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of it can be given. Some observers with good eyes still report it to be visible from time to time.

The question of the constitution of this curious planet is still an unsettled one. There is no one hypothesis that readily explains all the facts, which suggest many points, but prove few, unless negatively

Perhaps the most remarkable feature of the planet is its small density. Its diameter is about eleven times that of the earth. It follows that, in volume, it must exceed the earth more than thirteen hundred times. But its mass is only a little more than three hundred times that of the earth. It follows from this that its density is much less than that of the earth; as a matter of fact, it is only about one third greater than the density of water. A simple computation shows that the force of gravity at its surface is between two and three times that at the surface of the earth. Under this enormous gravitation we might suppose its interior to be enormously compressed, and its density to be great in comparison. Such would certainly be the case were it made up of solid or fluid matter of the same kind that composes the surface of the earth. From this fact alone the conclusion would be that its outer portions at least were composed of aeriform matter. But how reconcile this form with the endurance of the red spot through twenty-five years? This is the real difficulty of the case.

Nevertheless, the hypothesis is one which we are forced to accept without great modification. Besides the evi-