Charting The Greatest River In The World*

It is perfectly logical to speak of the Atlantic Ocean as the greatest river in the world. It forms a gigantic pool with the Sargasso Sea roughly in the center, around which seventy-five million tons of water per second is transported in clockwise fashion. This cold mathematical statistic hides many meanings. Compared to the Mississippi in flood, the daily flow of the River is nearly one thousand times as great.

Even more important than its size, however, is its function. The circulation of this vast river of warm tropical waters, linked as it is with the North Atlantic climate, has affected both the history and the present pattern of western civilization. Life-giving heat equivalent to the absorption of three million square miles of ocean surface at the equator is carried in constant battle against the encroachment of Arctic waters upon northern Europe.

In his report on the Gulf Stream, written in 1890, Lieutenant John Elliott Pillsbury, of the U. S. Navy, the first to carry out an intensive scientific investigation on this portion of the Ocean River, says:

“Man stands with bowed head in the presence of nature’s visible grandeurs, such as towering mountains, precipices, or icebergs, forests of immense trees, grand rivers, or waterfalls. He realizes the force of waves that can sweep away lighthouses or toss an ocean steamer about like a cork. In a vessel floating on the Gulf Stream one sees nothing of the current and knows nothing but what experience tells him; but to be anchored in its depths far out of the sight of land, and to see the mighty torrent rushing past at a speed of three to four miles per hour, day after day and day after day, one begins to think that all the wonders of the earth combined can not equal this one river in the ocean.”

Only in recent times, since the advent of steam, have ocean travelers ceased to be at the mercy of the direction and force of ocean currents; but even today the steady pace of deep waters working on the hull of a steamer can set her back 50 miles in a day’s sailing along the axis of the Gulf Stream. This, of course, has a direct effect both on time and on fuel economy. So it is obvious that, even though we have largely graduated from the age of sail, a thorough knowledge of ocean currents is still of considerable importance for all types of vessels.

In the early days of sailing, especially with square-rigged ships, the importance of currents as an aid or an obstacle to navigation was paramount. Under conditions when wind and stream were both unfavorable, a ship might well be set back in its course for many miles, or, at worst, might be lost upon the rocks of a lee shore. Though this is still true for all types of sailing vessels, it was particularly true of the square-rigged type used in the early Atlantic crossings, because of their limited powers of sailing against the wind. Largely because of this, the existence of great currents, their position, and their strength have shaped the course of early explorations and the pattern of settlement of the Atlantic shores, first of all in the east and later in the west.

The influence of ocean currents on man, working in various and devious ways, has inspired scientists from many lands to inquire into their movements and to speculate as to their cause. But progress in our knowledge of their position, their strength, and their variations has been understandably slow. Man is not naturally a sea-going animal and he has only been able to live for extended periods on the surface of the ocean by the invention and development of artificial aids in the form of simple floats, sailing vessels, .and the elaborate mechanically propelled sea-going hotels of the machine age. It is a relatively simple matter to take measurements and observations upon the rigid platform of land, but to do this on the heaving deck of a small vessel at sea is a very different affair. The problem of measuring and observing the ocean at first hand is neither a simple nor comfortable exercise, even in today’s fast and seaworthy vessels. Hence our knowledge of the Gulf Stream, of the Atlantic Current system of which it is a part, and of the forces setting it in motion, has only in recent years made rapid progress.

The great Ocean River is, in essence, a vast circular swirl of water complicated by the inflow and outflow of counter currents, eddies, tributaries, and branches. North of the equator in latitude 15° N, the north equatorial current under the whip of constant trade winds, runs in a westerly direction towards the American continent, and would thus girdle the globe if it were not turned and channeled by the interposed continents and spun clockwise by the earth’s rotation.

The wind starts westward off the African coast behind which lies the great Sahara, and so the waters begin to move by the Canaries and Cape Verde Islands in a great wide flow. Then the west-driving current as it accumulates in its ocean passage is forced in a northerly direction and in a clockwise motion north of the equator. South of the equator is another westward-flowing current, and this spins south and to the left, for in southern latitudes the earth’s rotation works contrariwise.

