To understand the difficulties which the inventors of the telephone had to overcome, let us consider for a moment the nature of the sound waves which represent speech. Spoken language consists of vibrations of the air. These vibrations are very small and very quick puffs of air, which we produce by means of our throat and mouth. Now if we were to try to give a very rapid succession of puffs, we could not give more than about ten per second. But if we had a machine which could puff much more quickly than that, we should find that after the puffs had become more than twenty per second, there would be a kind of hum. This result can be illustrated by means of a toothed wheel such as we find in clocks. When such a toothed wheel is turned quickly, and we hold the edge of a piece of paper against it, then, as it is made to turn more and more quickly, the little ticks of the paper merge more and more into a hum, and finally become a musical note. The faster the wheel, the higher the pitch of the note. Every note has its own pitch, that is to say, it has a certain number of vibrations per second. The notes in the middle of a piano have from two hundred to five hundred vibrations per second. The same note may be either loud or soft, just as a wave of water may be either high or low. The high wave corresponds to a loud sound. Very small ripples on the surface of water correspond to a very high, but faint, note of music, like the singing of an insect. Each note has its own pitch and its own loudness. In addition to that, it also has a certain “quality” of its own. We may, for instance, play the same note on a violin, a trumpet, or an organ. They may have the same pitch and the same loudness, and yet we can distinguish them from each other easily. What is this “quality” which enables us to distinguish the note of one instrument from the same note played on another instrument? It must be something in the position of the waves which travel through the air and reach our ear. This question long remained in doubt, until people found out that no musical sound was quite simple. Even in water one usually sees little ripples on the top of big waves. A wave with a perfectly smooth surface is very rare. In the same way, a perfectly simple and pure sound wave is hardly ever produced. Each wave has little ripples on it. The wave from a piano has one kind of ripples, the wave from an organ has another kind, and the wave from a trumpet--which is particularly shrill and penetrating--has a great number of very tiny ripples, which mean very high notes. These facts can be proved by playing a soft note on an organ, and playing a number of very high notes at the same time. If the high notes are properly chosen, the sound becomes shrill and penetrating, like the sound of a trumpet.