In our younger days most of us have amused ourselves with a toy telephone consisting of a long piece of string having each end passed through the bottom of a little cardboard box, and secured by a knot. If the string is stretched tightly this arrangement enables whispered words to be heard at a distance of 20 or 30 yards. Simple as is this little toy, yet it is probable that many people would be rather nonplussed if asked suddenly to explain how the sounds travel along the string from one box to the other. If the toy had some complicated mechanism most likely every one would want to know how it worked, but the whole thing is so extremely simple that generally it is dismissed without a thought. If we strike a tuning-fork and then hold it close to the ear, we hear that it produces a sound, and at the same time, from a slight sensation in the hand, we become aware that the fork is in vibration. As the fork vibrates it disturbs the tiny particles of air round it and sets them vibrating, and these vibrations are communicated from one particle to another until they reach the drum of the ear, when that also begins to vibrate and we hear a sound. This is only another way of saying that the disturbances of the air caused by the vibrations of the tuning-fork are propagated in a series of waves, which we call “sound waves.” Sound is transmitted better through liquids than through the air, and better still through solids, and this is why words spoken so softly as to be inaudible through the air at a distance of, say, 100 feet, can be heard fairly distinctly at that distance by means of the string telephone. The sound reaches us along the string in exactly the same way as through the air, that is, by means of minute impulses passed on from particle to particle. A more satisfactory arrangement than the string telephone consists of two thin plates of metal connected by a wire which is stretched very tightly. Words spoken close to one plate are heard by a listener at the other plate up to a considerable distance. Let us try to see exactly what takes place when this apparatus is used. In the act of speaking, vibrations are set up in the air, and these in turn set up vibrations in the metal plate. The vibrations are then communicated to the wire and to the metal plate at the other end, and finally the vibrations of this plate produce vibrations in the air between the plate and the listener, and the sound reaches the ear.