An open organ-pipe yields a note an octave higher than that of a closed pipe of the same length. When a stopped organ-pipe sounds its fundamental note, the column of air is undivided by any node; but the closed end will always be a node, because the air particles at that part are necessarily at rest. When an open pipe sounds its fundamental note, the column is divided by a node at its centre. The open pipe really consists of two stopped pipes with a common base.

The existence of nodes and vibrating segments within an organ pipe may be shown by lowering into the pipe a thin membrane stretched over a frame, with some fine, dry sand sprinkled on its surface. The front of the pipe is of glass, so that we can see any body in it. When the sand is in a segment it will be agitated, but when it is in a node it will remain at rest.

If a node is connected with KOENIG'S capsule, the flame is more violently agitated than when a segment is joined. This is owing to the continual change in the density of the air taking place at the node, while at a segment the density is not sensibly changed, although the air is in a state of vibration.

Fig. 179 represents the form of the mouth-piece of the flageolet, and it will be seen that it bears a close resemblance to the pipes already explained. In the flute, an opening is made in the side of the pipe, which changes the length of the segments of the columns of air that are vibrating, and thus determines the pitch of the tone. The arrest and flow of the current are effected by the arrangement of the lips of the player.

254. Reed Pipes. In REED PIPES the mouth-piece is provided with a vibrating tongue, called a reed, by means of which the air is put in vibration. To this class belong the clarinet, the hautboy, and the like. The reed may be so arranged as to beat against the sides of the opening, or it may play freely through the opening in the tube.

Figs. 180 and 181 show the arrangement of a reed of the first kind. A piece of metal, a, shaped like a spoon, is fitted with an elastic tongue, 7, which can completely close the opening. A piece of metal,, which may be elevated or depressed by a rod, b, serves to lengthen or shorten the vibrating part of the reed. This arrangement enables us to diminish or increase the rapidity of vibration at pleasure.

The inouth-piece, as described, connects with the tube, T, and is set in a rectangular box, K N, which is in communication with a bellows, from which the wind is supplied. For the purpose of class demonstration, the upper part of the tube, KN, has glass windows on three sides to show the motion of the reed.

When a current of air is forced into the tube, K N, the reed is set in rapid vibration, causing a succession of rarefactions and conden

sations in the air of the pipe, T, and causing it to emit a sound. The air entering the tube, K N, first closes the opening by pressing the reed against it; the reed then recoils by virtue of its elasticity, permitting a portion of condensed air to enter the pipe, when the reed is again pressed against the opening, and so on as long as the current of air is kept up. It is evident that the rapidity of vibration will be increased by increasing the tension of the air from the bellows, and also by shortening the vibrating part of the reed.

Fig. 182 shows the arrangement of the free reed. The vibrating plate, 1, is placed so as to pass backwards through an opening in the side of the tube, ca, alternately closing and opening a communication between the tube and the air from the bellows. The regulator, r, is entirely similar to that shown in Figs. 180 and 181, as are the remaining parts of the arrangement. The explanation of the action of this species of reed is entirely similar to that already described.

255. Wind Instruments. -WIND INSTRUMENTS of music consist of pipes, either straight or curved, which are made to sound by a current of air properly directed.

Fig. 184.

Fig. 183.

In some, the current of air is directed by the mouth upon an opening made in the side, as in the flute. In others, the current of air is made to enter through a mouth-piece, as in the flageolet. In others, a reed is used, as in the clarinet. In the organ there is a collection of tubes, similar to those shown in Figs. 175 and 177. In some instruments, as the trumpet and the horn, a conical mouth-piece is used, of the form shown in Fig. 183, within which the lips of the musician vibrate in place of the reed. The rapidity of vibration can be regulated at will.

256. Sounding Flames. When a gas-flame is enclosed in a tube, open at both ends, the passage of the air over it is generally sufficient to produce the

necessary rhythmic action, and to cause it to give out a musical tone. Fig. 184 represents such a tube firmly held in position by clamps, which are fastened by screws to a stand.

By means of the paper slider, s, the tube may be lengthened or shortened. While the flame is sounding, raise the slider, and the pitch falls; lower it, and the pitch rises.

By sounding the same note with the voice or any musical instrument, the singing of the flame may be interrupted, or caused to cease entirely; or, when silent, to begin again.

257. Sensitive Flames. - Flames are affected by soundwaves from musical tones even when not enclosed in tubes. The action of musical sounds upon such flames is shown by the vibrations of the gas-lights in unison with certain pulsations of the music at some instrumental concert. This phenomenon does not take place unless the pressure of gas is sufficiently great to keep the flames on the verge of flaring.

A long flame may be shortened and a short one lengthened by sonorous vibrations. Suppose we have a long smoky flame and a short, forked, and bright one, both on the point of flaring, and both issuing from a very small orifice, like a pin-hole in a tube. On sounding a whistle, their sensitiveness to the sound vibrations is at once apparent. The long flame becomes short, forked, and brilliant; and the forked, long and smoky. A flame may be shortened half its length by striking two pieces of wood or iron together.

258. The Human Voice. The most perfect reed instrument is the human voice. Across the top of the trachea, or windpipe, are stretched two elastic bands, called vocal chords; through the space between the chords the air passes in and out of the lungs.

During speaking and singing the space between the chords is less than in ordinary breathing. The voice is produced by the air, which, driven from the lungs and striking against the chords, causes them to vibrate. The greater the tension of the chords the higher the pitch.

The mouth, by its resonance, reinforces the sound given out by the vibrating chords. By changing its shape it can be made to

resound to the fundamental tone, or any of the overtones of the vocal chords.

259. The Human Ear. -A section of the ear is seen in Fig. 185. It consists of the external ear, so formed as to enable it to catch the sound-waves. B represents the auditory canal, about an inch in length. A circular membrane, called the membrane of the tympanum, closes the lower end of it.

The drum of the ear, or the tympanum, is the cavity behind this membrane. Beyond the drum is the labyrinth. It consists of a

E

B

Fig. 185.

small rounded chamber, A, called the vestibule; from it open three semicircular canals, D, and a spiral canal, E, called the cochlea, from its resemblance to a snail-shell.

Through these canals the auditory nerve is distributed. From the membrane of the tympanum to the membrane of the vestibule a chain of three bones is stretched, the hammer attached to the membrane of the tympanum, the anvil, and the stirrup connected with the membrane of the vestibule. The vibrations of the atmosphere strike against the membrane of the tympanum, and are conducted through the chain of bones to the second membrane, and thence, by the auditory nerve, to the brain. The Eustachian tube, G, admits air to the drum, and thus keeps the density within the same as the external air.