SOUND. 185 



of air. Every sonorous, elastic body can be thrown into oscillations, 

 if the air surrounding it be made to tremble. Thus, if we sound a 

 note near a piano-forte, whose dampers are raised so as to admit of 

 free vibration, the string, that is in unison with the tone produced, will 

 vibrate by reciprocation ; and a wine-glass or goblet may, according to 

 Dr. Arnott, be made to tremble, and even to fall from a table, by 

 sounding on a violoncello near it the note that accords with its own. 

 The strata of air, in proximity with the sonorous body, receive the first 

 impulses ; and from these they are successively propagated to others ; 

 much in the same manner as the undulations extend from the place in 

 which a stone is cast on a surface of smooth water ; except that the 

 aerial undulations extend in every direction, whilst the aqueous proceed 

 only horizontally. In this propagation from stratum to stratum a por- 

 tion of the sound is necessarily lost ; so that the loudest sound is heard 

 only within certain limits ; and, in all cases, its intensity is inversely 

 as the square of the distance from the sonorous body. 



By causing the sonorous undulations to proceed entirely in one di- 

 rection, and preventing their escape in every other, sound may be ren- 

 dered audible at a much greater distance. M. Biot found, that when 

 he spoke in a whisper at one extremity of a cylinder upwards of one 

 thousand yards long, he was distinctly heard at the other. In many 

 large manufactories the knowledge of this fact is turned to good ac- 

 count. By having numerous tubes communicating with the different 

 rooms of the establishment, and terminating in the office of the princi- 

 pal, he is enabled to have his directions readily conveyed, and to receive 

 information without the slightest inconvenience. 



The velocity with which sound proceeds admits of easy calculation. 

 Light passes with such rapidity, that it may be regarded as proceeding 

 instantaneously from objects on the earth to the eye. The velocity of 

 sound is incomparably less. We see the flash of a gun at a distance ; 

 and, some time afterwards, hear the report. Considering the light 

 then to have reached the eye instantaneously, if we know the distance 

 of the gun, and note the time that elapsed between the appearance of 

 the flash and the report, we can calculate accurately the rapidity of 

 sound. This is found to be about eleven hundred and forty-two feet 

 in a second. We can, in this manner, estimate the distance of a 

 thunder-cloud, by noting the time of the flash, and the interval that 

 elapses before hearing the clap. If it be thirty-seconds, the cloud is at 

 a distance of thirty times eleven hundred and forty-two feet, or six 

 miles and a half. The velocity is the same for all kinds of sounds. 

 M. Biot found, on playing a flute at the end of the tube, above referred 

 to, that the tones arrived at the ear placed at the other extremity in 

 due succession; so that their velocity must have been uniform. 



When aerial oscillations meet with a resisting body of a regular sur- 

 face, as A B, Fig. 75, they are reflected at an angle equal to the angle 

 of incidence: consequently, an ear, placed in the course of the reflected 

 waves as at C, will refer the sound to a distance as far behind the point 

 of reflection, and in the direction of the reflected ray, as the sonorous 

 body is from the point of reflection. It will seem to be at E. The ear 

 at C will, however, receive the direct oscillations from the bell D, as 



