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THE ATMOSPHERE AND FOG-SIGNALLING.
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 aërial pulses transmits it. A thick soap-film produces an obvious effect upon the sensitive flame, a very thin one does not. The augmentation
Apparatus for showing the influence of a non-homogenous Atmosphere on the transmission of sound.
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
