re
of the system was a perfect circle, and if its polar axis lay at right
angles to the plane of its journey, the share of light and heat which
would fall upon any one point on the sphere would be perfectly
uniform. There would be no variations in the length of day or night;
no changes in the seasons; the winds everywhere would blow with
exceeding steadiness--in fact, the present atmospheric confusion would
be reduced to something like order. From age to age, except so far as
the sun itself might vary in the amount of energy which it radiated,
or lands rose up into the air or sunk down toward the sea level, the
climate of each region would be perfectly stable. In the existing
conditions the influences bring about unending variety. First of all,
the inclined position of the polar axis causes the sun apparently to
move across the heavens, so that it comes in an overhead position once
or twice in the year in quite half the area of the lands and seas.
This apparent swaying to and fro of the sun, due to the inclination of
the axis of rotation, also affects the width of the climatal belts on
either side of the equator, so that all parts of the earth receive a
considerable share of the sun's influence. If the axis of the earth's
rotation were at right angles to the plane of its orbit, there would
be a narrow belt of high temperature about the equator, north and
south of which the heat would grade off until at about the parallels
of fifty degrees we should find a cold so considerable and uniform
that life would probably fade away; and from those parallels to the
poles the conditions would be those of permanent frost, and of days
which would darken into the enduring night or twilight in the realm
of the far north and south. Thus the wide habitability of the earth is
an effect arising from the inclination of its polar axis.

[Illustration: Fig. 3.--Inclination of Planetary Orbits (from
Chambers).]

As the most valuable impression which the student can receive from his
study of Nature is that sense of the order which has made possible all
life, including his own, it will be well for him to imagine, as he may
readily do, what would be the effect arising from changes in relations
of earth and sun. Bringing the earth's axis in imagination into a
position at right angles to the plane of the orbit, he will see that
the effect would be to intensify the equatorial heat, and to rob the
high latitudes of the share which they now have. On