hereafter.

The difference in the size of the planets is very noticeable. If
we represent the sun by a gilded globe two feet in diameter, we
must represent Vulcan and Mercury by mustard-seeds; Venus, by a
pea; Earth, by another; Mars, by one-half the size; Asteroids, by
the motes in a sunbeam; Jupiter, by a small-sized orange; Saturn,
by a smaller one; Uranus, by a cherry; and Neptune, by one a little
larger.

Apply the principle that attraction is in proportion to the mass,
and a man who weighs one hundred and fifty pounds on the earth
weighs three hundred and ninety-six on Jupiter, and only fifty-eight
on Mars; while on the Asteroids he could play with bowlders for
marbles, hurl hills like Milton's angels, leap into the fifth-story
windows with ease, tumble over precipices without harm, and go
around the little worlds in seven jumps.

[Illustration: Fig. 39.--Orbit of Earth, showing Parallelism of
Axis and Seasons.]

The seasons of a planet are caused by the inclination of its axis
to the plane of its orbit. In Fig. 39 the rotating earth is seen
at A, with its northern pole turning in constant sunlight, and
its southern pole in constant darkness; everywhere south of the
equator is more darkness than day, and hence winter. Passing on
to B, the world is seen illuminated equally on each side of the
equator. Every place has its twelve hours' darkness and light at
each revolution. But at C--the axis of the earth always preserving
the same direction--the northern pole is shrouded in continual