[Illustration: FIG. 167.--Elastic balls.]
[Illustration: FIG. 168.--Suspended billiard balls.]
If a number of balls are arranged in line as in Figure 168 or Figure
169, and the end ball is raised and then allowed to fall, or if _A_ is
pushed against _C_, the last ball _B_ will move outward alone, with a
force nearly equal to that originally possessed by _A_ and to a
distance nearly equal to that through which _A_ moved. But there will
be no _visible_ motion of the intervening balls. The force of the
moving ball _A_ is given to the second ball, and the second ball in
turn gives the motion to the third, and so on throughout the entire
number, until _B_ is reached. But _B_ has no ball to give its motion
to, hence _B_ itself moves outward, and moves with a force nearly
equal to that originally imparted by _A_ and to a distance nearly
equal to that through which _A_ fell. Motion at _A_ is transmitted to
_B_ without any perceptible motion of the balls lying between these
points. Similarly the particles of air set into motion by a sounding
body impart their motion to each other, the motion being transmitted
onward without any perceptible motion of the air itself. When this
motion reaches the ear, it sets the drum of the ear into vibration,
and these vibrations are in turn transmitted to the auditory nerves,
which interpret the motion as sound.
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