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CONDITION OF CONDUCTING MEDIA.

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mission of sound. Every one is acquainted with the appearance produced by the fall of stone into water. Many circular waves are produced, flowing from the centre, and gradually dying away as the distance increases. Now the successive impulses of a vibrating body act in precisely the same manner on any medium in which the vibrations are produced. Let us take the case of a tense string stretched by weights at both ends; for although bells, plates, and solids of other forms, act in the same manner, they would not perhaps afford so simple an illustration.

Imagine the string

с

d

VIBRATING STRING.

a b, to be drawn out of the straight line to point c, and then left at liberty to vibrate to d, and from thence to c again, and immediately after to come to rest in the line a b. The effect of this would be a series of waves in the atmosphere around the string, which would move, to use a common expression, backwards and forwards for a short time, and then die away in the calm uniformity. But instead of this single vibration, we may suppose the string to pass many times from c to d, and gradually taking a less arc, come to rest by slow and almost imperceptible degrees. If these vibrations be performed in equal times, that is to say, if an equal portion of time be occupied in its successive arrivals on the sides c and d, a regular series of waves will be produced, and a continuous musical sound will be the result; but if on the other hand the impulses be irregular,

the waves will be so also, and either the sound will be stifled, or a noise must be the result.

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The notion which people generally have of a wave is by no means correct, and it will be necessary that we should attach a right meaning to the term before we proceed. "A wave is not," says a celebrated writer, a progressive, moving body, but an advancing form." This is directly opposed to the common notion. Language is rightly employed when we say that a wave is approaching; but it is erroneous to suppose that the medium on which it is formed has a progressive motion. If we stand on the sea-shore at a time when the surface of the water is agitated, it will appear as if one mass of the liquid was advancing after another, and that we should soon be overwhelmed with the flood. however, is but an optical deception.

This,

We may perhaps

see at no great distance a boat, or a floating fragment of wood or sea-weed; at one moment it is on the top of the wave, and at the next hidden from sight in a valley, and makes no progress towards the shore. If the water were advancing, this would not be the case; it would occupy the summit or depression of a wave, as it might be accidentally placed at first, and move on with the mass, without any vertical motion, towards the shore.

A wave then is a form, and not a thing. It is true that the thing is in motion, but in a direction at right angles to the wave. In a string put into vibration by a bow, we may observe a system of waves, and they are moving in the direction of its length, but the string cannot do so, for it is fastened at both ends; its motion, in fact, is transverse to that of the waves.

UNDULATIONS IN ELASTIC MEDIA.

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If we have succeeded in giving an accurate and definite idea of what is meant by a wave or undulation, the reader who had previously entertained an erroneous opinion, will find many difficulties inseparably connected with his former supposition entirely removed. Should he have confined his attention to the propagation of sound in air, as he must have done, or would not have continued in error, many objections to the theory of undulations would be suggested to his mind. We can imagine an intelligent inquirer failing on this point, and being consequently involved in an ocean of difficulties. The impossibility of conceiving a mass of air in a state of undulation to be projected by every impulse of a sounding body, and to impinge upon the organ of hearing, would induce him at once to resist the theory as in the highest degree absurd. But at this moment he would be met by the recollection that all philosophers had given their assent to the accuracy of the theory of undulations. urged in one direction by the conviction of his mind, and in the other by the authority of great names, he would probably be kept in a state of indecision, and perhaps of indifference. Put this individual in possession of the fact, that a wave is a moving form and not a moving thing, and his objections are instantly removed. He can then perceive that there is nothing difficult of belief in supposing the undulations of the atmosphere to be the means of conducting sound.

III. THE ORGAN OF HEARING.

Thus

Having explained the action of a sounding body, and the conducting medium, we will endeavour to describe the anatomical construction of the organ of hearing.

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The human car is a very beautiful and complete arrangement of canals and orifices for the transmission of sound. It is divided by anatomists into two parts, one of which is called the external and the other the internal ear. The former includes all the parts of the organ which are without the membrane commonly known as the tympanum or the drum of the ear, and the latter all those which are within. Mr. Tod in his excellent Treatise on the Organ of Hearing," a work to which we are indebted for many of the facts to be mentioned in this part of our book, very properly objects to this division as arbitrary and defective, though he adopts it because generally employed in anatomical descriptions. The ear may be, he thinks, separated into three parts-an external, middle, and internal; and this division is to be preferred, not only because it gives the student a better idea of its construction; but also from the circumstance that it distinguishes between the several functions for which the ear is in fact constructed.

THE EXTERNAL EAR.

The external ear, or auricle, may be divided into two portions-a large superior, which is called the ala or pinna; and a small inferior, called the lobus. The auricle is attached to the temporal bone, and is of an irregular oval form, curiously constructed.

The ala, or pinna, is formed of cartilage, and to the several eminences and cavities names have been given by anatomists, such as the helix, antihelix, tragus, antitragus, cavitas innominata, scapha, or fossa navicularis, and the concha; all of which are represented in the following design.

ANATOMY OF THE EXTERNAL EAR.

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"The helix (a) is the large folded margin, or curved border, which commences at the posterior superior part of the lobus, and from thence ascends and forms the margin or border which surrounds the upper part of the ala, and subsequently descends and terminates anteriorly, nearly opposite to its beginning, in a ridge which divides the cavity called the concha into two unequal chambers.

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"The antihelix (b) is the large oblong eminence, which begins near the posterior extremity of the helix, and from thence extends obliquely forwards and upwards, and terminates by dividing into two parts, a superior and an inferior, of which the latter assists in forming the superior part of the brim of the concha.

"The tragus (c) is the small eminence situated below the anterior extremity of the helix. In advanced age it is generally covered with hairs.

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