Irregularities of the Earth's Surface. 151 



the mean line of the earth's irregularities, and consequently 

 the true circumference and line of gravity regulating the 

 invariable diurnal revolution of the spheroid. Or, to extend 

 the expression of the formula used by Humboldt, if the whole 

 irregularities of the earth's surface were levelled down to 

 one uniformity, the earth's circumference line would occupy 

 a position, bb, nearly 5000 feet below the present ocean level. 

 But as water is only about half the specific gravity of the 

 materials composing the superficial strata of the earth, twice 

 the volume of water is thus necessary to fill up the depres- 

 sions and to bring about that equilibrium which is required. 

 In looking, therefore, at a section of the earth's surface, con- 



Diagram shewing the relations of Land and Water on the Surface of the Earth. 



aa, present surface of ocean ; bb, mean line of earth's irregularities ; cc, mean 

 of elevation of land, 1000 feet; dd, mean depth of ocean, 10,360 feet. 



structed according to the above measurements (see woodcut), 

 it will at once be seen how small a proportion the mean 

 elevation of land bears to the mean depth of the ocean — that 

 while the extremes of both are nearly equal, the extreme 

 elevation of mountains equalling the extreme depressions of 

 ocean, the ocean surface occupies a level half-way between 

 these extremes, and thus becomes the actual line of circum- 

 ference. But then its surface is thus raised from the true 

 central line of the earth's inequalities, in consequence of the 

 less relative specific gravity which water bears to the super- 

 ficial strata of the earth's crust. In this approximate view 

 which I have attempted to suggest, I have left out of view 

 the zone of atmospheric air ; which, however, must form 

 an element in any rigid calculation of the true line of cir- 

 cumference which regulates the diurnal revolution of the 

 earth. 



