Soundifig Board erected in Altercliffe Church. 25 



Let B remain in the situation last supposed, and let A place 

 his face towards the parabolic surface, and his back towards 

 B ; let A now speak, having his mouth in the situation of the 

 focus, and he will be heard as distinctly as when his face was 

 turned towards B. 



If the mouth of the speaker is placed much within or with- 

 out, above or below the focus, the eflTect is proportionably di- 

 minished. It has been asked if the speaker must necessai-ily 

 be confined to one point : to this it may be replied certainly 

 not. He may consult his ease, and will still find the advantage 

 of the canopy over his head ; but as his mouth approaches 

 the focus, an attentive hearer will perceive an effect that may 

 not unaptly be compared to the gentle swell of an organ {j)ar- 

 vis componere mag7ia). The greater the distance between the 

 focus and the vertex, the less will this variation be perceived. 



This sounding board is equally well adapted for a strong 

 or weak voice; the latter acquires strength, whilst in both cases 

 distinctness of articulation is preserved : this may perhaps in 

 some measure be accounted for thus. Assuming that the 

 sound issuing from the focus is reflected in a direction parallel 

 to the axis ; assuming also that the velocity of sound is uni- 

 form ; then the vibrations of the air pi'oceeding from the focus 

 and striking the parabolic sm-face, at whatever point, will ar- 

 rive at the same moment of time at a plane perpendicular to 

 the axis. For (according to the properties of the parabola) 

 the sums of the distances (from the focus to the paraboloid, 

 and from the paraboloid to the plane so situated) are always 

 equal to each other : it must however be admitted, that the 

 velocity of sound is too great to allow much dependence to 

 be placed on this conclusion ; but it is here proved beyond 

 dispute, that a parabolic surface is capable of being success- 

 fully applied to the purpose of a sounding board : whether 

 other concave surfaces similarly situated would be equally 

 successful*, or other materials better adapted to answer the 

 end than pinef, it might be worth while by experiment to 

 ascertain. It is clear that unless the sounding board be con- 

 structed with mathematical nicety and placed with mathe- 

 matical precision, much of the effect will be lost. 



Whilst the figure of the canopy remained perfect, the 

 effect was most complete : perhaps it might be improved if 

 constructed larger, or in other words, if continued further in 



* Many persons have expressed a preference for tlie hypcrbolcid, as giving 

 a divergency to the rays: one fricn<i has proposed a logaritlimic curve. 



+ Some have suggested stone, or a frame-work covered with Roman ce- 

 ment; because such a piece of work would not vibrate, and consequently 

 would not counteract the vibration of the air, on whicli the sound depends. 

 MS. Vol. G. No. 31. J///7y 1829. E advance; 



