Lord Rayleigh's Acoustical Observations. 



281 



set into rapid rotation when exposed to wind. The phenome- 

 non is of the same character as the rotation of a slip of paper 

 falling freely in air, which was discussed a long while since by 

 Prof. Maxwell*. In both cases the rotation may occur in 

 either direction, proving that its cause is not to be looked for 

 in any want of symmetry. But the view expressed by Max- 

 well does not appear to apply to the pivoted blade ; and I 

 think that the real explanation is yet to be discovered. At 

 present, however, I am concerned merely with an applica- 

 tion. Fig, 2. 











F 



)) 



G 





K 



^ 



A 

 E 



R 



/ 







V 



N 









In fig. 2, A is a blade cut out of sheet brass and provided 

 with sharp projecting points, which bear in hollows at the 

 ends of the screws B, C. These screws pass through a small 

 wooden frame F G, and are adjusted until the blade can turn 

 freely but without perceptible shake. D and E are pieces of 

 cardboard or sheet metal, fitting pretty closely to the blade 

 when in the same plane with them, so that in this position of 

 the blade the passage through the frame is almost closed. As 

 the blade turns, it acts the part of a revolving stopcock. 



In the summer of 1878 1 made several sirens on this plan, 

 which performed well. One of them is represented about 

 full size in the figure. If the wind from the bellows is ad- 

 mitted symmetrically, they will revolve in either direction, 

 and soon acquire sufficient speed to give a note of moderate 

 pitch. The position of maximum obstruction is for small dis- 

 placements a position of stable equilibrium. Tf a larger dis- 

 placement is given, the vibration tends of itself to increase up 

 to a certain point, or even to pass into continuous rotation ; 

 but the precise behaviour in this respect probably depends 

 upon the details of construction. 



The Acoustical Shadow of a Circular Dish 



In a well-known experiment, suggested by Poisson, a blight 



point is observed in the centre of the shadow of a circular 



disk on which waves of light are directly incident. It is some 



years since I rirst attempted to obtain the acoustical analogue 



* Cambridge and Dublin Math, Journal, 1854. 



