SOUND. 
369 
for the production of different tones by any hypothesis; it 
being clear that if the velocity of vibration was the cause 
of particular tones; and if velocity of vibration was syno¬ 
nymous with velocity of transmission, treble sounds should 
travel farther in a given time than bass, which is contrary to 
experiment. 
It cannot be asserted that sound is propagated in different 
bodies with a velocity proportionate to their density, but it 
is certain that this propagation is more rapid through solids 
than fluids. Nor can it be shewn that in all instances the 
loudness of a sound is exactly in a direct ratio to the density 
of its medium. But Priestley’s experiments shew, that as 
far as regards gaseous bodies, the lighter the medium the 
feebler the sound, and that the denser this medium (whether 
from its original constitution or from mechanical pressure) 
the louder the sound emitted. In inflammable air the sound 
of the bell he used was hardly to be distinguished from the 
same in a pretty good vacuum; and this air is ten times 
rarer than common air. In fixed air the sound was much 
louder than in common air, so as to be heard about half as far 
again ; and this air is in about the same proportion, denser 
than common air. In dephlogisticated air the sound was 
also sensibly louder than in common air .—Experiments and 
Observations, &c., vol. v. p. 296, &c. 
According to the experiments of Dr. Derham, it does not 
appear that the temperature, luminousness or other state of 
composition in the atmosphere alters the velocity of the 
transmission of sound: that the direction of the sounding 
body (as whether the muzzle or butt of a discharged gun be 
turned towards the hearer) has no such influence. That in 
cases where the earth forms the medium of vibration, no 
difference is apparent whether its surface be acclivous or 
declivous: the loudness of the sound has no influence in the 
rapidity of its transmission, nor does the composition of the 
sounding body whether metal or wood or what not, make 
any difference. The blowing of the wind only was found 
to retard the velocity of the sound slightly. 
As to the direction whence sounds arrive, it is obvious that 
if the particular vibration began first to excite the hinder 
part of an auditory nerve, and afterwards to be propagated 
to its anterior, or vice versa, that to apprehend the direc¬ 
tion of a sound, would be easy without any reference 
either to its tone or loudness. The tympanum is, however, 
so small a part, that one can scarcely believe any nerve 
could exist with so fine a sense as to ascertain whether this 
membrane was impinged from behind forwards or from 
before backwards, from above downwards, &c. It would, 
we say, be next to impossible to believe such minute im¬ 
pressions capable of being perceived. Hence it would be 
far more rational to suppose, (and there are numerous cir¬ 
cumstances rendering it probable) that the portio dura 
stretched over the face and side of the head, performs the 
office of acknowledging the direction of a sound. At all 
events it is manifest that the only way in which a nerve can 
ascertain the direction of an impinging sound, is by consi¬ 
dering whether it first impinged on this side or the other. 
There is much difficulty in explaining how we calculate 
the distance whence a sound is transmitted to us. As in the 
eye, so in the ear, we are no doubt helped in this respect by 
the other senses. Still it is impossible to overlook the fact 
that we can to a degree distinguish a remote from a near 
sound, even without reference to their loudness. It would 
seem that vibrations become less frequent by distance (con¬ 
sequently the loudness of a peculiar sound is declared), and 
that along with this their vibrations become clearer. Thus 
we find, that in the organ the stops pulled out to produce 
the illusion of distant music, cause two effects. They lower 
the loudness of the sound, and they also render it extremely 
clear and melodious. On this point, however, there is room 
for reflexion and experiment. 
With regard to the properties of sound, these facts are 
established.—The gravity of a string is in direct proportion 
to its length. 2d. A'string sounded near several othersjwill cause 
in those others or parts of them, a sound identical, as to tone, 
Vol, XXIII. No. 1576. 
with itself, or else one bearing a mathematical ratio to it, and 
this effect is produced without any regard to the usual sound 
of the strings in question. 3d. As the sound of a string 
or other body dies away, it is heard to make ascents, each of 
which bear the same ratio to the primitive sounds. They 
run into octave 12th and 17th. These very curious circum¬ 
stances can be accounted for with difficulty. 
The last circumstance is, especially, difficult of explana¬ 
tion, since it cannot be shewn why the intermediate or dis¬ 
cordant vibrations are not heard equally with those which 
are in tune with the primitive sound. Otherwise a string 
double the length of another, is twice as grave, because its 
vibrations must be twice as large; and as to the second 
point, a vibrating string may naturally be supposed to 
excite its own vibration only in other bodies the same 
as we know it does in the air. 
The apparatus of the ear is very complicated: we shall 
not insist on anatomical details, but confine ourselves to such 
a description as is necessary for understanding the use of 
the different parts which constitute this organ. The external 
ear is composed of a cartilaginous conch, which is usually 
supposed to colleet and reflect inwards the waves of sound. 
Its removal does not, however, impair hearing, until some 
days after the operation. An opening of an elliptical form, 
lined with hairs, at first gradually contracting, and about 
its middle beginning to dilate again, passes into the head 
from this conch, until it is stopped by a membrane called the 
tympanum, which completely closes it. This canal is of 
course for the transmission of sound. 
The middle ear comprehends the cavity of the tympa¬ 
num, the little auditory bones, the mastoid cells and 
Eustachian tube. The cavity of the tympanum has 
opening into it internally the foramen rotundum closed by a 
membrane; the foramen ovale also closed by a membrane- 
posteriorly the mastoid cells (irregular cavities filled with 
air); anteriorly the Eustachian tube communicating with the 
pharynx. The cavity of the tympanum is occupied by 
four small bones connected with the membrane of the tym¬ 
panum on the one hand, and the membrane of the fenestra 
oval is on the other, so as to form a bony chain between the 
two. These bones are capable of being pulled in various 
directions by small muscles, so as to stretch or relax the 
membranes. 
The internal ear, or labyrinth, is composed of the cochlea 
the semicircular canals and the vestibule. The cochlea is 
a bony canal lined with a membrane of a spiral form • it is 
divided into two turns of the spiral by an osseo-cartilaginous 
partition. The outer spiral communicates with the tympa¬ 
num by the closed fenestra rotundum; the inner with the 
vestibule. 
The semicircular canals are three cylindric cavities, curved 
in a half circle, of which two are divided horizontally, while 
the third is vertical: these canals terminate at their extremities 
in the vestibule ; they contain bodies of a grey colour which 
terminate at their extremities by enlargements. The vesti¬ 
bule is a central cavity, the point of union of the other- 
cavities. It communicates with the drum through the fenestra 
ovale; with the internal spiral of the cochlea, with the semi¬ 
circular canals, and witn the internal auditory passage, by 
a great number of small holes. All the cavities of the 
internal ear are contained in the petrous portion of the 
temporal bone, and are lined with a membrane extremely 
thin, and filled with a limpid liquid, called the liquid of 
Cotunnius, of which a little flows by two passages, known 
by the name of aqueducts of the cochlea and vestibule- 
moreover the ear contains the auditory nerves. The auditory 
nerves arise from the fourth ventricle; they enter into the 
labyrinth by the foramen, which is seen at the bottom of 
the internal auditory canal; arriving at the vestibule, they di¬ 
vide into many branches, one of whieh remains in the 
vestibule, another in the cochlea, and two are destined for 
the semicircular canals. 
In crai/-fish, and the sepia:, the organ is most simple, 
consisting of a small cavity or vestibule and a single mem- 
5 B branous 
