76 Cincinnati Society of Natural History. 



48 minutes, 55 seconds and 4-10 of a second. The precise 

 moments of its minima have been calculated by Chandler for sev- 

 eral years to come. 



" During most of the time," says Prof. Young, " the star remains 

 •of the second magnitude. At the time of obscuration it loses 

 about five-sixths of its light, falling to the fourth magnitude in 

 about 4^ hours, remaining at the minimum for about twenty min- 

 utes, and then in $ l /> hours recovering its original condition. 

 During the minimum the spectrum undergoes no considerable 

 •change."' Its light curve may be represented thus: 



(11 Vo ll. 



/2 2<1 mag. 



4th mag. 



You will notice that while the light curve of Mir .1 is upward, as 

 if produced by a temporary outburst of hydrogen, that of Algol is 

 downward, as if caused by the periodical intervention of a dark 

 body. Are we warranted in drawing these inferences from the 

 simple phenomena we have described? 



The variability of Algol was noticed as long ago as 1669 by Mon- 

 tanari, and its period determined in 1782 by Goodricke. Since 

 that date some fifty astronomers have taken part in Us observations 

 and nearly 700 minima have been recorded. 



But it seems highly probable the Arabian astronomers had 

 observed these rapid variations thousands of years ago. whence 

 their significant name, Al Gol, the demon. 



Though the precise period of Algol at the present time is given 

 to the fraction of a second, yet these long series of observations, 

 extending over more than a century, have developed t^vo inequali- 

 ties in the Algol period. Chandler finds that the first inequality 

 has a period of 141. 3 years, with a variation of 173.3 minutes, 

 and the second a period of 37.7 years with a variation of t8 

 minutes. 



There are but few variable stars of the Algol type. Of these 

 some are entirely telescopic, and others have but a slight range of 

 brightness. For instance, a newly discovered variable in Cygnus 

 has a maximum magnitude of 7.1 and a minimum of 7.9 Its 

 period is 1 day, n hours, 56 minutes and 48 seconds It is 

 stationary at its maximum brilliancy for 28 hours. It then decreases 

 4 hours and increases 4 hours. 



