Page:The American Cyclopædia (1879) Volume XV.djvu/191

 SOUND 183 thereby has succeeded in measuring directly in the vibrating air the length of sonorous waves, and has determined in the air surrounding the vibrating body the form of the wave surface. (" American Journal of Science," November, 1872.) It is evident that the ultimate effect of the passage of sonorous waves through the atmosphere will be to cause the molecules of the air to swing to and fro with the motions of pendulums. It is also apparent that all the characteristics of the periodic motion at the source of the sound will be impressed on the surrounding air and transmitted through it to a distance. Reflection of Sound. It fol- lows from the very nature of sound pulses that if a sonorous wave meet a hard smooth. sur- face, or encounter the surface of separation of two media of unequal elasticity, reflection of sound will take place, and the laws of reflec- tion will be the same as in the case of light, viz. : the angle of reflection will equal the an- gle of incidence, and both the incident and reflected ray will lie in the same plane, which is at right angles to the reflecting surface. These laws admit of a ready experimental proof. If two concave parabolic mirrors, formed of metal backed with hard wood or plaster of Paris, be placed opposite each other at a distance of 10 or 15 ft. with the axis of the mirrors in the same line, and a watch be placed in the focus of one of the mirrors, it will be found that the sonorous pulses emana- ting from the watch will be reflected from the first mirror upon the surface of the second mirror, and here by a second reflection will be conveyed to the focus. This fact can be ascertained by leading to the focus a tube ter- minated at one end by a small funnel, while the ear is applied to the other end of the tube. In the article OPTICS it has been shown that the action just described is a necessary conse- quence of the laws of reflection given above. Refraction of Sound. Sound waves are also refracted, and their refraction is due to the same cause which produces refraction of the rays of light ; i. e., to the change in velocity which occurs when the sonorous beam enters a refracting medium. When the sonorous wave surface falls upon the refracting medi- um so that it is parallel to the refracting sur- face, there will be no refraction, or change in the direction of the sound, but only a change of velocity. But when the sonorous wave surface forms an angle with the surface of the refracting medium, the change in velo- city causes the refraction of the sonorous beam, so that if the velocity of the sound is less in the refracting medium than it was before it entered it, the sound will be re- fracted toward the perpendicular to the re- fracting surface. The refraction will be away from the perpendicular when the velocity of the sound is greater in the refracting medium than it was before it entered it. It follows from the above action, that for the same me- dia there will be a constant ratio existing be- tween the sines of the angles of incidence and refraction, and also that the incident and re- fracted ray will be in the same plane at .right angles to the refracting surface. (See LIGHT, vol. x., p. 439.) The experimental verification of these laws, however, is not so easy as in the similar phenomena of light. The experiment FIG. 17. best adapted for this purpose is one devised by Sondhaus and represented in fig. IT. He constructed a lens, L, of sheets of collodion, having the form of portions of a sphere, and united these sheets to the opposite sides of a metal ring. On inflating the envelope thus formed with carbonic acid gas, a lenticular form was given to it. A watch was placed at W, on the axis of the lens, and it was found that the sound waves were refracted to the 'conjugate focus of the lens at F. If at F we place a bent pipe with a funnel-shaped mouth, and replace the watch at W by a small organ pipe, the refraction is detected by seeing grains of a light powder dance on the membrane clo- sing the upper mouth of the bent pipe at c. Interference of Sound. Another necessary consequence of the nature of sound vibrations and of the manner of their propagation is, that if the condensed half of a sonorous wave meet the rarefied half of another sonorous wave, and these waves have the same length and the same energy of vibration, there can be no vibratory motion at their place of meeting, for the directions of the vibrations in the two half waves are opposed, and the intensities of these opposed vibratory motions are equal. These conditions are fulfilled in various well known experiments, and it is one of the best established facts in acoustics that two sound vibrations may meet and produce silence at the place of their meeting ; this is known as the phenomenon of the interference of sound. Dr. Thomas Young studied this phenomenon attentively, and its contemplation led to his great discovery of the similar phenomena of the interference of light, which formed the basis of his reasoning in establishing the undulatory theory of light. To Dr. Young we owe one of the simplest known means of ex- hibiting and studying the phenomena of inter- ference of sound. If a vibrating tuning fork be held in a vertical position at a short dis- tance from the ear, and then rotated around its vertical axis, it may be observed, when the