Sounding Board erected in Attercliffe Church. 27 



intended depth. Divide AB into 20 parts, and draw perpen- 

 diculars from each division. Consider the depth CD as equal 

 to 100. Make the next ordinate, or perpendicular, or either 

 side equal to 9 multiplied by 11, that is to say 99, by the same 

 scale; which is easily done by the line of lines on a sector rule: 

 the next ordinate on either side equal to 8 multiplied by 12, that 

 is to say 96 ; and so on. These numbers being respectively as 

 the rectangles of the segments into which CD is divided." 



A scaffolding was made of three semicircles (fig. 4.), KL, 

 MN, PZ; fixed perpendicular to the axis of the parabola ; the 

 axis passing through their centres. This done, three para- 

 bolic sectors (fig. 1.), AB, AC, AD, were cut out of three-inch 

 pine and placed as in fig. 1., pointed at the bottom ; these were 

 let into two cross ribs, EF, GH, cut out of tough wood natu- 

 rally bent (to avoid crossing the grain), and dovetailed at each 

 end to keep the three sectors firm and in their place: the spaces 

 between IC, CB, BD, DK, were filled up with sectors cut out 

 of 1 ^ inch wood, nailed and glued well together : lastly, the 

 inside was cleaned off and proved by the sector (fig. 6). At 

 regular distances, three iron plates or bands were let in (both 

 inside and outside), well fastened with screws. The horizontal 

 edge lAK was finished with two sectors of hard wood ; and 

 the back strengthened where the points chiefly met, with a 

 tough piece of inch board ; where also the sounding board 

 was fastened to the pulpit-back with screw-bolts and nuts, 

 being further sapported near the centre of gravity by an iron 

 rod suspended from the ceiling above. 



The wood was well seasoned, and placed beside a furnace 

 for six weeks; the sounding board has been fixed for nine 

 months, and has not been affected by weather. 



Figs. 2 & 3 represent the cross ribs EF, GH in fig. 1. 



Fig. 5 represents a parabolic sector whose concave is that 

 required. 



Fig. 6 represents a parabolic sector whose convex answers 

 to the concave of fig. 5 ; and is used to prove the 

 work when done, being applied at the point A, and 

 turned on its axis AZ. 



Atterclijfe Parsonage, near ShcJJicldy 

 February 28, 1829. 



E 2 V. An 



