204 



Mr. J. N. Lockyer. On the Classification [Jan. 10, 



perihelion distance, the greater fraction of it is the diameter of the 

 swarm, and the greater therefore the differential attraction. ;;> 



The initial movements of the individual members of the swarm, 

 and these superadded by tidal action, may be denned as producing 

 internal work. 



If all the heat of a comet is produced by such internal work, it is 

 clear that the temperature of the comet will depend (1) upon the 

 velocity of orbital motion of the particles, (2) upon the size of the 

 swarm of which it is composed, and (3) upon its perihelion distance. 

 It will practically be independent of the velocity of the comet in its 

 orbit round the sun. 



While some comets at perihelion give such high temperature 

 phenomena as were observed in Comet III, 1881, Wells's Comet, and 

 the Great Comet of 1882, others, like Winnecke's Comet, 1868, give 

 only the spectrum of carbon. 



These differences are what we should expect from the known peri- 

 helion distances, and it must be understood that the four stages into 

 which the different degrees of activity in a comet have been divided 

 in this paper are those which occur in a comet with a short perihelion 

 distance. In comets with a long one, perihelion effects may only be 

 equivalent to mean distance effects in comets with short perihelion 

 distances. 



I have prepared the following list of the perihelion distances of the 

 comets which have been discussed, the distances being given in 

 terms of the astronomical unit, derived from the data given in the 

 1 Ammaire du Bureau des Longitudes.' 



In the various tables which precede, for each comet the date of 

 observation, perihelion passage, and perihelion distance are stated. 