We spoke of continental interference. The west-driving south equatorial current only in part turns into the southern gyre. The huge shoulder of Brazil acts as a baffle which catches a goodly portion of it and shunts it north across the equator to join the north equatorial current on the Atlantic side of the Antilles or Windward Islands of the West Indies. Here again part of this great accumulation of warm waters, known as the Antilles Current, passes northward and westward outside the Windward Islands and the Bahamas until, beyond Cape Canaveral on the east Florida coast, it meets the Florida stream.

Another part of the equatorial drift of waters enters the Caribbean Sea and finds its way through the Straits of Yucatan between Mexico and Cuba and then through the Straits of Florida between Florida and the Bahamas. Here we call it the Florida Current. A side branch of this Caribbean stream varying in magnitude enters the Gulf of Mexico from the Straits of Yucatan and returns again near the Florida Keys.

North of the Bahamas Islands these two great streams, the Florida Current and the Antilles Current, reunite to form the Gulf Stream proper, which soon turns eastward towards the European continent. As it approaches Europe, south of Greenland along the edge of the north continental shelf and what is called the Telegraph Plateau, a broad diffuse portion of the Ocean River turns full circle down by France, Spain, the Azores, and back to the African coast, but part bathes the British Isles in a warmth that belies the fact that they stand in the same latitude as Labrador.

Now just north from Scotland, stretching northwest to Greenland, a rise of undersea land—the Wyville-Thompson ridge—comes near enough to the surface to stop the deeper southerly drift of heavy, ice-cold arctic waters but permits about two per cent of the warmer surface waters of the now reduced Atlantic stream to seep over into the Norwegian Sea. This mild current keeps open Norwegian ports in the same latitude as the Greenland ice-cap and likewise makes Iceland a habitable place and even affects the shallow Baltic Sea.

The River of the Ocean is not alone. Satellite rivers and counter-currents, clinging to the skirts of the great circular stream, are thrown off from the outer border of the flowing water, while eddies swirl along the inner boundary, small patterns of the Great Gyre itself. Most important of the satellite rivers is a cold mass of arctic water which flows down, as the Labrador Current, between the River and the eastern shores of the United States until it finally disappears beneath the warmer and lighter waters of the Gulf Stream.

Today’s charts of the great Atlantic River and its currents were not easily plotted. More than 400 years elapsed after the first recorded Atlantic crossing before even the general outlines appeared in print. Before Columbus’s first voyage a few serious speculations had appeared regarding the Atlantic Ocean; but mostly myth had grown up around it, and only an adventurous man would dare advance into the sea of mystery. And yet, as we have seen, the conception of the Atlantic as a river is a very ancient one. The Chaldeans imagined that the Earth floated on eternal waters, and that a river perpetually flowed in a ditch around it. The Egyptians added their own embellishment to this by picturing a boat which supported the sun, floating in the encircling stream.

The Phoenicians ventured as far afield as the British Isles, but like so many others who followed them in the slow charting of the Atlantic River they guarded their knowledge of navigation as a trade secret and left no maps behind them. With few preconceptions to hamper them, the Greeks who followed were able to speculate without restraint upon the nature of the world. Homer considered the earth to be flat and to consist only of the Mediterranean countries he knew. Outside of this he imagined the Ocean River, as the Greeks so aptly named it, to flow ceaselessly around the world, a living moat of danger and destruction.

The notion of a spherical earth came more than a thousand years before the time of Columbus. Ptolemy had conceived this idea long before and by the fifth century A.D. his successors were aware that oceans lay to the west and to the south. The map which he prepared in 150 A.D. showed only the western ocean between Africa and the Orient; the New World and the Pacific Ocean were still undreamed of.

But even this limited knowledge was, for all practical purposes, lost when superstition and ignorance again took hold during the Fifth and Sixth Centuries. And so, the maps of Bishop Isidore of Seville in the seventh century were far cruder even than the one Homer had made nearly 1700 years earlier. Bishop Isidore’s concept was simple in the extreme. He drew a circle for the Earth; cut it into three portions, separated by the Mediterranean Sea and by the two great rivers, the Nile and the Don; and resurrected the Ocean River to form a circular canal connecting the sea and the rivers and embracing the whole.

