The Plan of the Earth and its Causes. — Gregory. 135 
earth were quite homogeneous. But the earth is not homo- 
genous ; it varies in strength and density, and an unequal load 
on the earth in any area leads to a divergence there from the 
circular shape. It is, I believe, now universally admitted that 
the earth is flattened laterally at the equator as well as at the 
poles. The question was long disputed between the astrono- 
mers, who, from theoretical considerations, declared what the 
shape of the world ought to be, and the geographers, whose 
measurements showed what the shape actually was. There is 
now a general agreement that the geographers were right ; that 
the equatorial section of the earth is elliptical, similar to a sec- 
tion through the earth passing across the poles. The earth is 
therefore not a true spheroid, and it was accordingly regarded 
as an ellipsoid with three unequal axes. But there is good rea- 
son to believe that the earth is not even an ellipsoid ; for the 
northern and southern hemispheres are unlike, and the earth 
is therefore shaped like a peg-top. This is shown in two ways. 
It is a well known property of the ellipse that degrees meas- 
ured along the flatter side are longer than degrees measured 
near the sharper end. It was by proving that a, degree of lat- 
itude in Lapland is longer than a degree of latitude in Ecuador 
that the French astronomers in the seventeenth century defi- 
nitely proved the earth's flattening at the poles. In continua- 
tion of these observations. La Caille, in 1751, measured the 
length of a degree at the Cape of Good Hope. His measure- 
ments showed that the southern hemisphere was also flattened, 
but to a different extent than the northern hemisphere. This 
anomalous result of La Caille's was confirmed and extended by 
Maclear. 
The inequality of the two hemispheres has also been shown 
by the variations of gravity in the two hemispheres, which, as it 
is more easily tested, has been more widely applied. The prin- 
ciple is simple. A pendulum swings more rapidly the nearer 
it is brought to the centre of the earth. A pendulum swings 
more slowly on a mountain-top than at sea-level. It was be- 
cause Richer, in 1672, found that a clock which kept correct 
time at Paris lost two minutes a day in French Guiana that the 
polar flattening was first suspected. So many observations 
have been made that maps have been compiled showing the va- 
riation of the force of gravity throughout the globe. Fig. 6 is a 
