ACOUSTICS. 



189 



Tlow may the 

 Bound-vibra- 

 tions ill solid 

 bodies be ren- 

 dered visible ? 



^ ., ,^ If, however, the motions of the vibrating body are of such a 



Dpscnbe the , ^ • , 



Hill 11 re of a character that the agitatiou proceeds froin one particle to au- 



pr.igrcssiTe vi- other, SO that each makes the same vibration, or oscillation, 

 as the preceding one, with tlie sole exception of the motion 

 beginning later, we liave what is called progressive vibrations. Tims if we 

 fasten a cord at one end, and move the other end up and down, a wave, or 

 progressive vibration, is produced. 



As the clearest conception can be formed of vibrations by comparing them 

 to tho waves prod^iced by throwing a stone into smooth water, the term ul- 

 dulatory, or wave movement, has been adopted in general to express tha 

 phenomena of vibrations. 



405. Daily experience teaches us that almost every motion of bodies in our 

 vicinity is accompanied by a noise perceptible to our ears. All such sounds 

 are the result of the vibrations of a portion of matter, and the nature of the tone, 

 or sound, depends only on the manner in which these vibrations originate. 



406. Sound-vibrations in solid bodies may be rendered vis- 

 ible by many simple contrivances. If we attach a ball by 

 means of a string to a bell, and strike the bell, the baU will 

 vibrate so long as the bell continues to sound. When a bell 

 is sounding, also, the tremiolous motion of its particles may be 

 perceived by gently touching it with the finger. If the finger is pressed 

 firmly against the bell, the sound is stopped, because the vibrations are in- 

 terrupted. When sounds are produced by drawing the wet finger around the 

 edge of a glass containing water, waves wiU be seen undulating from the sides 

 toward the center of the glass. 



When a tuning-fork is struck and made to sormd, its vibrations Fig. 1S5. 

 are clearly visible, both branches alternately approacliing and re- 

 receding from each other, as is represented in Fig. 185. 



If we strike a tuaing-fork, and then touch the surface of mercury 

 with one of its extremities, the surface of the mercury will exhibit 

 little undulations or waves. 



„ ^. The most interesting method of exhibiting the 



IIow are the ° ° 



so-eaiicd acous- character of sound is by means of the so-called 



duced'?"^^ ^'^°' "acoustic figures," which may be produced in the 



following manner: — Sprinkle some fine sand over a 



square or round piece of thin glass or metal, and holding the plate 



firmly by means of a pair of pincers, draw a vioUn bow down the edge ; the 



Band is put in motion, and finally arranges itself along those parts of the surface 



which have the least vi- 



Fig. 186. 



assume various interestmg figures, as is represented in Fig. 186. 



bratory motion. By chang- 

 ing the point by which 

 the plate is held, or by 

 varying the parts to which 

 the viohn bow is applied, 

 the sand may be made to 



