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



advance ; but the distance from the focus to the vertex (which 

 regulates the curve) must depend on the supposed situation 

 of the speaker, which will vary according to the diameter of 

 the pulpit. 



The outline, No. 2, represents an improved parabolic sound- 

 ing board, fonned by an entire revolution of the parabola on 

 its axis, with pulpit, reading-desk, and clerk's-desk, according 

 to a model designed and arranged by the writer of this paper, 

 and deposited with the Society of Arts, together with a model 

 of sounding board. No. 1 . 



The ornamental parts may, of course, be adapted to the 

 character of the building in which it stands : the altar table 

 might be placed in front. 



The reflection of sound from the lower part would take the 

 same direction as that from the upper, viz. parallel to the 

 axis : and the effect would probably be much more than 

 double that produced by sounding board, No. 1. Many im- 

 provements may still doubtless be suggested. 



In erecting a new church, might it not be found most ad- 

 vantageous to give to the east end of the building itself the 

 form of a paraboloidal concave, and to place the pulpit in the 

 focus ? 



The sounding board. No. 1, was thus constructed. The 

 curve was first drawn according to the following method: — 

 On the straight line LN (fig. 6.) m ake LA = AS = SN. At the 

 point A draw AB perpendicular and equal to AL. Join LB. 

 Produce LB to C. Divide AN into any number of equal 

 parts in a, b, c, &c. ; and at a, b, c, &c. draw aa, bb, cc, &c. 

 parallel to AB, and meeting LC in a, b, c, &c. Let straight 

 lines = AB, aa, bb, cc, &c. revolve round S as a centre, inter- 

 secting AB, aa, bb, cc, &c. respectively in A, p, q, r, &c. Join 

 A, p, q, r, s, t, &c., and the curve traced out will be a para- 

 bola ; of which A will be the vertex, S the focus, AN the axis. 

 The distance between the speaker's mouth and the back of 

 the pulpit being 2 feet = AS = SN = AL. 



Another method is subjoined, being taken from the Me- 

 chanics' Weekly Journal, No. XXIV. 



" The parabola being the curve that is best adapted for the 

 reflection of heat, and of course requisite for the formation 

 of metallic mirrors, covings of chimneys, and cupolas of melt- 

 ing furnaces ; an easy method of describing it, adapted to the 

 comprehension of workmen, was wanted. 



" Mr. Leslie, in his ' Enquiry into the Nature and Propa- 

 gation of Heat,' having occasion for metallic mirrors, de- 

 scribed the gauges for them in the following manner. 



" Let AB (fig. 7.) denote the extreme breadth, and CD the 



intended 



