161. What is said of the disposition of motion to be straight? Why is motion never straight, so far as we know? How can we make motion very nearly straight? Give the illustration of the bullet in full. 162. Give the illustration represented in Fig. 141. What is compound motion? Illustrate straight compound motion. 163. Explain Fig. 143. 164. Explain what is represented Figs. 144, 145, and 146. 165. Explain Fig. 147. How is curved motion produced? Give the illustration of the ball and string. What are centrifugal and centripetal forces? 166. What are these two forces in the revolution of the earth around the sun? Give various illustrations of the operation of centrifugal force. 167. What is said of the formation of bends in rivers? 168. Show how eddies and whirlpools are formed. How is the centrifugal force used in the art of pottery. How in making window-glass? 169. Describe and explain the operation of the steam-governor. 170. What is said of the agency of the centrifugal force in shaping the earth? 171. Explain the operation of the apparatus represented in Fig. 154. What are the forces which act on a projectile? What is said of balls thrown horizontally from cannon with different velocities? 172. Show by Fig. 155 why a ball dropped from the mouth of a cannon will fall to the ground in the same time that one fired from it will. By What two forces is a falling body acted upon? 173. Explain Fig. 156. What is the course of a ball dropped from a railway car or from a mast-head? Give the comparison between the cannon-ball and the moon. 174. What is said of the velocities of the heavenly bodies? CHAPTER XI. 174. What are the Mechanical Powers? Why is the term power not strictly proper? 175. Explain the terms power, weight, and fulcrum. What is said of the use of the lever? What is the lever of the first kind? What is said of its force? 176. What is said of scales? What of steelyards? 177. Give examples of the first kind of lever. Show by Fig. 159 that there is no gain of power in this lever. 178. Give the illustration of the see-saw. What is said of Archimedes's lever? 179. State the analogy between this lever and the Hydrostatic Bellows, Bramah's Press, etc. What is the lever of the second kind? Apply the rule of equilibrium to it. Show how the common wheel-barrow is a lever of this kind. 180. Give other examples of the second kind of lever. What is lever of the third kind? How does this differ from the other two kinds? Apply the rule of equilibrium to it. 181. Give examples of the third kind of lever. Show how it acts at a mechanical disadvantage in the different examples mentioned. State in full what is said of muscular action. 182. Explain by the figures the operation of compound levers. 183. State the comparison between the lever and the wheel and axle. What is said of the common windlass? 184. Describe and explain the capstan. What are its chief uses? What is said of the fusee of a watch?