84 Mr. MVheatstona on Sound, _ fAuci 



When this bent plate is excited by percussion, the particles, 

 before their disappearance, will assume an apparent rotatory 

 motion, on account of the force exerted, and its susceptibility of 

 continuing the vibrations. Employing a parallelopedal rod, the 

 appearances of the higher modes of subdivisions are particularly 

 neat ; the entire vibrating parts between the nodes form ellipses, 

 and the semi-part at the free end, a regular half of the same 

 figure. It is important to remark, that the crispations of the 

 water only appear on the sides in the plane of oscillation ; the 

 other two sides, on one of which the exciter must be applied, do 

 not show similar appearances. 



I have also rendered the phonic molecular vibrations visible, 

 when produced by the longitudinal oscillations of a column of 

 air ; the following were the means employed : I placed the open 

 end of the head of a flute or flagiolet on the surface of a vessel 

 of water, and on blowing to produce the sound, I observed 

 similar crispations to those described above, forming a circle 

 round the end of the tube, and afterwards appearing to radiate 

 in right hues ; on the harmonics of the tube being sounded, the 

 crispations were correspondently diminished in magnitude. 

 These phenomena will be more evident if the tube be raised a 

 little from the surface of the liquid and a thin connecting film 

 be left surrounding it ; the vibrating particles will then occupy 

 a greater space, and be more sensible. 



The existence of the molecular vibrations being now com- 

 pletely established, it becomes a critical question, in what 

 manner the sensible oscillations induce these vibrating particles. 

 I do not know whether what I am now going to adduce will be 

 admitted as the right explanation, but it is certainly analogous, 

 80 far as the superficial and transversal linear oscillations are 

 concerned. A flexible surface, covered with a coat of resinous 

 varnish, being made to assume any curve, the cohesion of the 

 varnish will be destroyed in certain parts, and a number of cracks 

 will be observed more regularly disposed as the force inducing 

 the curve has been more regularly applied ; when the original 

 position of the surface is restored, the cracks will be impercepti- 

 ole, but will again appear at every subsequent motion. Be this 

 as it may, these particles are invariable concomitants of the sen- 

 sible oscillations, and there is no reason to suppose otherwise 

 than that their vibrations are isochronous with them. To avoid 

 confusion, I have restricted the word vibrations to the motions 

 of the more minute parts, and the term oscillations to those of 

 the sensible divisions. We may reasonably suppose that the 

 molecular vibrations pervade the entire substance of a phonic ; 

 their excursions, however, are not the same in all parts, and 

 they can only be rendered visible, when these excursions are 

 large ; they may be so few in number as to be entirely inaudible, 

 as in their transmission through linear conductors ; but however 

 feW; when they are properly directed, they induce the mechani- 



