But there are other forces at work. In the first place, sudden changes of temperature have a destructive influence. If the sun shines brightly by day, the rocks--especially in higher mountain regions--are considerably expanded by the heat they receive; and if a hot day is followed by a clear sky at night, the free radiation of heat into space (see chap. ii., p. 39) causes them to become very cold, and in cooling down they contract. In this way an internal strain is set up which is often greater than they can bear, and so they split and crack. Thus small pieces of rock are detached from a mountain-side. An Alpine traveller told the writer that one night when sleeping on a mountain-side, he heard stones rattling down at frequent intervals. Livingstone records in his journal that when in the desert he frequently heard stones splitting at night with a report like that of a pistol. But sometimes the expansion by day is sufficient to cause fragments of rock to be broken off. Frost, however, is responsible for a vast amount of destruction among rocks. When water freezes, it expands with tremendous force; and this is the reason why water-pipes so frequently burst during a frost, though we don't find it out until the thaw comes,--followed by long plumbers' bills. Rocks, being traversed in several directions by cracks, allow the water to get into them, and this in freezing acts like a very powerful wedge; and so the rocks on the higher parts of the mountains are continually being split up by Nature's ice-wedge. The amount of rock broken up in this way every year is enormous. Stone walls and buildings often suffer greatly from this cause during a long frost, especially if the stone be of a more than usually porous kind, that can take up a good deal of rain water. Where trees, shrubs, etc., grow on rocks, the roots find their way into its natural divisions, widened by the action of rain soaking down into them; and as they grow, they slowly widen them, and in time portions are actually detached in this manner. Moreover, the roots and rootlets guide the rain water down into the cracks, or joints, as they are called. Even the ivy that creeps over old ruined walls has a decidedly destructive effect. At the base of every steep mountain may be seen heaps of loose angular stones; sometimes these are covered with soil, and form long slopes on which trees and shrubs grow. Every one of the numerous little gullies that furrow the mountain-sides has at its lower end a similar little heap of stones. Sometimes a valley among the mountains seems half choked with rocky fragments; and if these were all removed, the valley would be deeper than it is. In some hot countries, where the streams only flow in winter, this is especially the case; for example, every valley, or "wady," in the region of Mount Sinai and Mount Horeb is more or less choked up with boulders and stones of every size, because the stones come down faster than they can be carried away.