24 Rev. J. Blackburn's Description of a Parabolic 



This, however, gives an appearance that is not inelegant; 

 and the outer edge being orna- 

 mented with ci'ockets and leaves r^,--— 1 



and with a finial at the highest ,^^7T — — _/ 



point, and the concave surface /f — T^f— - j 



being painted in imitation of a /^ I / l 



groined oak canopy, the effect / 1/ / / 



of the whole is pleasing to the ^^~~^X__J / 



eye. A curtain is suspended ~ .^J^IiC:r::nn::r=J.... 



from the lower edge of the ca- 

 nopy for about 18 inches on each side; the object of this is to 

 intercept the sound which would pass beneath the sounding 

 board, and might create a confusion behind : but to press it into 

 the service as proposed hereafter is of course to be preferred. 



By means of this erection the volume of sound is increased 

 in a very considerable ratio, and is thrown powerfully, as well 

 as distinctly, to the most distant pai'ts of the church; so that 

 whereas formerly the difficulty of hearing an intelligible sound 

 was very great, now that difficulty is effectually removed. 



The preacher was scarcely audible even in the pews near 

 the pulpit, and not at all in those more remote : he may now 

 be heard in every part. 



It should seem that the voice is reflected in a direction 

 parallel to the axis; for let A stand in the pulpit, and B 

 stand first in the west gallery opposite to the pulpit, and then 

 in the side galleries ; though B is much nearer to A in the 

 latter case than in the former, he can yet hear with decided 

 advantage when opposite to A (i. e. at the greater distance 

 from him). 



The side galleries appear to be benefited rather by the in- 

 creased volume of sound, and by the secondary vibrations 

 excited in a lateral direction. 



It appears also that vibrations proceeding from a distant 

 point and moving in the direction of the axis, are reflected 

 from the parabolic surface towards the focus. For let A 

 stand in the pulpit as before, and B in a distant point oppo- 

 site to A, A can then converse with B in a whisper; whilst C, 

 standing at an intermediate point, cannot at all distinguish 

 the words spoken by B ; he can however hear what is said by 

 A. Also if B, at a distance, opposite to the sounding board, 

 speaks ; whilst A places one ear in the focus of the parabola 

 and one ear towards B, the effect produced is that of a voice 

 close to the ear, and in a direction the reverse of that from 

 which it really proceeds. 



The converse of this also appears true from the following 

 experiment. 



Let 



