252.  How Sound is transmitted through Air.  We saw in Section 250 that sound can always be traced to the motion or vibration of matter.  It is impossible to conceive of an object being set into sudden and continued motion without disturbing 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 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.