Take about seven feet of strong wire, and bend it into a circular form,
as _abcd_, which being viewed obliquely, appears elliptical as in the
Figure. Place a lighted candle on a table, and having fixed one end of a
silk thread _K_, to the north pole of a small terrestrial Globe _H_,
about three inches diameter, cause another person to hold the wire
circle so that it may be parallel to the table, and as high as the flame
of the candle _I_, which should be in or near the center. Then, having
twisted the thread as towards the left hand, that by untwisting it may
turn the Globe round eastward, or contrary to the way that the hands of
a watch move; hang the Globe by the thread within this circle, almost
contiguous to it; and as the thread untwists, the Globe (which is
enlightened half round by the candle as the Earth is by the Sun) will
turn round it’s Axis, and the different places upon it will be carried
through the light and dark Hemispheres, and have the appearance of a
regular succession of days and nights, as our Earth has in reality by
such a motion. As the Globe turns, move your hand slowly so as to carry
the Globe round the candle according to the order of the letters _abcd_,
keeping it’s center even with the wire circle; and you will perceive,
that the candle being still perpendicular to the Equator will enlighten
the Globe from pole to pole in it’s motion round the circle; and that
every place on the Globe goes equally through the light and the dark, as
it turns round by the untwisting of the thread, and therefore has a
perpetual Equinox. The Globe thus turning round represents the Earth
turning round it’s Axis; and the motion of the Globe round the candle
represents the Earth’s annual motion round the Sun, and shews, that if
the Earth’s Orbit had no inclination to it’s Axis, all the days and
nights of the year would be equally long, and there would be no
different seasons. But now, desire the person who holds the wire to hold
it obliquely in the position _ABCD_, raising the side ♋ just as much as
he depresses the side ♑, that the flame may be still in the plane of the
circle; and twisting the thread as before, that the Globe may turn round
it’s Axis the same way as you carry it round the candle; that is, from
west to east, let the Globe down into the lowermost part of the wire
circle at ♑, and if the circles be properly inclined, the candle will
shine perpendicularly on the Tropic of Cancer, and the _frigid Zone_,
lying within the _arctic_ or _north polar Circle_, will be all in the
light, as in the Figure; and will keep in the light let the Globe turn
round it’s Axis ever so often. From the Equator to the north polar
Circle all the places have longer days and shorter nights; but from the
Equator to the south polar Circle just the reverse. The Sun does not set
to any part of the north frigid Zone, as shewn by the candle’s shining
on it so that the motion of the Globe can carry no place of that Zone
into the dark: and at the same time the _south frigid Zone_ is involved
in darkness, and the turning of the Globe brings none of it’s places
into the light. If the Earth were to continue in the like part of it’s
Orbit, the Sun would never set to the inhabitants of the north frigid
Zone, nor rise to those of the south. At the Equator it would be always
equal day and night; and as the places are gradually more and more
distant from the Equator, towards the arctic Circle, they would have
longer days and shorter nights, whilst those on the south side of the
Equator would have their nights longer than their days. In this case
there would be continual summer on the north side of the Equator, and
continual winter on the south side of it.
[Illustration: Plate V.
_J. Ferguson delin._ _J. Mynde Sc._]
[Sidenote: PLATE IV.