whose walls are formed of bone. In it are situated the Ear-bones, three in number, called the Hammer (Malleus), the Anvil (Incus), and the Stirrup (Stapes). The hollow opens inwards and leads to a canal, the Eustachian Tube, E, which is extended in a trumpet-like form and opens into the Pharynx. On the inner wall of the tympanic cavity there are two openings closed with membranes, an oval aperture, o, Fenestra Ovalis, on which the base of the stirrup rests, and a circular aperture, r, Fenestra Rotunda, the membrane of which is free.

The two openings lead to the Labyrinth, A, a peculiar, coiled, bony cavity, the walls of which are clothed with membrane and remarkable organs, and the interior filled with the labyrinthine fluid. One side of it is distinguished by the name of the Cochlea, and the other, the Semicircular Canals.

From this rapid view of the essential parts of the ear, we find that the Auditory Canal, the Tympanic Membrane, and the Tympanic Cavity with the Ear-bones, only serve to receive the sound from without and to convey it to the interior, that it must there be communicated to the Labyrinthine Fluid, and that in the Labyrinth the proper physiological process of hearing commences, since at this point the apparatus for the transfer of sound terminates. In fact, the Auditory Nerve here penetrates the solid structure of the skull, and through its connection with most curiously formed terminal organs, changes the dead sound into a living sensation.

Although we can form no conception of sound, as such, except through the excitement of our auditory organs, nevertheless its origin and propagation in nature were recognised by physicists long before anything was

known of its physiological action. In this respect physical acoustics were, to a certain extent, just as independent of physiological knowledge, as was the case with optics; since, in the latter, the undulatory theory was developed, without any definite knowledge of the constitution of the retina, and of the excitement of the nerves situated in it. It must be granted that the exclusively objective physical study of sound and light must necessarily have made some progress to enable us to consider the functions of the sensory organs physiologically; and it has been shown, that many of the inventions which have been laboriously made by the ingenuity of man during the last century, are to be found in the greatest perfection already existing in the Sensory Organs. Nevertheless, in the present century, the science of physics has reached the point at which it may be considerably advanced by the physiological study of the sensory sensations, which study may provide fresh material for future investigations.

Among the sensations of sound, there is one kind of sensation different from all others, and which has a dennite character of its own. This is the sensation of musical sound, which is distinguished by the terms tone and note. All other impressions of sound which do not possess this character may be included under the term noise.

A tone is produced as soon as any elastic body is set in rapid vibration. If, for instance, a thin piece of metal, or a knitting-needle, is fixed firmly at one end and its free end struck, it is then set in vibration, which is accompanied by a kind of hum or tone, and the shorter the part in vibration, the higher is the tone. The tone

ner.

of a tuning-fork is produced in precisely the same manImagine the tuning-fork, in fig. 54, in vibration. The branches of the fork at each vibration approach first each other and then recede, an action which is not apparent to the eye, in the tuning-forks used for musical purposes, since the vibrations are too small and follow each other too rapidly. By means of the following contrivance the vibrations can be made self-registering. On one branch a pencil b is fixed, which makes

a mark upon a plate B B, drawn rapidly before it. An undulating line is produced which gives the number of vibrations, if the rate of the motion of the plate is known. Fig. 55 represents an apparatus, the Phonautograph, with which such an experiment can be performed. The point, r, of the tuning-fork writes upon a cylinder, round which is rolled a sheet of paper covered with lamp-black. The cylinder can be set in rapid rotation by means of a handle, a screw, at A, giving it a gradual forward motion during the revolutions. Two wires lead from the fork and the cylinder to a

strong induction coil, which, by means of clockwork, gives an electric spark every second, which makes a hole in the blackened paper. By this means the number of vibrations in a second can easily be counted.

Physicists have agreed to understand by the word vibration, a forward and backward movement of the vibrating body, so that a single vibration is represented by a curve of the undulating line, first in an upward, and

then in a downward direction. The body makes an excursion or elongation from its position of rest first to one side, then swings past its position of rest to the other side, and then returns to its position of rest again, after which the movement is periodically repeated in the The time occupied by the entire vibration is called a period of vibration, and its extent the amplitude of the vibration.

same manner.

In all musical instruments the tones are produced by some such action as this. A stretched string, which produces the tone in pianos and stringed instruments, vibrates in the manner shown in fig. 56, by springing first to one side and then to the other. A glass plate or a bell sounds when it is struck, in consequence of the vibrations, which its particles make at right angles to its

surface. Such vibrations, which are made in a direction perpendicular to the length of the body, or better, perpendicular to the direction of the propagation of the vibrations, are called transverse vibrations. Longitudinal vibrations, however, can also produce tones; for instance, metallic rods when struck upon their terminal surfaces, or wooden rods when rubbed in the direction of their length.

In the examples given the tone is caused by the vibrations of a solid elastic body. A tone, however, can be produced directly by vibrations of the air, when, for instance, we blow across the mouth of a hollow ball, of a bottle, or of a hollow cylinder. An instrument which rests on this principle is the mouth-pipe, which can be used in the organ, or as a flute. Fig. 57 represents two such organ-pipes. Air streams up from below into the chamber K from bellows, and is directed by the triangular piece of wood d, through a small cleft c, against the lip a b, by which it is made to blow against the column of air contained in the cylinder R R, and throws it into vibration. In this action the air is com