OF THE COSMOS. PERIODICALLY VARIABLE STARS. 157 



server, Heis at Aix la Chapelle, is only from the 3'4m. to 

 the 4*5 magnitude. 



Mira Ceti shews great differences of maximum brightness : 

 for example, on the 6th of November, 1779, it was only a 

 little inferior to Aldebaran, and it has not infrequently been 

 brighter than stars of the 2nd magnitude ; whilst at other 

 times it has not even attained the brightness of S Ceti, which 

 is of the 4th magnitude. Its mean brightness is equal to 

 that of y Ceti (3rd magnitude). If we represent the light 

 of the faintest star visible to the naked eye by 0, and that of 

 Aldebaran by 50, then Mira has fluctuated, in its maximum, 

 between 20 and 47. Its probable brightness would be ex- 

 pressed by 30, and it is oftener below than above this limit : 

 when it exceeds it, however, the excess is much greater in 

 amount than is the defect when it falls below it. No de- 

 cided period in these oscillations has yet been discovered, but 

 there are indications of a period of 40, and of one of 1 60 years. 



The periods of variation differ in different stars as much 

 1 : 250. The period of ft Persei of 68 hours 49 minutes is 

 unquestionably the shortest, supposing that of Polaris, of 

 less than 2 days, not to be confirmed. Next to ft Persei 

 follow successively $ Cephei (5d. 8h. 49m.), 77 Aquilae 

 (7d. 4h. 14m.), and Geminorum (lOd. 3h. 85m.) The 

 variable stars of which the period has the longest duration are : 

 30 Hydrse Hevelii, 495 days ; x Cygni, 406 days ; Yariabilis 

 Aquarii, 388 days ; Serpentis S. 367 days ; and Mira Ceti, 

 332 days. In several variable stars it is certain that the 

 light increases more rapidly than it decreases : this phe- 

 nomenon shews itself in the most striking manner in 

 Cephei. Other stars have equal times of increasing and 

 decreasing light (ex. gr. ft Lyrse) . A difference in this 

 respect is sometimes found in the same star. As a general 



