106 Mr Fisher, Densities of the Earth's Crust 



Densities of the Earth's Crust beneath continents and oceans 

 compared. By Rev. 0. Fisher, M.A., F.G.S., Hon. Fellow of Jesus 

 College. 



[Received 10 March 1905.] 



The following paper refers to the subject treated of with 

 Mr Brill's assistance in the second edition of my Physics of the 

 Earth's Crust*. Our conclusions were criticised by Mr Blake "j", 

 while Mr Jukes-Browne desired further conditions to be taken 

 into account than we had considered %. Since I referred to our 

 results in a paper at the British Association 1904, during the 

 discussion of the causes of compression of the earth's crust§, I 

 have been led to go through the problem afresh, making, as I 

 think, considerable improvements in the demonstration, and ob- 

 taining some additional results which are of interest. 



Neglecting the centrifugal effect, let us regard the earth as 

 a sphere, whose surface consists of flat land and water. If it is 

 spherical, gravity must be constant all over its surface. Observa- 

 tion upon the force of gravity upon islands, as well as Hecker's 

 direct observations in crossing the Atlantic ||, confirm this view. 



Let us therefore assume the earth to consist centrically of a 

 nucleus of concentric layers each of uniform density, covered with 

 a concentric shell varying from place to place in the number, 

 density, and thickness, of the layers of which it is composed; but 

 whose normal attraction over its outer spherical surface is uniform. 

 This attraction, combined with that of the nucleus, will cause 

 gravity over the surface to be uniform. 



Since the density of the ocean is less than that of the land, 

 and its depth variable, there must be some arrangement of the 

 densities and thicknesses of the solid crust to compensate for these 

 variations, so as to render the normal attraction constant; and our 

 enquiry consists in seeking some law which will effect this object. 



The form being considered spherical, and the normal attraction 

 of the solid crust and the ocean as constant over the surface, it is 

 equivalent to that of some hypothetical uniform shell. If we then 



* Physics of the Earth's Crust, 2nd ed., ch. xvn. and Appendix, ch. xxvi. 



t Annals of British Geology, 1892. 



j Nature, Nov. 21, 1889, Vol. xli. p. 54. 



§ Geological Magazine, Decade v. Vol. i. p. 495, 1904. 



|| O. Hecker. Bestimmung der Schwerkraft auf dem Atlantischen Ozean sowie 

 in Bio de Janeiro, Lissabon und Madrid. Veroffentlichungen des K. Preuszischen 

 Geodatischen Institutes. Neue Folge. No. 11. 



