198. If the reader be hitherto unacquainted with the principal circles
of the Globe, he should now learn to know them; which he may do
sufficiently for his present purpose in a quarter of an hour, if he sets
the ball of a terrestrial Globe before him, or looks at the Figure of
it, wherein these circles are drawn and named. The _Equator_ is that
great circle which divides the northern half of the Earth from the
southern. The _Tropics_ are lesser circles parallel to the Equator, and
each of them is 23-1/2 degrees from it; a degree in this sense being the
360th part of any great circle which divides the Earth into two equal
parts. The _Tropic of Cancer_ lies on the north side of the Equator, and
the _Tropic of Capricorn_ on the south. The _Arctic Circle_ has the
_North Pole_ for it’s center, and is just as far from the north Pole as
the Tropics are from the Equator: and the _Antarctic Circle_ (hid by the
supposed convexity of the Figure) is just as far from the _South Pole_,
every way round it. These Poles are the very north and south points of
the Globe: and all other places are denominated _northward_ or
_southward_ according to the side of the Equator they lie on, and the
Pole to which they are nearest. The Earth’s _Axis_ is a straight line
passing through the center of the Earth, perpendicular to the Equator,
and terminating in the Poles at it’s surface. This, in the real Earth
and Planets is only an imaginary line; but in artificial Globes or
Planets it is a wire by which they are supported, and turned round in
_Orreries_, or such like machines, by wheel-work. The circles 12. 1. 2.
3. 4, _&c._ are Meridians to all places they pass through; and we must
suppose thousands more to be drawn, because every place that is ever so
little to the east or west of any other place, has a different Meridian
from that other place. All the Meridians meet in the Poles; and whenever
the Sun’s center is passing over any Meridian, in his apparent motion
round the Earth, it is mid-day or noon to all places on that Meridian.
[Sidenote: Zones.]
199. The _broad Space_ lying between the Tropics, like a girdle
surrounding the Globe, is called the _torrid Zone_, of which the Equator
is in the middle, all around. The _Space_ between the Tropic of Cancer
and Arctic Circle is called the _North temperate Zone_. _That_ between
the Tropic of Capricorn and the Antarctic Circle, the _South temperate
Zone_. And the two _circular Spaces_ bounded by the Polar Circles are
the two _Frigid Zones_; denominated _north_ or _south_, from that Pole
which is in the center of the one or the other of them.
200. Having acquired this easy branch of knowledge, the learner may
proceed to make the following experiment with his terrestrial ball;
which will give him a plain idea of the diurnal and annual motions of
the Earth, together with the different lengths of days and nights, and
all the beautiful variety of seasons, depending on those motions.
[Sidenote: Fig. III.
A pleasant experiment shewing the different lengths of days
and nights, and the variety of seasons.