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 mercu