690 
THE FIGURE OF THE EARTH. 
GEOLOGY, 
FIGURE AND MAGNITUDE OF THE EARTH, 
ITS MEAN DENSITY, SUPERFICIAL 
CONFORMATION AND STRUCTURE. 
{Continued from page 474.) 
Befere we proceed to describe the pre- 
sent state of the crust of the earth, and 
investigate the probable causes of its origin 
and structure, with the nature of the strata 
or more irregular masses of which it is 
composed, it will be requisite to notice those 
facts concerning the general figure, dimen- 
sions, and density of the terrestrial globe, 
and of the contour of its surface, as a body 
of land and water, for a knowledge of which 
we are indebted to the researches of astro- 
nomers and geographers. 
That the figure of the earth is sphei'i- 
cal, or rather spheroidal, though a matter of 
dispute among ancient pJiilosophers,and still 
disbelieved by the vulgar, is now admitted 
as an incontestable truth by all well inform- 
ed persons. The curved surface of the sea 
when viewed from the shore, and the obser- 
vation that the upper rigging of an approach- 
ing ship becomes visible to a distant spec- 
tator before the hull comes in sight, while 
the hull first disappears when the vessel is 
receding, prove that the object in question 
must be moving in the circumference of a 
g reat circle. 
A similar conclusion may be drawn 
from the changing aspect of the heavens to 
an observer travelling fi*om north to south. 
For though the stars and the constellations 
they form will be found to maintain the 
same relative positions with respect to those 
around them, and the points on which the 
celestial dome appears to revolve remain 
unaltered, yet the angle which its axis of 
revolution forms with the horizon continually 
lessens; and thus any star, which at the 
place whence it started, seemed to the ob- 
server to have reached its greatest elevation 
to the south of the point directly above his 
head, now that he has altered his position, 
will appear, when highest, on the north 
of that point ; clearly indicating that his 
path on the earth’s surface has not been a 
right line, but a curve, of which the con- 
vexity is turned towards the sky, correspond- 
ing, in fact, more or less, with a meridian of 
longitude. The appearance of the moon when 
eclipsed, likewise furnishes demonstrative 
proof of the spheroidal figure of the earth, 
for lunar eclipses are caused by its circular 
shadow intercepting the light of the sun from 
the moon’s disk. 
It has been found, however, both from 
astronomical and geodesical observations, 
that the earth is not a perfect sphere, but 
that its figure is that of an oblate spheroid, 
or such a solid as would be foniQed by the 
revolution ^of a fluid mass in open space. 
Huygens and Newton deduced the real figure 
of the earth frota the doctrine of central for- 
ces of bodies revolving in circles, and their 
conclusions were subsequently verified by ac 
tual measurements of degrees of the meridiau 
in various latitudes. It appears, however, 
that though the polar diameter of the earth 
is certainly smaller than its equatorial diame- 
ter, the exact difference between them has 
not yet been accurately ascertained. It has 
been estimated by some at 1 -305th part of 
the equatorial axis, by others at 1-31 0th 
part ; Wt Professor Wallace says : “ We may 
assume, without sensible error, that the 
equatorial axis is to the polar as 334 to 333 ; 
the difference, therefore, of the semiaxes, 
compared with the equatorial radius, will be 
1 part in 334. The fraction of 1-334, that is, 
the difference of the semiaxes divided by the 
equatorial radius, is called the compression of 
the earth at the poles. ”* 
The determination of the figure of the 
eai’th leads to conclusions respecting its 
mean density, which also has within certain 
limits been sufficiently ascertained. Sir 
Isaac Newton, reasoning on the supposition 
of uniform density in the earth, estimated 
its compression at the poles as 1-230 of its 
diameter. Now, since experiment has de- 
monstrated that the compression is less, 
amounting at most to 1-305, it may be con- 
cluded from the observations of Clairault, 
that if the earth is a spheroid of equilibra- 
tion, it is denser in the interior than at its 
surface; and from the experiments of Dr. 
Maskelyne and Mr. H. Cavendish,* it has 
been inferred that the mean density of the 
earth is about five times that of water, and 
therefore double that of the substances which 
compose the crust of the earth, collectively 
considered. 
{To he continued.) 
♦Murray’s Encyclopaedia ofGeograpliy,1834, 
part ii. t». i. ch. 19, p. 128. 
“ A.S the earth has a movement of rotation 
about its axis, all its parts will l)e animated 
with a certain degree of centrifugal force, 
which must be more or less considerable as 
the parts approach or are distant from the 
axis. Under the equator will be the points 
of greatest distance from the axis, and the 
centrifugal force directly opposed to that of 
weight or gravitation, ought to reduce the 
latter there more than at any other place; 
and at parts intermediate between the poles 
and the equator, the diminution of weight 
ought to become less sensible, in propertion 
as they are nearer the poles. At either pole 
the centrifugal force will vanish, and bodies 
will have the same weight as if the earth 
were at rest. 
“ As gravity must be normal at the surface 
of the sea, and as it is the resultant of terres- 
trial attraction and centrifugal force, it will 
be obvious that it must vary at different places; 
and that if the earth was originally a fluid, 
it could not, in consequence of its rotation, 
preserve the form of a sphere, but that it 
must assume that of a flattened spheroid, 
which would be generated by the revolution 
of an ellipsis round its smaller axis. This 
also is demonstrated by experience, and that 
the flattening at the poles renders the axis 
l-310th less than the diameter at the equa- 
tor .” — FranccBur Traite de Mecanique Etemen- 
teire,l825, pp.287, 288. See Scientific Class Book 
pt. i. Mechanics, "S os. 106, 107, and 114 to 123. 
♦ See Scientific Class Book, pt, l,pp. 40, 41 
