We have now seen our drop of water under all its various forms of
invisible gas, visible steam, cloud, dew, hoar-frost, snow, and
ice, and we have only time shortly to see it on its travels, not
merely up and down, as hitherto, but round the world.

We must first go to the sea as the distillery, or the place from
which water is drawn up invisibly, in its purest state, into the
air; and we must go chiefly to the seas of the tropics, because
here the sun shines most directly all the year round, sending
heat-waves to shake the water-particles asunder. It has been
found by experiment that, in order to turn 1 lb. of water into
vapour, as much heat must be used as is required to melt 5 lbs.
of iron; and if you consider for a moment how difficult iron is
to melt, and how we can keep an iron poker in a hot fire and yet
it remains solid, this will help you to realize how much heat the
sun must pour down in order to carry off such a constant supply
of vapour from the tropical seas.

Now, when all this vapour is drawn up into the air, we know that
some of it will form into clouds as it gets chilled high up in
the sky, and then it will pour down again in those tremendous
floods of rain which occur in the tropics.

But the sun and air will not let it all fall down at once, and
the winds which are blowing from the equator to the poles carry
large masses of it away with them. Then, as you know, it will
depend on many things how far this vapour is carried. Some of it,
chilled by cold blasts, or by striking on cold mountain tops, as
it travels northwards, will fall in rain in Europe and Asia, while