our visible sun, but its mass is so much less that its weight is only one-half that of our sun, so that its body is probably gaseous. The author concludes, "No one, however, will be likely to doubt that it is the law of gravitation, pure and simple, which prevails in the celestial spaces, and consequently we are able to make use of it to explain the circumstances attending the movements of Algol's dark companion. This body is the smaller of the two, and the speed with which it moves is double as great as that of Algol, so that it travels over as many miles in a second as an express train can get over in an hour. The companion of Algol is about the same size as our sun, but has a mass only one-fourth as great. This indicates a globe of matter which must be largely in the gaseous state, but which, nevertheless, seems to be devoid of intrinsic luminosity. Their distance [apart] is always some three million miles. This is, however, an unusually short distance when compared with the dimensions of the two globes themselves." With this exception, the author says, "the movements of Algol and its companion are not very dissimilar to movements in the solar system with which we are already familiar." It will be seen that the want of luminosity in the dark companion of Algol finds a ready explanation in the fact that it is a planet, acting precisely as our own planets do, and that the luminosity of Algol itself is directly attributable to the electricity developed by the presence of this planet rotating axially and orbitally around it, and the darkness of the planet itself is the necessary correlative of the heat and light of its sun. The planet has about one-half the density of Saturn, while Algol has one-half the density of the sun, and hence we should expect to find on Algol an atmosphere largely composed of glowing hydrogen, and on its planet an atmosphere largely composed of oxygen, in which, doubtless, float enormous clouds of aqueous vapor. The interpretation is direct and conclusive, and upon no other hypothesis can the facts be explained, for their close connection with each other demonstrates their common origin, and their masses are not so different one from the other as to permit, on any theory of their coequal origin as suns, one to glow with the fires of youth and energy and the other to have grown dark and dead from old age and exhaustion, and especially so if still in its gaseous stage, which is that which must characterize its highest state of incandescent energy from the most active condensation of its volume, if the nebular hypothesis has any validity whatever. In fact, this example alone, if the constitution of Algol's dark satellite is really gaseous, must go very far to throw the gravest doubt, in itself, on the validity of this hypothesis.