of Processes in the Fine and Useful Arts. 357 



of an iron retort, as ascertained by the weight of pure mercury which it 



contained at the temperature of 80°. The actual contents were as follows : — 



Before the first experiment, 9.13 cubic inches. 



After the first fire, - 9.64 



After three fires, - 10.16 



But what is more remarkable still, the augrnentation of the bulk of the re- 

 tort exceeds the dilatation due to the temperature to which it was exposed. 

 For as iron expands 0.0105 by 180° of Fahrenheit, the increase of bulk 

 upon 10 cubic inches should be 0.105 X 3 = 0.315 at 1800° of Fahren- 

 heit, or even the melting heat of silver. Hence we may conclude that the 

 dilatation of iron is not equable, a result formerly obtained by Messrs 

 Dulong and Petit. 



4. Description of a Sounding-Board in Attercliffe Church, invented by 

 the Rev. John Blackburn, Minister of AtterclifFe-cum-Darnall, Shef- 

 field. (From the Phil. Trans. 1828, p. 361.) 



This sounding board is represented in Plate II. fig, 9, 10. 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 is =2 feet 



The length of the abscissa is =4 feet 



The length of the ordinate to the axis Vsi feet 



= nearly 5.7 

 = Had. of outer circle. 



The axis is inclined forward to the plane of the floor, see Fig. 10, at 

 an angle ACD of about 10 or 15 degrees, and elevated so that the speaker's 

 mouth may be in the focus. 



A small curvilineal section is taken away on each side from beneath, that 

 the view of the preacher from the north and south galleries may not be in- 

 terrupted ; whence the outer semicircle is imperfect. 



This, however, gives an appearance that is not inelegant, and the outer 

 edge being ornamented with crockets and leaves, and with a pinnacle at the 

 highest point, and the concave surface being painted in imitation of a ground 

 oak canopy, the effect of the whole is pleasing to the eye. 



A curtain is suspended from the lower edge of the canopy for about 18 

 inches on each side. 



1. By means of this erection the volume of sound is increased in a very 

 considerable ratio, (perhaps as 5 : 1), and is thrown powerfully, as well as 

 distinctly, to the most distant parts of the church ; so that whereas for- 

 merly the difficulty of hearing an intelligible sound was very great, now 

 that difficulty is effectually removed, e. g. 



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, 

 and nowhere more distinctly than in the west gallery, or under it, on the 

 ground floor. 



2. 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 



