2 



The Right Hon. Lord Rayleigh 



[Jan. 17, 



plate of the bird-call. So far there is no difficulty ; but if the space 

 between the plates be boxed in air-tight, the action of the call is in- 

 terfered with. To meet this objection a tin-plate box is soldered 

 air-tight to A, and is stuifed with cotton- wool kept in position by a 

 loosely fitting lid at C. In this way very little sound can escape 

 except through the tube A, and yet the call speaks much as usual. 

 The manometer is connected at the side tube D. The wind is best 

 supplied from a gas-holder. 



With the steadily maintained sound of the bird-call there is no 

 difficulty in measuring accurately the wave-lengths by the method of 

 nodes and loops. A glass plate behind the flame, and mounted so 

 as to be capable of sliding backwards and forwards, serves as reflect- 

 ing wall. At the plate, and at any distance from it measured by an 





Fig. 1. 



even -number of quarter wave-lengths there are nodes, where the 

 flame does not respond. At intermediate distances, equal to odd 

 multiples of the quarter wave-length, the effect upon the flame is a 

 maximum. For the present 2)urpose it is best to use nodes, so adjust- 

 ing the sensitiveness of the flame that it only just recovers its height 

 at the minimum. The movement of the screen required to j^ass over 

 ten intervals from minimum to minimum may be measured, and gives 

 at once the length of five complete progressive waves. For the bird- 

 call used in the experiments of this lecture the wave length is 2 cm. 

 very nearly. 



When the sound whose wave length is required is not maintained, 

 the application of the method is, of course, more difficult. Never- 

 theless, results of considerable accuracy may be arrived at. A steel 

 bar, about 22 cm. long, was so mounted as to be struck longitudinally 



