The Sea Around Us (24 page)

If the history of the earth's tides should one day be written by some observer of the universe, it would no doubt be said that they reached their greatest grandeur and power in the younger days of Earth, and that they slowly grew feebler and less imposing until one day they ceased to be. For the tides were not always as they are today, and as with all that is earthly, their days are numbered.

In the days when the earth was young, the coming in of the tide must have been a stupendous event. If the moon was, as we have supposed in an earlier chapter, formed by the tearing away of a part of the outer crust of the earth, it must have remained for a time very close to its parent. Its present position is the consequence of being pushed farther and farther away from the earth for some 2 billion years. When it was half its present distance from the earth, its power over the ocean tides was eight times as great as now, and the tidal range may even then have been several hundred feet on certain shores. But when the earth was only a few million years old, assuming that the deep ocean basins were then formed, the sweep of the tides must have been beyond all comprehension. Twice each day, the fury of the incoming waters would inundate all the margins of the continents. The range of the surf must have been enormously extended by the reach of the tides, so that the waves would batter the crests of high cliffs and sweep inland to erode the continents. The fury of such tides would contribute not a little to the general bleakness and grimness and uninhabitability of the young earth.

Under such conditions, no living thing could exist on the shores or pass beyond them, and, had conditions not changed, it is reasonable to suppose that life would have evolved no further than the fishes. But over the millions of years the moon has receded, driven away by the friction of the tides it creates. The very movement of the water over the bed of the ocean, over the shallow edges of the continents, and over the inland seas carries within itself the power that is slowly destroying the tides, for tidal friction is gradually slowing down the rotation of the earth. In those early days we have spoken of, it took the earth a much shorter time—perhaps only about 4 hours—to make a complete rotation on its axis. Since then, the spinning of the globe has been so greatly slowed that a rotation now requires, as everyone knows, about 24 hours. This retarding will continue, according to mathematicians, until the day is about 50 times as long as it is now.

And all the while the tidal friction will be exerting a second effect, pushing the moon father way, just as it has already pushed it out more than 200,000 miles. (According to the laws of mechanics, as the rotation of the earth is retarded, that of the moon must be accelerated, and centrifugal force will carry it farther away.) As the moon recedes, it will, of course, have less power over the tides and they will grow weaker. It will also take the moon longer to complete its orbit around the earth. When finally the length of the day and of the month coincide, the moon will no longer rotate relatively to the earth, and there will be no lunar tides.

All this, of course, will require time on a scale the mind finds it difficult to conceive, and before it happens it is quite probable that the human race will have vanished from the earth. This may seem, then, like a Wellsian fantasy of a world so remote that we may dismiss it from our thoughts. But already, even in our allotted fraction of earthly time, we can see some of the effects of these cosmic processes. Our day is believed to be several seconds longer than that of Babylonian times. Britain's Astronomer Royal recently called the attention of the American Philosophical Society to the fact that the world will soon have to choose between two kinds of time. The tide-induced lengthening of the day has already complicated the problems of human systems of keeping time. Conventional clocks, geared to the earth's rotation, do not show the effect of the lengthening days. New atomic clocks now being constructed will show actual time and will differ from other clocks.

Although the tides have become tamer, and their range is now measured in tens instead of hundreds of feet, mariners are nevertheless greatly concerned not only with the stages of the tide and the set of the tidal currents, but with the many violent movements and disturbances of the sea that are indirectly related to the tides. Nothing the human mind has invented can tame a tide rip or control the rhythm of the water's ebb and flow, and the most modern instruments cannot carry a vessel over a shoal until the tide has brought a sufficient depth of water over it. Even the
Queen Mary
waits for slack water to come to her pier in New York; otherwise the set of the tidal current might swing her against the pier with enough force to crush it. On the Bay of Fundy, because of the great range of tide, harbor activities in some of the ports follow a pattern as rhythmic as the tides themselves, for vessels can come to the docks to take on or discharge cargo during only a few hours on each tide, leaving promptly to avoid being stranded in mud at low water.

