intervene before the separation of another annular mass." None of these rings could long remain of uniform thickness. Each would attenuate in some part, and finally rupture, resolving itself into a mass that would possess a rotary motion, the direction of which would be determined by the relation of the velocities of the outer and inner zones of the ring.
Part II, "Planetology," occupies about half the volume. In the first chapter of it, certain observed phenomena of the solar system are enumerated which accord with the requirements of the nebular theory, and objections to the theory are answered. For the retrograde motions of the satellites of Uranus and Neptune our author advances several explanations: 1. It is entirely conceivable that both the Uranian and Neptunian systems should have suffered a tilting through more than a right angle by the influence of some powerfully attracting body passing in the neighborhood. 2. The coalescence of two or more spheroids may have tilted the axis of the resultant planet, and its whole system of satellites would be correspondingly tilted. 3. Certain relations of density, distance from the center of the nebulous mass, breadth of ring, and velocity might cause retrograde motion in the earlier stages of the evolution of a nebula of a certain magnitude. The next chapter describes the passage of a gaseous planet to the molten phase, the solidification of its core from pressure of the superincumbent portions, the incrustation of its surface, and the transformations of this crust. A large influence on planetary history is ascribed to tidal action, a tide being defined as "the prolateness of a body resulting from the attraction of another body." Coming to some special considerations of the planetary bodies in the solar system, Professor Winchell mentions three independent conceivable causes for the molten condition in which a part of the earth's substance evidently is: "There may be a zone too deep for solidification by cooling, and too shallow for solidification by pressure. . . . In the next place, we may suppose that at all depths beneath the surface the pressure is such that the fusing-point is higher than the actual temperature, so that a state of solidity exists. . . . We may conceive that heat and fusion result from some mechanical crushing pressure." In regard to this last theory he says, further: "But a cause of crushing pressure which seems to me more adequate than secular cooling is suggested by Sir William Thomson's and Archdeacon Pratt's, and, we may add, Professor G. H. Darwin's, demonstrations of tidal effects in a globe as rigid as steel or glass. May not the tidal deformations of the earth's crust be the source of the internal heat which manifests itself in fluidity? The whole value of the lunar tidal oscillation in a yielding globe should be about fifty-eight inches. In a globe as rigid as glass it should, therefore, be about 34·8 inches, and, in one as rigid as steel, 19·33 inches. The whole tidal oscillation under the joint maximum influence of the sun and moon in a perfectly yielding globe would be about 81·2 inches. The amount in a globe of glass would, therefore, be, when at a maximum, 48·72 inches, and, in a globe of steel, 27·06 inches. Should the terrestrial globe yield to the extent of any one of these amounts, the crushing effect experienced by the superior zones of the crust would not be uniformly distributed, since variations in structure and hardness and surface configuration would preserve certain portions from any change, and the whole amount of the interstitial displacements would be accumulated in the remaining portions. It does not seem at all improbable that the transformation of such enormous mechanical force into heat should suffice to bring to a state of fusion volumes considerable enough to answer all the requirements of the thermal manifestations of modern times, as well as the terrestrial movements of modern earthquakes." From an examination of the planetology of the moon he concludes that "lunar history must have presented characteristics widely divergent from those of terrestrial history; and in this divergence the tenuity of the moon's atmosphere has performed a part quite comparable with the energetic work of the tides. . . .
"The question of the habitability of other worlds has generally been discussed from the assumption that all other corporeal beings must be clothed in flesh and bones similar to those of terrestrial animals, and must be adapted to a similar physical envi-
