230 ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 



From these experiments we may infer that, if a room were lined 

 with wainscot of thin boards, and a space left between the Avail and 

 the wood, the loudness of the echo of a single noise would be in- 

 creased, while the duration of the resonance would be diminished. If, 

 however, the thin board were glued or cemented in solid connexion 

 to the wall, or imbedded in the mortar, then the effect would be a 

 feeble echo, and a long continued resonance, similar to that from the 

 slab of marble. This was proved by first determining the length of 

 continuance of the vibrations of a tuning-fork on a thin board, which 

 was afterwards cemented to a flat piece of marble. 



A series of experiments were next commenced with reference to the 

 actual reflection of sound. For this purpose a parabolic mirror was 

 employed, and the sound from a watch received on the mouth of a 

 hearing trumpet, furnished with a tube for each ear. The focus was 

 near the apex of the parabola, and when the watch was suspended 

 at this point it was six inches within the plane of the outer circle of 

 the mirror. In this case the sound was confined at its origin, and 

 prevented from expanding. No conjugate focus was produced, but, on 

 the contrary, the rays of light, when a candle was introduced, con- 

 stantly diverged. The ticking of the watch could not be heard at all 

 when the ear was applied to the outside of the mirror, while directly 

 in front it was distinctly heard at the distance of thirty feet, and with 

 the assistance of the ear trumpet at more than double that distance. 

 When the watch was removed from t>e focus, the sound ceased to be 

 audible. This method of experimenting admits of considerable pre- 

 cision, and enables us directly to verify, by means of sound transmit- 

 ted through air, the results anticipated in the previous experiments, 

 A piece of tissue-paper placed within the mirror, and surrounding 

 the watch without touching it, slightly diminished the reflection.- A 

 single curtain of flannel produced a somewhat greater effect, though 

 the reflecting power of the metallic parabola was not entirely masked 

 by three thicknesses of flannel ; and, I presume, very little change 

 would have been perceived, had the reflector been lined with flannel 

 glued to the surface of the metal. The sound was also audible at the 

 distance of ten feet, when a large felt hat, without stiffening, was inter- 

 posed between the watch and the mirror. Care was taken in these 

 experiments so to surround the watch that no ray of sound could 

 pass directly from it to the reflecting surface. 



With a cylindrical mirror, having a parabolic base, very little in- 

 creased reflection was perceived. The converging beams in this case 

 were merely in a single plane, perpendicular to the mirror, and pass- 

 ing through the ear, while to the focal point of the spherical mirror a 

 solid cone of rays was sent. 



The reflection from the cylindrical mirror forms what is called a 

 caustic in optics, while that from a spherical mirror gives a true focus, 

 or, in other words, collects the sounds from all parts of the surface, 

 and conveys them to one point "of space. These facts furnish a ready 

 explanation of the confusion experienced in the Hall of Representa- 

 tives, which is surmounted by a dome, the under surface of which acts 

 as an immense concave mirror, reflecting to a focus every sound which 



