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



quired such aid,) but its general eiFect was so decidedly dis- 

 advantageous, that it was again taken down *. 



The desired object was to convey a distinct sound to remote 

 parts of the church : under the impression that this might 

 be attained by intercepting so much of the sound as escaped 

 behind and echoed in this part of the vaulted roof; as also by 

 giving it a right direction ; and conceiving that a parabolic 

 figure might be so applied as to answer these ends, the writer 

 of this paper made the trial ; and the issue has more than 

 realized his hopes and expectations. 



Grateful for this result, believing that the applicatiofi of the 

 principle was new, hoping to awaken the attention of others 

 better qualified than himself (for he does not pretend to much 

 mathematical science or knowledge of acoustics), and induce 

 them to pursue the investigation of this interesting and im- 

 portant, but perhaps neglected, subject, — he addressed a paper 

 to the Royal Society, which was kindly received and honoured 

 with a place among the records of that distinguished insti- 

 tution. Since this communication to the Royal Society, in- 

 quiries having been made in every variety of form as to the 

 construction and effect of the parabolic sounding board, a 

 more detailed account is now presented to the public, as the 

 most effectual means of suj^plying the information required. 

 Those who wish to satisfy themselves by personal view will 

 receive every attention. 



The material is pine wood. The surface is concave, and is 

 generated by half a revolution of one branch of a parabola on 

 its axis. 



The distance from the focus to the vertex = 2 feet. 



The length of the abscissa is . . . . = 4 feet. 



Tlie length of the ordinate to the axis . = 'V/32 feet. 



= nearly 5.7 

 = rad. of outer 

 circle. 



The axis is inclintjd forwards to the plane of the floor at an 

 angle of about 10 or 15 degrees, and elevated so as that the 

 speaker's mouth may be in the focus. 



A small curvilinear section is taken away on each side from 

 beneath, that the view of the preacher from the north and 

 south galleries may not be intercepted; whence the outer 

 semicircle is imperfect. 



* This follows of course, on the principle that the angle of incidence is 

 equal to the angle of rcllection ; and yet the horizontal board is that always 

 in use : whence many persons h;u e assumed that »io sounding board could 

 be adopted with advantage in anv church. 



No. 1. 



