Fig. 197.
281. Currents in the Air from Heat.—Heat is the grand mover of the atmosphere. Any portion of it that becomes warmer than surrounding portions rises, or rather is pushed up, for the same reason that a hot-air balloon rises, the only difference between the two cases being that in the one the air is confined, and in the other is left free, and so becomes diffused. And it is this rising of the air from expansion that causes nearly all the movements that we witness in the air. We see this exemplified in various ways wherever there is a fire. The air that is heated by the fire is forced upward by the colder air, which, on the principles of specific gravity, seeks to get below the warmer and lighter air. The hot air that comes through the registers of a furnace is pushed up by colder air below. For the same reason the heated air around a stove-pipe is constantly going upward. This is very prettily shown by the toy represented in Fig. 197 (p. 218), which is a paper cut spirally, and suspended, as you see, upon the point of a wire. The upward current makes the paper revolve rapidly around the wire. It is from the rising of warm air that the galleries of a church are warmer than the space below. In a common room the disposition of the air is continually to have its warmest portions above and the colder below. It is for this reason that we have our arrangements for producing or introducing heat at as low a point as possible.
282. Chimneys.—We speak of the draught of a chimney, and we say of one that does not smoke that it draws well, as if the smoke were in some way actually drawn up. But the same principles apply here as are developed in § 281. The smoke, which is a combination of heated air and gases with some solid matters in a fine state, is forced up the chimney. When a chimney does not draw well we open a door or a window for a little while until the fire gets thoroughly agoing. Why is this? It is that we may have denser air than there is in the room, so that the smoke may be pushed up more forcibly. When the chimney becomes well heated there is ordinarily no difficulty, because then the smoke in it is not obliged to part with much of its heat to the walls of the chimney, and therefore is so much lighter than the air in the room that it is very easily forced upward. The principal reason that a stove-pipe generally draws better than a chimney is that there is much less heat expended in establishing and maintaining the upward current. Especially is this true if the chimney be a large one. In such a case there are both a great extent of brick and a large body of air to be heated to establish the upward current, and these must be kept warm in order to maintain it.[3]
Fig. 198.
Fig. 199.
283. Winds.—If you open a door of a heated room a candle held near the floor will have its flame blown inward, while one held near the top of the door will have its flame blown toward the cold entry. Here you have a good illustration of the manner in which winds are produced. Wherever the wind blows it is air pushing out of the way other air that is warmer, in order that it may, in obedience to gravitation, get as near the earth as possible. Take, for example, the land and sea breezes, as they are called. During a hot summer's day the sun heats the earth powerfully, while the ocean receives but little of its heat. The heated land heats the air above it; and as the air over the ocean is cooler, and therefore heavier, it pushes upward the air of the land, for the same reason that water pushes up oil; and as this goes on continuously a regular current is established. The wind blows in upon the land, as represented in Fig. 198, while the warmer air passes upward into the higher regions of the atmosphere, and turns toward the sea. The arrows show the course of the currents. The resemblance of all this to the effect upon the candle held near the open door is very obvious, the cold air from the entry blowing in below representing the breeze from the ocean, and the warm air of the room blowing out above representing the passage of the warm air of the land out toward the ocean. At night this is apt to be reversed. The earth becomes cooled, and with it the air that is over it. The result is that the cooled air of the land now pushes upward the warmer air of the sea, as seen in Fig. 199.