13S AMERICAN ASSOCIATION TOE THE 



circle of the mirror. In this case the sound was confined at its origin and prevent- 

 ed from expanding. No conjugate focus was produced, but, on the contrary, 

 the rays of light, when a candle was introduced, constantly diverged. The tick- 

 ing of the watch could not be heard at all when the ear was applied to the out- 

 side 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 the focus the sound ceased to 

 be audible. This method of experimenting admits of considerable precision, and 

 enables us to directly verify, by means of sound transmitted through air, the re- 

 sults anticipated in the previous experiments. A piece of tissue paper placed 

 within the mirror, and surrounding the watch without touching it, slightly dimin- 

 ished the reflection. A simple curtain of flannel produced a somewhat greater 

 effect, though the reflecting power of the metallic parabola was not entirely mask- 

 ed by the 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 interposed 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 with parabolic base very little increased reflection 

 was perceived. The converging beams were merely in this case in a simple plane 

 perpendicular to the mirror, and passing 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 sound 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 Representatives, which is surmounted by a dome, the under surface 

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

 which ascends to it, leaving other points of space deficient in sonorous impulses. 



Water and other liquids, which offer great resistance to compression, are good 

 reflectors of sound. This may be shown by the following experiment : — When 

 water is gradually poured into an upright cylindrical vessel, over the mouth of 

 which a tuning fork is vibrated until it comes within a certain distance of the 

 mouth, it will reflect an echo in unison with the vibrations of the fork, and pro- 

 duce a loud resonance. This result explnins the fact, which had been observed 

 with some surprise, that the duration of the resonance of a newly plastered room 

 was not perceptibly less than that of one which had been thoroughly dried. 



There is another principle of acoustics which has a bearing on this subject. I 

 allude to the refraction of sound. It is well known that when a ray of sound 

 passes from one medium to another, change in velocity takes place, and consequent- 

 ly a change in the direction or refraction must be produied. The amount of this 

 can readily be calculated where the relative velocities are known. In rooms heated 

 by furnaces, and in wh'ch streams of heated air passed up between the audience 

 and speaker, a confusion has been supposed to be produced, and distinct hearing 

 interfered with by this course. Since tbe velocity of sound in air at 32 degrees 

 of Fahrenheit has been found to be 1,090 feet in a second, and since the velocity 

 increases 1.14 feet for every degree of Fahrenheit, if we know the temperature of 

 the room and that of the heated current, the amount of angular refraction can be 



