252 On Sonorous Vilrations. 



covery of the celebrated Cbladni, who taught the art of pro- 

 ducinsz; certain figures, by covering with sand plates ot" 

 metal or of glass, and then rubbing them upon the edge with 

 a violin bow. The sand is put in motion by the elTect of 

 the oscillations of, the sonorous body, and the grains of it, 

 quitting certain parts of the surface, heap themselves up upon 

 the others, by forming lines and figures of different forms. 

 The lines into -which the grains of sand heap themselves up, 

 are without doubt the points of rest between the portions of 

 the surface put in vibration by the rubbing of the bow upon 

 the edges of the plates. 



Modern philosopliers think they have found, in these curi- 

 ous results, a confirmation of the opinion which tends to 

 exclude those internal vibrations, formerly supposed to be 

 the cause of the phsenoniena of sound, and to refer the latter 

 merely to those oscillations which are visible and appreciable 

 by their mechanical effects, of which the experiment upon 

 sand gives an example. 



It appears -to me, however, that the antient hypothesis has 

 been abandoned too easilvj at least my experiments have 

 rather led me towards that hypothesis, than removed me 

 from it. The following are the observations which these 

 experiments have suggested to me : 



It is allowed that in common air, a fluid eminently elastic, 

 a sudden compression is followed by a reaction which pro- 

 duces a dilatation, which immediately compresses the 

 neighbouring particles, and the effects of a first concussion 

 propagate themselves in this manner to an indefinite extent. 

 But I do not see why this docs not take place also in elastic 

 solids, when they are made to produce a sound. The ex- 

 periments of Chladni, although in other respects very curi- 

 ous, are not proper to prove the small vibrations which con-r 

 cur to form the undulations from which the figures in ques- 

 tion result, and for the knowledge of which we are indebted 

 to him. Sand, which is made use of by him, is too large 

 grained to indicate the nature of the movement of the solid 

 moleculte which tremble under it ; and what is still worse, 

 ^ese grains are elastic, so that they do not remain where 



they 



