Page:Popular Science Monthly Volume 7.djvu/431
ON THE MOTIONS OF SOUND.
librium of temperature would be established. The non-homogeneous air surrounding Villejuif is experimentally typified by our screen with the source of sound close behind it, the upper edge of the screen representing the place where equilibrium of temperature was established in the atmosphere above the station. In virtue of its proximity to the screen, the echoes from our sounding-reed would, in the case here supposed, so blend with the direct sound as to be practically indistinguishable from it, as the echoes at Villejuif followed the direct sound so hotly, and vanished so rapidly, that they escaped observation. And as our sensitive flame, at a distance, failed to be affected by the sounding body placed close behind the cardboard screen, so, I take it, did the observer at Montlhéry fail to hear the sounds of the Villejuif gun.
Something further may be done toward the experimental elucidation of this subject. The facility with which sounds pass through textile fabrics has been already illustrated, a layer of cambric or calico, or even of thick flannel or baize being found competent to intercept but a fraction of the sound from a vibrating reed. Such a layer of calico may be taken to represent a layer of air differentiated from its neighbors by temperature or moisture; while a succession of such sheets of calico may be taken to represent successive layers of non-homogeneous air.
Two tin tubes (M N and O P, Fig. 3) with open ends are placed so as to form an acute angle with each other. At the end of one is the vibrating reed r, opposite the end of the other and in the prolongation of P O is the sensitive flame f, a second sensitive flame (f' ) being placed in the continuation of the axis of M N. On sounding the reed, the direct sound through M N agitates the flame f'. Introducing the square of calico a b at the proper angle, a slight decrease of the action on f' is noticed, and the feeble echo from a b produces a barely perceptible agitation of the flame f. Adding another square, c d, the sound transmitted by a b impinges on c d; it is partially echoed, returns through a b, passes along P O, and still further agitates the flame f. Adding a third square, e f, the reflected sound is still further augmented, every accession to the echo being accompanied by a corre-
- "Philosophical Transactions," 1874, Part I., p. 208.