14 OF THE EARTH AS A MASS. 
But it is not a regular and symmetrical figure of any kind. The 
elevation of the continents, and especially of the table lands that 
traverse them, produces one kind of inequality, and another is 
created by the irregular distribution of the denser masses of which 
it is composed. A large body of rocks of high specific gravity 
rising nearly to the surface in any part of the ocean,, will cause a 
bulging out of the water around that spot, and the same cause 
probably operates in modifying the figure of the exterior surface, 
even in the interior of extensive continents. Leaving out of the 
account the inequalities produced by the elevation of the land 
above the ocean, it is not certain that the excess of the equatorial 
over the polar diameter just stated, is accurate, though it is 
supposed not to differ widely from the truth, or that the curvature 
is regular along any given meridian. Observations with the pen- 
dulum and the actual measurement of arcs in different latitudes 
indicate, not only a small variation in the law of ellipticity at 
unequal distances from the equator, but also an inequality of size, 
and dissimilarity of form in the two hemispheres on the opposite 
sides of it. 
7. Density of the Earth. — The mean density of the earth is 
about five and a half, that of water being one. But the mean den- 
sity of the rocks at its surface, is about two and a half. A mass 
of granite, slate, or limestone, weighs, about two and a half times as 
much as an equal bulk of water. As the mean density of the 
earth is therefore, about double that of the common rocks, it fol- 
lows that it cannot be composed of those rocks in the state in 
which they exist at the surface, but if the material of which it is 
made be nearly the same in every part, that which is near the 
centre must be condensed by the pressure of the superincumbent 
mass into somewhat less than half the bulk it would occupy if it 
were at the surface. 
The density of the earth was first investigated by Cavendish, 
by means of a large torsion balance, and afterwards deduced by 
Maskelyne from the effect of a mountain in Scotland, in with- 
drawing the plumb line from the perpendicular. Cavendish states 
it at five and a half. Playfair, repeating Maskelyne's calculations, 
and applying some corrections that were neglected in the first in- 
stance, found it to be 4.71. Laplace prefers the determination 
of Cavendish. 
When it was ascertained that the globe taken as a mass, so 
much exceeds in density the rocks upon its surface, equalling in 
specific gravity many of the metallic ores, men ventured to draw 
the inference that it is a metallic body, enveloped in a covering 
of soil and rock. They then busied themselves with conjectur- 
ing what might be the nature of this supposed metallic nucleus, 
observing that if it were a metal of mean density, it must consti- 
tute between a quarter and a third of the whole mass, and if 
iron, the half. Bakewell suggested that iron nearly in the me- 
tallic state may be one of its constituents, and that to this the 
earth owes its magnetic polarity., 
