illustrations of motions and forces. Thus, all falling and pressing bodies
exhibit _attraction_ in its simplest form. _Repulsion_ is instanced in
explosion, steam, the action of springs, &c. Explosion of gunpowder is
repulsion among the particles when assuming the form of air. Steam, by the
repulsion among its particles, moves the piston of the steam-engine. All
elasticity, as seen in springs, collision, &c. belongs chiefly to
repulsion. A spring is often, as it were, a reservoir of force, kept ready
charged for a purpose; as when a gun-lock is cocked, a watch wound up, &c.

Why does a billiard ball stop when it strikes directly another ball of
equal size, and the second ball proceed with the whole velocity which the
first had?

Because the action which imparts the new motion is equal to the re-action
which destroys the old. Although the transference of motion, in such a
case, seems to be instantaneous, the change is really progressive, and is
as follows:--The approaching ball, at a certain point of time, has just
given half of its motion to the other equal ball; and if both were of soft
clay, they would then proceed together with half the original velocity;
but, as they are elastic, the touching parts at the moment supposed, are
compressed like a spring between the balls; and by their expanding, and
exerting force equally both ways, they double the velocity of the foremost
ball, and destroy altogether the motion in the other.

Why does a walking stick help a man on a journey?

Because he pushes against the ground with the stick, which may be
considered as compressing a spring between the earth and the end of his
stick, which spring is therefore pushing up as much as he pushes down; and
if, at the time, he were balanced in the scales of a weighing beam, he