It stands firm when it has a broad base, and its centre of gravity is near the earth. What effect is produced by elevating the centre of gravity? The higher the centre of gravity in any body is raised above the earth, the greater is its tendency to fall. Why is a load of hay more easily overturned than a load of iron, when upon the same wagon? Because the centre of gravity in the load of hay is much higher than it is in the load of iron. Why do passengers on top of a stage cause it to overturn more easily? Because their weight on the top raises the centre of gravity in the stage, and it is more readily overturned. Why is a person more likely to fall when standing up than when sitting down? Because the centre of gravity is higher in a person when standing than when sitting. Why is a boy more likely to fall when raised on stilts? Because the centre of gravity in the boy is farther from the earth when he is raised on stilts. When will a load of hay be overturned? It will be overturned whenever the line from the centre of gravity to the earth falls outside of the wheels, as is shown in Fig. 36. [Illustration: Fig. 36.] Why should lamps have large bottoms? Lamps should have large bottoms so that the line of gravity may not fall outside of the bottom. Where must the line of gravity in a body always fall? It must always fall within the base of that body, or the body will be overturned. How must a leaning tower be built so as not to fall? It must be built so that the line of gravity will fall within its base. Why do we lean back when carrying a load in front of us? We lean back so that the line of gravity may be kept directly over our feet. Why does a person standing up in a wagon fall when the wagon is suddenly moved? Because the feet, being in contact with the wagon, move with it before the motion extends to the rest of the body, thus throwing the line of gravity behind the feet; hence, the person must fall, unless supported. Why does a person standing up in a wagon fall when the wagon is suddenly stopped? Because the feet, being in contact with the wagon, are suddenly stopped, while the body still moves forward, thus throwing the line of gravity ahead of the feet; hence, the person must fall, unless supported. Why do animals with _four_ feet stand firmer than those with only _two_ feet? Because the four feet afford a larger base to stand upon than the two feet do, and, therefore, support the centre of gravity better. Why can a horse or a cow remain standing for many hours without seeming to be weary? Because their four feet afford a good support to the centre of gravity. Why does a person who is standing soon become weary? Because it requires a constant effort to keep the line of gravity over the feet, and this soon produces weariness. Why can a person sit longer than he can stand?