Natural Philosophy. — Acoustics. 19B 
therefore that its primitive form is either the cube, the regular 
octohedron, or the rhomboidal dodecahedron. Some of the cry- 
stals have obviously the form of the rhomboidal dodecahedron. 
The specimens which Dr Brewster had formerly examined, had 
received a polarising structure from fusion and rapid cooling. 
' ACOUSTICS. 
4. Account of the Sirene.) an Acoustic Instrument. — This in- 
strument, which has received the name of Sirene, from giving 
out its sounds under water, has been proposed by Baron Cogniard 
de la Tour. It consists of a circular copper box, four inches in 
diameter. The upper surface is pierced with 100 oblique aper- 
tures, each a quarter of a line in width, and two lines long, ar- 
ranged in ,a circle round the axis. In the centre of this surface 
is an axle, on which a circular plate turns by a current of air, 
or by means of a simple mechanism. This plate has also 100 
apertures corresponding to those in the surface of the box be- 
low it, having the same obliquity, but in the opposite direction. 
The obliquity of these apertures is not necessary to the produc- 
tion of the sounds, but serves to give motion to the circular 
plate, by the impulse it receives from the currents of air which 
issue from the apertures in the box. The wind of a pair of bel- 
lows being conveyed to the box by means of a tube, the circular 
plate is set in motion, and the apertures in the surface of the 
box are alternately open and shut to the passage of the air, by 
which means a regular series of blows is given to the external 
air, and a sound analogous to that of the human voice is pro- 
duced, becoming more or less acute according to the greater or 
less velocity of the plate. When water, in place of air, is made 
to pass through the apertures, sounds are equally produced even 
when it is entirely plunged in the fluid, and the same number . 
of concussions produces the same sound as in the air. — Ann. de 
Chim. xii. 167. This instrument is nearly the same as the one 
invented by the late Dr Robison, and described in the old 
Suppl. to da^Pncyc. Brit. Art. Tr:MPERAMENT, or in his wqrks^ 
now printing, vol. iv. p. 404. “ The intelligent reader,” says 
Dr Robison, “ will see here an opening made to great additions 
to practical music, and the means of producing musical sounds, 
of which we have at present scarcely any conception ; and this 
VOL, III. NO. 6. JULY 1820. N 
