When they wish to descend, what do they do? They open a valve at the top of the balloon, which permits some of the gas to escape; then it will sink towards the earth. How is the valve at the top of the balloon opened and closed? It is opened and closed by means of small ropes, reaching from the valve to the car beneath. Can the balloon be made to go in any one direction? No; it is carried about by the currents of air, and those in the car cannot control its course; therefore, it has never been of much use to man. Winds. What is wind? _Wind_ is air moving from one place to another. Does the air move in every direction? It does move in every direction, upward, downward, and to every side. When the wind blows from the north towards the south, what is it called? It is called a north wind. What is it called, when it blows from other directions? When it blows from the east, it is called an east wind; from the south, a south wind; and from the west, a west wind. What causes the air to move from place to place? Heat causes the air to move from place to place. When air is heated, what change takes place? The heated air ascends, and colder air moves in to fill its place; thus causing the wind to blow. What illustration have we of this? When a candle is held in the open door of a heated room, its flame is blown outwards if placed at the top of the door, and inwards if placed at the bottom of the door; thus showing two currents of air. Why is this? The heated air escapes from the top of the room, so as to ascend higher, while the colder air comes in at the bottom, to fill its place. Why does the wind blow towards a fire in the open air? Because the air near the fire becomes heated, and ascends rapidly, so that the air rushes in from every direction to fill its place. What does the earth do with the heat it receives from the sun? The earth gives off its heat to the air nearest to it. What becomes of this heated air? This heated air ascends, and colder air comes to take its place; thus producing winds. Are all parts of the earth heated alike by the sun? No; they are not all heated alike. How does this unequal heating affect the air? It causes currents in the air all over the world. Does water give off heat as rapidly as the earth does? No; water is a poorer radiator of heat than the earth is. On which will the air be the cooler when the sun shines, on the earth, or on the water? It will be cooler on the water than on the earth. Why does a sea-breeze feel cool in the daytime? Because the air over the water, in the daytime, is less heated than the air over the land; hence, the sea-breeze feels cool. Why does the earth become colder than the water at night? Because the earth gives off its heat more rapidly than the water, and as it receives no heat from the sun at night, it soon becomes colder than the water. Why does a land-breeze feel cool at night?