NOTE ON THE PRESSURE WITHIN THE EARTH. 



It is the object of the present paper to briefly consider the 

 magnitude of the pressures within the earth- spheroid, especially 

 as influenced by the changes that have been brought about in 

 the ellipticity of the earth's figure by its changing rotation 

 period. 



Darwin, in considering the stability of the moon-earth couple, 

 says it seems improbable that a rotation of the earth in a little over 

 five hours, with an ellipticity of y 1 ^-, would render the system 

 unstable, and it hardly seems likely that better data and more 

 perfect solution would largely affect the result, so as to make the 

 period of revolution of the two bodies in the initial configura- 

 tion very much less than five hours. 1 If the earth be assumed 

 homogeneous throughout, as was done by Darwin in his investi- 

 gations, with a density equal to the present mean density, it is a 

 simple matter to calculate the pressures within the earth for any 

 given eccentricity of its outer crust ; and these eccentricities 

 are, in turn, easily deducible from a knowledge of the rotation 

 period. A table on page 327 of Part II of Thompson and Tait's 

 Natural Philosophy gives us at once the rotation periods corre- 

 sponding to various values of the eccentricity. We there find 

 that 



<?=.5 corresponds to a rotation period of 15,730 seconds or 4^ hours. 

 ^=.4 corresponds to a rotation period of 19,780 seconds or 53^ hours. 



I have assumed that the separation of the moon-earth couple 

 took place at a time when the rotation period of the earth was 

 intermediate to the values just given, and that it would be 

 sufficient for the purposes of geology to trace, from the epoch 

 indicated, the changes in pressure that have taken place in the 

 earth's interior. If it be assumed that the spheroids of eccen- 



1 Phil. Trans., 1879, Part 2, p. 536. 



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