460 Lord Rayleigh's Acoustical Observations. 



forks were placed at a distance of about ten yards apart, and 

 were provided with suitably tuned resonators by which their 

 sounds were reinforced. The pitch of both forks is necessarily 

 identical, since the vibrations are forced by electromagnetic 

 forces of absolutely the same period. The arrangement was 

 successful; and with a battery-power of two Grove cells sounds 

 of fair intensity were obtained. With one ear closed it was 

 possible to define the places of silence with considerable accu- 

 racy, a motion of about an inch being sufficient to produce a 

 marked revival of sound. At a point of silence, from which 

 the line joining the forks subtended an angle of about 60°, the 

 apparent striking up of one fork, when the other was stopped, 

 had a very peculiar effect. 



Symmetrical Bell. 



I do not know whether it has ever been noticed that there 

 ought to be no sound emitted along the axis of a symmetrical 

 bell. It is easy to see that at any point of the axis any effect, 

 whether condensation or rarefaction, which may be produced 

 by one part of the surface of the bell must be neutralized by 

 other parts, and that therefore on the whole there can be 

 no variation of pressure during the vibration. The experi- 

 ment may be made with a large glass bell (such as those used 

 with air-pumps), set into vibration by friction with the wetted 

 finger carried round the circumference. If the axis of the 

 vibrating bell be turned exactly towards the observer, the 

 sound is feeble as compared with that heard when the position 

 of the bell is altered. The residual sound may be due to want 

 of symmetry, or more probably to reflexion from the ground, 

 which last cause of error it is almost impossible to get rid of. 



Octave from Tuning-forks. 



When a vibrating fork is held over an air-resonator in tune 

 with itself, the sound emitted is very approximately a pure 

 tone ; but when the fork is placed in contact with a sounding- 

 board, the octave may generally be perceived by a practised 

 ear, and is often of remarkable loudness. By means of a reso- 

 nator tuned to the octave the fact may be made apparent to 

 any one. This result need not surprise us. By the construc- 

 tion of a fork the moving parts are carefully balanced, and the 

 motion is approximately isolated. In the ideal tuning-fork, 

 composed of equal masses moving to and fro in a straight line, 

 the isolation would be complete, and there would be no ten- 

 dency whatever to communicate motion to surrounding bodies. 

 In an actual fork, however, even if the direction of motion of 

 the masses were as nearly as possible perpendicular to the 



