DENSITY OF THE EARTH. 105 



original form, however determined, can not have been retained ; for 

 there are causes in operation which have tended constantly to modify 

 it. If abrasion can produce, it can also modify the form of the earth. 

 If the form of the earth is a form of equilibrium, then a change in the 

 rate of rotation will produce a change in the degree of oblateness or 

 ellipticity. Now, when the earth first solidified from an incandescent 

 liquid condition, it had a certain degree of ellipticity determined by 

 its rate of rotation ; but this rate of rotation has not been constant. 

 The earth, from that time until now, has been cooling and contracting ; 

 and contraction would tend to accelerate rotation and increase ellip- 

 ticity. But, also, ever since an ocean was first formed by precipita- 

 tion on the cooling earth, tides have been formed by the moon and 

 sun, and the friction of the dragging tides would tend to retard rota- 

 tion and decrease ellipticity. At first, doubtless, the contractional 

 acceleration prevailed and ellipticity increased ; but now tidal retarda- 

 tion prevails, and ellipticity is probably decreasing. 



2. ; — Density of the Earth. 



The mean density of the earth, as determined by several independent 

 methods, is about 5 -6. The density of the materials of the earth-sur- 

 face, leaving out water, is only about 2 to 2 # 5. It is evident, therefore, 

 that the density of the central portions must be much more than 5*6. 

 This great interior density may be the result — 1. Of a difference of ma- 

 terial. It is not improbable that the surface of the earth has become 

 oxidized by contact with the atmosphere, and that at great depths the 

 earth may consist largely of metallic masses. Or the great interior 

 density may be the result. — 2. Of condensation 

 by the immense pressure of the superincumbent 

 mass. In either case the tendency of increasing 

 heat would be to diminish the increasing densi- 

 ty. But how much of the greater density is due 

 to difference of material, and how much to in- 

 creasing pressure, and how much these are coun- 

 terbalanced by expansion due to increasing heat, 

 it is impossible to determine. 



The increase of density has been somewhat 



, .. ., , . „ ,, .., ,. i n Fig. 130.— Diagram ilhistrat- 



arbitranly assumed to iollow an arithmetical law. ing the increasing Densi- 

 Under this condition a density equal to the mean 



density would be found at \ radius from the surface, and taking the 

 surface density at 2, and the mean density at 5*5, the central density 

 would be 16. In the diagram (Fig. 130), if a c = radius, the ordinate 

 a x = surface density = 2, and b y = mean density = 5*5, then c z, 

 the central density, will be = 16. 



It is needless to say that this result (Plana's) is unreliable. 



