^9o6] CLARKE— STATISTICAL METHOD 15 



I am indebted to Dr. R. S. Woodward for data relative to the 

 volume of matter which is thus taken into account. The volume of 

 the ten-mile rocky crust, including the mean elevation of the con- 

 tinents above the sea, is 1,633,000,000 cubic miles; and to this ma- 

 terial we may assign a mean density not lower than 2.5, nor much 

 higher than 2.7. The volume of the ocean is put at 302,000,000^ 

 cubic miles, and I have given it a density of 1.03, which is a trifle 

 too high. The mass of the atmosphere, so far as it can be deter- 

 mined, is equivalent to that of 1,268,000 cubic miles of water, the 

 unit of density. Combining these data, we get the following ex- 

 pression for the composition of the known matter of our globe. 



Density of Crust. Density of Crust. 

 2.5. 2.7. 



Percentage of atmosphere 03 .03 



Percentage of ocean 7.08 6.58 



Percentage of solid crust 92.89 93.39 



100.00 100.00 



In short, we can regard the surface layer of the earth, to a depth 

 of ten miles, as consisting very nearly of 93 per cent, solid and 7 

 per cent, liquid matter, treating the atmosphere as a small correction 

 to be applied when needed.^ The figure thus assigned to the ocean 

 is probably a little too high, but its adoption makes an allowance 

 for the fresh waters of the globe, which are too small in amount to 

 be estimable directly. Their insignificance may be inferred from 

 the fact that a section of the ten-mile crust having the surface area 

 of the United States, represents only about 1.5 per cent, of the entire 

 mass of matter under consideration. Even the mass of Lake Su- 

 perior thus bcomes a negligible quantity. 



The composition of the ocean is easily determined from the data 



^ Sir John Murray, Scottish Geograph. Mag., 1888, p. 39, estimates the volume 

 of the ocean at 323,722,150 cubic miles. Karstens, more recently, puts it at 

 1,285,935,211 cubic kilometres, or 307,496,000 cubic miles. " Eine neue Berechnung 

 der mittleren Tiefen der Oceane," Inaitg. Diss., Kiel, 1894. Karstens gives a good 

 summary of previous estimates, which vary widely. To change the figure given 

 in my original essay would be a straining after unattainable precision. 



The adoption of Murray's figure for the volume of the ocean would raise 

 its percentage to from 7.12 to 7.88, according to the density, 2.5 or 2.7, assigned 

 to the lithosphere. 



