254 On Sonorous Vilrationf. 



the other drives them to the two sides. I was anxious to ex- 

 amine, as niiiiutelv as possible, each of these movements by 

 itself, and the following is what I remarked. I held a square 

 of glass in such a manner that two or three of the edges of 

 it were in contact with my fingers, I struck the edge which 

 I did not touch, with a piece of smooth wood, in such 

 a manner that every point received the shock at the same 

 instant. In this case, the powder arranges itself in lines 

 parallel to the edge struck upon. These lines are rarely 

 straisiht, because these edges have always irregularities; the 

 straiu;htest I ever saw were produced upon an iron rule, 

 struck with a smooth piece of the same metal : in order to 

 the better success of this experiment, it is necessary that the 

 square or the rule should rest upon a smooth table. If I do 

 not strike the blow upon the whole edge, but only upon 

 some of its points, other lines are formed parallel to the 

 direction of the blow, and perpendicular to the edge struck. 

 These lines seem to be composed of a crowd of small eleva- 

 tions, less regular than those produced when plates covered 

 with the dust are struck with the fiddle bow. This experi- 

 ment succeeds with difficulty when the whole of the plate is 

 allowed to rest upon a table, because this circumstance pre- 

 vents us from striking a good blow. In short, if I struck 

 the plate in a directi<in perpendicular to its plane, the dust 

 arranged itself in little regular heaps. In the former case 

 the undulations proceed forward ; in the second case, they 

 take place both in a forward and a lateral direction ; in the 

 third case, thev proceed in every direction, because the com- 

 pression produced by the blow perpendicular to the plane, 

 propagates itself as well horizontally as vertically. The 

 tone produced in the first case is very low ; that of the 

 second is much more elevated ; but that of the third is still 

 much more elevated ; and it is the only one which deserves 

 to be called a tone, or a sound appreciable in the musical 

 scale. 



One would suppose that the change produced in elastic 

 bodies, by tl^e communication of motion, could scarcely be 

 limited to the simple mechanical displacement of the part, 

 but that in this modification it ought to have some other 



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