GEOLOGY. 



367 



been inferred from an accretional formation. This prompted a search 

 for evidences bearing on the total shrinkage of the earth, and the 

 comparison of the earth with its neighbors suggested itself. 



THE SHRINKAGE OF THE EARTH DEDUCED FROM A COMPARISON 



WITH ITS NEIGHBORS. 



The study began by a comparison of the respective volumes, densi- 

 ties, and masses of the earth with the same properties of the next outer 

 planet, Mars, of the next inner planet, Venus, and of its own satellite. 

 These four bodies form a little astronomic group occupying less than 

 3 per cent of the outward reach of the sun's sphere of control. Their 

 dynamic environments, while in process of formation, were therefore 

 so similar that they can be compared with advantage. The method 

 consisted first in building up theoretical earths by adding to each of the 

 three other bodies matter of its own kind and density until the whole 

 equaled the mass of the earth, and computing the respective volumes 

 thus attained. Then, having shrunk each body to the volume of the 

 earth, the amount of shrinkage involved was determined. The 

 results are shown in table 1 . 



Table 1.* 



*The parity-earths may be derived either from the relative densities or the relative masses. The 

 results, however, are not strictly identical in all cases, doubtless because the figures adopted are 

 the weighted means of different methods of determining the masses and densities and these thus 

 lose strict consistency with one another. The differences are not enough to affect seriously the 

 order of magnitude of the shrinkage results. 



These results exceed any shrinkage heretofore postulated, so far as 

 known. They are abundantly ample to meet the unexpectedly large 

 shrinkage requii-ements that had given rise to the inquiry. 



The rest of the study was an effort to see whether these results were 

 subject to serious question or required any radical qualifications. 

 The reasonableness of trying to account for these results by arbitrary 

 assignments of intrinsically lighter and heavier material was con- 

 sidered, but the conditions of the case were found not only to lend no 

 justification to this, but to clearly imply that the more massive and 

 denser bodies contained the smaller proportions of inherently heavy 

 material. As all four bodies are essentially solid, and there was found 

 to be not only an increase but a growing rate of increase of mean 



