326 OUR PHYSICAL WORLD 



modified by the resonance of the body of the instrument and the 

 air in its chambers. 



Sound is due to the vibration of the body that initiates it, 

 and these vibrations pass out as pulses into the surrounding 

 medium. Strike a gong or bell and hold against its edge a ball 

 made of the pith of the elderberry stem, or a tissue-paper wad 

 suspended by a string, and the ball flies off from the bell 

 repeatedly, impelled by the push of the oscillating particles. 

 One can see the vibrations of a taut string, for, when plucked, 

 it is a blur, widest usually at its central region, where it is swing- 

 ing back and forth with the greatest amplitude (Fig. 165). 



Sound, like light, is a form of wave-motion. The vibrating 

 particles of the substance that carries the sound move back 

 and forth in the same direction the sound is traveling; while, 



FIG. 165. Vibration of a taut string 



in the case of light, this oscillation is transverse to the line of 

 propagation. The sound waves move out in all directions in 

 air, for instance, from the sounding body as concentric spheres 

 that are alternately dense and rare (Fig. 166). Sound, therefore, 

 like light travels from point to point in straight lines, the radii 

 of these concentric spheres. 



Its rate of propagation is relatively slow. In air it goes 

 about 1,100 feet per second, while light in the same time travels 

 186,300 miles. In general this discrepancy in the rates of move- 

 ment of sound and light is familiar from commonplace experi- 

 ences, even if the exact difference is unknown. You see the puff 

 of steam from a distant locomotive whistle long before you 

 hear the toot. You see a distant woodchopper, or a section 

 hand driving a spike into the ties, deliver a stroke and straighten 

 up ready for the next one before you hear the sound of his blow. 



