The Optical Magic Lantern Journal (May 1895)

82 The Optical Magic Lantern Journal and Photographic Enlarger. Then whisper some question—it will be unheard by the audience, but very distinctly audible at the end of the rod; he will answer correctly. Hold the rod horizontally, and pressing one end lightly against the panel of a door, vibrate the fork, and press its handle against the free end. The door panel sings out loudly ; take away the fork the sound from the panel stops instantly. Remind them of the row of boys; the little bits of woods in this latter case swing to and fro, and pass the sound on. Now bring out a toy telephone, which consists of two short tubes of tin or pasteboard, each covered at one end with parchment, a long string being fixed to the middle of each diaphragm. The sounds spoken into the drum at one end when the string is stretched are transmitted along it to the diaphragm at the other end, and thence to the listening ear. So far all is clear to the children. Come back now to the old point. How does the sound from this whistle get to the back of the room, there being neither string nor board along which it can be passed on. Various answers will be given. You might proceed thus, addressing yourself to the child at the end of the room. If I were to offer you a halfpenny and kept it in my pocket, it is clear that you would not get it. . If I held it out to you, and you remained where you now are, you still would be unable to receive it. " How do you propose to get it? (Suggested.) “IT would come for it.” ‘You would throw it.”’ No, I won’t do either. the boy next me, and he will pass to the boy behind him, and so on till it reaches you. Now I will pass it to you have it. I ring this bell by setting it | swinging to and fro, and as you say you hear it, it is evident there is some busybody or bodies : at work between me and you, passing on those swings till they reached your ear. Now I want you to think. When they have decided correctly, you must resume. It is quite true that the air can be set swinging, though it is too large to have a vibratory rate of its own. not often that a considerable portion is set going. Yetin storms no doubt you have felt the house rock, and seen broken windows. Sometimes large trees are uprooted by gusts timed to the vibratory rate of the tree. “A large iron bridge was being built at Colebrooke Dale in America, when by came a fiddler and told the workmen that he could fiddle their bridge down. The workmen laughed at him, and told him he might fiddle away as hard as he liked they didn't care. (Fine slide effect.) One note after another was struck upon the strings until one was found with which the bridge was in sympathy. Then the bridge began to shake violently, and the alarmed workmen bade the fiddler stop. No doubt you have been in a church or other building when a massive bell or organ has been pealing. Ifso you will have felt the trembling of the air and the seats, and if you could have touched the windows you would have been even more surprised. Small portions of air, such as little boxes or tubes of air, are readily set in motion. A very simple experiment shows this. Four cardboard tubes, 1 in. diam.,and 16,12, 10, and 8 in. long respectively, have the one end covered with a cardboard disc. Resting the closed ends on the table, they are allowed to fall one after another, and the four notes (doh, me, soh, doh), due to the air in the tubes vibrating, will be readily distinguished. But though air is capable of conveying pulsations, it is not by any means so good as some other substances, some of which we have already tried. Water for example is much better. Place a tumbler of water on an empty cigar box (one short end having been removed). A small fork stuck in a block of wood is excited, and the block lowered in the water. The note will be heard proceeding out of the box; the vibrations have been passed on from the fork to the block, thence to the water, and from the water to the interior of the box. Refer to the earth as a good transmitter. Instance the blind man finding a watch by putting his ear to the ground. Take a child’s wooden whistle, cut off just behind the first finger hole, and fasten it into a glass tube three times its length. Close the end with a cork. Introduce into the tube a small quantity of very fine precipitated silica. Place the tube horizontally on the vertical attach _ ment, and by playing soundly the scale upon it, It is | a note will easily be found which acts as 4 command to the fine plates of silica, which thereupon stand erect in groups, separated from each other, and make a very striking show upon the screen. j If two tuning forks have the same pitch, and one of them be caused to sound, the other, though several feet away, will sound also. In this case the air acts upon the second fork, just as the one who pushed the swing in our first experiment. He timed his own movement to that of the swing. Here the air took its time from the movement of the swinging fork, A going clock may be made to set another precisely similar clock going by the same means. Press down gently a bass C on a piano, so as not to make it sound. Now strike the C above it, holding down the key for a moment, and