[Illustration: THE WEST WINDS AND THE RAINS
On the western slopes of this mountain the trees, with the help of the winds and the rain, climb to the very summit, while the other side of the mountain remains only a barren rock. The moisture-laden winds from the west glide up the slope, the air expands as it rises, the expansion cools it and down comes the rain! But the eastern slope gets little or none of it.]
The higher Nilus swells The more it promises; as it ebbs, the seedsman Upon the slime and ooze scatters his grain, And shortly comes the harvest.
All that wind was bound to blow up rain. I said so at the time. And, sure enough, here it is; right where we want it, at the beginning of April, a month famous for its rains.
The work of the rains is going to make one of the most interesting chapters in the long story of the dust. At least I hope so. But don't think I intend to tell it all. Why, it would make a whole book in itself. But you can believe every single thing I do tell, no matter how it makes you open your eyes; for, if I've helped it rain once I've helped it rain a million times!
It's this way: You remember how you can "see your breath," as we say, on a cold morning? Well, that's because the moisture in your breath is condensed by the cold. Now as the waters of the earth--the seas, lakes, rivers, ponds, and so on--are warmed by the sun, the air above them is filled with moisture, for the heating of the air causes it to expand and draw in moisture from the water like a sponge. Expansion makes it lighter also, and it rises. Rising, it turns cooler, and the moisture condenses and comes down as rain. Mountains usually have clouds around them because moist air striking the mountainside is driven up the slope, cooling as it rises. So rain and snow fall often in mountain regions, and that's why so many rivers rise in mountains. The moist air is also condensed when it meets other and cooler air currents. But right here is where the work of the dust comes in. For to make rain you've got to have clouds, and clouds are due to this moisture collecting around the little particles of dust of which the air is full. When these little motes of matter become cooler than the air that touches them the moisture in the air condenses into a film of water around them. Fairy worlds with fairy oceans floating in the sky!