THE ATMOSPHERE AND FOG-SIGNALING. 561 



through a tin tube, 38 inches long, and If inch diameter, in the direc- 

 tion B A, the action of the sound heing rendered manifest by its caus- 

 ing 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 upward across the end A of the tin tube. 

 A portion of the sound issuing from the tube was reflected at the limit- 

 ing surfaces of the heated layer, the part transmitted being 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 was sent through a second tin tube, A F 

 (of the same dimensions as B A). Its action was rendered visible by 

 causing a second sensitive flame placed at the end of the tube i^to be- 

 come violently affected. This echo continued active 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 vio- 

 lently agitated, and at the same moment the sensitive flame at F, ceas- 

 ino- to be affected by the echo, 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 gas, the flame at F' can be ren- 

 dered so moderately sensitive to the direct sound-wave that the portion 

 transmitted through the reflecting layer shall be incompetent to affect 

 the flame. Then by the introduction and withdrawal of the bat's-wing 

 flame the two sensitive flames can be rendered alternately quiescent 

 and strongly agitated. 



"An illustration is here afforded of the perfect analogy between 

 light and sound ; for if a beam of light be projected from B to F', and 

 a plate of glass be introduced at A in the exact position of the reflect- 

 ing layer of gas, the beam will be divided, one portion being reflected 

 in the direction A F, and the other portion transmitted through the 

 glass toward F\ exactly as the sound-wave is divided into a reflected 

 and transmitted portion by the layer of heated gas or flame." 



Thus far, therefore, we have placed our subject in the firm grasp 

 of experiment ; nor shall we find this test failing us further on. Con- 

 temporary Beview. 



vol. vi. 36 



