[ 139 ] 



v ~-t^ 



XXII. Modification of the usual Trombone Apparatus for showing 

 the Interference of Sound-bearing Waves. By W. F. Barrett, 

 F.R.S.E. $c, Professor of Physics in the Royal College of 

 Science, Dublin*. 



A SIMPLE apparatus for showing the interference of sound- 

 bearing waves may be made by employing a circular 

 arrangement of tubes, one sliding within the other. One tube, 

 A, to' which the mouthpiece M is fixed, is three fourths of a 

 circle : the other tube, B, to which the nozzle N is attached, is 

 half a circle, and of such diameter that it slides freely over the 

 tube A. 



When the nozzle is diame- 

 trically opposite the mouth- 

 piece, the path of the sound- 

 waves is of equal length, and 

 hence the sound from any 

 convenient source placed near 

 to or within the mouthpiece 

 is distinctly heard. By turn- 

 ing the nozzle towards N',in the 

 direction shown by the dotted 

 lines, one limb of the tube 

 is lengthened whilst the other 

 is correspondingly shortened; 

 the path of the waves being 

 now unequal, a point is soon 

 reached where the sound is nearly obliterated. 



Employing a suitable source of sound, and a sensitive flame 

 or a resonant jar as a phonoscope, an audience can perceive at 

 once the gradual destruction of the sonorous pulses ; and more- 

 over the relative lengths of the two branches of the tube clearly 

 indicate the principle of interference thus illustrated. 



One instrument I made was 2 feet in diameter, of 1 -inch- 

 square zinc tubing ; another and better instrument (skilfully 

 made by Mr. R. H. Ridout) was of brass tubing, 1 foot in 

 diameter, the one limb being -^-inch, the other f-inch tube. 

 About 18 inches in diameter would probably be the best and 

 most convenient size. In making the experiment, care should be 

 taken to avoid (a.) the conduction of sound to the ear by the 

 metal substance of the instrument; (/3) the direct transmis- 

 sion of sound through the surrounding air. The latter can be 

 overcome by attaching a sufficiently long gutta-percha tube to M, 



* Read before the Physical Society, June 20, 18/4. 

 by the Society. 



Communicated 



