186 Transactions of the American Institute. 



SCIENTIFIC LECTURE— IV. 



SOUND: THE VOICE AND THE EAR 



By Professor Ogden N. Rood, op Columbia College, Dec. 28, 1871. 



Probably every person who is present to-night has at some time 

 stood on a sea beach, and watched the long lines of advancing waves 

 as they swept inward, only to be followed by others, in a perpetual 

 monotonous succession, so that it seemed as though the ocean was 

 actually sending toward the coast vast masses of water from its inex- 

 haustible magazines. But this beautiful appearance, like so much, 

 else in our world, is partly illusory ; the water is not transported 

 toward the coast, and, if the tide happens to be ebbing, it may have a 

 real though slow motion away from it. The waves are produced 

 merely by a momentary heaping up of the drops of water, along a 

 great line parallel to the shore ; an instant afterward the drops fall to 

 their old level, and you say the wave has passed that spot. 



Can, then, a mere up and down motion produce the appearance of 

 an advancing wave ? Yes, if that motion be executed in suitable 

 time, as I will show in an instant ; but let me first add, that the 

 motion we are considering can also be reversed ; the wave, after it 

 has struck on a precipitous rock, is driven back from the shore, 

 travels outward to the regions that gave it birth, its drops of water, 

 meanwhile, remaining near their original positions, moving, for the 

 most part, only upward and downward. When this reversal of the 

 wave's motion occurs, we say that the wave of water has been 

 reflected, just as the infinitely smaller waves we call light are 

 reflected from a mirror. 



Now, this transmission and reflection of a wave I can easily show 

 you, under circumstances which forbid the idea of any motion of trans- 

 portation. I have here a very elastic cord or rope, made of brass 

 wire wound in a long, thin spiral, and when I strike it with my hand 



