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. And what muscles Jack has! Freezing water exerts a pressure of 138 tons to the square foot; so there's no holding out against him once he gets his ice wedges in a good crack. He sends huge blocks tumbling down the mountainside. The larger blocks, striking against one another, break off smaller fragments. The smallest fragments the wind seizes. Others are washed down by the rains. The largest, carried away by mountain torrents, bump together as they thunder along, and so break off more fragments and grind them so small that the wind can pick them up along the banks when the torrents shrink, or in their beds when these sudden streams go dry. RUNNING WATER AND THE WINDS