THE ATMOSPHERE AND FOG-SIGNALING. 



689 



But, though oiled-silk, foreign post, and even gold-beater's-skin can 

 stop the sound, a film sufficiently thin to yield freely to the aerial 

 pulses transmits it. A thick soap-film produces an obvious effect 

 upon the sensitive flame, a very thin one does not. The augmentation 



of the transmitted sound may be observed simultaneously with the 



generation and brightening of the colors which indicate the increasing 



thinness of the film. A very thin collodion-film acts in the same way. 



Acquainted with the foregoing facts regarding the passage of 



vol. vi. 44 



