i?. S. Woodward — Mathematical Theories of the Earth. 343 



facts, are that the mass of the earth is very nearly symmetri- 

 cally disposed about its center of gravity, that pressure and 

 density except near the surface are mutually dependent, and 

 that the earth in reaching this stage has passed through the 

 fluid or quasi fluid state. 



Later writers have suggested other hypotheses for a con- 

 tinuous distribution of the earth's mass, but none of them can 

 be said to rival the hypothesis of Laplace. Their defects lie 

 either in not postulating a direct connection between density 

 and pressure or in postulating a connection which implies ex- 

 treme or impossible values for these and other mechanical 

 properties of the mass. 



It is clear, from the positiveness of his language in frequent 

 allusions to this conception of the earth, that Laplace was 

 deeply impressed with its essential correctness. " Observa- 

 tions,'' he says, " prove incontestably that the densities of the 

 strata (couches) of the terrestrial spheroid increase from the 

 surface to the center";* and " the regularity with which the 

 observed variation of a second's pendulum follows the law of 

 the squares of the sines of the latitudes, proves that the strata 

 are arraDged symmetrically about the center of gravity of the 

 earth. "f The more recent investigations of Stokes, to which 

 allusion has already been made, forbid our entertaining any- 

 thing like so confident an opinion of the earth's primitive 

 fluidity or of a symmetrical and continuous arrangement of its 

 strata. But, though it must be said that the sufficiency of 

 Laplace's arguments has been seriously impugned, we can 

 hardly think the probability of the correctness of his conclu- 

 sions has been proportionately diminished. 



Suppose, however, that we reject the idea of original fluidity. 

 Would not a rotating mass of the size of the earth assume 

 finally the same aspects and properties presented by our planet ? 

 Would not pressure and centrifugal force suffice to bring about 

 a central condensation and a symmetrical arrangement of strata 

 similar at least to that required by the Laplacian hypothesis ? 

 Categorical answers to these questions cannot be given at present. 

 But whatever may have been the antecedent condition of the 

 earth's mass the conclusion seems unavoidable that at no great 

 depth the pressure is sufficient to break down the structural 

 characteristics of all known substances, and hence to produce 



* Enfin il (Newton) regarde la Terre conime homogeue, ce qui est contraire aux 

 observations, que prouvent incontestablemeut que les densites des couches du 

 spheroide terrestre croissent de la surface au centre. Mecanique Celeste, Tome 

 5, p. 9. 



\ La regularity avec laquelle la variation observee des longueurs du pendule a 

 secondes suit la loi du carre du sinus de la latitude prouve que ces couches sont 

 disposees regulierement autour du centre de gravite de la Terre et que leur forme 

 est a peu pres elliptique et de revolution. Ibid, p. 17. 



