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Fig. 1 is the simplest figure, and represents the lowest note. The centre line of the vibrating segment is 45° from the node in each of the quadrants.

Fig. 12 represents the next figure and the next note above. The vibrating segment is 30° from the node. Fig. 13 gives a more complex figure, the segment being 22 from the node.

O

The disc may be further divided into any even number of segments. The larger the number, the more acute the note. In a disc the rate of vibration is dependent upon the thickness and diameter of the plates. It is directly proportional to the thickness, and inversely proportional to the squares of the diameters.

Bells have their nodes and ventral segments. Fig. 14 shows the transverse and longitudinal vibrations of

Fig. 14.

a bell which extend up the sides and over the bow of the bell. As in a disc, there may be any even number of vibrating segments.

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The lower line of numbers are those the squares of which are the ratios of the rates of vibration. For instance, suppose for the lowest note the number of vibration is 40, or 4 times 10; then for six or eight sections the rates will be 90, 160, and so on.

The Human Ear.-The waves of sound pass into the external ear, strike the tympanum or drum in the middle ear, and this motion is then communicated to the hammer bone, then to the anvil bone, and next to the stirrup bone. The motion of this is now communicated to the Fenestra ovalis, which communicates it to the fluid in the labyrinth, and by it the motion is continued to the auditory nerve. There is

a connecting passage between the middle ear and the
mouth called the Eustachian tube, which relieves the
external pressure on the tympanum.

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Fig. 15.-GENERAL VIEW OF THE HUMAN EAR.

d. Head of malleus, or hammer-bone.

e. Incus, or anvil-bone, at the end of which is the stirrup or
stapes.

f. Fenestra ovalis, or semicircular ducts.

o. Fenestra rotunda, or snail's shell.

The Human Voice.-This is one of the most perfect of reed instruments. As the air rushes upwards through the larynx it comes in contact with the vocal cords which stretch across the opening,

hi giving a slit-like passage called the glottis. It is the vibration of these cords which gives flexibility and variety of tone to the voice. The tighter we draw them across, the more acute will be the tone produced. Fig. 15a illustrates this part of the larynx; the other numbered parts representing cartilages by which we have command over the larynx and the vocal cords.

The sweetness and smoothness or the voice depend upon the comFig. 15a. plete closure of the glottis at regular intervals. The great resemblance between this apparatus and the syren before mentioned cannot but strike the thoughtful reader. The only difference is that in the voice the origin of the sound is the vibration of the cords imparted to the air, while in the syren the puffs of air are produced by cutting columns of air into small parts, and by their concussion setting air in vibration. For experiment, a short tube of wood, for blowing through with the upper part nearly closed by strips of thin leather, which may be tightened or relaxed, while the pipe is in the mouth, by means of two bits of cork as holders glued to the leather. This will give a fair illustration of the varieties of sound to be obtained by tightening or relaxing the membranes. (Fig. 16.)

Resonance. Take a long solution glass; pour in water and sound a tuning fork at the mouth until you notice a great addition to the sound of the fork this is called resonance. In the downward journey of the prong there is a condensation in the column of air which passes to the surface of the water and rises again,

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arriving at the prong just at the moment when it commences its upward motion; this gives a rarefaction in the air column which travels down and up again at the moment that the prong completes one part of its vibration. The other part gives the condensation which performs the same journey. Therefore one

complete vibration of the prong, or what is the same, the length of one sound wave, is exactly four times the length of the resounding column of air.

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From this we may ascertain the velocity of sound if we first note by the syren the number of vibrations per second-for every vibration is equal to four times the length of a resonant column of air. (See Exercise III.)

The mouth is a resonant cavity. It materially modifies the voice, and its effect is easily shown when we play the common instrument, a Jew's harp. The term resonance is also applied to the confused and

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