548 SCIENCE PROGRESS 



was shown by Bessel that some of the changes could be 

 explained by supposing that the comet was rotating in about 

 five days, and a similar suggestion was made for comet More- 

 house. A difficulty arises in supposing a body to rotate that 

 is neither a rigid body nor held together by a strong central 

 force. However, it seems that a rotating head would produce 

 a semblance of rotation in the tail streaming from it, although 

 the separate particles of the tail are not rotating, but simply 

 being driven outwards. 



We now enjoy a great advantage over the observers of 

 1835 in being able to apply photography to the study of comets' 

 tails. This was first rendered possible in 1881 by the invention 

 of the dry plate, the wet plate formerly in use being too slow 

 for such faint objects. An almost continuous series of photo- 

 graphs was obtained of comet Morehouse, through the 

 co-operation of several observatories, and it was possible to 

 trace the course and speed of the several portions of the tail, as 

 they were repelled from the head and from the sun. The 

 repulsive force is found to be considerably stronger than 

 gravitation, so that, obviously, the matter driven off is lost to 

 the comet, which must therefore undergo a continuous loss of 

 substance. It is not, therefore, difficult to understand that 

 Halley's comet has appeared much less brilliant at recent 

 returns than it seems to have been several centuries ago, when 

 it caused widespread terror. 



Another feature that Halley's comet in 1835 shared with 

 comet Morehouse was the fact that both, at times, lost their 

 tails for a short period. Since we may be sure that the sun's 

 repulsive action did not cease, we must suppose that this arose 

 from some change in the head of the comet, the nature of which 

 we cannot guess. Comet Morehouse seemed to have a fairly 

 regular cycle of changes, so that it was even possible to predict 

 when the tail was likely to disappear, and when a new outburst 

 was likely to occur. 



Before looking forward to the return of Halley's comet next 

 year, I propose to carry its story backwards as far as we can 

 go. Halley himself traced it as far back as 1456, and indeed 

 thought that he had traced it some fifteen hundred years further, 

 but he made an error in his identification before 1456, which affects 

 all the returns before that. Laugier was the first to carry it one 

 step further, showing that the comet observed in China in 1378 



