tells about what ploughing means to the soil and also:

Why good soil takes up more room than poor.

Why it is a good thing to plough deep, but a bad thing, if you don't do it just right.

And farther on there is a most inspiring poem about the history of the plough from the days of early Egypt to the present. It begins like this:

"From Egypt behind my oxen, With their stately step and slow, Northward and east and west I went, To the desert and the snow; Down through the centuries, one by one, Turning the clod to the shower, Till there's never a land beneath the sun But has blossomed behind my power."

The deserts have helped to make western China fertile. How did they do it? (Look at your geography map and remember that the prevailing winds of the world are westerly.)

You'll find many interesting things about the winds and the soil in Keffer's "Nature Studies on the Farm" and Shaler's "Outlines of Earth's History." Shaler's "Man and the Earth" says a single gale may blow away more soil from an unprotected field than could be made in a geological age, and an hour's rain may carry off more than would pa.s.s away in a thousand years if the land were in its natural state. He also tells what to do to prevent the best part of ploughed fields from being carried off by the wind.

Have you any idea how far seed may be carried by a hurricane?

Wallace, in his "Darwinism" deals with this question, and it's very important in the story of the earth. Beal's admirably written and ill.u.s.trated little book on "Seed Dispersal" tells a world of interesting things about the wind as a sower. For instance:

How pigweed seeds are built so that wind can help them toboggan on snow or float on water;

How wind and water work together in the distribution of seeds;

About seeds that ride in an ice-boat;

About the monoplane of the ba.s.swood;

About the "flail" of the b.u.t.tonwood, and how the wind helps it to whip out the seeds; and how the seeds then open their parachutes.

Dandelions go through quite a remarkable process in preparing for flight. I wonder if you have ever noticed it. Before the seeds get ripe Mother Dandelion blankets them at night and puts a rain-cloak on them on rainy days, and just won't let them get out, as shown on page 51. And do you know how she opens the flowers for the bees on sunshiny days?

There is no island, no matter how remote, that isn't supplied with insects. How do you suppose they get there? You may be sure the wind has something to do with it or I wouldn't mention the subject at the end of this chapter. (Wallace: "Darwinism.")

[Ill.u.s.tration: 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.]

CHAPTER IV

(APRIL)

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.

--_Shakespere: "Antony and Cleopatra."_

THE BOTTOM-LANDS

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!

I. THE MARCH DUST AND THE APRIL RAINS

HOW RAIN GOES UP BEFORE IT COMES DOWN

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!

Each of these baby worlds is falling toward the big world below. But very slowly; only a few feet a day, so that even if nothing happened it might be months--yes, years--before it would come to the ground, even in still air. But when air is very thick with moisture the water films on these dust particles grow rapidly, and thus increasing in weight, they fall faster and faster, and finally strike the earth as raindrops.

But here's another thing that helps. On the way down two or more raindrops, falling in with each other, will go into partnership--melt into one--and then they hurry down so much the faster. That's why the sky grows darker and darker just before a rain, and why the lower part of a rain-cloud is the darkest: the little raindrops are forming into bigger raindrops as they fall.

THE LITTLE ARTISTS THAT SHAPE THE CLOUDS

But the shapes of clouds are supposed to be due to another thing, the mysterious force we call electricity, and that other mysterious force we call gravity. Just as the worlds attract each other by gravity so these raindrops--or dust grains growing into raindrops--are drawn toward one another. Here's where Electricity steps in. These rain particles are full of electricity and when two of these electrified particles meet in the air--unless they strike one another in falling, in which case, as I said a moment ago, they blend into one--they get very close together and yet keep dancing around one another without touching! It is this dancing about that makes all those strange and beautiful and ever-changing forms in the vast picture-gallery of the sky.

Of course the wind currents help to change these shapes, but I'm talking about the original designs.

