222 Royal Society .— 



Feb. 12. — Joseph Dalton Hooker, C.B., President, in the Chair. 



The following communication was read : — 



" On the Division of a Sound-TVave by a Layer of Flame or heated 

 G-as into a reflected and a transmitted Ware.'' By John Cottrell, 

 Assistant in the Physical Laboratory of the Boyal Institution. 



The incompetency of a sound-pulse to pass through non-homo- 

 geneous air having been experimentally demonstrated by Dr. Tyn- 

 dall, and proved to be due to its successive partial reflections at the 

 limiting surfaces of layers of air or vapour of different density, 

 further experiments were conducted in order to render visible the 

 action of the reflected sound-wave. 



The most successful of the various methods contrived for this 

 purpose consists of the following arrangement. A vibrating bell 

 contained in a padded box was directed so as to send a sound- 

 wave through a tin tube, B A (38 inches long, 1| inch diameter), 

 in the direction BF', its action being rendered manifest by its 

 causing a sensitive flame placed at F' to become violently agitated. 



The invisible heated layer immediately above the luminous por- 

 tion of an ignited coal-gas flame issuing from an ordinary bat's- 

 wing burner was allowed to stream upwards across the end of the 

 tin tube B A at A. A portion of the sound-wave issuing from the 

 tube was reflected at the limiting surfaces of the heated layer ; 

 and a part being transmitted through it, was now only competent 

 to slightly agitate the sensitive flame at F'. 



The heated layer was then placed at such an angle that the re- 

 flected portion of the sound-wave was sent through a second tin 

 tube, A F (of the same dimensions as B A), its action being ren- 

 dered visible by its causing a second sensitive flame placed at the 

 end of the tube at F to become violently affected. This action 

 continued so long as the heated layer intervened; but upon its 

 withdrawal the sensitive flame placed at F', receiving the whole of 

 the direct pulse, became again violently agitated, and at the same 

 moment the sensitive flame at F, ceasing to be affected, resumed 

 its former tranquillity. 



Exactly the same action takes place when the luminous portion 

 of a gas-flame is made the reflecting layer ; but in the experiments 

 above described, the invisible layer above the flame only was used. 

 By proper adjustment of the pressure of the gas, the flame at F' 

 can be rendered so moderately sensitive to the direct sound-wave, 



