4 THE POPULAR SCIENCE MONTHLY. 



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 vibra- 

 tions 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. 



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 uj;> 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 dif- 

 ferent way. A vibrating body throws the contiguous air into move- 

 ment, 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 longitu- 

 dinal. 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 them be withdrawn with the hand and lightly struck against its neigh- 

 bor, the motion imparted to the first ball is delivered up to the second, 



