EXPERIMENTS IN SOUND. 65 



EXPEKIMENTS IN SOUND. ^ 



SOUND is the sensation peculiar to the ear. This sensation is caused 

 by rapidly-succeeding to-and-fro motions of the air which touches 

 the outside surface of the drum-skin of the ear. These to-and-fro mo- 

 tions may be given to the air by a distant body, like a string of a vio- 

 lin. The string moves to and fro, that is, it vibrates. These vibra- 

 tions of the string act on the bridge of the violin, which rests on the 

 belly or sounding-board of the instrument. The surface of the sound- 

 ing-board is thus set trembling, and these tremors, or vibrations, spread 

 through the air in all directions around the instrument, somewhat in the 

 manner that water-waves spread around the place where a stone has 

 been dropped into a quiet pond. These tremors of the air, however, 

 are not sound, but the cause of sound. Sound, as we have said, is a 

 sensation ; but, as the cause of this sensation is always vibration, we 

 call those vibrations which give this sensation sonorous vibrations. 

 Thus, if we examine attentively the vibrating string of the violin, we 

 shall see that it looks like a shadowy spindle, showing that the string 

 swings quickly to and fro ; but, on closing the ears, the sensation of 

 sound disappears, and there remains to us only the sight of the quick 

 to-and-fro motion which the moment before caused the sound. 



Behind the drum-skin of the ear is a jointed chain of three little 

 bones. The one, IToi Fig. 4, attached to the drum-skin, is called the 

 hamtner ; the next. A, is called the anvil; the third, S, has the exact 

 form of a stirrup, and is called the stirrup-bone. This last bone of the 

 chain is attached to an oval membrane, which is a little larger than 

 the foot of the stirrup. This oval membrane closes a hole opening into 

 the cavity forming the mner ear y a cavity tunneled out of the hardest 

 bone of the head, and having a very complex form. The oval hole 

 just spoken of opens into a globular portion of the cavity, known as the 

 vestibule ; and from this lead three semicircular canals, SC, and also a 

 caAdty, C, of such a marked resemblance to a snail's shell that it is 

 called cochlea, the Latin word* for that object. The cavity of the inner 

 ear is filled with a liquid, in which spread out the delicate fibres of the 

 auditory nerve. 



Let us consider how this wonderful little instrument acts when 

 sonorous vibrations reach it. Imagine the violin-string vibrating 500 

 times in one second. The sounding-board also makes 500 vibrations 

 in a second. The air touching the violin is set trembling with 500 

 tremors a second, and these tremors speed with a velocity of 1,100 feet 



' From " Sound : A Series of Simple, Entertaining, and Inexpensive Experiments in 

 the Phenomena of Sound, for the Use of Students of Eve,ry Age." By Alfred Marshall 

 Mayer, Professor of Physics in the Stevens Institute of Technology. " Experimental 

 Science Series for Beginners, No. II." New York : D. Appleton & Co. 



VOL. XIV. — 5 



