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
would find that he weighed just as much less as he were pressing with his
stick.

Why is sea-sickness produced on shipboard?

Because man, strictly to maintain his perpendicularity, that is, to keep
the centre of gravity always over the support of his body, requires
standards of comparison, which he obtains chiefly by the perpendicularity
or known position of things about him, as on land; but on shipboard, where
the lines of the masts, windows, furniture, &c. are constantly changing,
his standards of comparison are soon lost or disturbed. Hence, also, the
reason why persons unaccustomed to the motion of a ship, often find relief
by keeping their eyes directed to the fixed shore, where it is visible, or
by lying on their backs, and shutting their eyes; and, on the other hand,
the ill-effects of looking over the side of the vessel at the restless
waves of the sea.

Why is the pendulum a time-keeper?

Because the times of the vibrations are very nearly equal, whether it be
moving much or little; that is to say, whether the arc described by it be
large or small. A common clock is merely a pendulum, with wheel-work
attached to it, to record the number of the vibrations; and with a weight
or spring, having force enough to counteract the retarding effects of
friction and the resistance of the air. The wheels show how many swings or
beats of the pendulum have taken place, because at every beat, a tooth of
the last wheel is allowed to pass. Now, if this wheel has sixty teeth, as
is common, it will just turn round once for sixty beats of the pendulum, or
seconds; and a hand fixed on its axis, projecting through the dial-plate,
will be the second hand of the clock. The other wheels are so connected
with this first, and the numbers of the teeth on them so proportioned, that
one turns sixty times slower than the first, to fit its axis to carry a
minute hand; and another, by moving twelve times slower still, is fitted to
carry an hour-hand.--_A