185. Describe the arrangement of the fixed pulley. What are its uses? 186. Describe the arrangement of the movable pulley. Show how the relation of the power to the weight is estimated in the case of compound pulleys. 187. Explain the mechanical advantage of the inclined plane. Give examples of it. 188. What is said of roads? Give the comparison of the wedge to the inclined plane. How is the power of the wedge estimated? Give examples of the wedge. 189. What is said of the screw? Show by Fig. 180 how the force of the screw is estimated. What are some of the uses of the screw? 190. Give the estimate of the power of the screw and lever as used together. How can you show that there are really but three mechanical powers? What is said of these as composing tools and machinery? What is said of friction in machinery? 191. What is the first advantage of the mechanical powers which is mentioned? Give the illustrations. What is the second advantage? Give the illustrations. 192. What is the third advantage? Give examples. How is the velocity of motion in machinery usually varied? What is the fourth advantage? Mention examples. Describe the instrument called a Lewis. 193. What is said of the title by which Aristotle distinguished man from other animals? CHAPTER XII. 194. What is sound? What relation has sound to rapidity of vibration? Mention cases in which the vibration of sounding bodies is manifest to the sight and touch. What is said of wind instruments? 195. State the analogy of a sounding body to a pendulum. Describe the process by which the sensation of sound is produced. Where does the vibration caused by the sounding body stop in the ear? What is transmitted from thence to the brain? 196. Give examples of the transmission of sound through various substances? State the experiment by which it is shown that sound is not transmitted through a vacuum. What is said of sound at great heights? 197. How far has the sound of a volcano been heard? If the same sound were made in space at that distance from the earth why could not the inhabitants hear it? What is the cause of the noise of bodies passing through the air? Why do the heavenly bodies, moving so rapidly, produce no sound? Cite examples showing the different velocities of sound in different media. 198. What is said of the uniformity of the velocity of sound? Show how we can measure distances by sound as compared with light in velocity. Upon what does the loudness of sound depend? Illustrate this point. 199. What is said of the diffusion of sound? What of its reflection? What of echoes? 200. What is said of multiplied and mingled reflections of sound? Explain the operation of whispering galleries by Fig. 187. 201. Explain the operation of the speaking-trumpet. Give other examples of the concentration of sonorous vibrations.