turbing the air immediately surrounding
it. A sounding body always disturbs and throws into vibration the air
around it, and the air particles which receive motion from a sounding
body transmit their motion to neighboring particles, these in turn to
the next adjacent particles, and so on until the motion has traveled
to very great distances. The manner in which vibratory motion is
transmitted by the atmosphere must be unusual in character, since no
motion of the air is apparent, and since in the stillness of night
when "not a breath of air" is stirring, the shriek of a railroad
whistle miles distant may be heard with perfect clearness. Moreover,
the most delicate notes of a violin can be heard in the remotest
corners of a concert hall, when not the slightest motion of the air
can be seen or felt.

In our study of the atmosphere we saw that air can be compressed and
rarefied; in other words, we saw that air is very elastic. It can be
shown experimentally that whenever an elastic body in motion comes in
contact with a body at rest, the moving body transfers its motion to
the second body and then comes to rest itself. Let two billiard balls
be suspended in the manner indicated in Figure 167. If one of the
balls is drawn aside and is then allowed to fall against the other,
the second ball is driven outward to practically the height from which
the first ball fell and the first ball comes to rest.

[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 s