keeping up the pulls you may be able to swing it through a foot or more. But if your pulls on the thread are not in time with the swings of the scuttle you will not be able to swing it.

EXPERIMENT 99.-Now that the scuttle is swinging through a foot or so, hold the thread fast so that it cannot follow the scuttle. It snaps asunder and the scuttle goes on its way, as far as you can see, as though it had received no check at all on its speed. This is so because the strongest pull you can give through the thread to stop the scuttle is only a very small part of the force you have already given the scuttle by your many small pulls through the thread.

As the many feeble pulls of the delicate thread at length made the scuttle swing with great force, so the many feeble pushes and pulls of the delicate air brought the fork into a state of vibration powerful enough to make all the air in the room tremble.

This co-vibration may be found wherever two bodies in tune are placed near each other. Co-vibration explains why the tuning-forks sounded so much louder when placed on the resonant boxes. The volume of air inside the box is in tune with the fork, and it takes up the vibrations sent through the box, and, vibrating with the fork, the united vibrations make the sound so much the louder. The air in the tumbler and bottles of Experiments 43 and 63, and the air inclosed in glass tubes, as in Experiments 78 and 81, also move by co-vibration. It is also by the co-vibration of resonant pipes that the feeble notes on the lips and reeds of organ-pipes are made full and powerful.

CHAPTER XV.

ON CHANGES IN THE PITCH OF A VIBRATING BODY CAUSED BY ITS MOTION.

EXPERIMENT IN WHICH THE PITCH OF A WHISTLE IS CHANGED BY SWINGING IT ROUND IN A CIRCLE.

EXPERIMENT 100.-Take the piece of rubber tubing used in Experiment 32 and fit it over the mouth of the whistle used in Experiment 33. Let some one go to the end of the room or stand at a distance out-of-doors. Then, by the tube, swing the whistle round in a vertical circle, and at the same time blow through the tube so that the whistle will sound. The observer will see the whistle alternately coming toward him and going away from him, and with these motions he will hear the pitch of the whistle rising and falling.

In this experiment the sounding body is moving, and its movements cause a change in the number of vibrations which the ear receives in a given time. When the whistle swings toward the observer the sonorous waves are crowded together, and they reach the ear in greater number than when the whistle is at rest, and the note appears to have a higher pitch. On the other hand, when the whistle moves away from the observer, its backward movement draws out the sonorous waves, and fewer vibrations are

given to the ear than when the whistle is at rest. Thus we see how the motion of a vibrating body changes the pitch of its sound.

EXPERIMENT 101.-You must notice the same thing on the railway, where the sound of the whistle or bell of a moving locomotive appears to change in pitch as the engine draws swiftly near and then passes quickly away

from us.

CHAPTER XVI.

ON THE QUALITY OF SOUNDS.

WHEN you hear the sound of a violin, flute, clarinet, trumpet, piano, or organ, you readily recognize the sound of each instrument though it may not be in sight. Some one sings or speaks, and another person sings near him or after him, and we at once recognize each singer's or speaker's voice; and if we are familiar with his voice we can give his name, even if we do not happen to see him. This leads us to think that there must be some other characteristic of sounds besides pitch and intensity.

The flute, the violin, the clarinet, the singer, may each give the same note, and with equal power, yet the note of each has a character of its own, a peculiar something that distinguishes it from the same note given by the other instruments or singers. This we call the quality of the sound (sometimes called timbre, or character). Let us now make some

EXPERIMENTS ON THE QUALITY OF SOUNDS.

The experiments now to be made are of a peculiarly fascinating character. They will have for their object the discovery of what gives to sounds their various quali

ties. These experiments will lead us to the understanding of some of the laws of music.

All sounds may be divided into two great divisions. They are either simple sounds or compound sounds.

EXPERIMENT 102.-A simple sound is one in which the ear cannot distinguish two or more sounds differing in pitch. Hold one of your vibrating forks over the resonant tumbler, tuned by partly closing its mouth with the glass plate, as shown in Experiment 43. You now are listening to a simple sound, a sound in which the ear can detect only a sound of one pitch. A wide closed organ-pipe also gives a simple sound. All simple sounds have necssarily the same quality, for they differ only in pitch and intensity.

A compound sound is a sound formed of two or more simple sounds, all differing in pitch. Such is the sound given by a piano-wire. It may surprise you to learn that more than one sound is heard when you strike a piano-key which can vibrate only one wire. Yet this is so.

EXPERIMENT 103.-Strike the c"-key of the piano and sound your c"-fork. Though the same note in written music stands for each of these sounds, yet your ear at once perceives a marked difference in them. Now fix your attention on the sound given by the fork alone. Remembering well this sound, strike the c" of the piano. You now recognize that the c" of the fork is in the sound of the piano c"; but after some practice the ear begins to hear other sounds in the piano c"-sounds which are evidently higher in pitch than the pitch of the fork's c".

The c" of the fork is certainly the loudest simple sound heard in the piano c", and it is also the gravest; therefore the compound sound given by the piano is given in written music by the same note which stands for the cʻ of the fork.