272 



EIGHTEENTH CENTURY. 



TT. III. 



FIG. 



that the water will be agitated in the form of four semi- 

 circles, a, b, c, d, while from 

 the centre to the four points, 

 71, ?i, 71, ;/, it will be still. The 

 reason of this is, that when the 

 glass is set vibrating, it assumes 

 a slightly oval shape first in 

 one direction Q, and then in 

 the other Q> ar) d these two move- 

 ments produce a figure such as 



. 



, is shown- much exaggerated by 



Water vibrating in glass bowl 

 which is giving out a musical the fine HnCS in Fig. 5 2. The 



place a, across which your bow 



was drawn will move most, and form the centre of a bulge, 

 so will the point c opposite to it, making the dotted oval 

 a, n, 71, c, 7t, n ; immediately after, 

 the other two points fr, d, will 

 bulge out in their turn, making an 

 oval in the opposite direction; and 

 so the water will alternately be 

 pushed together and spread out 

 at all these four places, but at ;/, 

 n, n, , it will not be disturbed, 

 and so the nodes are formed. 

 You can even produce this figure 

 by simply wetting your finger and passing it closely round 

 tlie rim of the glass, for whenever. the whole glass vibrates so 

 as to give its lowest note, it forms two alternating ovals, and 

 leaves the four points at rest. The sounds given out by 

 bells, gongs, cymbals, glasses, etc., are all produced by 

 this change of shape, which causes them to strike the air 

 first one way and then the other, and so produce the 

 series of condensations and rarefactions (see p. 168) which 



n 



FIG. 52. 



Changes of figure assumed by a 

 glass when sounding. 



