Fig. 147.
The pipe A B may be turned upside down, an orifice seen between A and B fitting on to the stand which supports the tube. The conduit t leads into a shallow rectangular box, which communicates by a series of transverse apertures with A B. When air, saturated with the vapor of a volatile liquid, is forced through these apertures, the atmosphere in A B is immediately rendered heterogeneous, the agitated flame being as rapidly stilled.
Fig. 148.
In the experiments at the South Foreland, not only was it proved that the acoustic clouds stopped the sound; but, in the proper position, the sounds which had been refused transmission were received by reflection. I wished very much to render this echoed sound evident experimentally; and stated to my assistant that we ought to be able to accomplish this. Mr. Cottrell met my desire by the following beautiful experiment, which has been thus described before the Royal Society:
A vibrating reed B (Fig. 148) was placed so as to send sound-waves through a tin tube, 38 inches long, and 1-3/4 inch diameter, in the direction B A, the action of the sound being rendered manifest by its causing a sensitive flame placed at F′ to become violently agitated.
“The invisible heated layer immediately above the luminous portion 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 limiting 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 reflected 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 at F to become violently affected. This echo continued active as 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 by the echo, resumed its former tranquillity.