But wind is not alone a carrier for other dust-makers; it has dust mills of its own. The greatest of these mills are away off among the mountains and in desert lands, but after making it in these distant factories the winds carry much of this fresh new soil material to lands of orchard and pasture and growing grain.
Not long ago two of the professors at the University of Wisconsin found a good illustration of what an immense amount of soil is distributed in this way, and what long distances it travels. Among the weather freaks of a March day was a fall of colored snow that, it was found, covered an area of 100,000 square miles, probably more. The color on the snow was made by dust blown clear from the dry plains of the Southwestern States, a thousand miles away. The whole of this dust amounted to at least a million tons; and may even have amounted to hundreds of millions of tons, so the professors think.
TYPES OF NATURE'S SCREW PROPELLERS
You can see for yourself (from the picture on the left) that long before man ever thought of driving his ships through the water with screw propellers or pulling his flying machines through the air by the whirligigs on the end of their noses, some flying seeds, such as those of the ash here, had screw propellers of their own. And do you know that Nature also employs the propeller principle, not only in the operation of the wings of birds but in the wing feathers themselves? The two pictures on the right show the action of the wing and the wing feathers when a bird is in flight.
LITTLE MILLSTONES IN BIG BUSINESS
For grinding rocks to get out ore, or for making cement in cement mills, men use big machines, somewhat on the style of a coffee-mill. These machines are called "crushers." The winds, in their enormous business of soil-grinding, however, stick to the idea you see so much in Nature, that of using little things to do big tasks; as in digging canyons and river beds, and spreading out vast alluvial plains by using raindrops made up into rivers; in working the wonders of the Ice Ages with snowflakes; and building the bones and bodies of those big early settlers, and of all animal life, and the giant trees of the forest out of little cells. For, what do you suppose the winds take for millstones in grinding down the mountains into dust? Little grains of sand!
And with the help of the sun and Jack Frost it makes these fairy millstones for itself. The outside of a big rock grows bigger under the warm sun, in the daytime, and then when the sun goes down and the rock cools off it shrinks, and this spreading and shrinking movement keeps cracking up and chipping off pieces of rock of various sizes. Up on the mountain tops, among the peaks, the change of temperature between night and day is very great, and even in midsummer you can always hear a rattling of stones at sunrise. The heat of the rising sun warms and expands the rock, and so loosens the pieces that Jack Frost has pried off with his ice wedges during the night.
Then also during periods of alternate freezing and thawing in Spring and Fall, the rock is slivered up. These changes in the weather as between one day and another are due to the winds. In January and February, for example, thaws and freezes are common. When the winds blow from the south, the snow melts, water runs into cracks in the rock and fills their pores; then a shift of the winds to the north, a freeze, and the water in the crevices and the pores turns to ice, expands, and breaks off more rock.