150 Lord Rayleigh's Acoustical Observations. 



from the cylindrical form to give a pure tone. For ready 

 speech, it is sometimes necessary to restrict the lower aperture, 

 e. g. by a bored disk of wood attached with wax. Another 

 plan which answers very well is to block the middle of a cy- 

 lindrical tube by a loosely fitting plug. The tubes that I used 

 are of cast iron ; and were plugged by rectangular pieces of 

 wood provided with springs of brass wire to keep them in 

 position. The length of the plug may be about two diameters 

 of the tube ; the length of the tube itself should be about 

 twelve diameters. In all cases the best result requires that 

 the tubes through which the hydrogen is supplied be of suitable 

 length, and be provided with suitable burners. These may 

 be made of glass, and are easily adjusted by trial. 



For ordinary purposes a common hydrogen-bottle is suffi- 

 cient ; but the note is rather more steady when the hydrogen 

 is supplied from a gas-holder. In this way I have obtained 

 pure tones, giving with tuning forks pretty steady beats of 

 more than two seconds' period. When the intensities are 

 nearly equal, the phase of approximate silence is very well 

 marked. 



Points of silence near a wall from ivhich a pure tone is reflected. 



On this subject there are two papers by N. Savart *, who 

 advances views very difficult of acceptance. A criticism of 

 some of Savart's positions was published soon after by Seebeck; 

 but the question does not appear to have been thoroughly 

 cleared up. 



One source of confusion is imperfect recognition of the 

 fact that the positions of the silences depend upon the nature 

 of the apparatus used for the investigation. In the case of 

 the ear a silence requires that there be no variation of pres- 

 sure at the open end of the ear-passage, whether it be in its 

 natural state, or prolonged by a tube fitted into the external 

 ear. The addition of a small cone or resonator will not affect 

 the truth of this statement. Thus, if the influence of the head 

 and body of the observer acting as simple obstacles be put out 

 of account (as may fairly be done when a tube is used), the 

 silences occur at distances from the wall which are odd mul- 

 tiples of the quarter wave-lengthf . On the other hand, if a 

 membrane simply stretched over a Loop and held parallel to 

 the wall be used as the indicator, the positions of zero dis- 

 turbance are at distances from the wall equal to even multiples 

 of the quarter wave-length. 



In the theory of organ-pipes the places of zero velocity 



* Ann. d. Ohim. lxxi. 1839, xl. 1845. 



t The waves are here supposed complete. Savart's " ondes " are only 

 half as long. 



