144. State and explain the experiment with the lead and feather. Explain the operation of the water-hammer. Show the relation of bulk to the resistance of liquids and gases. 145. Illustrate the relation of bulk to the motion of solids, produced by moving gases and liquids. 146. What is said of the opposition of gravitation to water and air in moving solids? What difference does the presence of obstacles make in the relation of force to velocity? 147. State the law of the relation of force to velocity, and illustrate by Fig. 136. What are some of the practical applications of this law? What is said of the relation of shape to velocity? What is said of the shape of fishes? 148. What is said of the shapes of boats? What of the management of the webbed feet of water-fowls? What of the wings of birds? What is said of friction as an obstacle to motion? What of it as a cause to motion? Illustrate fully in the case of the wheel. 149. What is said of the friction of liquids in tubes? What is the effect of sudden turns in pipes? What is the arrangement of arteries in the heads of grazing animals? Illustrate the difference of friction in small and large pipes by Fig. 131. 150. What is said of the effect of friction in brooks and rivers? In what part of a stream does the water move most rapidly? Explain the formation and breaking of the crest of waves rolling over a beach. What is said of the velocity of rivers as affected by friction? Explain the formation of waves. 151. What is it that really advances in the forward movement of a wave? Give the comparison mentioned. What is said of the height of waves? 152. What is momentum? Upon what two things does it depend? Illustrate this dependence. Explain Fig. 133. 153. Give the illustration of the musket-ball and cannon-ball. Give that of the plank. That of the candle. That of the air. 154. What is said of the expression, quantity of motion? Under what circumstances may a single impulse produce a great velocity? What examples have we of this? How is it with the motions that we see around us? What is said of the fall of bodies to the earth? 155. Give examples from muscular action. Give that of the arrow. Give that of gunpowder. What is said of the arrest of great velocities? Give the illustrations in regard to cannon-balls. 156. State and explain the feat of the anvil. Give examples from common efforts and labors. 157. Explain the communication of motion in the case of elastic bodies by Figs. 133 and 134. What is said of the reflection of motion? 158. What is said of the uniformity of motion? What of its uniformity in velocity? State by what means we calculate as to time. 159. What is said of the sun-dial? What of the hour-glass? What of Galileo and the pendulums? Explain the operation of the pendulum. 160. Explain Fig. 137. Explain the operation of the gridiron pendulum by Fig. 138.