1889.] of the various Species of Heavenly Bodies. 207 



no doubt the same in both cases. The rapidity with which the 

 comet cooled demonstrates that only small masses could be in 

 question. This took place whilst the comet was no less than 

 200 million miles from the sun. 



On October 15th there was a similar occurrence in the same comet, 

 and again, a more decided one on January 1st. In the latter case, in 

 less than four hours,* the comet had become an apparent star, and 

 again assumed the cometary form. 



In these cases, then, we have evidence that the luminosity of the 

 comets depends first upon its distance from the sun, and secondly upon 

 distribution of otber swarms along its path. 



It would appear that a further discussion from this point of view 

 might afford us interesting information on several points. 



X. ON. SOME EFFECTS OF COLLISIONS m COMETS. 



If we assume that the increased brightness of comets as the sun is 

 approached depends to any extent on collisions with meteorites 

 external to the swarm, we mast conclude that such meteorites exist 

 nearer together nearer the sun. The idea seems strengthened by the 

 great and irregular variations of intensity sometimes observed, as we 

 know that the meteorites which the comet is liable to meet are 

 not equally distributed. Such a variation was noticed in Sawerthal's 

 Comet in 1888, as I have already stated. 



Such variations, however, would be more likely to be observed in 

 the tails in consequence of the enormous dimensions of some of them. 

 Such variations have been observed from the time of Kepler. 



The fact that these variations so strongly resemble at times auroral 

 displays is an additional argument in favour of the meteoric origin of 

 the latter. 



Another result of a different order produced by a comet moving 

 through a meteoric plenum would be the gradual shortening of a 

 comet's periodic time as the result of collisions, and this shortening 

 should not be absolutely regular, as in a homogeneous gas, for the 

 reason that the meteorites are not equally distributed. 



That there is such a shortening was proved by Encke for the comet 

 which bears his name, and that there are irregularities the following 

 table will show, though how far they might have been due to pertur- 

 bations has not, I believe, been so far studied : 



* Dr. Miiller, ' Astr. Nachr.,' No. 2568. 



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