'Experiments en Vibrating Surfaces. 3 55 



the thinner it is will be fo much the fitter for thefe experi- 

 ments, though in this refpect there is a certain minimum. 

 In fmall plates, fuch as thofe that are circular, and not above 

 fix inches in diameter, the obfervation is general ; but in 

 larger plates too great thinnefs is prejudicial. Befides, it will 

 be found that verv thin glafs is commonly very uneven, and, 

 as has been already faid, mult therefore be unfit for the ex- 

 periments. If you have a plate, however, which is on one 

 fide thicker than on the other, you mull endeavour to produce 

 the figure on the plate in fuch a manner that you can nib the 

 violin bow againfi the thinncft part. 



In the laft place, in pra&iimg the experiments, you mull 

 have glafs plates of different lizes_, fuch, for example, as circu- 

 lar ones of from four to twelve inches in diameter : for, on 

 a fmall one of four inches in diameter, you cannot produce 

 three concentric circles with two or more diameters; and on 

 a larger, for example, of twelve inches of diameter, you can- 

 not fo eafily produce a fingle circle as on a circular plate of 

 three inches diameter. Sometimes it is difficult to make the 

 figure appear on a plate, while on another it is very eafy. 

 Sometimes it appears only when you hold the place in a cer- 

 tain place. Thefe are phenomena which arife partly from the 

 unlike thickntfs of the glafs, and partly from the ftripes and 

 flaws in it. 



You muff not employ too fine fand, but rather that which 

 is coarfer; becaule the former is eafier thrown from the vi- 

 brating parts, while the latter adheres better to them. It 

 mult be of fuch a nature that when you incline the glafs- 

 plate it may readily roll off; becaufe, in that cafe, it will be 

 eafily thrown from the vibrating parts. It will be of advan- 

 tage that it be mixed with line duft, which (hews peculiar 

 phenomena during the experiments, as it collects itfclf at one 

 place of the vibrating part. 



1 lie plate mud be equally beCtYewed with fand, and not 

 t ) thick, as the lines will then be exceedingly fine; and tlie 

 will acquire a better defined appearance. 



Thofe 



