Sept. 26, 1878] 



NATURE 



571 



ON THE NATURE OF VIBRATORY MOTIONS^ 

 On the Nature 0/ 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 motions may 

 be given to the air by a distant body, like a string of a 

 violin. The string moves to and fro, that is, it vibrates. 

 These vibrations 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 sounding-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 hare 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 vibrat- 

 ing 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, H of Fig. i, attached to the 



Fig. I. 



drum-skin, is called the haimnerj the next. A, is called 

 the anvil J the third, s, has the exact form of a stirrup, 

 and is called the stirrup-botie. 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 in7ter ear ; a 

 cavity tunnelled 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, 

 S C, and also a cavity, 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 U5 consider how this wonderful little instrument 

 acts when sonorous vibrations reach it. Imagine the 

 violin-string vibrating 500 times in one second. The 



' From a forthcoming work on " Sound : a Series of Simple, Entertaining, 

 and Inexpensive Experiments in the Phenomena of Sound, for the Use of 

 Students of every Age." By Alfred Marshall Mayer, Professor of Physics 

 in the Stevens Institute of Technology. Communicated by the author. 



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 in a second in all directions through the 

 surrounding air. They soon reach the drum-skin of the 

 ear. The latter, being elastic, moves in and out with the 

 air which touches it. Then this membrane, in its turn, 

 pushes and pulls the little ear-bones 500 times in a second. 

 The last bone, the little stirrup, finally receives the vibra- 

 tions sent from the violin-string, and sends them into the 

 fluid of the inner ear, where they shake the fibres of the 

 auditory nerve 500 times in a second. These tremors of 

 the nerve — how we know not — so affect the brain that we 

 have the sensation which we call sound. The description 

 we have just given is not that of a picture created by the 

 imagination, but is an account of what really exists, and 

 of what can actually be seen by the aid of the proper 

 instruments. 



A body may vibrate more or less frequently in a 

 second; it may swing over a greater or less space; and it 

 may have several minute tremors while it makes its main 

 swing. These differences in vibrations make sounds higher 

 or lower in pitch, loud or soft, simple or compound. It is 

 easy to say all this, but really to understand it, one must 

 make experiments and discover these facts for himself. 



On the Nature of Vibratory Motions 



The character of a sound depends on the nature of the 

 vibrations which cause it, therefore our first experiments 

 will be with vibrations which are so slow that we can 

 study the nature of these peculiar motions. These experi- 

 ments will be followed by others on vibrations of the same 

 kind, only differing in this — that they are so rapid and 

 frequent that they cause sounds. A correct knowledge of 

 the nature of these motions lies at the foundation of a 

 clear understanding of the nature of sound. We hope 

 that the student will make these experiments with care, 

 and keenly observe them. 



Experiment i. — At the toy-shops you can buy for a 

 few pence a wooden ball having a piece of elastic rubber 

 fastened to it. Take out the elastic and lay it aside, as 

 we shall need it in another experiment. Get a piece of fine 

 brass wire, about 2 feet (61 centimetres) long, and fasten 

 it to the ball. The weight of the ball should pull the wire 

 straight, and, if it does not, a finer wire must be used. 

 Hold the end of the wire in the left hand, and with the 

 right hand draw the ball to one side. Let it go, and it 

 will swing backward and forward like the pendulum of a 

 clock. This kind of movement we call a pendulous or 

 transverse vibration . 



Experiment 2. — Cut out a narrow triangle of paper, 

 4 inches (10 centimetres) long, and paste it to the bottom 

 of the ball. Twist the wire which supports the ball by 

 turning the latter half round, and watch the paper pointer 

 as it swings first one way and then the other. Here we 

 have another kind of vibration, a motion caused by the 

 twisting and untwisting of the wire. Such a motion is 

 called a torsional vibration. 



Experiment 3. — Take off the wire and the paper, and 

 put the elastic on the ball. Hold the end of the elastic 

 in one hand, and with the other pull the ball gently down- 

 ward, then let it go. It vibrates up and down in the 

 direction of the length of the elastic. Hence we call this 

 kind of motion a longitudinal vibration. 



These experiments show us the three kinds of vibra- 

 tions, transverse, torsional, and longitudinal. They differ 

 in direction, but all have the same manner of moving ; 

 for the different kinds of vibration, transverse, longitudi- 

 nal, and torsional, go through motions with the same 

 changes in velocity as take place in the swings of an 

 ordinary pendulum. These vibrations all start from a 

 position of momentary rest. The motion begins slowly, 

 and gets faster and faster till the body gains the position 

 it naturally has when it is at rest — at this point it has its 



