LISSAJOUS' figures can also be produced by means of the vibrations of a pendulum in a slower and easier way than by means of the tuning-fork.

249. Kaleidophone. -The optical study of vibrating rods can be made by means of an apparatus called the kaleidophone. This can be constructed by a very simple process. Insert, with the aid of an awl, a knitting-needle with a glass bead on the end, firmly in an inch board several inches

square.

Fig. 171.

Place the board on a table, and hold it tightly with the hand while the needle vibrates.

Allow the light of a lamp to fall upon the bead when still, we have a small spot on the screen intensely illuminated; now cause the needle to vibrate, and the spot will be drawn out into a brilliant line which will change into a circle; and thus the character of the vibrations is shown.

250. Koenig's Manometric Flames. - Other ingenious instruments have been constructed for illustrating the optical method. The apparatus of KOENIG transmits the movements

of the sound-waves to gas-flames, and these, by their pulsations, show the nature of the sound.

We have, in Fig. 172, a metal capsule, A, in section. This is divided into two compartments by a membrane of gold-beater's skin or thin rubber. Immediately below the section, A, is seen the capsule supported on a stand; on the right is the gas-jet, below it the tube for conveying the gas to the compartment at the right of the membrane; on the left is the tube for the sound-waves to reach the membrane. To this may be attached a rubber tube, which can terminate in a mouth-piece or be connected with an organ-pipe.

When the sound-waves enter the mouth-piece and tube, the thin membrane is set vibrating. The gas, while passing through the compartment at the left, is caused to vibrate in a corresponding way, and thus the flame itself is shaken up and down.

The changes in the length of the flame are scarcely perceptible when it is observed directly. But to make them distinctly visible they may be received on a mirror, M, with four faces. This is made to revolve by means of two cog-wheels and a handle.

While the flame burns steadily there appears in the mirror, when turned, a continuous band of light. But if the fundamental note is sounded in the tube on the left of the capsule, the image of the flame

takes the form represented in Fig. 173. If the octave be sounded, the image of the flame takes the form seen in Fig. 174.

Many varieties of forms can be produced when several sounds of different intensities enter the tube simultaneously.

These flames just described are called manometric flames. The succession of separate images of the flames, which we see on turning the mirror, is due to the fact that the image of an object remains on the retina for a little time after the object itself has been removed.

251. Stringed Instruments. All stringed instruments of music are constructed in accordance with the preceding laws. They are divided into instruments with fixed sounds, and instruments with variable sounds.

To the former class belong the piano, the harp, etc. They have a cord for each note, or else an arrangement is made so that by placing the finger at certain points, as in the guitar, the same cord may be made to render several notes in succession.

To the latter class belong the violin, the violoncello, etc. They are provided with cords of catgut, or sometimes of metal, put in vibration by a bow. Various arrangements

are made for regulating the notes, such as increasing the tension, placing the finger upon the cords, and the like. These instruments are difficult to play upon, and require great nicety of ear, but in the hands of skilful players they possess great power. They are the soul of the orchestra, and it is for them that the finest pieces of music have been composed.

252. Sound from Pipes. When the air in a pipe, or hollow tube, is put into vibration, it yields a sound. In this case it is the air which is the sonorous body, the nature of the sound depending upon the form of the pipe and the manner in which the vibrations of its contained air are produced.

To produce a sound from a pipe, the contained air must be thrown into a succession of rapid condensations and rarefactions, which is effected by introducing a current of air through a suitable mouthpiece. Two principal forms are given to the mouth-piece, in one of which the parts remain fixed, and in the other there is a movable tongue, called a reed.

253. Pipes with fixed Mouth-pieces. Pipes with fixed mouth-pieces are of wood or metal, rectangular or cylindrical, and always of considerable length compared with their cross section. To this class belong the flute, the organ pipe, and the like. Some of the forms given to pipes of this class are shown in Figs. 175–179.

Fig. 175 represents a rectangular pipe of wood, and Fig. 176 shows the form of its longitudinal section. P represents the tube through which air is forced into it. The air passes through a narrow opening, i, called the vent. Opposite the vent is an opening in the side of the pipe, called the mouth. The upper border, a, of the mouth is bevelled, and is called the upper lip; the lower border is not bevelled, and is called the lower lip.

The current of air forced through the vent strikes against the upper lip, is compressed, and by its elasticity, reacts upon the entering current, and for an instant arrests it. This stoppage is only for

an instant, for the compressed air finds an outlet through the mouth, again permitting the flow. No sooner has the flow commenced than it is a second time arrested as before, again to be resumed, and so on.

This continued arrest and release of the current gives rise to a succession of vibrations, which are propagated through the tube, causing alternate and rapid condensations and rarefactions, which

P

Fig. 175. Fig. 176. Fig. 177. Fig. 178.

Fig. 179.

result in a continuous sound. The vibrations are the more rapid as the current introduced is stronger, and as the upper lip approaches

nearer the vent.

Fig. 177 represents a second form of organ pipe, which is shown in section in Fig. 178. This is but a modification of the pipe already explained. The letters indicate the same parts as in the preceding figures.