y for the velocity to be
reduced, and consequently we believe that the body could journey on for
ever with unabated speed. No doubt such a statement seems at variance
with our ordinary experience. A sailing ship makes no progress on the
sea when the wind dies away. A train will gradually lose its velocity
when the steam has been turned off. A humming-top will slowly expend its
rotation and come to rest. From such instances it might be plausibly
argued that when the force has ceased to act, the motion that the force
generated gradually wanes, and ultimately vanishes. But in all these
cases it will be found, on reflection, that the decline of the motion is
to be attributed to the action of resisting forces. The sailing ship is
retarded by the rubbing of the water on its sides; the train is checked
by the friction of the wheels, and by the fact that it has to force its
way through the air; and the atmospheric resistance is mainly the cause
of the stopping of the humming-top, for if the air be withdrawn, by
making the experiment in a vacuum, the top will continue to spin for a
greatly lengthened period. We are thus led to admit that a body, once
projected freely in space and acted upon by no external resistance, will
continue to move on for ever in a straight line, and will preserve
unabated to the end of time the velocity with which it originally
started. This principle is known as the _first law of motion_.

Let us apply this principle to the important question of the movement of
the planets. Take, for instance, the case of our earth, and let us
discuss the consequences of the first law of motion. We know that the
earth is moving each moment with a velocity of about eighteen miles a
second, and the first law of motion assures us that if this globe were
submitted to no external force, it would for ever pursue a straight
track through the universe, nor would it depart from the precise
velocity which it possesses at the present moment. But is the earth
moving in this manner? Obviously not. We have already found that our
globe is moving round the sun, and the comprehensive laws of Kepler have
given to that motion the most perfect distinctness and precision. The
consequence is irresistible. The earth cannot be free from external
force. Some potent influence on our globe must be in ceaseless action.
That influence, whatever it may be, constantly deflects the earth from
the rectilinear path which it tends to pursue, and constrain