CHAPTER XXIV 

 SOUND 



139. Vibrations in Air. Sound, like light, consists of vibra- 

 tions which excite certain of our nerve endings and cause char- 

 acteristic sensations to be transmitted to the brain. In one 

 case, the vibrations fall on the retina producing light, vision, 

 and color; in the other, they fall on the ear and cause sensa- 

 tions of hearing. Sound differs from light in consisting of 

 waves in ordinary matter instead of in the ether, and in con- 

 sequence they move much more slowly and cannot cross a 



FIG. 60. Recording the vibrations of a tuning fork. 



vacuum. The speed of sound is also affected by the medium 

 in which it travels. In ordinary air, it moves about 1100 feet 

 a second, in water nearly five times as fast, and in iron or steel 

 fifteen times as fast. Even this latter speed is in marked con- 

 trast to the speed of light, which is nearly 186,000 miles a 

 second under all circumstances. In general, the denser the 

 substance, the more rapidly sound travels in it, though this 

 statement is subject to some modifications since the elasticity 

 of the substance in which it moves must also be taken into 



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