326 THE BINARY STARS. 



ments across our line of sight ; and if, therefore, Algol be really 

 moving at twenty-six miles a second round a nearly circular orbit 

 placed edgeways to us, this will explain perfectly the appearances 

 presented, as the spectroscope would only show the movement at 

 the sides of the orbit, when either approaching us or retreating from 

 us, and not at all when crossing our line of sight at the nearest 

 and the most distant points in the orbit. Vogel also found that 

 the time of this revolution of Algol in an orbit, as shown by the 

 spectroscope, corresponded exactly with the complete cycle of 

 variability, namely, 2 days 20 hours 48 minutes and 51 seconds. 

 It seemed certain, therefore, that a connection must exist between 

 the orbital revolution and the variability ; and to strengthen this 

 probability he also found that the minimum always occurred at the 

 conclusion of Algol's movement of retreat from the earth. 



These facts have furnished the mathematician with all the 

 data he requires for explaining the variability of Algol, and for 

 finding out a great many things about him. As the time he takes 

 to complete a revolution of his orbit and also his velocity are both 

 known, the size of his orbit is at once ascertained. This is found 

 to be a little more than six million miles in circumference, or, in 

 round numbers, two million miles in diameter. It is evident, 

 therefore, that there must be some invisible body of large mass 

 near to Algol, causing him to revolve in this orbit. The next 

 point for consideration is the amount of light lost when at mini- 

 mum, and this is about three-fifths of the whole, the dark body 

 being only able to cover three-fifths of Algol's surface, so that two- 

 fifths therefore remain uneclipsed. This minimum lasts for twenty 

 minutes, and then the star rather suddenly begins to brighten. 

 The dark satellite must therefore remain wholly in front of Algol 

 for twenty minutes, aud then begin to pass off The total time 

 which the two bodies take to pass one another completely must 

 now be taken into account. This is found, by careful observation, 

 to be about seven hours and a quarter. 



Now if we know the velocity with which each of the globes is 

 moving, we shall be able to arrive at their respective diameters, 

 seeing that one eclipses three-fifths of the other's light for twenty 

 minutes, and they take seven and a quarter hours to pass each 

 other. The velocity of Algol, the spectroscope tells us, is twenty- 