In the fifteenth century the spirit of exploration blossomed under the encouragement of Prince Henry the Navigator, and Ptolemy’s map was brought out of hiding. And so it came about that, by the time Columbus was ready to set out across the Ocean River, a great part of the coast of western Africa had been added to the charts. Finally, when Bartholomew Diaz rounded the Cape of Good Hope and found this connection between the Indian and Atlantic Oceans, the extent of the eastern shores was established. The stage was now set for the gradual unfolding of the story of the River itself.

The earlier navigators may not have known that there were steady currents well out to sea, but they can hardly have failed to notice the nearby currents where they ran parallel to the shore. During the century before the westward crossing of Columbus, for instance, the Portuguese had with great perseverance developed a trade route to Guinea along the West African coast; and they therefore knew by hard experience of the Guinea Current, which sets to the south and southeast around Cape Nun on the bulge of North Africa, in places as strongly as knots. Combined with the north and northeasterly winds, this was a great handicap even to the Portuguese caravels, which, with their fore-and-aft rigs, were much better equipped to sail against the wind than Columbus’ hermaphrodite-rigged craft.

Probably the first signs of far ranging currents in the open ocean were strange objects cast ashore—seeds, branches of trees, and pieces of wood—brought from the western world by the Ocean River to the shores of northern Europe, just as today we find upon the Atlantic beaches of North America the glass floats of Portuguese fishing nets, brought there by the Canaries Current, the equatorial drift, and the Florida Current.

The presence of these objects from the western world was certainly known to Columbus. The sea-bean—about the size and shape of a horse-chestnut—the seed of a West Indian plant, Entada gigas, is often thrown upon European shores, together with bamboo stems and even an occasional coconut. Particularly large quantities are found on the shores of the Faroe Islands, between Scotland and Iceland. Whether this exotic sea drift influenced the westward migrations of the Norsemen we do not know, but it is certain from their accounts that they met with currents off the coast of North America; names given by them to geographic features of their discoveries make this clear. Among them are Straumsfjord or Bay of Currents, Straummes, Cape of Currents, and Straumsoe, Island of Currents. Unfortunately their writings leave us no way of identifying the exact localities described.

The first well defined observation of the west-going limb of the Ocean River was probably made by Columbus during his first voyage when on September 19, 1492, he became aware of a westerly drift—the Canaries Current. The voyage might easily have resulted in disaster and the entire history of the Atlantic communities might have been changed had it not been for the benign flow of the River. The death of all hands from lack of food and water due to an overextended voyage was prevented by the fact that the Canaries Current and the equatorial current added at times as much as forty miles a day to his passage.

During the later voyages of Columbus he discovered the Atlantic Stream where it flows into the Caribbean Sea between the Windward Islands and again where it leaves this sea through the Yucatan Channel. He was greatly impressed by the Windward Island currents and considered them sufficiently powerful to have formed the islands by washing away the land between them: “ . . . I hold it for certain that the waters of the sea move from East to West with the sky and that in passing this track they hold a more rapid course and have thus carried away large tracts of land and that from here has resulted the great number of islands.”

The current flowing along the Honduras coast was an especially difficult one to navigate and, according to Peter Martyr, he found “ . . . the course of the waters so vehement and furious against the fore part of his ship that he could at no time touch the ground with his sounding plummet, but that the contrary violence of the waters would bear it up from the bottom. He affirmeth also that he could never in one day with a good wynde wynn one mile of the course of the waters.”

While Columbus was discovering the Caribbean currents, John and Sebastian Cabot crossed the Atlantic in 1497. They rediscovered the coast of Labrador and then sailed southwest until they reached land somewhere in the vicinity of Delaware. It was here that they noticed the counter- current running southward between the Gulf Stream and the coast, but even so they were apparently unaware of the Stream itself.

