e
of the Earth is that of an oblate spheroid; it is slightly flattened at
the poles and bulges at the equator. Its polar or shortest diameter is
7,899 miles, its equatorial diameter is 7,926 miles--greater than the
other by 27 miles. The circumference of the Earth at the equator is
24,899 miles, and the total area of its surface is 197,000,000 square
miles. Its mean density is 5-1/2 times greater than that of water.

The two principal motions performed by the Earth are: (1) Rotation on
its axis; (2) its annual revolution round the Sun. The Earth always
rotates in the same manner, and in the same direction, from west to
east. As the axis of rotation corresponds with the shortest diameter of
the planet, it affords strong evidence that the Earth assumed its
present shape whilst rapidly rotating round its axis when in a fluid or
plastic condition. This would accord with the nebular hypothesis. The
ends of the Earth's axis are called the poles of the Earth; one is the
north, the other the south pole. The north pole is directed towards a
star in the Lesser Bear called the Pole Star. The south pole is directed
to a corresponding opposite part of the heavens. The Earth's axis is
inclined 63 deg. 33' to the plane of the ecliptic, and is always directed to
the same point in the heavens. The Earth accomplishes a revolution on
its axis in 23 hours 56 minutes 4 seconds mean solar time, which is the
length of the sidereal day. This rate of rotation is invariable. At the
equator, where the circumference of the globe exceeds 24,000 miles, the
velocity of a point on its surface is upwards of 1,000 miles an hour,
but, as the poles are approached, the tangential velocity diminishes,
and at those points it is entirely absent. The Earth accomplishes a
revolution of her orbit in 365 days 6 hours 9 minutes; in her journey
round the Sun she travels a circuit of 580,000,000 miles at an average
pace of 66,000 miles an hour. The Earth has other slight motions called
_perturbations_, which are produced by the gravitational attraction of
other members of the solar system. The most important of these is
Precession of the Equinoxes, which is caused by the attraction of the
Sun, Moon, and planets, on the protuberant equatorial region of the
globe. This attraction has a tendency to turn the Earth's axis at right
angles to her orbit, but it only results in the slow rotation of the
pole of the equator round that of the ecliptic, which is occurring at
the