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 severaclass="underline" 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.