II. THE RAINDROPS AND THE RIVER MILLS

So much for the dust that helps make raindrops; now for the raindrops that help make dust. This the raindrops do in several ways. Falling on big rocks or decaying pebbles, for example, they pound loose with their patter, patter, patter, any little bits of soil and grains of sand that have been made by the other soil makers--the sun, the wind, the lichens, the chemists of the air, and so on. This soil and these sand particles, if there is already any depth of earth there, they carry down into the ground. Some of this soil, with various stops and mixings with other soils on the way, finally reaches the sea, where it helps to make the rich limestone soils for the Kentuckies of millenniums yet to be, by supplying food for sea creatures and lime for their sh.e.l.ls. For these sh.e.l.ls become limestone when the sh.e.l.l-fish are through with them.

Mother Nature, in addition to feeding her big, hungry families of to-day in the plant and animal world, is always laying by something for the future. But before it gets back to the sea, by far the greatest part of the ground-up soil the rivers carry is spread out in the lowlands in those "alluvial plains" your geography tells about and that make a large proportion of the fertile farms of the world. If the raindrops fall on comparatively barren rock--in the mountains, say--they carry some of this fresh soil to the mountain valleys below, and some of it they may spread in bottom-lands a thousand miles away, where the new soil helps feed the plants. The sand grains in it not only help the soil to get its breath by making little air s.p.a.ces, but these sand grains themselves slowly decay and so make more soil.

[Ill.u.s.tration: WHAT IRRIGATION DOES FOR DESERTS

It is such land as this, in the arid regions of the West, that irrigation converts from a desert to a garden of abundance. The soil is rich in all the substances that plant life needs.]

But it isn't alone that they carry away the soil already made and bury the sand grains. Some of the raindrops soak into cracks in stones and dissolve the material that binds the rock particles together, and so get them ready to give way under the fairy hammers of the next shower that comes along.

After Nature finally gets an original waste of barren rock all nicely set with gra.s.s and flowers and trees and things, the raindrops help to make soil in still another way. Soaking through the decaying leaves, they pick up acids which are just the thing for eating into rock and crumbling it into soil. To be sure, the water soaking into the soil and coming out of springs carries some plant food away with it; but it takes it to lands farther down the river valleys, and more than makes up for what it carries away by the new soil made by its acids from the rocks, as it soaks into their pores and runs among the cracks.

HOW RAINDROPS MANAGE TO GRIND UP THE ROCKS

Moreover, raindrops actually grind up rocks. In order to do this a lot of raindrops have to get together, to be sure, and become rivers; but after all it's the raindrops that do it. There'd never be any rivers if it weren't for the rains and, of course, the snows.

Well, anyhow, the rivers, besides running other people's mills, have mills of their own; and millstones. Most of these stones originally came from mountains and were brought into the milling business by mountain streams, with the help of Jack Frost. For the frost not only pries stones from the mountains and so sends them tumbling down the slopes, but it keeps edging them along and edging them along, farther down, after they have fallen. You'd hardly think that, would you? Yet it's simple enough. The water in the pores of the rock expands when it freezes and that makes the whole rock expand, for the time being. Then when the frozen water in the rock pores thaws out, the rock contracts, and this spreading out and pulling together, small as it is, causes the rock to keep hitching along down the incline; oh, say a fraction of an inch a year. But still, in the course of the ages, these inches foot up, and after a while this tortoise-like gait lands the stone--lands tens of thousands of such stones--in the beds of the mountain torrents that run along at the bottom of these inclines. There they get ground together and so grind out more soil material, particularly when the floods are on, with the melting of the snows in spring and the falling of the heavy and frequent rains.

[Ill.u.s.tration: AN OLD RIVER MILL

It used to do a lot of business--this old river mill. Its grist was ground-up rock that helped make fine farming land in the bottoms along the river's course. Such mills, called "pot holes," are found in the rocky floors of rapid streams, where the eddying current or the water of a waterfall wears depressions in the bed. Into these depressions stones are washed, and then by the whirl of the flowing water kept going round and round, grinding themselves away and grinding out the sides and bottom of the mill.]