68 



THE POPULAR SCIENCE MONTHLY. 



being then about equal to the faint stars near it. It remains thus 

 obscured for only a few minutes, and then begins to brighten again, 

 and in about four and a half hours more resumes its former brilliancy. 

 This phenomenon is very easily obsei'ved, for, as will be seen "by con- 

 sulting our little map, Algol can be readily found, and its changes are 



^Iap G— The Attendants of Alpha Persei. 



SO rapid that under favorable circumstances it can be seen in the 

 course of a single night to run through the whole gamut. Of course, 

 no oi)tical instrument whatever is needed to enable one to see these 

 changes of Algol for it is plainly visible to the naked eye throughout, 

 but it will be found interesting to watch the star with an opera-glass. 

 Its periodic time from minimum to minimum is two days, twenty 

 hours, and forty-nine minutes, lacking a few seconds. Any one can 

 calculate future minima for himself by adding the periodic time above 

 given to the time of any observed minimum. For instance, there will 

 be a minimum on November 12th at about 11.15 p. m., then the next 

 minimum will occur two days, twenty hours and forty-nine minutes 

 later, <»r at H.()4 p. m., on November Ifjth. 



While spots upon its surface may be the cause of the variations in 



