302 Royal Society: — Lord Rayleigh on the Application 



been drawn by Prof. Reynolds, does not interfere with the ap- 

 plication of the law. 



An experiment has, however, been brought forward by Prof. 

 Tyndall, in which there is an apparent failure of reciprocity not 

 referable to any motion of the medium*. The source of sound 

 is a very high-pitched reed mounted in a short tube and blown 

 from a small bellows with which it is connected by rubber tubing. 

 The variation of pressure at the second point is made apparent 

 by means of the sensitive flame, which has been used by Prof. 

 Tyndall with so much success on other occasions. Although the 

 flame itself, when unexcited, is 18 to 2-i inches high, it was proved 

 by a subsidiary experiment that the root of the flame, where it 

 issues from the burner, is the seat of sensitiveness. With this ar- 

 rangement the effect of a cardboard or glass screen interposed 

 between the reed and the flame was found to be different, ac- 

 cording as the screen was close to the flame or close to the reed. 

 In the former case the flame indicated the action of sound, but in 

 the latter remained uninfluenced. Since the motion of the screen is 

 plainly equivalent to an interchange of the reed and flame, there is 

 to all appearance a failure in the law of reciprocity. 



At first sight this experiment is difficult to reconcile with the- 

 oretical conclusions. It is true that the conditions under which 

 reciprocity is to be expected are not very perfectly realized, since 

 the flame ought not to be moved from one position to the other. 

 Although the seat of sensitiveness may be limited to the root 

 of the flame, the tall column of highly heated gas might not be 

 without effect ; and in fact it appeared to me possible that the 

 response of the flame, when close to the screen, might be due to 

 the conduction of sound downwards along it. Not feeling satisfied, 

 however, with this explanation, I determined to repeat the experi- 

 ment, and wrote to Prof. Tyndall, asking to be allowed to see 

 the apparatus. In reply he very kindly proposed to arrange a 

 repetition of the experiment at the Royal Institution for my bene- 

 fit, an offer which I gladly accepted. 



The effect itself was perfectly distinct, and, as it soon appeared, 

 was not to be explained in the manner just suggested, since the 

 response of the flame when close to the screen continued, even 

 when the upper part of the heated column w-as protected from the 

 direct action of the source by additional screens interposed. I w r as 

 more than ever puzzled, until Mr. Cottrell showed me another ex- 

 periment, in which, I believe, the key of the difficulty is to be found. 



"When the axis of the tube containing the reed is directed to- 

 wards the flame, situated at a moderate distance, there is a distinct 

 and immediate response ; but when the axis is turned away from 

 the flame through a comparatively small angle, the effect ceases, 

 although the distance is the same as before, and there are no ob- 

 stacles interposed. If now a card-board screen is held in the 

 prolongation of the axis of the reed, and at such an angle as to 



* Proceedings of the Koyal Institution, January 1875 ; also Prof, TyndaU's 

 work on Sound, 3rd edition, 



