158 SPECIAL RESULTS IN THE URANOLOGICAL PORTION 



rule, Mira Ceti (like 8 Cephei) increases faster 'than it 

 decreases ; but the contrary has also been observed. 



In regard to periods which are themselves subject to a 

 periodical variation, we find such decidedly in Algol, 

 Mira Ceti, and p Lyrse, and with much probability in 

 x Cygni. The decrease of the period of Algol is now 

 undoubted. Goodricke did not find it, but Argelander has 

 done so, having in 1842 been able to compare above 100 

 well-assured observations, of which the extremes are above 

 58 years apart, comprising 7600 periods. (Schumacher's 

 Astr. Nachr. No 472 and 624.) The decrease of duration 

 becomes more and more sensible. ( 277 ) For the periods of 

 maximum in Mira (taking in the maximum of brightness 

 observed by Fabricius in 1596), Argelander has given a 

 formula ( 278 ) by which all the maxima can be so deduced 

 that the probable error in a mean period of 33 Id. 8h. does 

 not exceed 7 days, whereas on the assumption of a uniform 

 period it would be 15 days. 



The double maximum and minimum of /3 Lyrae, in each 

 of its periods of almost 1 3 days, were already very correctly 

 recognised in 1784 by the discoverer Goodricke, but have 

 been placed still more beyond doubt by the most recent ob- 

 servations. ( 279 ) It is worthy of notice, that this star attains 

 the same degree of brightness in both its maxima, but at its 

 principal minimum it is half a magnitude less than at its 

 secondary minimum. From the earliest discovery of the 

 variability of ft Lyrse its period was probably lengthening, 

 but more and more slowly, until, between 1840 and 1844, 

 the period ceased to increase, and has since decreased. We 

 find something similar to the double maximum of ft Lyrse in 

 3 Cephei ; it has so far an inclination to a second maximum 

 that the decrease of light does not proceed uniformly, but, 



