WHY IT IS HARD TO UNDERSTAND A SPEAKER IN AN EMPTY HALL. A speaker can be heard much more easily in a room full of people than in an empty hall. The sound does not reflect well from the soft clothes of the audience and the uneven surfaces of their bodies, just as a rubber ball does not bounce well in sand. So the sound does not reverberate as in an empty hall. _APPLICATION 43._ Explain why a carpeted room is quieter than one with a bare floor; why you shout through your hands when you want to be heard at a distance. INFERENCE EXERCISE Explain the following: 261. It is harder to walk when you shuffle your feet. 262. The air over a lamp chimney, or over a register in a furnace-heated house, is moving upward rapidly. 263. The shooting of a gun sounds much louder within a room than it does outdoors. 264. A drum makes a loud, clear sound when the tightened head is struck. 265. The pull of the moon causes the ocean tides. 266. Sand is sometimes put in the bottom of vases to keep them from falling over. 267. It is difficult to understand clearly the words of one who is speaking in an almost empty hall. 268. The ridges in a washboard help to clean the clothes that are rubbed over them. 269. One kind of mechanical toy has a heavy lead wheel inside. When you start this to whirling, the toy runs for a long time. 270. If you raise your finger slightly after touching the surface of water, the water comes up with your finger. SECTION 30. _Pitch._ What makes the keys of a piano give different sounds? Why does the moving of your fingers up and down on a violin string make it play different notes? Why is the whistle of a peanut roaster so shrill, and why is the whistle of a boat so deep? Did you ever notice how tiresome the whistle on a peanut roaster gets? Well, suppose that whenever you spoke you had to utter your words in exactly that pitch; that every time a car came down the street its noise was like the whistle of the peanut roaster, only louder; that every step you took sounded like hitting a bell of the same pitch; that when you went to the moving-picture theater the orchestra played only the one note; that when any one sang, his voice did not rise and fall; in short, that all the sounds in the world were in one pitch. That is the way it would be if different kinds of air vibrations did not make different kinds of notes,--if there were no differences in pitch. PITCH DUE TO RAPIDITY OF VIBRATION. When air vibrations are slow,--far apart,--the sound is low; when they are faster, the sound is higher; when they are very quick indeed, the sound is very shrill and high. In various ways, as by people talking and walking and by the running of street cars and automobiles, all sorts of different vibrations are started, giving us a pleasant variety of high and low and medium pitches in the sounds of the world around us.