"Our lessons have shown us that the heat of the fire, the gas, or the lamp causes currents of air through the room or, in other words, sets the air of the room in motion," began Mr. Wilson, at the next meeting of the class.
"I want now to get outside the room. Let us imagine ourselves, some calm, still evening, in a field, where an immense bonfire is burning. Immediately the flames began to ascend, we should feel, as we stood round, currents of cold air rushing in from every quarter towards the fire. These currents would at once become apparent if we held over our heads some long, thin, paper streamers.
"The reason is clear. The immense heat from the fire expands the air round and above it, thus making it lighter, bulk for bulk, than the air in its vicinity. This thinner, lighter air is not able to resist the pressure of the denser, heavier air around. There is a momentary struggle between the two, but it has to give way to the greater pressure. The dense, heavy air rushes in from all quarters towards the fire, and by its pressure forces the lighter air upward.
"This movement of the air as the result of heat from the bonfire will illustrate to you the manner in which winds are caused. These currents of air setting in on all sides towards the fire are actually winds on a small scale.
"Wherever winds occur, something similar to this must have been going on. The air in some spot has been greatly heated, and of course expanded, and made lighter. It consequently ascends, while colder, heavier air rushes in from all sides to prevent a vacuum. This rushing air forms the wind.
"Land and sea breezes are the simplest illustration of the formation of winds. We will see how they are caused.
"Carry your minds back, in the first place, to our lesson on the radiation and absorption of heat. You remember, of course, the meaning of the two terms, and that good absorbers are always good radiators, bad absorbers bad radiators.
"The land is a better absorber and therefore a better radiator of heat than the sea. The sun shines all day with equal power on the land and on the adjoining sea, but the land takes in more heat than the water. Being a good radiator, however, it radiates this heat into the air as readily as it receives it; thus the air above the land becomes hotter than that over the sea. This heated air from the land expands, and becomes lighter, bulk for bulk, than the colder, denser air over the sea. It is unable to resist the pressure of this colder, heavier air; it givesway to that pressure, and ascends, owing to its lightness, the air from the sea rushing in to fill its place. Thus all day long we have a breeze blowing from the sea to the land.
"Now when the sun has set, the source of heat is gone. The land is no longer absorbing heat. Through-out the night, however, the land continues to give up by radiation the heat it has taken in during the day. It soon becomes cold-colder than the sea, which, although it has absorbed slowly, has also radiated slowly. Hence the air above the water is warmer, in comparison, than that over the land; warmer means also, as we have seen, lighter, bulk for bulk. "Heat has, in this case again, caused unequal pressure.
The rarefied, lighter air over the sea cannot resist the denser, heavier air from the land, which is pressing upon it. As before it gives way, and rises into the higher regions, forced to do so by the onrush of the air from the land. This gives a land breeze, which lasts till the sun next morning begins to assert its power, and then the breeze from the sea sets in once more.
"Think now of the tropical regions of the globe, and of the immense power of the sun in those parts. The heated air acts in exactly the same way and, in consequence, movements of colder air from the temperate and polar regions, north and south, set in towards the equator. The directions of these winds are modified from various causes, but their origin is always the same."