If the Corte-Reals noticed the Gulf Stream in their voyages from Labrador to Cuba between 1500 and 1502, they too failed to leave any account. It was characteristic of the early days of Atlantic exploration that navigators kept their observations to themselves, as the Phoenicians had before them, and rarely passed them on except by word of mouth, so that they did little to help the orderly development of permanent geographical knowledge.

For example, Sebastian de Ocampo circumnavigated Cuba in 1509, taking fully eight months to complete the voyage. It is hard to believe that he did not notice the Stream, because he was almost certainly forced to sail against it when he rounded the western point of San Antonio. Even so he makes no mention of it.

Once more we turn to Peter Martyr, who gave so much thought and scholarly speculation to the problem of Ocean currents. He realized that the stream of water entering the Caribbean must either pass still further west, through an opening in the mainland into what we now know as the Pacific, or else be diverted to the north. If neither took place then he reasoned that the water must accumulate continuously in the western part of the Atlantic. He mentions a more fantastic possibility, that the westward current might be piled high against the western shores or even absorbed into the depths of the earth, but dismisses it with the remark that such a place had not yet been found. Later he also described an argument between Andreas Moralis, the pilot, and Ouidas, who took opposing sides on the question of a westward passage to the Pacific or a northeastern flow of water back into mid-Atlantic where the currents were supposed to be completely absorbed:

“As we met thus together there arose a contention between them two as regarding this course of the ocean. They both agree that these landes and regions pertayning to the Dominion of Castile, do with one continuale tract and perpetual bond embrace as one whole firme lands or continent all the mayne lands lying to the north of Cuba and Hispaniola. Yet as touching the course of the water be received into the lappe of the supposed continent, which bendeth so much and extendeth so farre toward the north, and that by the object or resistence of the lande so bending and crooking the water as it were, rebounde in compasse and by the force thereof be driven about the north side of Cuba and the other islands excluded outside the circle called Tropicus Cancri, where the largeness of the sea may receive the waters falling from the narrow streams and thereby represse that inordinate course by reason that the sea is there very large and great.” Peter Martyr obviously had not heard of a discovery made by Ponce de Leon, or rather by his pilot, Antonio de Alaminos. Although Ponce de Leon is better known for the fruitless quest of his 1513 expedition, which set out to find the Fountain of Youth, he has greater claim to fame for leaving one of the first definite records of the Florida Current. He sailed from Puerto Rico along the north eastern side of the Bahamas, and somewhat to the north of Cape Canaveral he crossed westward into the stream. When he turned southwards along the Florida coast he had a good and favorable wind, but he was powerless to stem the powerful northerly flow. He tells us that when two of the vessels came to anchor one day, the third, in water too deep to do likewise, was rapidly carried out of sight to the north by the fierce pull of the waters.

The Gulf of Mexico was discovered in 1517 and explored the following year, and this finally disproved the theory of a break in the North American continent which would allow the current a westward outlet. Thus it became inevitable that the great flood of water passing north from Cuba should sooner or later be put to use by the Spanish vessels, which were plying in increasing numbers between the Old World and the New. Antonio de Alaminos, pilot first with Columbus and later with Ponce de Leon, was well acquainted at first hand with the Florida Current. He quickly took advantage of it. When sent by Cortez from Mexico to deliver dispatches and presents to Spain, he chose to make his way home by way of the Florida Straits so as to have the benefit of the Stream until he turned eastward towards Spain. From this time onward all sea traffic returning to Spain took advantage of the current.

In the wake of the great flood of trading voyages and exploring expeditions made during the next fifty years, knowledge of the directions and whereabouts of ocean currents began to pile up in the professional gossip of sailors but nothing further was published; though there were many detailed accounts of the newly discovered lands, people, and vegetation. Because of the value of cargoes carried by the Spanish vessels, their routes and general navigational knowledge were still held secret for security reasons. The sixteenth century had almost come to a close before Sir Humphrey Gilbert, Martin Frobisher, and others published their own observations and confirmed the earlier records of currents in the western ocean.

