39* Experiments on Vibrating Surfaces, 



points of the edge p and q> as may be feen fig. 5. and make 

 the plate to vibrate by rubbing the violin bow againft it at r. 

 There is ftill another convenient method of damping feveral 

 points at the edge when you employ large plates. Fig. i t 

 reprefents a ftrong fquare bit of metal a b, a line in circum- 

 ference, which is fcrewed to the edge of a table, or made faft 

 in any other manner ; and a notch about as broad as the edge 

 of the plate is cut into pne fide of it with a file. You then 

 hold the plate, rpfting againft this bit of metal by two or 

 more fingers when requifite, as at c and d\ by which means 

 the edge of the plate will be damped in three points d, c, e ; 

 and in this manner, by putting the plate in vibration atf, 

 you can produce fig, 10. In cafes of necefiity you may ufe 

 the edge of a table inftead of the bit of metal ; but it does not 

 anfwer the purpofe fo well. 



To produce the vibration at any required place, a common 

 violin bow rubbed with refin is the moft proper inftrument 

 to be employed. The hair muft not be too flack, becaufe it 

 is fometimes neceflary to prefs pretty hard on the plate, in 

 order to produce the tone fooner. 



When you wifh to produce any particular figure, you muft 

 firft form it in idea upon the plate, in order that you may be 

 able to determine where a line at reft and where a vibrating 

 part will occur. The greateftreft will always be where two 

 or more lines at reft interfeft each other ; and fuch places 

 jnuft, in particular, be damped. For example, in fig. 7. you 

 muft damp the part », and ftroke with {he bow in^>. Fig. 11, 

 may be produced with no lefs eafe, if you hold the plate 

 at g, and ftroke with the bow aty. The ftrongeft vibration, 

 feems always to be in that part of the edge whiph is bounded 

 by a curve: fat example, in fig. 8. and fig. 9. at a. To 

 produce thefe figures, therefore, ypu muft rub with the bow 

 at «, ami' not at r. 



You muft, however, damp not only thpfe points where 

 two lines interffeft each other, but endeavour to fupport at 

 leaft one which is fuited to that figure and to no other. 



Fo? 



