10 



THE INTERIOR OF THE EARTH. 



If the earth had throughout the same density as the rocks at its sur- 

 face, the pendulum would beat slower at the bottom of a mine than 

 at the top ; because the attraction of the earth is then that of a globe 

 lessened in radius by the distance which you descend towards its 

 centre ; and as the force of gravity thus becomes diminished with the 

 decrease of the mass, the pendulum swings more slowly. But at the 

 bottom of the mine the pendulum actually goes much faster than at 

 the top, showing that the interior of the earth is formed of materials 

 much more dense than the surface rocks. But the density found in 

 this way being 6 J, is so much greater than that found by other methods 

 that it is not usually regarded as quite so reliable, and the experiment 

 has not been repeated. The method of Cavendish, who used a torsion 

 balance, is of a different character. It has been repeated twice, and 

 gives the earth a density of 5^. This being the mean of the various 

 observations, is believed to be as near the true density as possible. It 

 however by no means follows that the interior of the earth consists of 

 substances which have a high density at its surface, since the compres- 

 sion due to gravity alone would greatly condense the materials forming 

 the earth, and give to the interior a high density, even though it con- 

 sisted throughout of such minerals as form the surface rocks. But 

 the effects of this pressure in condensing the interior of the globe 

 would be far more considerable than they are, were they not resisted 

 within by some general antagonist force, such as the expansive power 

 of heat, or an extraordinary want of compressibility among the parti- 

 cular substances operated on. It may be useful to compare with the 

 specific gravity of the earth itself the specific gravities of a few of its 

 constituent minerals and metals as shown in this table : 



Sulphur . 

 Dolomite 

 Calcspar 

 Felspar . 

 Quartz . 

 Mica 



Sp. gravity. 



7-5 



8. 5 

 II.O 

 I 3 .0 

 IQ.O 

 2O.O 



From such a list it is evident that the heavier metals can only form a 

 small portion of the interior of the earth. But if the high density of 

 the earth were supposed to be connected with an original fusion which 

 allowed heavy substances to gravitate towards the centre, then it must 

 be remembered that our only knowledge of the existence of such 

 substances in the earth at all is from their occurrence at the surface, 

 where on such a theory they ought not to be found. It is, therefore, 

 improbable that the earth's density is a result of the arrangement of 

 different mineral constituents in successive layers like the coats of an 

 onion, with the density decreasing towards the surface ; and density 

 probably gives no insight into the interior construction of the earth. 



The Figure of the Earth, as it is commonly named, is such as would 

 be ultimately attained by a rotating body, no matter what its original 

 form may have been. If the earth had ever been a sphere, then the 

 wearing and transporting power of water would gradually cut down the 