Now that the idea of a westward passage through the Gulf of Mexico to the rich Orient had finally been disproved, the twin stimuli of greed and curiosity drove merchants and governments to seek a northwest passage from the Atlantic to the Pacific; and the contemporary accounts of voyages reflected this new interest among the more adventurous sailormen. Gilbert, who argued that the Gulf Stream must find an outlet either to the northeast or to the northwest, wrote that the current “ . . . runs all along the eastern coast of that continent as far as Cape Freddo, being the farthest known place of the same continent toward the north ... it must either flow around the north of America into the South Sea or it must needs strike over upon the coasts of Iceland, Norway, and Finmark.” His desire to prove the existence of the Northwest Passage prejudiced him in favor of the first alternative. Even so he was careful enough observer to note that, in 50° N latitude a current carried ice to the southward—the Labrador Current.

Frobisher discovered the northeasterly pull of the Norwegian Current and also saw clearly that there had to be some kind of return path for the currents, a kind of circulating ocean river, but his little knowledge led him to a false conclusion when he wrote:

“Sayling toward the northwest parts of Ireland we mette with a great current from out of the southwest, which carried us (by our reckoning) one point toward the northeastward of our said course, which current seemed to us to continue itself toward Norway and other of the northeast parts of the world, whereby we may be induced to believe that this is the same which the Portuguese mette at Capo de Buong Speranza, where, stricking over from thence to the Straits of Magellan and finding no passage there for the narrowness of the sayde Straits, runneth alongue to the great Bay of Mexico, where also haveing a lot of land it is forced to strike back again toward the northeast, as we not only have but in another place also further northward by goode experience this year have found.”

Though he saw the logical necessity of linking currents into a continuous system, he was unaware that the main Atlantic stream runs southward when it approaches Europe and that the Norway Current is only a small branch. And so he produced the fanciful theory of a current which rounded the north of Norway and thence by some unexplained route reached the eastern coast of Africa. By what strange paths it could have reached the Capo de Buong Speranza, or Cape of Good Hope, and so back into the Atlantic, he does not say.

As new communities of the Ocean River began to appear in the west, more and more pieces of the great mosaic of drift and current fell into place. John White, once governor of the colony of Roanoke, repeating Cabot’s observation, wrote in 1590 of the satellite swirl which runs to the south and southwest between the Gulf Stream and the shore and which has been particularly puzzling to modern oceanographers. He discovered this counter current during a voyage from Florida to Virginia, when he found it necessary to stand well out to sea in order to stay within the Gulf Stream and avoid the contrary currents.

Shortly afterwards Lescarbot rediscovered the south-flowing Labrador Current. The Cabot brothers had noticed this icy stream a century earlier, but a strange result of its head-on collision with the Gulf Stream was not described until Lescarbot wrote in 1612: “I have found something remarkable upon which a natural philosopher should meditate ... for space of three days the water very warm, whilst the air was cold as before, but on the 21st of June quite suddenly we were surrounded by fogs and cold that we thought to be in the month of January and the sea was extremely cold.” The sudden, sharp boundary between the cold Labrador Current flowing to the southwest and the warm Gulf Stream flowing to the northeast is a most striking phenomenon to all who have witnessed it.

Most of the larger drifts and currents which go to make up the Atlantic swirl were now discovered; but they had not been linked together and no chart had yet been printed to show them. Even so, they were of increasing importance in determining the pattern of the colonization in North America during the early seventeenth century. The northern colonies—what is now New England—were reached by a northern route, for the experienced sailor, in order to avoid the main eastward flow of the River, sailed westward at a latitude of about 40°. The southern colonies, from North Carolina and Chesapeake Bay to the colony of New York, were approached by a southern route by way of the West Indies, to take full advantage of the equatorial drift and the trade winds. In this way the Ocean River continued to influence the practice of navigation to the extent of justifying a course to New York, which was nearly 1800 miles south of the route taken to the northern colonies only 100 miles away.

