58 C. R. VAN HISE 



earth from the interplanetary spaces represents the average 

 composition of that out of which the earth segregated, but I 

 can see no prospect that we shall be able to make any better con- 

 jecture of the average composition than that based upon meteoric 

 falls. As already noted, the average specific gravity of such 

 falls is 3.69, and the specific gravity of the earth is 5.67. Now, 

 if this increased density is due to pressure, notwithstanding the 

 high average temperature of the interior of the earth, it follows 

 that the volume of the earth, as a result of pressure, has been 

 reduced in the proportion of 5.67 to 3.69. The former number 

 is 53.65 per cent, greater than the latter. If it be supposed 

 that this percentage of expansion in volume would be inversely 

 as the pressure at the center, a decrease of pressure at the center 

 of 2*4 per cent, would represent an increase of volume of 1.34 

 per cent., and an increase of superficial area of about 1,650,000 

 square miles. It will be noted that the above numbers are so 

 manipulated as to give a minimum result. They could be handled 

 in a different way and give a larger contraction of the surface. 



The correspondence of this result with that obtained by Pro- 

 fessor Slichter, 1,700,000, by an entirely independent line of 

 calculation, is notable and suggests that Laplace's hypothesis 

 as to the relations of pressure and densities within the earth, 

 and the hypothesis that the average specific gravity of the earth 

 material at ordinary pressures and temperatures is 3.69, and that 

 the present density of the earth, 5.67, is due to pressure, may 

 possibly both be approximately true. 



The above calculations are based upon the hypothesis that 

 the matter of the earth remains in the same condition under all 

 pressures. It is subsequently seen that by a change from a 

 liquid to a solid crystalline condition there is an important con- 

 traction. The increased pressure due to lessening rotation may 

 have carried this change further than it would otherwise have 

 gone. Gilbert has suggested to me that a moderate change of 

 pressure within the earth may have acted similarly to the pres- 

 sure upon a spring. Until the pressure reaches a certain amount 

 but little deformation occurs, but at a certain stage a little 



