THE BINARY STARS. 327 



six miles per second, and, assuming the same density for both the 

 bodies (which is the only element of doubt in the calculation), it 

 has been shown that a dark companion of the required size, rela- 

 tively to Algol, would really have just about half his mass, and 

 must revolve in an orbit twice as large, or about four million miles 

 in diameter. The centre of gravity round which both the bodies 

 revolve must therefore lie at a point between them which is situ- 

 ated at twice as great a distance from the centre of the smaller 

 body (the dark companion) as it is from the centre of Algol ; and 

 as both must revolve round their common centre of gravity in the 

 same time, it follows that the dark companion, whose orbit is twice 

 as largQ as Algol's, must travel twice as fast, namely, fifty-two miles 

 per second. 



The orbital velocity of the companion having in this way been 

 ascertained, and that of Algol declared by the spectroscope to be 

 twenty-six miles per second, and also knowing that seven and a 

 quarter hours are required for the two bodies to pass one another, 

 it has therefore been possible, for the first time in the history of 

 astronomy, to say, with what must certainly be a very near approach 

 to the truth, what the size of one at least among the host of stars 

 is ; and Algol is declared to be just about a million miles in 

 diameter, or nearly twice as big in bulk as our sun, and his com- 

 panion about 800,000 miles, or very nearly the same size as the 

 sun. This is a noteworthy achievement in astronomy. We 

 have known for many years what the masses of many of the stars 

 are as explained in the former part of my paper, but no astronomer 

 has ever before been able to say what the actual size of any star 

 really is. The stars could, in many cases, be weighed, but it was 

 impossible to measure them. 



As I have said, the only element of doubt in this calculation 

 is that it assumes the same density for both Algol and his dark 

 companion. If this is not the case it will, of course, modify the 

 result somewhat, but this is a matter which it is impossible to 

 put to the test with the resources at present at the command of 

 science. Algol's companion is a body which has never been seen, 

 and I think I may safely say that it never can be seen from our 

 earth, and it is all the more wonderful therefore that science has 

 been able to teach us so much about a body which we can never 



