Lord Rayleigh : Acoustical Xotes. 323 



•cell. A sufficient mutual reaction could be obtained through 

 the air. To this end a large resonator was constructed by 

 cementing a wooden plate over the opening of a bell-glass. 

 In the plate were two similar apertures, to which the free 

 ends of the forks were presented, the pitch of the resonator 

 being equal of that of the forks (128). In this way an 

 adequate control was secured, but the margin was narrow. 



A more powerful controlling reaction accompanies a con- 

 nexion between the two forks by means of slender cotton 

 threads. The arrangement employed is indicated in the 

 figure. A and B are the free ends of the upper prongs seen 



Fig. 2. 



from above. To them is attached a Y-shaped thread 

 ABCD, the tension of which can be adjusted at D. When 

 the control is established, the Lissajous' ellipse is stationary 

 and usually open. An ellipse closed in upon its major axis 

 would indicate that the natural frequencies were identical, 

 independently of the control. By touching a fork judiciously 

 with the rubber tip of the exciting hammer, the phase may be 

 disturbed without stopping the electric maintenance. If one 

 fork be touched, the ellipse closes in, while a similar operation 

 upon the other fork opens it out. In a short time the ellipse 

 settles back, showing that the original phase-relationship is 

 recovered. 



Mutual Reaction of Singing Flames. 



In a former paper * I discussed the mutual influence of 



organ-pipes nearly in unison, showing that the disturbances 



depend upon the approximation of the open ends, and not 



sensibly upon the circumstance that they may take their wind 



* Phil. Mag. vii. p. 149 (1879) ; Scientific Papers, i. p. 409. 

 2A2 



