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the trumpet is sounding, twirl the fan slowly by the handle, and you will observe a change in the sound. In certain positions of the fan the trumpet will sound louder, and in other positions it will be softened. If you do not obtain this effect at once, try the fan in several positions as it stands upright, and, after a few trials, you will obtain a reflection of the sound from the surface of the fan. The sound of a locust on a warm day, or the beating of the surf on the shore, or the sound of a distant voice, may thus be caught on the fan and reflected into the ear.

Echoes are also reflections. The vibrations travel through the air and meet a building, then the side of a mountain or hill, and rebound or reëcho, perhaps many times.

EXPERIMENT 75.-You can readily find an echo anywhere in the country by walking near a barn or house and shouting or singing. The first trial may not bring out the echo, but, by changing your position, going nearer or walking farther away, and always standing squarely in front of the barn or other building, you will soon find the spot where an echo is heard. We already know that in winter, when the thermometer is at 32° Fahr., sound moves at the rate of 1,090 feet in a second. If you stand at 545 feet from the reflecting wall, and make a short, sharp sound, it will take one half second for it to go to the wall, and one half second to come back, and there will be one second between the sound and its echo.

In our experiments with the tuning-fork and two bottles (see Fig. 40), you remember, we put a piece of cardboard and a flat gas-flame before the mouth of one of the bottles. Here, also, we had a reflection of the sound from the cardboard, and even from the flame.

CHAPTER X.

ON THE PITCH OF SOUNDS.

EXPERIMENT 76.-Take one of the A-forks and the Cfork and stick them in the block of wood side by side, with the opposite prongs of the two forks inclined to each other, so that by drawing a rod between them they will be set vibrating at the same time. Stick a piece of copper-foil on the tips of the prongs nearest each other, and arrange the smoked glass and its guide as directed in Experiment 25. Vibrate the forks by drawing the rod between them, and obtain the traces of their vibrations on the smoked glass.

Take the smoked glass and carefully measure off an equal space on each trace, and then count the vibrations inclosed in this space. If the right forks have been selected it will be found that 17 vibrations of one fork cover as much space as 21 vibrations of the other. From this you readily see that, in the same time, one fork vibrates oftener than the other. Carefully notice which fork makes the greater number of vibrations. Bring one vibrating fork to the ear, and then the other, and you will observe that the C-fork gives a higher note than the A. The C-fork makes the greater number of vibrations (21) in a given length on the trace, and the A-fork makes the smaller number (171) in the same length. We are con

vinced by this experiment that a fork giving a high note vibrates oftener in a second than a fork giving a lower note. Experiments on all kinds of vibrating bodiessolids, liquids, and gases-have proved that the pitch of a sounding body rises with the increase in the number of its vibrations in a second. This fact may be stated thus: The pitch rises with the frequency of the vibrations. From the above fact it follows that the pitch of a sound rises with an increase in the number of sonorous waves that reach the ear in a second.

EXPERIMENTS WITH THE SIREN.

Fig. 43 shows an instrument called a siren. I will show you how to make several instructive and curious experiments with it. First, you will find out the number of vibrations made in a second by a sounding body like one of your tuning-forks; and, having found out this, you will use the fork to determine for yourself the velocity of sound. The siren will also tell you this important fact: That the numbers of vibrations per second which give the various notes of the gamut, or musical scale, bear to each other fixed numerical relations.

To make the siren, get a piece of cardboard, or millboard, and draw on it with a pair of dividers a circle 8 inches (21.6 centimetres) in diameter; then cut this circle out of the cardboard. Now draw four circles, the inner one with the legs of the dividers opened to 24 inches (5.73 centimetres), the next with a radius of 22 inches (6.99 centimetres), the third with 3 inches (8.26 centimetres), and the fourth with 34 inches (9.53 centimetres). Divide the circumference of the outer circle into 24 equal parts, and to each of these points of division draw a line from the centre, as shown in Fig. 44.

Divide the spaces

on the outer circle in halves; this will give 48 points on this circle. At each of these points cut a hole of about

[graphic][merged small]

inch (5 millimetres) in diameter with a punch. Then punch holes at the 24 points on the inner circle.

The student, on looking at Fig. 44, will see that, on the radii marked 1, 2, 3, 4, 5, and 6, the holes are all in a

line. These holes, thus in line, divide the circle into six equal parts. Divide each of these sixths on the second circle into five equal parts, and each sixth on the third circle into six equal parts, and through each of these points of division cut a hole with the punch. By following these

6

2

3

FIG. 44.

directions you will have made on the inner circle 24 holes, on the second 30, on the third 36, and on the fourth 48 holes.

Now cut a hole in the centre of the disk, so that it neatly fits on the screw of the small pulley of the rotator

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