1889.] On the Spectra of Meteor-sivarms. 381 



tinct and of another order from the great group of cosmical bodies to 

 which our sun and the fixed stars belong." 



With regard to the most generally accepted classification of stars, 

 that of Vogel, Duner (" Etoiles a Spectres de la 3me Classe ") writes, 

 " Selon la theorie il faudra que tot ou tard toutes les etoiles de la 

 premiere classe deviennent de la seconde, et celles-ci de la troisieme." 



Vogel, and before him, others, working on the assumption that all 

 the heavenly bodies were reducing their temperature, practically 

 included all stars between the hottest and the coldest in one class 

 (Class Ila of Vogel). 



In previous papers to the Royal Society I have adduced evidence to 

 show that all cosmical bodies are or have been meteor-swarms, that 

 at the present time some are increasing and some are reducing their 

 temperature. Thus, in the Bakerian Lecture, 1888, I demonstrated 

 that nebulae and stars of Group II (Class Ilia) are still increasing in 

 temperature by the condensation due to gravity, and that the red 

 stars of Group VI (Class III6) are at a nearly equal mean tempera- 

 ture to stars of Group II, but are cooling bodies. 



In these extreme cases the differentiation between the two groups 

 was comparatively easy. In the case of those stars which are a little 

 less hot than the hottest, whether they are getting hotter or cooler, 

 the spectral difference cannot nearly be so well marked, as both 

 classes will have line spectra ; but it was essential to my hypothesis 

 that these bodies should be resolvable into two groups, one increasing 

 and one decreasing in temperature, with spectra proper to each. 



The object of the present paper is to set forth the evidence which 

 shows that this differentiation is possible, and to suggest the lines 

 along which future researches on the subject might follow. 



In this paper, which is only to be regarded as preliminary, I pur- 

 pose to state the information already obtained with respect to 

 Group III, and its relation to the two groups which bound it, in 

 order that the validity of the distinction that I have drawn may be 

 further tested. At present the observations are not sufficiently 

 detailed to enable a classification into species to be made, as was done 

 for Group II, so that we have to be contented with a general state- 

 ment of the sequence of phenomena in passing from the early to the 

 later stages of the group. 



The observations lay no claim to great accuracy j only small disper- 

 sion has been employed, and only a reconnaissance has been attempted. 

 The general method has been first to observe the differences between 

 stars like Capella, which mostly resemble the sun, and those like 

 a. Cygni and a, Tauri, which show marked variations. In this way 

 the criteria which are hereafter enlarged upon were determined. 



Particular attention was directed to the manner in which the 

 fiutings which form the special characteristic of Group II died out in 



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