b GEOLOGY. 



According to the original Laplacian hypothesis, the earth, after 

 the separation of the moon-ring, was still a spheroid of hot gas which 

 continued to cool and shrink until finally its main mass passed into 

 a liquid and at length into its present state. 



Darwin's modification. — George H. Darwin x has proposed a modi- 

 fication of the theory, so far as relates to the moon, which he supposes 

 to have separated, not as a ring while the earth was still in an ex- 

 panded gaseous condition, but as a lump, or as a series of fragments, 

 after the earth had concentrated to a liquid or perhaps even an incipient 

 solid state. By a beautiful mathematical investigation of the effect 

 of a postulated body-tide of the earth upon the motion of the moon, 

 he concluded that the moon has been forced to retire gradually from 

 a position at or near the earth, to its present distance, in the course 

 of its history. The tidal protuberance of the earth is carried forward 

 by rotation, and so tends to draw the moon forward in its course. This 

 causes the moon to take a larger orbit, while the backward pull of 

 the moon on the tidal protuberance tends to retard the rotation of 

 the earth. That the tides tend to force the moon to retire in some 

 measure, is a sure deduction from the dynamics of tidal forces and 

 the assumption of a body-tide, and hence it seems probable that the 

 moon was originally nearer the earth than it is now; but how much 

 nearer remains an open question on which we have already touched. 2 

 On account of its close relations to the earth, the origin and history 

 of the moon are matters of great interest, but the different views that 

 have been entertained cannot be pursued into detail at this point. 



Supposed passage of the earth from the gaseous to the present con- 

 dition. — The Laplacian hypothesis assumes that the earth-matter 

 remained essentially in the gaseous condition until the heat of the 

 central portion fell to the temperature of liquefaction of the more 

 refractory matter, when a liquid nucleus formed. As cooling went 

 on, this grew until the chief rock-substances had concentrated into a 

 molten globe, while the more volatile material remained in a gaseous 

 state and formed a vast hot atmosphere. 



1 On the Secular Changes in the Elements of the Orbit of a Satellite Revolving about 

 a Tidally-distorted Planet, Phil. Trans. Roy. Soc., Pt. II, 1880. Also, On the Tidal 

 Friction of a Planet Attended by several Satellites and on the Evolution of the Solar 

 System, ditto, Pt. II, 1881. 



* Vol. I, p. 576. 



