270 NATURE OF SOUND. 



end to one side and let it go. Elasticity will return it to its position 

 of rest, kinetic energy will carry it beyond, and so on, a vibratory 

 motion being thus produced. When the spring is long, the vibra- 

 tions may be seen. By lowering the spring in the vise, the vibrating 

 part is shortened, the vibrations reduced in amplitude and increased 

 in rapidity. As the spring is shortened, the vibrations become 

 invisible but audible, showing that a sufficiently rapid vibratory 

 motion may produce a sound. 



(b.) Suspend a pith ball by a thread so that it shall hang lightly 

 against one prong of a tuning-fork. When the fork is sounded, the 

 pith ball will be thrown off by the vibrations of the prongs. Other 

 illustrations of the same truth will be observed as we go on. 



(c.) The vibrations of a tuning-fork may be made visible in the 

 following manner : A glass plate which has been blackened by 

 holding it in a petroleum flame is arranged so as to slide easily in 

 the grooved frame F. A pointed piece of metal is attached to one 



FIG. 212. 



of the prongs of the fork. When the fork is made to vibrate, the 

 point placed against the smoked plate and the plate drawn along 

 rapidly in the grooves, the point traces on the glass an undulating 

 line which represents fairly the vibratory movement of the prong. 



422. Propagation of Sound. Sound is ordi- 

 narily propagated through the air. Tracing the sound 

 from its source to the ear of the hearer, we may say that 

 the first layer of air is struck by the vibrating body. The 

 particles of this layer give their motion to the particles of 

 the next layer, and so on until the particles of the last 

 layer strike upon the drum of the ear. 



(a.) This idea is beautifully illustrated by Prof. Tyndall. He 



