Courtesy of "The Scientific American"
BEHIND THE SCENES
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 [Chagter I], of the down-wrinkling of the rocks under the weight of the sea.
In the very body of the rocks also is hidden a secret enemy; the salt left when they were made. And more salt is constantly being forced into the surface pores as the waves strike. This salt helps to dissolve and weaken the rock under the chemical action of the air, and the rains and the mechanical expansion and contraction of the surface with changes of temperature.
PLANING MILLS OF THE WINTER SEA
All the Great Powers of nature, "on land, on sea, and in the air," seem to be in open conspiracy against our peace. The evidence seems especially plain in late fall and winter, when the sea, contrary to the usual practice in war, carries on its most vigorous campaigns. Then come the winds for the great drives; then come the frosts that change the water wedges into expanding blocks of ice that, almost with the force of exploding shells, tear the walls apart. In winter are formed the great ice-fields that help in two ingenious ways to further the destructive action of the storm waves. In bays and smaller recesses in rocky shores, the ice has embedded in it fragments of stone which the sea has battered down. The constant plunge of the waves breaks up these ice-fields into sections which, with the embedded stones, become rude planing mills. Where a headland is sloping, these planers, driven back and forth by the waves, chisel the rock away as a planer chisels down a piece of steel upon which it has been set to work.
HOW STONES ARE CARRIED OUT TO SEA