Chap. 10.] Vibrations ofjonorous Bodies. 441 



as will be afterwards proved > and their found, as to 

 its force and duration, is proportionate to their vibra- 

 tions. 



Suppofe, for example, the bell of a clock to be ftruck 

 by any folid body, a kind of undulating or tremulous 

 motion is imparted to the minute particles ; and this 

 motion may be even perceived by the hand or fingers 

 when applied to the bell. 



To underftan4 this more completely, let us conceive 

 that a bell is compofed of .a feries of circular zones, 

 decreafing in diameter all the way to its top, each of 

 which may be confidered as a flat ring, compofed of 

 as many concentric circles as ics thicknefs will admit 

 of. If this ring is ftruck at the point a (Plate XXX. 

 Fig. 4.) the part fo ftruck tends towards g, and at the 

 feme time the parts b and d tend towards i and m, and 

 this action in thefe parts neceflarily caufes the point c 

 to approach towards e\ by their elaftic power, how- 

 ever, thefe parts prefently regain the pofition in which 

 they were before the bell was ftruck ; but as they re- 

 turn with an accelerated force, they generally go be- e 

 yond the point where they ought to reft. The part 

 a, therefore, after having returned from g to a, tends 

 towards /, the part c towards h, and the parts b and 

 d towards k and / j whence it happens that the bell, at 

 firft of a circular form, really becomes alternately oval 

 in two different directions j it follows then, that in 

 thofe parts where the curvature is the greateft, their 

 exterior points depart from each other. 



The fame circumftance happens to the mufical cord 

 of a harp, or other ftringed inftrument, when it is 

 touched, for, in order to become angular, as B C D 

 or B E D (Fig. 5.) it is necefiary that the firing mould 

 !>e ftretched or lengthened, and confequently its par- 

 ticles 



