20. The revolution of Algol seems to approach the nearest to a fact in favor of a star being the centre of a revolving system; and from the first, as we have said, the periodical change, and the sudden darkening and brightening of this luminary, suggested the supposition of an opaque body revolving about it. But this body cannot be a planet. The planets which revolve about our Sun are not, any of them, nor all of them together, large enough to produce a perceptible obscuration of his light, to a spectator outside the system. But in Algol, the phenomena are very different from this.[19] The star is usually visible as a star of the second magnitude; but during each period of 2 days 21 hours, (or 69 hours,) it suffers a kind of eclipse, which reduces it to a star of the fourth magnitude. During this eclipse, the star diminishes in splendor for 3-1/2 hours; is at its lowest brightness for a quarter of an hour; and then, in 3-1/2 hours more, is restored to its original splendor. According to these numbers, if the obscuration be produced by a dark body revolving round a central luminary, and describing a circular orbit, as the regular recurrence of the obscuration implies, the space of the orbit during which the eclipsing body is interposed must be about one-ninth of the circumference; for the obscuration occupies 7-1/4 hours out of 69. And therefore the space during which the eclipsing body obscures the central one, must be about one _sixth_ of the _diameter_ of its orbit. But in order that the revolving body may, through this space, obscure the central one, the latter must extend over this space, namely, one sixth of the diameter of the orbit. But we may remark that there is no proof, in the phenomena, that the darkening body is detached from the bright mass. The effect would be the same if the dark mass were a part of the revolving star itself. It may be that the star has not yet assumed a spherical form, but is an oblong nebular mass with one part (perhaps from being thinner in texture) cooled down and become opaque. And the amount of obscuration, reducing the star from the second to the fourth magnitude, implies that the obscuring mass is large (perhaps one half the diameter, or much more) compared with the luminous mass. If this be a probable hypothesis to account for the phenomena, they are much more against than for the supposition of the star being the centre of seats of habitation. And even if we have a planet nearly as large as its sun, revolving at the distance of only six of the sun's radii, how unlike is this to the solar system!