ORAVITATIONAL STABILITY OF THE EARTH. 217 



PART II. 



A Past State of Gravitational Instability as a Reason for the existing Distribution 



of Land and Water. 



44. Although the conclusion reached by JEANS (1903), that a spherical planet of 

 the same size, mass and elasticity as the Earth, in its present state, would be in 

 a condition of gravitational stability, is confirmed and strengthened by the present 

 investigation, it by no means follows that the Earth has always been in such a state 

 as it is now. The fact that the mean density of the Earth as a whole is greater than 

 the average density of surface rocks points to a concentration of mass towards the 

 centre, and suggests that such a concentration may have come about through the 

 elasticity having once been too small for a homogeneous state to be stable. We have 

 seen that this would have been the case if the modulus of compression was once as 

 small as, or smaller than, that of glass. But we also saw reason to think that, if the 

 mean modulus of compression was once decidedly less than that of glass, spherically 

 symmetrical states of aggregation would also have been unstable, and the body would 

 have existed in some other state. Further, we saw that, if the body was at rest, the 

 state in which it would have existed is that which we have described as a state of 

 lateral disturbance with a hemispherical distribution of density. The excess of 

 density in one hemisphere and defect in the antipodal hemisphere would have existed 

 alongside of the concentration of mass towards the centre. 



45. In the paper already cited JEANS (1903) struck out the idea that the 

 distribution of land and water on the surface of the globe is associated with a past 

 state of gravitational instability. He had found that such instability would manifest 

 itself in what has been called above a hemispherical distribution of density. When 

 the square of the irregularity is neglected, the figure of a planet at rest, with such 

 a distribution of density, is a sphere, but the centre of figure does not coincide with 

 the centre of gravity. On taking account of the square of the irregularity, JEANS 

 found that the surface of the planet, still supposed to be at rest, would be such as 

 can be described roughly as a nearly spherical ellipsoid of revolution, with one half 

 slightly flattened at the middle, and the other slightly tapered in the antipodal 

 direction. The figure was described as " pear-shaped," the " pear " having a blunt 

 end, a sharper end, and a waist. The waters of the ocean would presumably collect 

 in the hollow of the waist, and JEANS pointed out that there is some resemblance of 

 the shape of the Earth to this figure, although the " stalk " end of the "pear " was 

 difficult to discover. 



In the same year a paper was published by W. J. SOLLAS,* in which it was 

 concluded from a discussion of the geographical facts that the shape of the Earth 



* " The Figure of the Earth," ' Quart. J. Geol. Soc.,' 59 (1903), p. 180. 

 VOL. CCVII. A. 2 F 



