"Algol is another remarkable variable, passing, however, much more rapidly through all its changes. It is ordinarily a second-magnitude star, but during about seven hours in each period of sixty-nine hours its lustre first diminishes until the star is reduced to a fourth magnitude, and after it has remained twenty minutes at its minimum its lustre is gradually restored. It remains a second-magnitude star for about sixty-two hours in each period of sixty-nine hours. These changes seem to correspond to what might be expected if a large opaque orb is circling around this distant sun in a period of sixty-nine hours, transiting its disk at regular intervals." Of this star, Professor Ball says, "Applying the improved spectroscopic process to Algol, he [Vogel] determined on one night that Algol was retreating from the earth at a speed of twenty-six miles per second.... When Vogel came to repeat his observations, he found that Algol was again moving with the same velocity, but this time towards the earth instead of from it.... It appeared that the movements were strictly periodic; that is to say, for one day and ten hours the star is moving towards us, and then for a like time it moves from us, the maximum speed being ... twenty-six miles a second.... It is invariably found that every time the movement of retreat is concluded the star loses its brilliance, and regains it again at the commencement of the return movement.... The spectroscopic evidence admits of no other interpretation save that there must be another mighty body in the immediate vicinity of Algol.... Algol must be attended by a companion star which, if not absolutely as devoid of intrinsic light as the earth or the moon, is nevertheless dark relatively to Algol. Once in each period of revolution this obscure body intrudes itself between the earth and Algol, cutting off a portion of the direct light from the star and thus producing the well-known effect." This is, in fact, a periodic transit or eclipse of Algol by a planet, such as we see in eclipses of our own sun by the moon and the inner planets, except that Algol's planet is apparently single like our moon with reference to the earth, and that it is relatively much larger than any of our own planets, as we would necessarily suppose it to be, if solitary. Its mass has been computed by the effects which it produces, and we learn that it is not a dark sun with a brilliant planet, but a brilliant sun with a dark planet, just as our solar system presents. "Algol, at the moment of its greatest eclipse, has lost about three-fifths of its light; it therefore follows that the dark satellite must have covered three-fifths of the bright surface.... The period of maximum obscuration is about twenty minutes, and we know the velocity of the bright star, which, along with the period of revolution, gives the magnitude of the orbit." From these data it has been computed that the globe of Algol itself is about one-fourth larger than that of