If we heat an open jar there is no pressure developed, since the air simply expands and flows out of the neck. But the air that remains in the jar, being less in quantity than when it was not yet heated, weighs less, though occupying the same space as before. If we took a very thin bladder and filled it with hot air it would therefore float in colder air, proving that heated air, as we should expect, tends to rise. The fire-balloon employs this principle, the air inside the bag being kept artificially warm by a fire burning in some vessel attached below the open neck of the bag.
Now, the sun shines with different degrees of heating power at different parts of the world. Where its effect is greatest the air there is hottest. We will suppose, for the sake of argument, that, at a certain moment, the air envelope all round the globe is of equal temperature. Suddenly the sun shines out and heats the air at a point, A, till it is many degrees warmer than the surrounding air. The heated air expands, rises, and spreads out above the cold air. But, as a given depth of warm air has less weight than an equal depth of cold air, the cold air at once begins to rush towards B and squeeze the rest of the warm air out. We may therefore picture the atmosphere as made up of a number of colder currents passing along the surface of the earth to replace warm currents rising and spreading over the upper surface of the cold air. A similar circulation takes place in a vessel of heated water ([see p. 17]).
LAND AND SEA BREEZES.
A breeze which blows from the sea on to the land during the day often reverses its direction during the evening. Why is this? The earth grows hot or cold more rapidly than the sea. When the sun shines hotly, the land warms quickly and heats the air over it, which becomes light, and is displaced by the cooler air over the sea. When the sun sets, the earth and the air over it lose their warmth quickly, while the sea remains at practically the same temperature as before. So the balance is changed, the heavier air now lying over the land. It therefore flows seawards, and drives out the warmer air there.
LIGHT AIR AND MOISTURE.
Light, warm air absorbs moisture. As it cools, the moisture in it condenses. Breathe on a plate, and you notice that a watery film forms on it at once. The cold surface condenses the water suspended in the warm breath. If you wish to dry a damp room you heat it. Moisture then passes from the walls and objects in the room to the atmosphere.
THE BAROMETER.
This property of air is responsible for the changes in weather. Light, moisture-laden air meets cold, dry air, and the sudden cooling forces it to release its moisture, which falls as rain, or floats about as clouds. If only we are able to detect the presence of warm air-strata above us, we ought to be in a position to foretell the weather.
We can judge of the specific gravity of the air in our neighbourhood by means of the barometer, which means "weight-measurer." The normal air-pressure at sea-level on our bodies or any other objects is about 15 lbs. to the square inch—that is to say, if you could imprison and weigh a column of air one inch square in section and of the height of the world's atmospheric envelope, the scale would register 15 lbs. Many years ago (1643) Torricelli, a pupil of Galileo, first calculated the pressure by a very simple experiment. He took a long glass tube sealed at one end, filled it with mercury, and, closing the open end with the thumb, inverted the tube and plunged the open end below the surface of a tank of mercury. On removing his thumb he found that the mercury sank in the tube till the surface of the mercury in the tube was about 30 inches in a vertical direction above the surface of the mercury in the tank. Now, as the upper end was sealed, there must be a vacuum above the mercury. What supported the column? The atmosphere. So it was evident that the downward pressure of the mercury exactly counterbalanced the upward pressure of the air. As a mercury column 30 inches high and 1 inch square weighs 15 lbs., the air-pressure on a square inch obviously is the same.