SOUND 249 



another division of the scale be it one-twelfth or one-eighth 

 and strike another circle from the next point on the little 

 divided circle. Extend another division of the scale, and strike 

 from the next point of the circle in the same direction, and so on 

 round and round fig. 131, extending radius each time, till the 

 disc is filled, which will require going round fig. 131 two 01 

 three times. 



In using this disc, when finished and mounted, a cap or 

 plate is placed on the lantern with a horizontal slit 

 A B, and inspection will show how the revolution of 

 the disc with its eccentric circles, over such a slit, 

 will represent by the motion of bright dots focussed 

 upon the screen the successive condensations and FIG.ISI 

 expansions in a series of aerial waves. 



135. Bells and Liquids, The fact that a sounding bell 

 is in a state of vibration, is best projected by adjusting a thin 

 goblet in the field of the lantern, with a small pith ball hung 

 from a wire stand so as just to touch it. On now drawing a 

 violin bow across the edge of the goblet, the enlarged image 

 of the ball will be seen to be thrown away by the vibrating 

 glass. 



The thinnest glasses should be chosen for all acoustic ex- 

 periments. About 3 inches across is the best average size. 1 

 To ensure steadiness, a good plan is to cut off the feet, and 

 cement the stems into solid blocks about three inches square 

 and one inch deep, cast in lead or type-metal. This is easily 

 done with plaster of Paris mixed with a little powdered char- 

 coal, made up with gum-water. Fixed in such heavy feet, all 

 the phenomena are more easily obtained. 



The nodes of a bell are best shown by the ripples in con- 



1 For ripple phenomena next mentioned, some pains should be taken to 

 get good glasses. At any shop where high-class ware is sold, a certain num- 

 ber should be selected of various shapes, which are thinnest and seem to ring 

 most clearly. Then, by experiment before a window, those which give ripples 

 best and most readily are easily selected. As a rule quite different glasses 

 will give the best results with different liquids like mercury, water, and alcohol 



