While this has been (in my judgment) a defect in what may be called the Brewsterian theory of other worlds, a defect not altogether dissimilar has characterised the opposite or Whewellite theory. Very useful service was rendered to astronomy by Whewell's treatise upon, or rather against, the plurality of worlds, calling attention as it did to the utter feebleness of the arguments on which men had been content to accept the belief that other planets and other systems are inhabited. But some among the most powerfully urged arguments against that belief tacitly relied on the assumption of a similarity of general condition among the members of the solar system. For instance, the small mean density of Jupiter and Saturn had, on the Brewsterian theory, been explained as probably due to vast hollow spaces in those planets' interiors--an explanation which (if it could be admitted) would leave us free to believe that Jupiter and Saturn may be made of the same materials as our own earth. With this was pleasantly intermixed the conception that the inhabitant of these planets may have his 'home in subterranean cities warmed by central fires, or in crystal caves cooled by ocean tides, or may float with the Nereids upon the deep, or mount upon wings as eagles, or rise upon the pinions of the dove, that he may flee away and be at rest,' with much more in the same fanciful vein. We now know that there can be no cavities more than a few miles below the crust of a planet, simply because, under the enormous pressures which would exist, the most solid matter would be perfectly plastic. But while Whewell's general objection to the theory that Jupiter or Saturn is in the same condition as our earth thus acquires new force, the particular explanation which he gave of the planet's small density is open to precisely the same general objection. For he assumes that, because the planet's mean density is little greater than that of water, the planet is probably a world of water and ice with a cindery nucleus, or in fact just such a world as would be formed if a sufficient quantity of water in the same condition as the water of our seas were placed at Jupiter's greater distance from the sun, around a nucleus of earthy or cindery matter large enough to make the density of the entire planet thus formed equal to that of Jupiter, or about one-third greater than the density of water. In this argument there are in reality two assumptions, of precisely the same nature as those which Whewell set himself to combat. It is first assumed that some material existing on a large scale in our earth, and nearly of the same density as Jupiter, must constitute the chief bulk of that planet, and secondly that the temperature of Jupiter's globe must be that which a globe of such material would have if placed where Jupiter is. The possibility that Jupiter may be in an entirely different stage of planetary life--or, in other words, that the