would it go? What forms will it take before it reappears in the
rain-cloud, the river, or the sparkling dew?

These are questions we are going to try to answer to-day; and
first, before we can in the least understand how water travels,
we must call to mind what we have learnt about the sunbeams and
the air. We must have clearly pictured in our imagination those
countless sun-waves which are for ever crossing space, and
especially those larger and slower undulations, the dark heat-
waves; for it is these, you will remember, which force the air-
atoms apart and make the air light, and it is also these which
are most busy in sending water on its travels. But not these
alone. The sun-waves might shake the water-drops as much as they
liked and turn them into invisible vapour, but they could not
carry them over the earth if it were not for the winds and
currents of that aerial ocean which bears the vapour on its
bosom, and wafts it to different regions of the world.

Let us try to understand how these two invisible workers, the
sun-waves and the air, deal with the drops of water. I
have here a kettle (Fig. 18, p. 76) boiling over a spirit-lamp,
and I want you to follow minutely what is going on in it. First,
in the flame of the lamp, atoms of the spirit drawn up from below
are clashing with the oxygen-atoms in the air. This, as you know,
causes heat-waves and light-waves to move rapidly all round the
lamp. The light-waves cannot pass through the kettle, but the
heat-waves can, and as they enter the water inside they agitate
it violently. Quicker, and still more quickly, the particles of
water near the bottom of the kettle move to and fro and are
shaken apart; and as they become light they rise through the