Towards the end of the seventeenth century the growing spirit of scientific inquiry began to be felt, and so a number of theoretical treatises on ocean currents began to appear. One of these, written by Isaac Vossius in 1663, is worth our mention if only because he showed the true circulatory nature of the Atlantic River and so drew together the separate and unorganized observations of the 150 years which had gone by since Ponce de Leon discovered the Florida Current and of 650 years since Leif Ericson sighted North America. He describes it with admirable simplicity:

“With the general equatorial current, the waters run toward Brazil, along Guyana, and enter the Gulf of Mexico. From there, turning obliquely, they pass rapidly through the Straits of Bahama. On the one side they bathe the coasts of Florida and Virginia and the entire shore of North America, and on the other side they run directly east until they reach the opposite shores of Europe and Africa; from thence they run again to the south and join the first movement to the west, perpetually turning in this manner circuitously.”

About this time, too, Athanasius Kircher and Happelius published the first known charts of the Gulf Stream. They were not simple factual records of the known ocean currents, but included a number of greatly exaggerated and half legendary features such as the Maelstrom, the great whirlpool off the Lofoten Islands. Nor were they designed for the practical interests of navigators but rather to illustrate real or imaginary scientific questions of the day. But the time was not too distant when the Atlantic currents were to be mapped in careful fashion for the express use of shipmasters, with more attention to fact then to fable.

Most of the ships sailing coastwise along the Atlantic seaboard of North America at this time were delayed as much as three weeks because of their ignorance of the Gulf Stream limits and by not knowing of the counter-current closer to the shore. Many vessels voyaging between England and New York were likewise set back in their westward course. But gradually the regular coastwise captains were pooling their experiences and evolving a practical system of pilotage. The tremendous expansion of the whaling industry about this time also brought into the Western Atlantic a new kind of sailor whose voyages covered a wide area of the ocean and whose observations and experience added greatly to what was already known of the ocean currents. These whaling captains, who were in many ways the founders of the art of modern navigation, traveled in their search for whales from the Bahamas to the Arctic Sea and from the Carolinas to the Azores and their sealore quickly spread among the American trans-Atlantic shipmasters. As a result the merchant captains were able to plan a sailing route from Europe which avoided the easterly stream as far as possible and thus saved themselves as much as two weeks of fighting wind and stream in their westerly passage.

The reasons for selecting this westerly route were apparently unknown to the skippers of the English mail packets, who continued to use the more direct but much slower route. Complaints were made by the Boston Board of Customs to the Lords of the Treasury in London and this was brought to the attention of the Benjamin Franklin in 1769, at a time when he was Postmaster General for the colonies. He immediately made inquiries in order to discover the cause of the slowness of the mail packets and discussed the problem with a Nantucket whaler captain named Timothy Folger, who, like the majority of his fellows, was well acquainted with the currents of the Atlantic River. With Folger’s advice he prepared a chart of the Gulf Stream. This was the first current chart of the North Atlantic designed to aid navigation rather than to support or confound scientific speculation. It is traditionally true that sailormen are conservative and slow to adopt new ideas and the Falmouth sea captains characteristically refused to use Franklin’s chart when it was published in 1770. Franklin wrote:

“There happened then to be in London a Nantucket sea captain of my acquaintance, to whom I communicated the affair. He told me he believed the fact to be true, but the difference was owing to this, that the Rhode Island captains were acquainted with the Gulf Stream, while those of the English packets were not. We are well acquainted with that stream, because in our pursuit of whales, which keep near the sides of it but are not met within it, we run along the side and frequently cross it to change our side; and in crossing it have sometimes met and spoke with those packets who were in the middle of it and stemming it. We have informed them that they were stemming a current that was against them to the value of three miles an hour and advised them to cross it, but they were too wise to be coun- celled by simple American fishermen. When the winds are light,” he added, “they are carried back by the current more than they are forwarded by the wind, and if the wind be good the subtraction of seventy miles a day from their course is of some importance.”

Benjamin Franklin marked a turning point in the mapping of salt waters, the plotting of the Ocean River, when he published his chart for the General Post Office. A new impetus was beginning to be felt; and as time went on science was to go about the enquiry in its own logical and methodical way and to seek not only facts but causes and effects. Yet before we view the north Atlantic Ocean through the eyes of scientific inquiry, we can better understand the reasons for this impetus if we turn for a moment to the growing art of navigation. In coming of age, this art took on a greater accuracy and helped to speed up the ocean mapmaking and to usher in the infant science of oceanography.

