 



Independent lines of inquiry point to the extremely un- 

 substantial character of a comet, with the possible exception 

 of the bright central part or nucleus, which is nearly always 

 present. More than once, as in 1767 (chapter XL, 248), a 

 comet has passed close to some member of the solar system, 

 and has never been ascertained to affect its motion. The 

 mass of a comet is therefore very small, but its bulk or 

 volume, on the other hand, is in general very great, the tail 

 often being millions of miles in length ; so that the density 

 must be extremely small. Again, stars have often been ob- 

 served shining through a comet's tail (as shewn in fig. 99), 

 and even through the head at no great distance from the 

 nucleus, their brightness being only slightly, if at all, affected. 

 Twice at least (1819, 1861) the earth has passed through a 

 comet's tail, but we were so little affected that the fact was 

 only discovered by calculations made after the event. The 

 early observation (chapter HI., 69) that a comet's tail points 

 away from the sun has been abundantly verified ; and from 

 this it follows that very rapid changes irr-the position of the 

 tail must occur in some cases. For example, the comet of 

 1843 passed very close to the sun at such a rate that in 

 about two hours it had passed from one side of the sun to 

 the opposite ; it was then much too near the sun to be seen, 

 but if it followed the ordinary law its tail, which was unusually 

 long, must have entirely reversed its direction within this 

 short time. It is difficult to avoid the inference that the 

 tail is not a permanent part of the comet, but is a stream 

 of matter driven off from it in some way by the action of 

 the sun, and in this respect comparable with the smoke 



