When it has less specific gravity than water, what will it do? It will float on the surface of the water. Will all parts of the body be above the surface of the water? No; a part of it will sink into the water. How much of it will sink into the water? It will sink so as to displace water enough to balance its own weight. Why does a boat float on the water? Because it weighs less than a bulk of water of the same size as the boat. When a cargo is placed on the boat, why does it still float? Because it still weighs less than a bulk of water of the same size as the boat. How do we make use of this principle? We make use of it in the construction of vessels, so as to carry on trade with other parts of the world. Why can ducks and geese float on the water? Because their light, downy feathers make their specific gravity less than that of the water; hence, they must float on its surface. What may be said of the specific gravity of the human body? When the lungs are filled with air, the specific gravity of the human body is rather less than that of water. Can a person float in the water? Yes; if a person, who cannot swim, will throw his head back so that his mouth and nose only remain above the water, he may float in this position for some time. What is the specific gravity of fish? The specific gravity of fish is so nearly equal to that of water, that they can move about anywhere beneath its surface. How do fish rise and sink in the water? Fish rise and sink in the water by means of an air-bladder in their bodies. How does the air-bladder help the fish to rise or sink? When the bladder is filled with air, the specific gravity of the fish becomes lighter, and it rises towards the surface; and when the air is forced out, the specific gravity becomes heavier, and the fish sinks towards the bottom. Why does cream rise to the surface of milk? Because the specific gravity of the cream is less than that of the milk. Why does oil float on water? Because the specific gravity of the oil is less than that of the water. Why can a fat person float more easily in the water than a lean person? Because the fat reduces the specific gravity of the body, so that it floats more easily. Why can a person float more easily in salt water than in fresh water? Because salt increases the weight of the water, and, therefore, it buoys up with more force. What familiar example have we of this? An egg will sink when placed in fresh water; but if salt be dissolved in the water, the egg will float near the surface. [Illustration: Decoration.] [Illustration: Decoration.] CHAPTER IX. Motion. [Illustration: Illustrated Capital W] When is a body said to be in motion? A body is in _motion_ when it is moving from one place to another. When is a body said to be at rest? A body is at rest when it is not moving from one place to another.