Star.
London, England, 1830-12-22 00:00:00

LONDON AIECHANICS• INSTITUTION.

On Friday evening last Dr. Birkbeck delivered the first of • Course
of Lectures on Acoustics and the Theoryof Music, which he introduced
by observing that his audience had already been made acquainted with
the nature of the aerial fluid by which men and animals were sur-
rounded, and also with its motions, and the changes which they pro.
duced ; but there was another motion in the atmosphere, at least 20
times as rapid as that of the highest wind, which produced no tangible
effect, and was only rendered sensible by its impression on the organs of
hearing. He alluded to the motion of sound, the phenomena of which
constituted the science of acoustics, which was sometimes divided into
diacoustics, or transmitted sound, and catacoustics, or reflected sound.
He should first consider the nature of sound, and afterwards explain the
structure of the ear, at least till he arrived at the auditory nerves, beyond
which the investigation was physiological, and was obscure even to
physiologists.
Sound was said to be peculiar to elastic bodies, but an elastic rod
might be made to vibrate without returning any sound, if its vibrations
were not sufficiently rapid. Thus if a rod of metal, of such length that
its vibrations were visible, were put in motion, no sound would be din.
tinguished, while a shorter rod, the vibrations of which were so rapid
as to be invisible, would return a distinct musical tone, because it had
been ex/ erimentaliy amvtained that toeCiltee sound, it must vibrate
at least .qt times in Stet 11. When bodies were struck, they
retumet' those short, dull a, .nds generally called noises, while a blow
struck ,men a beil produced ontinuity of sound. Dr. Birkbeck then
explained, by referring to proper diagrams, the change of form which
takes place in a hie: wneld it is struck by the clapper, and showed that
the blow iu tht.'first vltee threw the bell into an elliptical or oval form,
elongating it both on the side where the blow was given and at the op.
posite side; that afterwards, in consequence of its elasticity, its edges
not only returned to their original situation, but as much nearer to the
centre of the bell as they had been driven beyond it, thus throwing it
into an oval form in the contrary direction; and that these alternate
changes of form contiuued till the vibrations ceased, spathe bell resumed
its circular figure. The lecturer next adverted to the vibrations of mu.
sisal strings, which return a variety of tones, according as they are
longer or shorter, and neatly vibrate with less or greater rapidity.
Air, be observed, was cipal conductor of sound, which, there-
fore, could not be heard In a vacuum, but other substances also con-
ducted it, such as a brick wall or a log of timber, and Dr. Franklin
had conveyed sound through the include fluid, water, to the distance
of half a mile.

After a variety of interesting remarks on the nature of echoes, the
eonstruction of ear.trumpets, spealting-trumpets, and other acoustical
instruments, Dr. Birkbeck referred to a series of beautiful transparent
diagrams, representing the curious and complicated internal structure of
the ear, of which be gave a minute and distinct anatomical description,
pointing out in succession the external auditory passage, the membrane
called the drum, or tympanum, the labyrinth, and the Eustachian tube
leading from the tympanum to the back of the mouth. He also exhi-
bited and described the curiously formed bones called the milieus or
hammer, the incus or anvil, and the stapes or stirrup, together with the
cochlea or convoluted cavity, resembling a scull-shell, and the
three semicircular bony canals communicating with the vestibule ;
alter which he concluded by intimating that in his next Lecture
be should advert to the construction of Musical Instruments, and the
principles which regulate the production of Melody and Harmony.
It was then announced that the Right Hon. Wilmot Horton would
deliver his second Lecture on Statistics and Political Economy, on
Wednesday evening next.