LAWS OF SONOROUS VIBRATIONS. 169 



portant discovery, that the simpler the relation of the two 

 parts into which the string was divided, the more perfect 

 was the harmony of the two sounds." ] These important- 

 facts were developed more than five hundred years before 

 the Christian era. 



Laws of Sonorous Vibrations. 



As we have already remarked, sound is produced by vibra- 

 tions in a ponderable medium. The sounds ordinarily heard 

 are transmitted to the ear by means of vibrations of the atmos- 

 phere. A simple and very common illustration of this fact 

 is afforded by the experiment of striking a bell carefully ar- 

 ranged in vacuo. Though the stroke and the vibration can 

 readily be seen, there is no sound ; and, if air be gradually 

 introduced, the sound will become appreciable and progres- 

 sively more intense as the surrounding medium is increased 

 in density. This interesting experiment was made by Hawks- 

 bee, in 1705 ; a but, to secure perfect success, it is necessary 

 to suspend the bell in the exhausted receiver by threads, 

 so that the vibrations cannot be conducted to the solid parts 

 of the apparatus. 



If we produce a single sound, or shock, in a free atmos- 

 phere, we may suppose that the waves are transmitted equally 

 in every direction ; and this is accomplished in the following 

 manner : An imaginary sphere of air receives an impulse, or 

 shock, from the body which produces the sound. This shock 

 is, in its turn, communicated to another spherical stratum of 

 air ; this, to a third, and so on. The elasticity of the air, 

 however, produces a recoil of each imaginary sphere of air, 

 and it is a portion of the last stratum which strikes the tym- 

 panum, throwing it into vibration. If but a single impulse 

 be given to the air, we may suppose that all of the different 



1 TYNDALL, Sound, London, 1867, p. 288. 



8 HAWKSBEE, An Experiment made at a Meeting of the Royal Society, touching 

 the Diminution of Sound in Air rarefijd. Philosophical Transactions, for the 

 Years 1704 and 1705, London, 1706, vol. xxiv., p. 1904. 



