This apparent war of the sea upon the land is a war of machinery whose workings are curiously like the ancient war machinery of men. Without tools the sea is almost as helpless as man himself; and, as in man's history, its use of tools begins with the Stone Age. Where there is no stone-strewn beach or underwater shelf extending out from a cliff, the waves do little damage. They give only a muffled and (to the poetic ear) a baffled roar. But a sloping shelf along a rocky shore not only makes a kind of scaling ladder on which the waves can climb to great heights, but these waves are pitched forward with terrific force as they reach it from the open sea. As they come on they seize huge stones which they hurl against the cliffs. Even amid the wild voices of tempests one hears the boulders crashing against the walls. In storms of sufficient energy rocks of three tons weight are driven forward like pebbles. The action against the upper part of a cliff may be compared to that of one of those great stone-throwing engines of the Romans, while on the lower portion the drive suggests the battering-ram. WHAT NEPTUNE KNOWS ABOUT WEDGES AND PNEUMATIC TOOLS Where the waves strike into narrowing crevices in the rocks they act as wedges, prying the walls apart. In this form of the sea's destructive work we find also an application of a motive power which has come to play so important a part in modern engineering; namely, compressed air. Waves strong enough to handle big rocks not only dash them against the cliff, while the waves themselves drive into the crevices like wedges, but in so doing they force air into the crevices and compress it. This air, expanding as the waves fall back, forces out great blocks of stone which, in turn, are also used as weapons of assault. And, as we look back in the history of the sea, we find that he long ago--the deep-laid schemer!--planted enemies within our very walls. Waves, even when armed with the heaviest missiles, can do comparatively little damage to walls in which there are no crevices. But there are few such walls. Usually even the hardest rocks have running through them those cracks which the geologists (with a fine sense of humor) call "joints"; or they have "bedding planes," the divisions between the rock beds. Both of these weaknesses in our defensive walls are, in a large degree, the handiwork of the sea; the bedding planes because rocks are so laid in the sea mills, and the joints because the wrinkling up and consequent cracking of the land rocks is the other end, as we learned in Chapter I, of the down-wrinkling of the rocks under the weight of the sea.