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, hh, 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. 
