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. [Illustration: FIG. 169.--Elastic balls transmit motion.] [Illustration: FIG. 170.--When a ball meets more than one ball, it divides its motion.] 253. Why Sound dies away with Distance. Since the last ball _B_ is driven outward with a force nearly equal to that possessed by _A_, it would seem that the effect on the ear drum should be independent of distance and that a sound should be heard as distinctly when remote as when near. But we know from experience that this is not true, because the more distant the source of sound, the fainter the impression; and finally, if the distance between the source of sound and the hearer becomes too great, the sound disappears entirely and nothing is heard. The explanation of this well-known fact is found in a further study of the elastic balls (Fig. 170). If _A_ hits two balls instead of one, the energy possessed by _A_ is given in part to one ball, and in part to the other, so that neither obtains the full amount. These balls, having each received less than the original energy, have less to transmit; each of these balls in turn meets with others, and hence the motion becomes more and more distributed, and distant balls receive less and less impetus. The energy finally given becomes too slight to affect neighboring balls, and the system comes to rest. This is what occurs in the atmosphere; a moving air particle meets not one but many adjacent air particles, and each of these receives a portion of the original energy and transmits a portion. When the original disturbance becomes scattered over a large number of air particles, the energy given to any one air particle becomes correspondingly small, and finally the energy becomes so small that further particles are not