230 COSMOS. 



the years 1840 and 1844. During that time its period \va? 

 nearly invariable ; at present it is again decidedly on the de- 

 crease. Something similar to the double maximum of /3 Lyrse 

 occurs in 6 Cephei. There is a tendency to a second maximum, 

 in so far as its diminution of light does not proceed uniformly ; 

 but after having been for some time tolerably rapid, it comes 

 'to a stand, or at least exhibits a very inconsiderable diminu- 

 tion which suddenly becomes rapid again. In some stars it 

 would almost appear as though the light were prevented from 

 fully attaining a second maximum. In x Cygni it is very 

 probable that two periods of variability prevail, a longer one 

 of 100 years, and a shorter one of 8^. 



The question whether, on the whole, there is greater 

 regularity in variable stars of very short than in those of 

 very long periods, is difficult to answer. The variations from 

 an uniform period can only be taken relatively ; t. e. in parts of 

 the period itself. To commence with long periods, ^ Cygni, 

 Mira Ceti, and 30 Hydra, must first of all be considered. In 

 jg Cygni, on the supposition of a uniform variability, the 

 deviations from a period of 406-0634 days, (which is the 

 most probable period,) amount to 39 -4 days. Even though a 

 portion of these deviations may be owing to errors of 

 observation, still at least 29 or 30 days remain beyond doubt; 

 t. e. one-fourteenth of the whole period. In the case of 

 Mira Ceti, 14 in a period of 331 '340 days, the deviations 

 amount to 55 -5 days, even if we do not reckon the 

 observations of David Fabricius. If, allowing for errors of 

 observation, we limit the estimate to 40 days, we still obtain 

 one-eighth; consequently, as compared w r ith % Cygni, nearly 



14 The work of Jacques Cassini (EUmens d 'Astronomic, 

 1740, pp. 66-69), belongs to the earliest systematic attempts 

 to investigate the mean duration of the period of the variation 

 of Mira Ceti. 