In the confinement of narrow passages or when opposed by contrary winds and swells, the tidal currents often move with uncontrollable violence, creating some of the most dangerous waterways of the world. It is only necessary to read the Coast Pilots and Sailing Directions for various parts of the world to understand the menace of such tidal currents to navigation.

‘Vessels around the Aleutians are in more danger from tidal currents than from any other cause, save the lack of surveys,' says the postwar edition of the
Alaska Pilot.
Through Unalga and Akutan passes, which are among the most-used routes for vessels entering Bering Sea from the Pacific, strong tidal currents pour, making their force felt well offshore and setting vessels unexpectedly against the rocks. Through Akun Strait the flood tide has the velocity of a mountain torrent, with dangerous swirls and overfalls. In each of these passes the tide will raise heavy, choppy seas if opposed by wind or swells. ‘Vessels must be prepared to take seas aboard,' warns the
Pilot,
for a 15-foot wave of a tide rip may suddenly rise and sweep across a vessel, and more than one man has been carried off to his death in this way.

On the opposite side of the world, the tide setting eastward from the open Atlantic presses between the islands of the Shetlands and Orkneys into the North Sea, and on the ebb returns through the same narrow passages. At certain stages of the tide these waters are dotted with dangerous eddies, with strange upward domings, or with sinister pits or depressions. Even in calm weather boats are warned to avoid the eddies of Pentland Firth, which are known as the Swilkie; and with an ebb tide and a northwest wind the heavy breaking seas of the Swilkie are a menace to vessels ‘which few, having once experienced, would be rash enough to encounter a second time.'

Edgar Allan Poe, in his ‘Descent into the Maelstrom,' converted one of the more evil manifestations of the tide into literature. Few who have read the story will forget its drama—how the old man led his companion to a mountain cliff high above the sea and let him watch the water far below in the narrow passageway between the islands, with its sinister foam and scum, its uneasy bubbling and boiling, until suddenly the whirlpool was formed before his eyes and rushed with an appalling sound through the narrow waterway. Then the old man told the story of his own descent into the whirlpool and of his miraculous escape. Most of us have wondered how much of the story was fact, how much the creation of Poe's fertile imagination. There actually is a Maelstrom and it exists where Poe placed it, between two of the islands of the Lofoten group off the west coast of Norway. It is, as he described it, a gigantic whirlpool or series of whirlpools, and men with their boats have actually been drawn down into these spinning funnels of water. Although Poe's account exaggerates certain details, the essential facts on which he based his narrative are verified in the
Sailing Directions for the Northwest and North Coasts of Norway,
a practical and circumstantial document:

Though rumor has greatly exaggerated the importance of the Malström, or more properly Moskenstraumen, which runs between Mosken and Lofotodden, it is still the most dangerous tideway in Lofoten, its violence being due, in great measure, to the irregularity of the ground … As the strength of the tide increases the sea becomes heavier and the current more irregular, forming extensive eddies or whirlpools (Malström). During such periods no vessel should enter the Moskenstraumen.

These whirlpools are cavities in the form of an inverted bell, wide and rounded at the mouth and narrower toward the bottom; they are largest when first formed and are carried along with the current, diminishing gradually until they disappear; before the extinction of one, two or three more will appear, following each other like so many pits in the sea … Fishermen affirm that if they are aware of their approach to a whirlpool and have time to throw an oar or any other bulky body into it they will get over it safely; the reason is that when the continuity is broken and the whirling motion of the sea interrupted by something thrown into it the water must rush suddenly in on all sides and fill up the cavity. For the same reason, in strong breezes, when the waves break, though there may be a whirling round, there can be no cavity. In the Saltström boats and men have been drawn down by these vortices, and much loss of life has resulted.

