actually complete
eighteen miles every second, as otherwise it would not finish its
journey within the allotted time.

[Illustration: Fig. 31.--The Orbits of the Four Interior Planets.]

Pause for a moment to think what a velocity of eighteen miles a second
really implies. Can we realise a speed so tremendous? Let us compare it
with our ordinary types of rapid movement. Look at that express train
how it crashes under the bridge, how, in another moment, it is lost to
view! Can any velocity be greater than that? Let us try it by figures.
The train moves a mile a minute; multiply that velocity by eighteen and
it becomes eighteen miles a _minute_, but we must further multiply it
by sixty to make it eighteen miles a _second_. The velocity of the
express train is not even the thousandth part of the velocity of the
earth. Let us take another illustration. We stand at the rifle ranges to
see a rifle fired at a target 1,000 feet away, and we find that a second
or two is sufficient to carry the bullet over that distance. The earth
moves nearly one hundred times as fast as the rifle bullet.

[Illustration: Fig. 32.--The Earth's Movement.]

Viewed in another way, the stupendous speed of the earth does not seem
immoderate. The earth is a mighty globe, so great indeed that even when
moving at this speed it takes almost eight minutes to pass over its own
diameter. If a steamer required eight minutes to traverse a distance
equal to its own length, its pace would be less than a mile an hour. To
illustrate this method of considering the subject, we show here a view
of the progress made by the earth (Fig. 32). The distance between the
centres of these circles is about six times the diameter; and,
accordingly, if they be taken to represent the earth, the time required
to pass from one position to the other is about forty-eight minutes.

Outside the path of the earth, we come to the orbit of the fourth
planet, Mars, which requires 687 days, or nearly two years, to complete
its circuit round the sun. With our arrival at Mars we have gained the
limit to the inner portion of the solar system.

The four planets we have mentioned form a group in themselves,
distinguished by their comparative nearness to the sun. They are all
bodies of moderate dimensions. Venus and the Earth are globes of about
the same size. Mercury and Mars are both smaller objects which lie, so
far as bulk is concerned, between the earth and the moon. The four
planets w