Even today most of the charted facts about the Ocean River are compiled from entries in ships’ logs that show how far vessels are carried from their courses by the force of water currents. When the shipmaster knows the direction of his course and the speed of his ships he is able to calculate the apparent position at sea at any time, assuming that no currents are acting. This is his “dead reckoning.” If he now finds that his true position is not the same as the dead reckoning, then the difference must be due to the flow of current. Thus, to be able to chart ocean currents we must first be able to measure the speed of a ship and then to find with accuracy its true position.

When Columbus sailed beyond known soundings the crudest kind of dead reckoning prevailed, and men set their course “by God and by guess.” The first rude beginnings of measuring speed at sea were by watching the time by sandglass of a chip of wood released at the bow of a vessel as it drifted astern.

There were other means of detecting and measuring the pulse of the ocean currents which early explorers could well have used. During his first voyage Columbus, by accident rather than design, made use of a simple but effective way of measuring the surface stream. On September 19, 1492, while becalmed in the southwesterly current, he sounded with the deep sea leadline. It is not surprising to us that it recorded no bottom at 200 fathoms, since on modern charts the sounding nearest to his probable position is 2290 fathoms, or over 12,000 feet deeper than the length of his line.

Nevertheless, at 200 fathoms the water in this locality is relatively motionless. As a result the weighted end of the line entering the still water below was held back by friction drag while the ship itself, being becalmed, drifted with the surface current. Consequently the leadline was pulled a considerable distance from the vertical. If Columbus had had sufficient knowledge of physics, lie could have calculated from this the actual rate of flow at the surface in relation to the deeper levels. All he actually did was note the presence of a current carrying him to the southwest.

Two important instruments, the modern sextant and the chronometer, which came into use in the latter part of the eighteenth century, provided navigators and the oceanographers who came later with tools of greater accuracy for their difficult task of measuring the Atlantic current. The ship’s chronometer, which became generally available about 1775, was invented by John Harrison and later developed by Thomas Earnshaw. Before this it had been impossible to determine longitude with accuracy when out of sight of land, because this depends upon a very accurate measurement of time as well as upon nautical astronomy.

The clock used for timing the stars, the sun, and the planets could not be an ordinary clock. The earth revolves once every twenty- four hours, and the circumference is about 21,600 nautical miles, so that at the equator a mistake of one minute in time is equal to an error of as much as 15 miles. As far as land clocks were concerned the problem was solved by Galileo when he discovered the simple truth that a pendulum of a given length always takes the same time to complete its swing. Using this principle, Christian Huyghens in 1656 constructed the first reasonably accurate clock. But the pendulum principle, like most simple truths, is a good deal more complicated than it first appeared and it was soon discovered that the time of swing, and therefore the speed of the clock, was different at different parts of the earth and also changed with changing temperature.

Moreover, the rolling and pitching of a ship interferes with the movement of a pendulum so that this type of clock was unsuitable for a marine chronometer.

The matter was brought to a head at the beginning of the eighteenth century when an admiral and his fleet were lost by a gross error in navigation. Sir Cloudesley Shovel, returning from Gibraltar to England, ran ashore on the rocky Scilly Isles, at a time when his navigators thought they were at least a hundred miles farther east in longitude. The wreck resulted in the loss of four ships and over 2,000 seamen besides that of the admiral.

John Harrison eventually produced the first marine chronometer in 1736. It was far from pretty and weighed seventy-two pounds, but it did keep good time at sea. After working for over thirty years he finally produced his fifth clock and this well justified his long labors for it had the surprisingly small error of less than five seconds in a period of ten weeks.

The development of chronometers and of more accurate instruments for measuring altitude, such as the modern sextant, greatly aided navigators in determining exact positions and so helped in the accurate plotting of the direction and strength of currents. With the aid of these the charting of the Ocean River became more of a science and less an affair of general observation, imagination, and intuition.