Page:Popular Science Monthly Volume 3.djvu/14

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heard about equally well at a distance of 100 feet in front, 75 feet on each side, and 30 feet behind. When an obstacle checks a sound in one direction it can be heard farther in others, because, as a given amount of force produces a given amount of motion, if the motion is arrested in some directions, it is increased in others.

We have now seen that air is the common vehicle of sound, and that the sound-impulse moves in all directions at a high speed/ But what is it that actually moves? The particles of air are certainly not shot from the vibrating body to the ear, for then we should live in the midst of storms ten times more violent than tropical cyclones. The wonderful elastic properties of gases here come into play. The vibrations of bodies produce waves or pulses in the air. It is the same in effect as with water-waves. When we throw a stone into a quiet pool, the ripples chase each other in circles to the shore, but the water itself

Fig. 3.
PSM V03 D014 Vibrations of a clamped plate.jpg
Vibrations or a Clamped Plate.

does not move forward. The floating straw is not borne along, but merely rises and falls in its place, and so the particles of water only oscillate up and down in circles, and, communicating their motion to the adjacent particles, there is an outward transference of force by wave-action, and the water-particles move up and down while the wave moves forward. Air-waves exemplify the same principle, but in a different way. A vibrating body throws the contiguous air into movement, and produces the wave. But the air-particles oscillate backward and forward or in the same direction as the advancing wave. The oscillations in water are transversal; in air, they are said to be longitudinal. The mode of movement may be rudely illustrated by a row of glass balls such as are employed in the game of "Solitaire." If a dozen of them are placed in a groove in contact (Fig. 5), and one of them be withdrawn with the hand and lightly struck against its neighbor, the motion imparted to the first ball is delivered up to the second,