In changing rock into soil, running water and the winds each have an advantage over the other. Water weighs a great deal more than air--over 800 times as much--and so grinds faster with its tools of pebbles and sand. The winds, on the other hand, get over a great deal more territory, and they, like the lichens, understand chemistry. Two of the gases they always carry right with them--carbon dioxide and oxygen--help decay the rocks. As I said, the winds do most work in dry and desert regions, but when you remember that over a fifth of the globe is just that--dry as a bone most of the time--you see this is a great field. It has been so from the beginning, for it is thought probable that there was always about the same proportion of desert lands. Night and day the winds have been busy through all these ages. Dust is carried up by ascending air currents. Then the same force that keeps the earth in its orbit--gravity--pulls down on a grain of dust. But its fall is checked by the friction of the air. You see there's a lot of mechanics involved in moving a grain of dust; and Nature goes about it as if it were the most serious business in the world; handles every grain as if the future of the universe depended on it. In the case of sand or coarse dust, unless the winds are very strong, gravity soon gets the best of it, and down the dust grain comes to the ground again; then up with another current, then down again--carried far by stiff breezes, only a short distance by puffs--a kind of hop, skip, and jump. But fine dust getting a good lift into the upper currents at the start may stay in the air for weeks. [Illustration: _Courtesy of The Dunham Company._ TO KEEP MOISTURE AND SOIL AT HOME In the broad fields of the West, where "dry-farming" is practised, they have these huge machines. They are called "Cultipackers." They are cultivators with big, broad-brimmed wheels that pack the surface of the soil after the blades of the cultivator have stirred it. This not only prevents the moisture in the soil from evaporating as fast as it would otherwise do, but keeps the winds from carrying away the soil itself.] In very wild wind-storms it has been figured out that there may be as much as 126,000 tons of dust per cubic mile; several good farms in the air at once, over every square mile of the earth below! III. THE STORM PLOUGHS OF THE WIND TWO KINDS OF WOODEN PLOUGHS They use wooden ploughs, these winds, just as primitive man did, and as primitive peoples do now; but not quite in the same way, and the ploughing they do is much better. For man's wooden plough is a crooked stick made from the branches of a tree while the winds use the whole tree--roots and all, and both on mountainsides and on level lands the amount of ploughing they do is immense.