Among unusual creations of the tide, perhaps the best known are the bores. The world possesses half a dozen or more famous ones. A bore is created when a great part of the flood tide enters a river as a single wave, or at most two or three waves, with a steep and high front. The conditions that produce bores are several: there must be a considerable range of tide, combined with sand bars and other obstructions in the mouth of the river, so that the tide is hindered and held back, until it finally gathers itself together and rushes through. The Amazon is remarkable for the distance its bore travels upstream—some 200 miles—with the result that the bores of as many as 5 flood tides may actually be moving up the river at one time.

On the Tsientang River, which empties into the China Sea, all shipping is controlled by the bore—the largest, most dangerous, and best known in the world. The ancient Chinese used to throw offerings into the river to appease the angry spirit of this bore, whose size and fury appear to have varied from century to century, or perhaps even from decade to decade, as the silting of the estuary has shifted and changed. During most of the month the bore now advances up the river in a wave 8 to 11 feet high, moving at a speed of 12 to 13 knots, its front ‘a sloping cascade of bubbling foam, falling forward and pounding on itself and on the river.' Its full ferocity is reserved for the spring tides of the full moon and the new moon, at which times the crest of the advancing wave is said to rise 25 feet above the surface of the river.

There are bores, though none so spectacular, in North America. There is one at Moncton, on New Brunswick's Petitcodiac River, but it is impressive only on the spring tides of the full or new moon. At Turnagain Arm in Cook Inlet, Alaska, where the tides are high and the currents strong, the flood tide under certain conditions comes in as a bore. Its advancing front may be four to six feet high and is recognized as being so dangerous to small craft that boats are beached well above the level of the flats when the bore is approaching. It can be heard about half an hour before its arrival at any point, traveling slowly with a sound as of breakers on a beach.

The influence of the tide over the affairs of sea creatures as well as men may be seen all over the world. The billions upon billions of sessile animals, like oysters, mussels, and barnacles, owe their very existence to the sweep of the tides, which brings them the food which they are unable to go in search of. By marvelous adaptations of form and structure, the inhabitants of the world between the tide lines are enabled to live in a zone where the danger of being dried up is matched against the danger of being washed away, where for every enemy that comes by sea there is another that comes by land, and where the most delicate of living tissues must somehow withstand the assault of storm waves that have the power to shift tons of rock or to crack the hardest granite.

The most curious and incredibly delicate adaptations, however, are the ones by which the breeding rhythm of certain marine animals is timed to coincide with the phases of the moon and the stages of the tide. In Europe it has been well established that the spawning activities of oysters reach their peak on the spring tides, which are about two days after the full or the new moon. In the waters of northern Africa there is a sea urchin that, on the nights when the moon is full and apparently only then, releases its reproductive cells into the sea. And in tropical waters in many parts of the world there are small marine worms whose spawning behavior is so precisely adjusted to the tidal calendar that, merely from observing them, one could tell the month, the day, and often the time of day as well.

Near Samoa in the Pacific, the palolo worm lives out its life on the bottom of the shallow sea, in holes in the rocks and among the masses of corals. Twice each year, during the neap tides of the moon's last quarter in October and November, the worms forsake their burrows and rise to the surface in swarms that cover the water. For this purpose, each worm has literally broken its body in two, half to remain in its rocky tunnel, half to carry the reproductive products to the surface and there to liberate the cells. This happens at dawn on the day before the moon reaches its last quarter, and again on the following day; on the second day of the spawning the quantity of eggs liberated is so great that the sea is discolored.

The Fijians, whose waters have a similar worm, call them ‘Mbalolo' and have designated the periods of their spawning ‘Mbalolo lailai' (little) for October and ‘Mbalolo levu' (large) for November. Similar forms near the Gilbert Islands respond to certain phases of the moon in June and July; in the Malay Archipelago a related worm swarms at the surface on the second and third nights after the full moon of March and April, when the tides are running highest. A Japanese palolo swarms after the new moon and again after the full moon in October and November.