GEOGRAPHY. 
may find the difference of the inclination of 
the surfaces at both points ; of course, sup- 
posing the earth a sphere, this difference in 
latitude will be the angle, subtended at its 
centre by the given portion of the surface, 
•whence the whole circumference may be 
determined ; and on these principles the 
earliest measurements of the earth were 
conducted. The first of these, which can 
be considered as accurate, was executed by 
Picart, in France, towards the end of the 
seventeenth century. 
But the spherical form is only an approxi- 
mation to the truth. It was calculated by 
Newton, and ascertained experimentally 
by the French academicians, sent to the 
equator and to the polar circle ; that, in 
order to represent the earth, the sphere 
must be flattened at the poles, and promi- 
nent at the equator. We may therefore 
consider the earth as an oblate elliptic 
spheriod ; the curvature being greater and 
consequently every degree shorter at the 
equator, than nearer the poles. If the den- 
sity of the earth were uniform throughout, 
its ellipticity, or the difference of the 
length of its diameters would be ^ of the 
whole ; on the other hand, if it consisted of 
matter of inconsiderable density, attracted 
by an infinite force in the centre, the ellip- 
ticity would be only and whatever may 
be the internal structure of the earth, its 
form must be between these limits, since 
its internal parts must necessarily be denser 
than those parts which are nearer the sur- 
face. If, indeed, the earth consisted of 
water or ice, equally compressible with 
common water or ice, and following the 
same laws of compression with elastic 
fluids, its density would be several thousand 
times greater at the centre than at the sur- 
face ; and even steel would be compressed 
into one-fourth of its bulk, and stone into 
one-eighth, if it were continued to the 
earth’s centre; so that there can be no 
doubt but that the central parts of the 
earth must be much more dense than the 
superficial. 
Whatever this difference may be, it has 
been demonstrated by Clairaut, that the 
fractions expressing the ellipticity and the 
apparent diminution of gravity at the 
equator, must always make together 
and it has been found, by the most accurate 
observations on the lengths of pendulums 
in different latitudes, that the force of gra- 
vity is less powerful by -fa at the equator 
than at the pole, whence the ellipticity is 
found to be -fa of the equatorial diameter; 
the form being the same as would be pro- 
duced, if about three-eighths of the wdiole 
force of gravity were- directed towards a 
central particle, the density of the rest of 
the earth being uniform. 
This method of determining the general 
form of the earth is much less liable to 
error and irregularity, than the measure- 
ment of the lengths of degrees in various 
parts, since the accidental variations of cur- 
vature produced by local differences of 
density, and even by superficial elevations, 
may often produce considerable errors in 
the inferences which might be deduced 
from these measurements. For example, a 
degree measured at the Cape of Good 
Hope, in latitude 33° south, was found to 
be longer than a degree in France, in lati- 
tude 46° north, and the measurements in 
Austria, in North America, and in England, 
have all exhibited signs of similar irregu- 
larities. There appears also to be some 
difference in the length of degrees under 
the same latitude, and in different longi- 
tudes. We may, however, imagine a regu- 
lar elliptic spheroid to coincide very nearly 
with any small portion of the earth’s sur- 
face, although its form must be somewhat 
different for different parts : thus, for the 
greater part of Europe, that is, for England, 
France, Italy, and Austria, if the measure- 
ments have been correct, this oscillating 
spheroid must have an ellipticity of fa. 
The earth is astronomically divided into 
zones, and into climates. The torrid zone 
is limited by the tropics, at the distance of 
23° 28' on each side of the equator, con- 
taining all "such places as have the sun 
sometimes vertical, or immediately over 
them : the frigid zones are within the polar 
circles, at the same distance from the 
'poles, including all places which remain 
annually within the limit of light and dark- 
ness, for a whole diurnal rotation of the 
earth, or longer : the temperate zones, be- 
tween these, have an uninterrupted alter- 
nation of day and night, but are never sub- 
jected to the sun’s vertical rays. At the 
equator, therefore, the sun is vertical at the 
equinoxes, his least meridian altitude is at 
the solstices, when it is 66° 32', that is, 
more than with us at Midsummer ; and this 
happens once on the north, and once on 
the south side of the hemisphere. Between 
the equator and the tropics he is vertical 
twice in the year, when his declination is 
equal to the latitude of the place, and his 
least meridian altitudes, whieh are unequal 
between themselves, are at the solstices. 
