Experiments on Vibraifiij* Surfases. 391 



of glafs, though the experiments will fucceed equally well 

 with plates of metal, or pieces of board, of a line or t\v6 in 

 thicknefs. One might now believe that to produce fig. 1. 

 (See Plate VIII.) it would be neceflary to damp, in particular, 

 even' point of the part to be kept at reft, viz. the two concen- 

 tric circles and the diameter, and to put in motion every part 

 intended to be made to vibrate. This, however, is not the 

 cafe; for you need damp only the points a and b, and caufe to 

 vibrate one part c at the edge of the plate, for the movement 

 is foon communicated to the other parts which you with to 

 \ibrate, and the required figure will in this manner be pro- 

 duced. How and why the movement takes place in this 

 manner, and in no other, is difficult to be explained, and 

 would lead me too far, as I with merely to give a few rules 

 for making the experiments. 



The damping may be belt effected by laying hold of the 

 place to be damped between two figures, or by fupporting it 

 only bv one finger. This will be comprehended more clearly 

 by turning to fig. 6. where the hand is represented in that 

 pofition neceffary to hold the plate. In order to produce fig. 3. 

 you muft hold the plate horizontally, placing the thumb 

 above at a, with the fecond finger directly below it; and, be- 

 fides this, you muft fupport the point b on the under fide of 

 the plate. If you then rub the bow of a violin againft the 

 plate at <r, you will produce on the glafs the figure which is 

 delineated figj 3. When the point to be fupported or damp- 

 ed lies too near the centre of the plate, you may reft it on a 

 cork, not too broad at the end, in contact with the glttfs, in 

 order to fupply the place of the finger. It is convenient alfo, 

 when you with to damp feveral points at the circumference 

 of the glafs, to place your thumb on the cork, and ufe the 

 reft of your linsrers for touching the parts which you wifh to 

 keep at reft. For example, if you wifh to produce fig. 4. on 

 ah elliptic plate, the larger axis of which is to the lelTer :>s 4 

 to 3, vou muft place the cork under the centre c of the plate} 

 but your thumb upon this point, and tin n dtfrhp life two 

 C c 4 pointa 



