THE PRESSURE WITHIN THE EARTH 77 



Alcohol ---... 37. 



Water ------ 29. 



Mercury ------ 27. 



Glass - - - - 5,0 



Copper - - - - - - 8.1 



Iron - - - - - 4.1 



The comparison, they remark, may well be considered as 

 decidedly not adverse to Laplace's law. We may thus infer 

 that it is far from unreasonable to hold that the high density of 

 the interior of the earth is entirely due to the enormous pres- 

 sures there present and hence it is not unreasonable to hold that 

 the diminution in the earth's surface, pointed out above, has 

 actually taken place. Yet, whatever future experiments may 

 show in regard to the compressibility of melted rock, we cer- 

 tainly must believe that the enormous pressure at the center of 

 the earth does have some effect, and, indeed, a large effect, in 

 making the density high in that part of the interior. 



In addition to the results which must follow from a former 

 high rotation period of the earth and large ellipticity, important 

 increments to the internal pressures must have taken place, if 

 any change in the interior from homogeneity to heterogeneity 

 has occurred. Notwithstanding the current computed results for 

 the cooling of the earth, it seems reasonable to suppose that the 

 energy in the interior of the earth, was, within geological times, 

 distributed with greater homogeneity than it is at present. If 

 any such change in the distribution of energy has taken place, 

 then the density of the earth's interior has likewise progressed 

 from homogeneity to heterogeneity. The curves given in Fig. 2 

 show that the pressure at the center of a homogeneous 

 spheroid is only about half the pressure at the center of 

 the present earth. Therefore, any progress that has been made 



each quotient thus obtained be multiplied by 981 times the density of the substance, 

 the result will be the volume elasticity in dynes per sq. cm. If the reciprocal of this 

 last result be multiplied by io 6 , the result will be the compression per atmosphere. The 

 numbers given in the table divided into the radius of the earth give what Thompson 

 and Tait call the "lengths of the moduli of compression." See Thompson and Tait, 

 II, p. 225, § 689. 



