358 History of Mechanical Inventions and 



nearer to A in the latter case than in the former, he can yet h^ar with de- 

 cided advantage when opposite to A {i. e, at the greater distance from him.) 

 The side galleries appear to be benefited rather by the increased volume 

 of sound, and by the secondary vibrations excited in a lateral direction. 



3. It appears also that vibrations, proceeding from a distant point and 

 moving in the direction of the axis, are reflected from the parabolic sur- 

 face towards the focus. For let A stand in the pulpit, as before, and B 

 in a distant point opposite to it, 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 on the focus of the parabola, and one ear towards B, the effect pro- 

 duced is that of a voice close to the ear, and in a direction the reverse ol? 

 that from which it really proceeds. 



4. The converse of this also appears true from the following experiment. 



Let B remain in the situation last supposed, and let A place his face to- 

 wards the parabolic surface, and his back towards B ; let A now speak, 

 having his mouth in the situation of the focus, and he will be heard as 

 distinctly as when his face was turned towards B. 



5. If the mouth of the speaker is placed much within oi* without, above 

 or below the focus, the effect is proportionally diminished. 



6. While the figure of the canopy remained perfect, the effect was more 

 complete ; perhaps it might be improved if constructed longer, or in 

 other words if continued farther; but the distance of the focus S to the 

 vertex A, fig. 10, which regulates the curve must depend on the supposed si- 

 tuation of the speaker, which will vary with the diameter of the pulpit. 



Fig. 10 represents a section of the parabolic sounding-board, which is 

 shown by the line A P B. The axis of the parabola A C is inclined as 

 shown on the figure, to the horizon. The mouth of the speaker would be 

 about the focus S, If S />, S P are the directions of the sound incident upon 

 the board, p gr, P Q will be its direction after reflexion. 



5. Account of a Process for producing a beautiful Blue Colour. By M. 

 Braconnot. 

 Six parts of sulphate of copper were dissolved in a small quantity of 

 water ; also, six parts of white arsenic with eight parts of potash of com- 

 merce, were boiled in water until no further quantity of carbonic acid was 

 disengaged. This hot solution was gradually mixed with the first, conti- 

 nually agitating until effervescence ceased. An abundant dull yellowish 

 green precipitate was formed. About three parts of acetic acid were then 

 added, or such a quantity that a slight excess was sensible to the smell. 

 Gradually the precipitate diminished in volume, and in some hours a 

 slightly crystalline powder was deposited at the bottom of an entirely 

 colourless solution. The fluid was poured off" as soon as possible ; and the 

 powder, washed with plenty of water to remove the last portions of arsenic, 

 was then of a brilliant blue colour. 



Care must be taken not to add to the cupreous solution an excess of ar- 

 seniate of potash, as it causes waste of the acetic acid afterwards added, as 

 the latter must be in excess. In repeating the process in the large way, an 



