the glass tube C. Then bore another-inch hole directly into the shallow hole in B. Put a glass tube in a gas or spirit flame and heat it red-hot at a place about two inches from its end. a narrow neck. FIG. 51. Then draw the tube out at this place into Make a cut with the edge of a file across this narrow neck, and the tube will readily snap asunder at this mark. Then heat a place on the tube in a flame, and here bend it into a right angle, as shown at D, Fig. 51. Now fit this tube into the hole just made, as shown at D. These tubes may be firmly and tightly fitted by wrapping their ends with paper coated with glue before they are forced into their holes. Get a small piece of the thinnest sheet rubber you can find, or a piece of thin linen paper, and, having rubbed glue on the board A around the shallow hole, stretch the thin rubber, or paper, over this hole and glue it there. Then rub glue on the block B, and place the shallow hole in this block directly over the shallow hole in A, and fasten B to A by wrapping twine around these blocks. Thus you will have made a little box divided into two compartments by a partition of thin rubber. Fasten the rod A to the side of a small board, so that it may stand upright. Attach a piece of large-sized rubber tube to the glass tube E, and into the other end of the tube stick a cone, made by rolling up a piece of cardboard so as to form a cone 8 inches long and with a mouth 2 inches (51 millimetres) in diameter. Now get a piece of wood 1 foot long, 4 inches wide, and inch thick. Out of this cut the square, with the two rods projecting from it, as shown at M. The lower of these rods is short, the one above the square is long. Cut the end of the shorter rod to a blunt point, and with this point make a very shallow pit in the piece of flat wood K for the rod and square to twirl in. Glue the piece of wood K on the end of a brick, L. Get two pieces of thin silvered glass, each 4 inches square, and, placing one on each side of the square M, fasten them there by winding twine around the top and bottom borders of the mirrors. EXPERIMENT 112.-Through a rubber tube lead gas to C. It will go into the left-hand partition of the box and will come out at F, where you will light it. Now place the mirror-rod in the shallow pit in K, and hold the mirror upright so that you may see the flame Freflected from its centre. Hold the rod upright and twirl it slowly between the thumb and forefinger, which should point downward and not horizontally, as shown in the figure. The flame appears in the mirror drawn out into a band of light with a smooth top-border. While twirling the mirror put the cone to your mouth and sing into it. The sonorous vibrations enter the side A of the box, and, striking on the thin rubber, force this in and out. When it goes in, a puff of gas is driven out of the other partition, B, of the box, and the flame F jumps up. When the sheet of rubber vibrates outward, it sucks the gas into the box B, and the flame F jumps down. Therefore, on singing into the funnel, you will see in the mirror the smooth top-border of the luminous band broken up into little tongues or teeth of flame, each tooth standing for one vibration of the voice on the rubber partition. Place a lamp-chimney around the flame, should the wind from the twirling mirror agitate it, and be careful not to have the flame too high. EXPERIMENT 113.-Another way of showing the vibrations of the flame is to burn the jet of gas at the end of a glass tube stuck into the end of a rubber tube attached to F. Now sling the tube round in a vertical circle, and you have an unbroken luminous ring; but as soon as you sing into the cone this ring breaks up into a circle of beads of light, or sometimes changes into a wreath of beautiful little luminous flowers, like forget-me-nots. To make this experiment, you will be obliged to have a tube with a larger opening than that at F. This instrument will afford you many an hour of instruction and amusement. We have only space to show you a few experiments. Others will suggest themselves whenever you use it. EXPERIMENT 114.-Sing into the funnel the sound of oo as in pool. After a few trials you will get a pure simple sound, and the flame will appear as in Fig. 52. Some voices get this figure more readily by singing E. EXPERIMENT 115.-Twirling the mirror with the same velocity, gradually lower the pitch of the oo sound till your voice falls to its lower octave, when the flame will appear as in Fig. 53, with half the number of teeth in Fig. 52, because the lower octave of a sound is given by half the number of vibrations. EXPERIMENT 116.—Sing the vowel sound o on the note and you will see Fig. 54 in the mirror. This evidently is not the figure that would have been made by a simple vibration. It shows that this o sound is compound, and formed of two simple sounds, one the octave of the other. The larger teeth are made by every alternate vibration of the higher simple sound acting with a vibration of the lower, and thus making the flame jump higher by their combined action on the membrane. EXPERIMENT 117.-Fig. 55 appears on the mirror when we sing the English vowel a on the note f. EXPERIMENT 118.-Fig. 56 appears on the mirror when we sing the English vowel a on the note c. |