:9o8.] OF DANIEL'S COMET. 15 



tion, and that some of the particles must move outward from the 

 sun very much faster than others of the same stream. This 

 was shown in the formation of the new tail of the above-named 

 comet on July 24. In the tail on that date the later photographs 

 showed that the end of the new tail was increasing its distance from 

 the head much faster than the end of the receding disconnected tail. 

 But in this case the conditions were different ; the supply of matter 

 forming the outgoing stream had suddently been stopped and the 

 stream itself continued to move out bodily into space until it was 

 dissipated. The apparent velocity was then the velocity of the 

 stream of particles. In the case of Daniel's comet a denser mass of 

 particles differing from the general streams that formed the tail was 

 separated from the main body. This would naturally leave the 

 comet slowly and continue to partake of the original motion. Still 

 another case was that of Brook's comet of 1889 (comet V., 1889) 

 where the masses thrown off were so dense that they traveled v/ith 

 the parent comet for months as individual companions before finally 

 disappearing.^ And yet another case, that of Biela's comet which 

 separated into two masses that remained individually distinct for 

 some years and then entirely disintegrated. The motion of a dense 

 mass thrown off from a comet would not therefore be a criterion for 

 the determination of the velocity in general of the particles of the 

 tail of such a comet. 



The plate of September 8 is introduced, not from any scientific 

 value it may have, but from an artistic standpoint and from its 

 unique character. So far as I know this is the only comet, or star 

 photograph, on which clouds are actually shown. The exposure was 

 very short, for the comet was visible for only a few minutes in a 

 break. The clouds stand out black and distinct on the dawn-lit sky. 

 To the eye it was a beautiful and striking scene — the comet in pale 

 but clear relief on the dawn-whitening sky, the dark clouds, through 

 a break in which the comet shone, and the solemn stillness of the 

 morning, made it a picture not soon to be forgotten. The photo- 

 graph rather faithfully records the appearance of the comet and 

 clouds and dawn-lit sky, but the reproduction cannot do justice to the 

 ^ See Astronomische Nachrichtcn, nos. 2914, 2919, 2988 and 2998. 



