trade winds, blowing towards the equator, is that the sun is very
hot at the equator, and hot air is always rising there and making
room for colder air to rush in. We have not time to travel
farther with the moving air, though its journeys are extremely
interesting; but if, when you read about the trade and other
winds, you will always picture to yourselves warm air made light
by the heat rising up into space and cold air expanding and
rushing in to fill its place, I can promise you that you will not
find the study of aerial currents so dry as many people imagine
it to be.

We are now able to form some picture of our aerial ocean. We can
imagine the active atoms of oxygen floating in the sluggish
nitrogen, and being used up in every candle-flame, gas-jet and
fire, and in the breath of all living beings; and coming out
again tied fast to atoms of carbon and making carbonic acid.
Then we can turn to trees and plants, and see them tearing these
two apart again, holding the carbon fast and sending the
invisible atoms of oxygen bounding back again into the air, ready
to recommence work. We can picture all these air-atoms, whether
of oxygen or nitrogen, packed close together on the surface of
the earth, and lying gradually farther and farther apart, as they
have less weight above them, till they become so scattered that
we can only detect them as they rub against the flying meteors
which flash into light. We can feel this great weight of air
pressing the limpet on to the rock; and we can see it pressing up
the mercury in the barometer and so enabling us to measure its
weight. Lastly, every breath of wind that blows past us tells us
how this aerial ocean is always moving to and fro on the face of
the earth; and if we think for a moment how much bad air and bad