394 Experiments an Vibrating Surfaces. 



damped; and you may mark thefe points by making a fcratch 



on the plate with a bit of flint. 



When the plate lias acquired the proper vibration, you 

 muft endeavour to keep it in that ftate for fome feconds, 

 which can be belt done by rubbing the bow feveral times in 

 fucceffion. By thefe means the land will be formed much 

 more accurately. 



Any fort of glafs may be employed for thefe experiments, 

 provided its furface be fmooth ; otherw ife the fand will fall 

 into the hollow parts, or be thrown about in an irregular 

 manner. The furface of the common green glafs is, how-* 

 «'ver, not very fmooth, and often full of cavities. The fi- 

 gures, therefore, formed upon it are not fo clear and accurate 

 as thofe formed upon fmooth white glafs, which is the kind I 

 employ. 



When there are feveral places on the plate where there 

 are ftripes in the mafs of the glafs, they hurt the experiments, 

 especially in producing thofe figures where a vibrating part 

 falls upon feveral of thefe ftripes. In that cafe the figures 

 are not fo fymmetric as on other plates. I have in my pof- 

 feffion, for example, a circular plate of glafs 20 inches in 

 diameter, on which I was never able to produce a perfect 

 circle as on other plates, the circle alwavs appearing like a 

 very long ellipfe. This plate contains a great many of the 

 above-mentioned ftripes. The flaws and knots which occur 

 in c'lafs are not very hurtful. 



Common glafs plates, when cut with a none, are vcrv 

 fliarp on the edge, and would foon deftroy the hair of the 

 violin bow : on this account the edge mull be rendered fome- 

 what fmooth. For that purpofe I employ a file or a piece of 

 coarfehard free (lone. It is of no great confequence, accord- 

 ing !o my obfervations, whether the edges be very fmooth of 

 roughly ground, provided they be in fuch a ftate as not to 

 injure the hair of the bow. 



Vou mult endeavour to procure fuch plates as are pretty 

 eqi sal in thiekncls. It may be f;:id. in gaietaj, thai • plate 



the 



