SOUND 255 



a fourth of 258 giving two superior beats. The essential point 

 is exactness in the tuning of these forks and their cases. It was 

 found quite insufficient to tune two forks in apparent unison, 

 as one has the power to some extent of coercing the other to 

 its own note. The only effectual plan was found to be, tuning 

 both forks so as to give the same number of beats per minute 

 with another slightly different, which can easily be done. The 

 boxes can of course be tuned by measurement. With forks 

 thus tuned, and turning the open ends of the cases towards 

 each other, one fork will excite the other and throw the ball 

 off, when as much as GO feet apart. The ball should be 5 or 

 6 mm. in diameter, smoothly fashioned out of cork, and 

 varnished, which gives it more ' life.' It is suspended by a 

 fine silk thread, and pains taken to suspend it accurately, so 

 that however it may revolve by torsion, it may hang nicely 

 against the fork. The frame should also be fixed steadily. 



Nothing more need be said as to the effect of unisonal 

 vibrations ; and the illustrations of Doppler's principle will 

 almost suggest themselves. 



a. Taking the unisonal fork, and walking steadily up with 

 it from a distance (Prof. Mayer detached the fork from case, 

 striking it, and then placing it on case for the resonance ; but 

 it may be more convenient to keep it on the case, and excite 

 with a bow), the approach is sufficient to so destroy unison 

 that the fork in the lantern does not respond until the demon- 

 strator stops. The same in receding. 



b. Taking the 254 fork and exciting at some distance, there 

 is no response. There is still none when slowly walking. 

 But on swinging the fork and case towards the lantern with 

 approximately the right velocity between 8 and 9 feet per 

 second for 2 beats difference the fork responds and the ball 

 flies off. 



c. A backward swing produces the same effects with the 

 258 fork. 



140. Lissajous' Method. Ever since it was devised by 



