202. What is the difference between a musical sound and a noise? What is said of the exact regularity of musical vibrations? How are different notes produced in stringed instruments? Upon what does the note depend in wind-instruments? 203. Explain the operation of the organ-pipe represented in Fig. 190. What is said of the notes of bells and of musical glasses? Explain the mechanism of the human voice. 204. What is harmony? Upon what does it depend? Between what two notes of the scale is there the greatest harmony? What note next to the octave harmonizes best with the fundamental note? And what note next? Show why the second note, in contrast with the octave, is so discordant with the fundamental note. 205. State the proportions between the numbers of the vibrations in the different notes. If you know the number of vibrations of the fundamental note in a second, how may you determine the number of vibrations in the other notes? What is said of the number of notes in the diatonic scale? What of the proportionate lengths of strings for different notes? What is said of tuning instruments? 206. What is meant by saying that a note is too sharp or too flat? State in full what is said about the mysteries of sound and hearing. CHAPTER XII. 207. State the experiment of the three vessels, and the inference from it. 208. What other facts sustain this inference? How did Sir Humphrey Davy prove that there is heat in ice? What are the two theories of heat? What is the chief source of heat for the earth? What is said of the heat of the sun itself? 209. What is said of the universal influence of the heat of the sun in the earth? What of the heat supplied from within the earth itself? What of electricity as a source of heat? 210. What is said of chemical action as a source of heat? Give examples of the production of heat by mechanical action. What is said of the relations of heat and light? 211. Show the expansive influence of heat by describing the experiment represented in Fig. 192. Give familiar examples of this expansion. 212. How can you loosen a stopper stuck fast in a bottle? Give the anecdote about the _Persia_. Give the statement about the building in Paris. 213. What is said of the expansion of liquids by heat? How may the influence of this expansion upon specific gravity be shown? 214. What is said of thermometers? What of the invention of the thermometer? 215. State the plan of Fahrenheit's thermometer. Give the plans of other thermometers. 216. Why is Fahrenheit's thermometer, on the whole, the best? What is said of the expansion of gases by heat? State experiments in illustration. 217. What is said of balloons? What of the influence of heat on the atmosphere? Give examples of this influence. 218. Why, in heating apartments, do we have the heat created or introduced at as low a place as possible? Explain the _draught_ of a chimney. Why does a stove-pipe generally draw better than a chimney?